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Environmental, energetic, and economic comparisons of organic and conventional farming systems

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Abstract

Various organic technologies have been utilized for about 6000 years to make agriculture sustainable while conserving soil, water, energy, and biological resources. Among the benefits of organic technologies are higher soil organic matter and nitrogen, lower fossil energy inputs, yields similar to those of conventional systems, and conservation of soil moisture and water resources (especially advantageous under drought conditions). Conventional agriculture can be made more sustainable and ecologically sound by adopting some traditional organic farming technologies.
Environmental, Energetic, and Economic Comparisons
of Organic and Conventional Farming Systems
1. David Pimentel 1
" ,
2. Paul Hepperly 2
" ,
3. James Hanson 3
" ,
4. David Douds 4
" and
5. Rita Seidel 2
1 David Pimentel (e-mail: dp18@cornell.edu) works in the Department of Entomology, College of
Agriculture and Life Sciences, at Cornell University, Ithaca, NY 14853.
2 Paul Hepperly and Rita Seidel are with the Rodale Institute, 611 Siegfriedale Road, Kutztown, PA
19530.
3 James Hanson works in the Department of Agricultural and Resource Economics at the University
of Maryland, College Park, MD 20742.
4 David Douds is with the USDA Agricultural Research Service, Eastern Regional Research Center,
600 E. Mermaid Lane, Wyndmoor, PA 19038.
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Abstract
Various organic technologies have been utilized for about 6000 years to make agriculture sustainable while
conserving soil, water, energy, and biological resources. Among the benefits of organic technologies are higher
soil organic matter and nitrogen, lower fossil energy inputs, yields similar to those of conventional systems,
and conservation of soil moisture and water resources (especially advantageous under drought conditions).
Conventional agriculture can be made more sustainable and ecologically sound by adopting some traditional
organic farming technologies.
Key words
organic
agriculture
conventional
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Heavy agricultural reliance on synthetic chemical fertilizers and pesticides is having serious impacts on
public health and the environment (Pimentel et al. 2005). For example, more than 90% of US corn farmers rely
on herbicides for weed control (Pimentel et al. 1993), and one of the most widely used of those herbicides,
atrazine, is also one of the most commonly found pesticides in streams and ground-water (USGS 2001). The
estimated environmental and health care costs of pesticide use at recommended levels in the United States run
about $12 billion every year (Pimentel 2005).
Other aspects of conventional agriculture also have adverse effects on environmental and human health, as
well as a high price tag. Nutrients from fertilizer and animal manure have been associated with the
deterioration of some large fisheries in North America (Frankenberger and Turco 2003), and runoff of soil and
nitrogen fertilizer from agricultural production in the Corn Belt has contributed to the “dead zone” in the Gulf
of Mexico. The National Research Council (BANR/NRC 2003) reports that the cost of excessive fertilizer use
—that is, fertilizer inputs that exceed the amount crops can use—is $2.5 billion per year. Modern agricultural
practices can also contribute to the erosion of soil. The estimated annual costs of public and environmental
health losses related to soil erosion exceed $45 billion (Pimentel et. al. 1995).
Integrated pest and nutrient management systems and certified organic agriculture can reduce reliance on agro-
chemical inputs as well as make agriculture environmentally and economically sound. Pimentel and Pimentel
(1996) and the National Research Council (BANR/NRC 2003) have demonstrated that sound management
practices can reduce pesticide inputs while maintaining high crop yields and improving farm economics. Some
government programs in Sweden, Canada, and Indonesia have demonstrated that pesticide use can be reduced
by 50% to 65% without sacrificing high crop yields and quality (BANR/NRC 2003).
The aim of organic agriculture is to augment ecological processes that foster plant nutrition yet conserve soil
and water resources. Organic systems eliminate agrochemicals and reduce other external inputs to improve the
environment and farm economics. The National Organic Program (a program of the USDA Agricultural
Marketing Service; 7 CFR pt. 205 [2002]) codifies organic production methods that are based on certified
practices verified by independent third-party reviewers. These systems give consumers assurance of how their
food is produced and enable consumers to choose foods on the basis of the methods by which they were
produced. The National Organic Standards Program prohibits the use of synthetic chemicals, genetically
modified organisms, and sewage sludge in organically certified production.
Organic agriculture is a fast-growing agricultural section in the United States. Dimitri and Greene
(2002) report a doubling of area in organic production from 1992 to 1997, currently more than 500,000
hectares (ha). Organic food sales total more than $7 billion per year and are growing at double-digit rates
(Greene 2000, 2004, ERS 2003). With continuing consumer concerns about the environment and the chemicals
used in food production, and with the growing availability of certified organic production, the outlook for
continuing growth of organic production is bright (Dimitri and Greene 2002).
Since 1981, the Rodale Institute Farming Systems Trial (FST) has compared organic and conventional grain-
based farming systems. The results presented here represent a 22-year study of these farming systems, based
on environmental impacts, economic feasibility, energetic efficiency, soil quality, and other performance
criteria. The information from this trial can be a tool for developing agricultural policies more in tune with the
environment while increasing energy efficiency and economic returns.
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The Rodale Institute Farming Systems Trial
From 1981 through 2002, field investigations were conducted at the Rodale Institute FST in Kutztown,
Pennsylvania, on 6.1 ha. The soil at the study site is a moderately well-drained Comly silt loam. The growing
climate is subhumid temperate (average temperature is 12.4 degrees Celsius and average rainfall is 1105
millimeters [mm] per year).
The experimental design included three cropping systems (main plots). These systems, detailed below,
included (a) conventional, (b) animal manure and legume-based organic (hereafter organic animal-based), and
(c) legume-based organic systems. The main plots were 18 × 92 meters (m), and these were split into three 6 ×
92 m subplots, which allowed for same-crop comparisons in any one year. The main plots were separated with
a 1.5-m grass strip to minimize cross movement of soil, fertilizers, and pesticides. The subplots were large
enough that farm-scale equipment could be used for operations and harvesting. Each of the three cropping
systems was replicated eight times.
Conventional cropping
The conventional cropping system, based on synthetic fertilizer and herbicide use, represented a typical cash-
grain, row-crop farming unit and used a simple 5-year crop rotation (corn, corn, soybeans, corn, soybeans) that
reflects commercial conventional operations in the region and throughout the Midwest (more than 40 million
ha are in this production system in North America; USDA 2003). Fertilizer and pesticide applications for corn
and soybeans followed Pennsylvania State University Cooperative Extension recommendations. Crop residues
were left on the surface of the land to conserve soil and water resources. Thus, during the growing season, the
conventional system had no more exposed soil than in either the organic animal-based or the organic legume-
based systems. However, it did not have cover crops during the nongrowing season.
Organic animal-based cropping
This system represented a typical livestock operation in which grain crops were grown for animal feed, not
cash sale. This rotation was more complex than the rotation used in the conventional system. The grain-
rotation system included corn, soybeans, corn silage, wheat, and red clover–alfalfa hay, as well as a rye cover
crop before corn silage and soybeans.
Aged cattle manure served as the nitrogen source and was applied at a rate of 5.6 metric tons (t) per ha (dry), 2
years out of every 5, immediately before plowing the soil for corn. Additional nitrogen was supplied by the
plow-down of legume–hay crops. The total nitrogen applied per ha with the combined sources was about 40
kilograms (kg) per year (or 198 kg per ha for any given year with a corn crop). The system did not use
herbicides for weed control; it relied instead on mechanical cultivation, weed-suppressing crop rotations, and
relay cropping, in which one crop acted as a living mulch for another.
Organic legume-based cropping
This system represented a cash grain operation, without livestock. Like the conventional system, it produced a
cash grain crop every year; however, it used no commercial synthetic fertilizers, relying instead on nitrogen-
fixing green manure crops as the nitrogen source. The final rotation system included hairy vetch (winter cover
crop used as a green manure), corn, rye (winter cover crop), soybeans, and winter wheat. The hairy vetch
winter cover crop was incorporated before corn planting as a green manure. The initial 5-year crop rotation in
the legume-based system was modified twice to improve the rotation. The total nitrogen added to this system
per ha per year averaged 49 kg (or 140 kg per ha for any given year with a corn crop). Both organic systems
(animal based and legume based) included a small grain, such as wheat, grown alone or interseeded with a
legume. Weed control practices were similar in both organic systems, neither of which used herbicides for
weed control.
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Measurements recorded in the experimental treatments
Cover crop biomass, crop biomass, weed biomass, grain yields, nitrate leaching, herbicide leaching, percolated
water volumes, soil carbon, soil nitrogen, and soil water content were measured in all systems. In addition,
seasonal total rainfall, energy inputs and returns, and economic inputs and returns were determined.
A lysimeter, a steel cylinder 76 centimeters (cm) long by 76 cm in diameter, was installed in four of the eight
replications in each cropping system in fall 1990 to enable the collection of percolated water. The top of each
lysimeter was approximately 36 cm below the soil surface to allow normal field operations to be carried out
directly over the lysimeters. Water could not escape from the lysimeter system, and leachate samples were
collected throughout the year.
Levels of nitrogen as nitrate in leachate samples were determined by the Soil and Plant Nutrient Laboratory at
Michigan State University in East Lansing. Herbicides in leachate samples were analyzed by M. J. Reider
Associates, Reading, Pennsylvania. Total soil carbon and nitrogen were determined by the Agricultural
Analytical Services Laboratory at Pennsylvania State University in University Park. Soil water content was
determined gravimetrically on sieved soil (particles 2 mm in diameter). Statistical analyses were carried out
using SPSS version 10.1.3 General Linear Model Univariate Analysis of Variance.
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Results
We examined the data from the 22-year experiments carried out at the Rodale Institute, which compared the
organic animal-based, organic legume-based, and conventional systems. The following data were considered
for all three systems: crop yields for corn and soybeans, impacts of drought on crop yields, fossil energy
requirements, economic costs and benefits, soil carbon (organic matter) changes over time, and nitrogen
accumulation and nitrate leaching.
Crop yields under normal rainfall
For the first 5 years of the experiment (1981–1985), corn grain yields averaged 4222, 4743, and 5903 kg per ha
for the organic animal, organic legume, and conventional systems, respectively, with the yields for the
conventional system being significantly higher than for the two organic systems. After this transition period,
corn grain yields were similar for all systems: 6431, 6368, and 6553 kg per ha for the organic animal, organic
legume, and conventional systems, respectively (Pimentel et al. 2005). Overall, soybean yields from 1981
through 2001 were 2461, 2235, and 2546 kg per ha for the organic animal, organic legume, and conventional
systems, respectively (Pimentel et al. 2005). The lower yield for the organic legume system is attributable to
the failure of the soybean crop in 1988, when climate conditions were too dry to support relay intercropping of
barley and soybeans. If 1988 is taken out of the analysis, soybean yields are similar for all systems.
Crop yields under drought conditions
The 10-year period from 1988 to 1998 had 5 years in which the total rainfall from April to August was less
than 350 mm (compared with 500 mm in average years). Average corn yields in those 5 dry years were
significantly higher (28% to 34%) in the two organic systems: 6938 and 7235 kg per ha in the organic animal
and the organic legume systems, respectively, compared with 5333 kg per ha in the conventional system.
During the dry years, the two organic systems were not statistically different from each other in terms of corn
yields.
During the extreme drought of 1999 (total rainfall between April and August was only 224 mm compared with
the normal average of 500 mm), the organic animal system had significantly higher corn yields (1511 kg per
ha) than either the organic legume (421 kg per ha) or the conventional system (1100 kg per ha). Crop yields in
the organic legume system were much lower in 1999 because the high biomass of the hairy vetch winter cover
crop used up a large amount of the soil water (Lotter et al. 2003).
Soybean yields responded differently than the corn during the 1999 drought. Specifically, soybean yields were
about 1800, 1400, and 900 kg per ha for the organic legume, the organic animal, and the conventional systems,
respectively. These treatments were significantly different (p = 0.05) from each other (Pimentel et al. 2005).
Over a 12-year period, water volumes percolating through each system (collected in lysimeters) were 15% and
20% higher in the organic legume and organic animal systems, respectively, than in the conventional system.
This indicated an increased groundwater recharge and reduced runoff in the organic systems compared with the
conventional system. During the growing seasons of 1995, 1996, 1998, and 1999, soil water content was
measured for the organic legume and conventional systems. The measurements showed significantly more
water in the soil farmed using the organic legume system than in the conventional system (Pimentel et al.
2005). This accounted for the higher soybean yields in the organic legume system in 1999 (Pimentel et al.
2005).
Energy inputs
The energy inputs in the organic animal, organic legume, and conventional corn production systems were
assessed. The inputs included fossil fuels for farm machinery, fertilizers, seeds, and herbicides. About 5.2
million kilocalories (kcal) of energy per ha were invested in the production of corn in the conventional system.
The energy inputs for the organic animal and organic legume systems were 28% and 32% less than those of the
conventional system, respectively (figure 1). Commercial fertilizers for the conventional system were
produced employing fossil energy, whereas the nitrogen nutrients for the organic systems were obtained from
legumes or cattle manure, or both. The intensive reliance on fossil fuel energy in the conventional corn
production system is why that system requires more overall energy inputs than do organic production systems.
Fossil energy inputs were also required to transport and apply the manure to the field.
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Figure 1. Average energy inputs for corn and soybeans (in millions of kilocalories per hectare per year) in the
three systems used in the Rodale Institute Farming Systems Trial from 1991 to 2000: organic animal-based
cropping, organic legume-based cropping, and conventional cropping.
The energy inputs for soybean production in the organic animal, organic legume, and conventional systems
were similar: 2.3 million kcal, 2.3 million kcal, and 2.1 million kcal per ha, respectively (figure 1).
Economics
Two economic studies of the FST were completed, evaluating its first 9 years (Hanson et al. 1990) and first 15
years of operation (Hanson et al. 1997). These two studies captured the experiences of organic farmers as they
develop over time a rotation that best fits their farm. With the development of the final rotation, a third
evaluation was completed comparing this rotation with its conventional alternative (Hanson and Musser 2003).
Many organic grain farmers in the mid-Atlantic region have been adopting this “Rodale rotation” on their
farms, and there was strong interest in an economic evaluation of this rotation alone (i.e., without the transition
period or learning curve).
The third economic comparison of the organic corn–soybean rotation and conventional corn–soybean systems
covered the period 1991 to 2001(figure 2). Without price premiums for the organic rotation, the net returns for
both rotations were similar. The annual net return for the conventional system averaged about $184 per ha,
while the organic legume system for cash grain production averaged $176 per ha. The annual costs per ha for
the conventional versus organic rotations, respectively, were (a) seed, $73 versus $103; (b) fertilizers and lime,
$79 versus $18; (c) pesticides, $76 versus $0; (d) machinery costs, $117 versus $154; and (e) hired labor, $9
versus $6. Similar revenue comparisons are $538 per ha and $457 per ha (conventional versus organic). The
net returns for the conventional rotation were more variable (i.e., risky). The standard deviation for net returns
over the 10-year period was $127 for the conventional rotation and $109 for the organic rotation.
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Figure 2. Average net returns per hectare (ha) for a 400-ha farm for the organic legume and conventional grain
rotations in the Rodale Institute Farming Systems Trial from 1991 to 2001. NR I = revenue minus explicit
costs; NR II = NR I minus transitional costs; NR III = NR II minus all labor costs. Source: Hanson and Musser
(2003).
When the costs of the biological transition for the organic rotation (1982–1984) were included, the net returns
for the organic rotation were reduced to $162 per ha, while the conventional net returns remained unchanged.
Including the costs of family labor for both rotations reduced the net returns of conventional farming to $162
and organic farming to $127. However, even with the inclusion of the biological transition and family labor
costs, the organic price premium required to equalize the organic and conventional returns was only 10%
above the conventional product. Throughout the 1990s, the organic price premium for grains has exceeded this
level, and premiums now range between 65% and 140% (New Farm Organization 2003).
The organic system requires 35% more labor, but because it is spread out over the growing season, the hired
labor costs per ha are about equal between the two systems. Each system was allowed 250 hours of “free”
family labor per month. When labor requirements exceeded this level, labor was hired at $13 per hour. With
the organic system, the farmer was busy throughout the summer with the wheat crop, hairy vetch cover crop,
and mechanical weed control (but worked less than 250 hours per month). In contrast, the conventional farmer
had large labor requirements in the spring and fall, planting and harvesting, but little in the summer months.
This may have implications for the growing number of part-time farmers for whom the availability of family
farm labor is severely limited. Other organic systems have been shown to require more labor per hectare than
conventional crop production. On average, organic systems require about 15% more labor (Sorby
2002, Granatstein 2003), but the increase in labor input may range from 7% (Brumfield et al. 2000) to a high
of 75% (Karlen et al. 1995, Nguyen and Haynes 1995).
Over the 10-year period, organic corn (without price premiums) was 25% more profitable than conventional
corn ($221 per ha versus $178 per ha). This was possible because organic corn yields were only 3% less than
conventional yields (5843 kg per ha versus 6011 kg per ha), while costs were 15% less ($351 per ha versus
$412 per ha). However, the organic grain rotation required a legume cover crop before the corn. This was
established after the wheat harvest. Thus, corn was grown 60% of the time in the conventional rotation, but
only 33% of the time in the organic rotation. Stated in another way, the yields per ha between organic and
conventional corn for grain may be similar within a given year; however, overall production of organic corn is
diminished over a multiple-year period because it is grown less frequently. On the other hand, the reduced
amount of corn grown in the organic rotation is partly compensated for with the additional crop of wheat.
Soil carbon
Soil carbon, which correlates with soil organic matter levels, was measured in 1981 and 2002 (figure 3). In
1981, soil carbon levels were not different (p = 0.05) between the three systems. In 2002, however, soil carbon
levels in the organic animal and organic legume systems were significantly higher than in the conventional
system: 2.5% and 2.4% versus 2.0%, respectively (figure 3). The annual net aboveground carbon input (based
on plant biomass and manure) was the same in the organic legume system and the conventional system (about
9000 kg per ha) but close to 12% higher in the organic animal system (about 10,000 kg per ha). However, the
two organic systems retained more of that carbon in the soil, resulting in an annual soil carbon increase of 981
and 574 kg per ha in the organic animal and organic legume systems, compared with only 293 kg per ha in the
conventional system (calculated on the basis of about 4 million kg per ha of soil in the top 30 cm). The
increased carbon was also associated with higher water content of the soils in these systems compared with the
conventional system. The higher soil water content in the organic systems accounted for the higher corn and
soybean yields in the drought years in these systems compared with the conventional system (Lotter et al.
2003).
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Figure 3. Percentage of soil carbon (above) and nitrogen (below) for the three systems of the Rodale Institute
Farming Systems Trial in 1981 and 2002 (organic animal-based cropping, organic legume-based cropping, and
conventional cropping). Different letters indicate statistically significant differences according to Duncan's
multiple range test; p < 0.05. NSD = not significantly different.
Soil nitrogen
Soil nitrogen levels were measured in 1981 and 2002 in the organic animal, organic legume, and conventional
systems (figure 3). Initially the three systems had similar percentages of soil nitrogen, or approximately 0.31%.
By 2002, the conventional system remained unchanged at 0.31%, while nitrogen levels in the organic animal
and organic legume systems significantly increased to 0.35% and 0.33%, respectively. Harris and colleagues
(1994) used 15N (nitrogen-15) to demonstrate that 47%, 38%, and 17% of the nitrogen from the organic animal,
organic legume, and conventional systems, respectively, was retained in the soil a year after application.
Nitrate leaching
Overall, the concentrations of nitrogen as nitrate in leachates from the farming systems varied between 0 and
28 parts per million (ppm) throughout the year (Pimentel et al. 2005). Leachate concentrations were usually
highest in June and July, shortly after applying fertilizer in the conventional systems or plowing down the
animal manure and legume cover crop. In all systems, increased soil microbial activity during the growing
season appears to have contributed to increased nitrate leaching.
Water leachate samples from the conventional system sometimes exceeded the regulatory limit of 10 ppm for
nitrate concentration in drinking water. A total of 20% of the conventional system samples were above the 10-
ppm limit, while 10% and 16% of the samples from the organic animal and organic legume systems,
respectively, exceeded the nitrate limit.
Over the 12-year period of monitoring (1991–2002), all three systems leached between 16 and 18 kg of
nitrogen as nitrate per ha per year. These rates were low compared with the results from other experiments with
similar nitrogen inputs, in which leaching of nitrogen as nitrate ranged from 30 to 146 kg per ha per year (Fox
et al. 2001, Power et al. 2001). When measuring these nitrogen losses as a percentage of the nitrogen originally
applied to the crops in each system, the organic animal, organic legume, and conventional systems lost about
20%, 32%, and 20%, respectively, of the total nitrogen as nitrate.
The high nitrate leaching in the organic legume system was not steady over the entire period of the study;
instead, it occurred sporadically, especially during a few years of extreme weather. For example, in 1995 and
1999, the hairy vetch green manure supplied approximately twice as much nitrogen as needed for the corn crop
that followed, contributing excess nitrogen to the soil and making it available for leaching. In 1999, the heavy
nitrogen input from hairy vetch was followed by a severe drought that stunted corn growth and reduced the
corn's demand for nitrogen. In both years, these nitrogen-rich soils were also subjected to unusually heavy fall
and winter rains that leached the excess nitrogen into the lower soil layers. Monitoring of soil nitrogen and
cover crop production is needed to manage the potential for excessive nitrate in all systems.
These data contrast with the results of experiments in Denmark, which indicated that nitrogen leaching from
the conventional treatments was twice that in the organic agricultural systems (Hansen et al. 2001). Overall,
nitrogen leaching levels were lower in the FST rotation study than in those reported by Hansen and others.
Herbicide leaching
Four herbicides were applied in the conventional system: atrazine (to corn), pendimethalin (to corn),
metolachlor (to corn and soybeans), and metribuzin (to soybeans). From 2001 to 2003, atrazine and
metolachlor were only detected in water leachate samples collected from the conventional system. No
metribuzin or pendimethalin were detected after application (Pimentel et al. 2005).
In the conventional plots where corn was planted after corn, and atrazine was applied two years in a row,
atrazine in the leachate sometimes exceeded 3 parts per billion (ppb), the maximum contaminant level (MCL)
set by the US Environmental Protection Agency (EPA) for drinking water. These atrazine levels were higher
than those in the corn-after-soybean treatment (Pimentel et al. 2005). In the conventional system, metolachlor
was also detected at 0.2 to 0.6 ppb. When metolachlor was applied two years in a row in a corn-after-corn
treatment, it peaked at 3 ppb (Pimentel et al. 2005). The EPA has not yet established an MCL for meto-lachlor
in drinking water.
Soil biology
Among the natural biological processes on which the organic rotations depend is symbiosis of arbuscular
mycorrhizae, and this aspect was investigated in the FST experiments. Arbuscular mycorrhizal (AM) fungi are
beneficial and indigenous to most soils. They colonize the roots of most crop plants, forming a mutualistic
symbiosis (the mycorrhiza). The fungus receives sugars from the root of the host plant, and the plant benefits
primarily from enhanced nutrient uptake from the fungus. The extraradical mycelia of the AM fungi act, in
effect, as extensions of the root system, more thoroughly exploring the soil for immobile mineral nutrients
such as phosphate (Smith and Read 1997). Arbuscular mycorrhizae have been shown to enhance disease
resistance, improve water relations, and increase soil aggregation (Miller and Jastrow 1990, Hooker et al.
1994,Wright et al. 1999, Augé 2000). Efficient utilization of this symbiosis contributes to the success of
organic production systems.
Soils of the Rodale Institute FST have been sampled to study the impact of conventional and organic
agricultural management on indigenous populations of AM fungi. Soils farmed with the two organic systems
had greater populations of spores of AM fungi and produced greater colonization of plant roots than in the
conventional system (Douds et al. 1993). Most of this difference was ascribed to greater plant cover (70%) on
the organic systems compared with the conventional corn–soybean rotation (40%). This was due to
overwintering cover crops in the organic rotation (Galvez et al. 1995). In addition to fixing or retaining soil
nitrogen, these cover crops provide roots for the AM fungi to colonize and maintain the fungi's viability during
the interval from cash crop senescence to next year's planting. Though levels of AM fungi were greater in the
organically farmed soils, indices of ecological species diversity were similar in the farming systems (Franke-
Snyder et al. 2001).
Wander and colleagues (1994) demonstrated that soil respiration was 50% higher in the organic animal system,
compared with the conventional system, 10 years after initiation of the Rodale Institute FST. Microbial activity
in the organic soils may be higher than in the conventional system's soils and hence could explain the higher
metabolism rates in the organic systems (Lavelle and Spain 2001).
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Discussion
The crop yields and economics of organic systems, compared with conventional systems, appear to vary based
on the crops, regions, and technologies employed in the studies. However, the environmental benefits
attributable to reduced chemical inputs, less soil erosion, water conservation, and improved soil organic matter
and biodiversity were consistently greater in the organic systems than in the conventional systems.
Soil organic matter and biodiversity
Soil organic matter provides the base for productive organic farming and sustainable agriculture. After 22 years
of separate management, soil carbon (soil organic matter) was significantly higher in both the organic animal
and the organic legume systems than in the conventional system. Soil carbon increased 27.9%, 15.1%, and
8.6% in the organic animal, organic legume, and conventional systems, respectively (figure 3).
The amount of organic matter in the upper 15 cm of soil in the organic farming systems was approximately
110,000 kg per ha. The soil of the upper 15 cm weighed about 2.2 million kg per ha. Approximately 41% of
the volume of the organic matter in the organic systems consisted of water, compared with only 35% in the
conventional systems (Sullivan 2002). The amount of water held in both of the organic systems is estimated at
816,000 liters per ha. The large amount of soil organic matter present in the organic systems aided in making
the systems more tolerant of droughts, such as those that occurred in 1999 and other drought years.
Large amounts of biomass (soil organic matter) are expected to significantly increase soil biodiversity
(Pimentel et al. 1992, Troeh and Thompson 1993, Lavelle and Spain 2001, Mader et al. 2002). The arthropods
per ha can number from two million to five million, and earthworms from one million to five million (Lavelle
and Spain 2001, Gray 2003). The microarthropods and earthworms were reported to be twice as abundant in
organic versus conventional agricultural systems in Denmark (Hansen et al. 2001). The weight of the
earthworms per ha in agricultural soils can range from 2000 to 4000 kg (Lavelle and Spain 2001). There can be
as many as 1000 earthworm and insect holes per m2 of land. Earthworms and insects are particularly helpful in
constructing large holes in the soil that increase the percolation of water into the soil and decrease runoff.
Soil organic matter is an important source of nutrients and can help increase biodiversity, which provides vital
ecological services, including crop protection (Pimentel et al. 2005). For example, adding compost and other
organic matter reduces crop diseases (Cook 1988, Hoitink et al. 1991) and increases the number of species of
microbes in the agroecosystem (van Elsen 2000). In addition, in the organic systems, not using synthetic
pesticides and commercial fertilizers minimizes the harmful effects of these chemicals on nontarget organisms
(Pimentel 2005).
In conventional crop management in New Zealand, Nguyen and Haynes (1995) did not report any adverse
effect on soil microbial activity. These conventional systems, however, were part of a rotation pastoral–arable
system with a relatively high level of soil organic matter (carbon content of the soil ranged from 2.9% to
3.5%).
Overall, environmental damage from agricultural chemicals was reduced in the organic systems because no
commercial fertilizers or pesticides were applied to the organic systems. As a result, overall public health and
ecological integrity could be improved through the adoption of these practices, which decrease the quantities
of pesticides and commercial fertilizers applied in agriculture (BANR/NRC 2003, Pimentel 2005).
Oil and natural gas inputs
Significantly less fossil energy was expended to produce corn in the Rodale Institute's organic animal and
organic legume systems than in the conventional production system (figure 1). There was little difference in
energy input between the different treatments for producing soybeans. In the organic systems, synthetic
fertilizers and pesticides were generally not used. Other investigators have reported similar findings (Karlen et
al. 1995, Smolik et al. 1995, Dalgaard et al. 2001, Mader et al. 2002, Core 4 2003, Pimentel et al. 2005). In
general, the use of less fossil energy by organic agricultural systems reduces the amount of carbon dioxide
released to the atmosphere, and therefore the problem of global climate change (FAO 2003).
Crop yields and economics
Except for the 1999 drought year, the crop yields for corn and soybeans were similar in the organic animal,
organic legume, and conventional farming systems. In contrast, Smolik and colleagues (1995) found that corn
yields in South Dakota were somewhat higher in the conventional system, with an average yield of 5708 kg per
ha, compared with an average of 4767 kg per ha for the organic legume system. However, the soybean yields
in both systems were similar at 1814 kg per ha. In a second study comparing wheat and soybean yields, the
wheat yields were fairly similar, averaging 2600 kg per ha in the conventional system and 2822 kg per ha in
the organic legume system. Soybean yields were 1949 kg per ha and 2016 kg per ha for the conventional and
the organic legume systems, respectively (Smolik et al. 1995). In the Rodale experiments, corn, soybean, and
wheat yields were considerably higher than those reported in South Dakota. These results might be expected,
given the shorter growing season (146 days) and lower precipitation (460 mm) in South Dakota.
European field tests indicate that yields of organically grown wheat and other cereal grains average from 30%
to 50% lower than conventional cereal grain production (Mader et al. 2002). The lower yields for the organic
system in these experiments, compared with the conventional system, appear to be caused by lower nitrogen-
nutrient inputs in the organic system. In New Zealand, wheat yields were reported to average 38% lower than
those in the conventional system, a finding similar to the results in Europe (Nguyen and Haynes 1995). In New
Jersey, organically produced sweet corn yields were reported to be 7% lower than in a conventional system
(Brumfield et al. 2000). In the Rodale experiments, nitrogen levels in the organic systems have improved and
have not limited the crop yields except for the first 3 years. In the short term, organic systems may create
nitrogen shortages that reduce crop yields temporarily, but these can be eliminated by raising the soil nitrogen
level through the use of animal manure or legume cropping systems, or both.
In a subsequent field test in South Dakota, corn yields in the conventional system and the organic alternative
system were 7652 and 7276 kg per ha, respectively (Dobbs and Smolik 1996). Soybean yields were
significantly higher in the conventional system, averaging 2486 kg per ha, compared with only 1919 kg per ha
in the organic alternative system.
The Rodale crop yields were similar to the results in the conventional and organic legume farming system
experiments conducted in Iowa (Delate et al. 2002). In the Iowa experiments, corn yields were 8655 and 8342
kg per ha for the conventional and organic legume systems, respectively. Soybean yields averaged 2890 kg per
ha for the conventional farming system and 2957 kg per ha for the organic legume system.
Although the inputs for the organic legume and conventional farming systems were quite different, the overall
economic net returns were similar without premiums (figure 2). Comparative net returns in the Rodale
experiments differ from those of Dobbs and Smolik (1996), who reported a 38% higher gross income for the
conventional than for the organic alternative system. However, Smolik and colleagues (1995) reported higher
net returns for the organic alternative system in their study with alfalfa and nearly equal returns in the green
manure treatment.
Prices for organic corn and soybeans in the marketplace often range from 20% to 140% higher than for
conventional corn, soybeans, and other grains (Dobbs 1998, Bertramsen and Dobbs 2002, New Farm
Organization 2003). Thus, when the market price differential was factored in, the differences between the
organic alternative and conventional farming would be relatively small, and in most cases the returns on the
organic produce would be higher, as in the results here for the FST.
In contrast to these results for corn and soybeans, the economic returns (dollar return per unit) for organic
sweet corn production in New Jersey were slightly higher (2%) than for conventional sweet corn production
(Brumfield et al. 2000). In the Netherlands, organic agricultural systems producing cereal grains, legumes, and
sugar beets reported a net return of EUR 953 per ha, compared with conventional agricultural systems
producing the same crops that reported EUR 902 per ha (Pacini et al. 2003).
In a California investigation of four crops (tomato, soybean, safflower, and corn) grown organically and
conventionally, production costs for all four crops were 53% higher in the organic system than in the
conventional system (Sean et al. 1999). However, the profits for the four crops were only 25% higher in the
conventional system compared with the organic system. If the 44% price advantage of the four organically
grown crops were included, the organic crops would be slightly more profitable than the conventional ones
(Sean et al. 1999).
One of the longest-running organic agricultural trials (ongoing for more than 150 years) is the Broadbalk
experiment at Rothamsted (formerly the Rothamsted Experimental Station) in the United Kingdom. The trials
compared a manure-based organic farming system with a system based on synthetic chemical fertilizer. Wheat
yields were slightly higher on average in the manured organic plots (3.45 t per ha) than in the plots receiving
chemical fertilizers (3.40 t per ha). The soil quality improved more in the manured plots than in those receiving
chemical fertilizer, based on greater accumulations of soil carbon (Vasilikiotis 2004).
Challenges for organic agriculture
Two primary problems with the organic system in the California study were nitrogen deficiency and weed
competition (Sean et al. 1999). This was also noted for the organic faming systems in the US Midwest.
Although the Rodale experiment overcame nitrogen deficiency challenges through legume cover crop
management, other researchers have been less successful in maintaining and improving soil fertility levels in
organic systems. Rodale's results could also be influenced by geographical soil characteristics and may not be
universally applicable.
In organic production systems, pest control can be of heightened importance and impact. Weed control is
frequently a problem in organic crops because the farmer is limited to mechanical and biological weed control,
whereas under conventional production mechanical, biological, and chemical weed control options often are
employed. Also, weather conditions can limit the efficacy of weed control. Mechanical weed control is usually
more effective than chemical weed control under dry conditions, while the reverse holds true under wet
conditions. In the Rodale experiments, only the organic soybeans suffered negative impacts from weed
competition.
Insect pests and plant pathogens can be effectively controlled in corn and soybean production by employing
crop rotations. Some insect pests can be effectively controlled by an increase in parasitoids; reports in organic
tomato production indicate nearly twice as many parasitoids in the organic compared with the conventional
system (Letourneau and Goldstein 2001). However, increased plant diversity in tomato production was found
to increase the incidence of plant disease (Kotcon et al. 2001). With other crops, like potatoes and apples,
dealing with pest insects and plant pathogens that adversely affect yields is a major problem in organic crop
production.
Adoption of organic technologies
Several organic technologies, if adopted in current conventional production systems, would most likely be
beneficial. These include (a) employing off-season cover crops; (b) using more extended crop rotations, which
act both to conserve soil and water resources and also to reduce insect, disease, and weed problems; (c)
increasing the level of soil organic matter, which helps conserve water resources and mitigates drought effects
on crops; and (d) employing natural biodiversity to reduce or eliminate the use of nitrogen fertilizers,
herbicides, insecticides, and fungicides. Some or all of these technologies have the potential to increase the
ecological, energetic, and economic sustainability of all agricultural cropping systems, not only organic
systems.
Previous Section Next Section
Conclusions
Various organic agricultural technologies have been used for about 6000 years to make agriculture sustainable
while conserving soil, water, energy, and biological resources. The following are some of the benefits of
organic technologies identified in this investigation:
Soil organic matter (soil carbon) and nitrogen were higher in the organic farming systems, providing
many benefits to the overall sustainability of organic agriculture.
Although higher soil organic matter and nitrogen levels were identified for the organic systems,
similar rates of nitrate leaching were found to those in conventional corn and soybean production.
The high levels of soil organic matter helped conserve soil and water resources and proved beneficial
during drought years.
Fossil energy inputs for organic crop production were about 30% lower than for conventionally
produced corn.
Depending on the crop, soil, and weather conditions, organically managed crop yields on a per-ha
basis can equal those from conventional agriculture, although it is likely that organic cash crops cannot be
grown as frequently over time because of the dependence on cultural practices to supply nutrients and
control pests.
Although labor inputs average about 15% higher in organic farming systems (ranging from 7% to 75%
higher), they are more evenly distributed over the year in organic farming systems than in conventional
production systems.
Because organic foods frequently bring higher prices in the marketplace, the net economic return per
ha is often equal to or higher than that of conventionally produced crops.
Crop rotations and cover cropping typical of organic agriculture reduce soil erosion, pest problems,
and pesticide use.
The recycling of livestock wastes reduces pollution while benefiting organic agriculture.
Abundant biomass both above and below the ground (soil organic matter) also increases biodiversity,
which helps in the biological control of pests and increases crop pollination by insects.
Traditional organic farming technologies may be adopted in conventional agriculture to make it more
sustainable and ecologically sound.
Previous Section Next Section
Acknowledgements
We thank the following people for reading a draft of this article and for their many helpful suggestions: Robin
G. Brumfield, Rutgers, The State University of New Jersey; Wen Dazhong, Institute of Applied Ecology,
Academia Sinica, Shenyang, China; Tomek De Ponti, Visiting Fulbright Scholar, Cornell University; Andrew
R. B. Ferguson, Optimum Population Trust, Oxon, United Kingdom; Long Nguyen, National Institute of Water
and Atmospheric Research, Auckland, New Zealand; Maurizio Paoletti, Università di Padova, Italy; James
Smolik, South Dakota State University; Chris Wien, Cornell University.
© 2005 American Institute of Biological Sciences
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... Manure-based organic system Soil C increase 981 kg/ha Soil (Pimentel et al., 2005) Legume-based system C increase 574 kg/ha United States farming trials -14 % higher soil organic C inorganic than in (Marriott and Wander, 2006) conventional systems ...
... Organic agriculture reduces the direct and indirect use of energy in agriculture. It is reported that organic farming systems use 20-50% less energy than the conventional farming system (Pimentel et al., 2005;Schader et al., 2011;Muller et al., 2012). In the USA, high-input industrialized systems consumed 22-120% more energy than sustainable and organic systems and achieved similar yields (Pretty and Ball, 2001). ...
... The reduced dependency on energy inputs in organic agriculture reduces vulnerability to rising energy prices, and hence volatility of agricultural input prices. Pimentel et al. (2005) reported that, organic no-till practice saved 61% fuel/ha compared to conventional notill corn-production practice and organic tillage farming practice saved 47% fuel/ha as compared to the conventional tillage practice. Results from the Rodale Institute Farm Systems Trials (21 years, 1981 to 2002) showed that, fossil-fuel energy inputs for organic corn production were about 30% lower as compared to conventional corn production. ...
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Estimated anthropogenic global warming is currently increasing at 0.2°C (likely between 0.1°C and 0.3°C) per decade due to past and ongoing emissions. Climate change can affect food security of billions of people. The agriculture sector is the main source of CH 4 and N 2 O emissions. Agriculture and related land-use emissions accounted for 17% of global GHG emissions from all sectors. Agriculture accounted to around 14% of the total GHGs emissions of India. There is a need to identify and adopt best agricultural practices to minimize the adverse impacts of climate change. Organic agriculture has many positive effects in mitigating climate change, largely owing to its greater ability to reduce emissions of greenhouse gases (GHGs) such as nitrous oxide (N 2 O) and methane (CH 4) and also sequestering carbon in soils as compared to conventional agriculture. Organic agriculture promotes ecological resilience, improved biodiversity, healthy management of farms and the surrounding environment, and builds on community knowledge and strength. Hence, organic agriculture has been proved to be effective for enhanced adaptive capacity of farmers adversely affected by climate change. More research is needed for improving yields under organic agriculture and enhancing its potential for mitigating climate change. For further popularization of organic farming in view of adapting and mitigating effect of climate change, there is a need to demonstrate the value of analyzing the problem of GHG emissions from agriculture at different stages along food-supply chains and at different scales of food systems. Based on the current evidences, a transition to organic farming at a national or global scale without other substantial changes in the food system is unlikely to reduce GHG emissions.There is a need to estimate the breakeven point of profitability to farmers vis a vis food security over time and space, to upscale the organic agriculture in view of changing climate.
... Such food is commonly referred to as organic (Ramesh et al., 2009). Various organic technologies have been utilized for about 600 years to make agriculture sustainable while conserving soil, water energy and biological resources (Pimentel et al., 2005). Organic farming is gaining gradual momentum across the International J. Seed Spices 6(2), July 2016:84-87 *Corresponding author email: mittalgkumar@gmail.com ...
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The field experiment was conducted in All India Coordinate Research Project on Spices at S. K. N. College of Agriculture, Jobner, India during rabi seasons of 2010-11, 2011-12, 2012-13 and 2013-14 for the organic management of diseases and pest in cumin. The pooled data over four years revealed that all the organic management practices for diseases and pest in cumin resulted in significantly low incidence of wilt, blight and aphid and significantly maximized seed yield and benefit: cost ratio over the check. The pooled analysis of data further revealed that minimum incidence of wilt (10.60%), Blight (5.90%) and Aphid (26.20%) and maximum seed yield (242.0 kg ha-1) and benefit : cost ratio (1.78) was recorded with soil application of vermi compost @ 2.5.t ha-1 + seed treatment with neem seed kernel extract (NSKE) @ 5.0 percent + spray of neem seed kernel extract (NSKE) @ 5.0 percent. Control treatment resulted in maximum incidence of wilt (24.20%), blight (29.24%) and aphid (81.30%) and minimum seed yield (137.0 kg ha-1) and benefit: cost ratio (1.03).
... Contamination by organic-approved pesticides [see list here: (PAN UK, 2022)], which at the moment is not monitored, should be counted too as well as organic-approved fertilizers and plastics debris due to the use of plastic mulching (Wu and Sardo, 2010). Most of the indicators used to compare organic and conventional farming perform better or much better in organic farming, except for productivity (Gomiero et al., 2011;Knapp and van der Heijden, 2018;Seufert et al., 2012), which can be economically balanced or surpassed by lower input costs, higher market prices of organic food and premiums (Nemes, 2009;Pimentel et al., 2005). However, several 6 researchers believe that rigid organic farming principles are not necessarily compatible with the needs of farmers nor sustainable agriculture, stressing the need of more flexible, site/custom-tailored managements (Deguine et al., Gibson et al., 2007;Gomiero et al., 2011;Wu and Sardo, 2010). ...
... With the steady increase in world population, the need to supply food to a large community has become a challenge, specifically requiring the agriculture sector to adopt farming practices that ensure better yields and crop production. For the sustainability of agricultural land and better crop production management, efforts are required to comprehend the complex processes related to soil biological systems and agro-ecosystems (Pimentel et al., 2005;Moonen and Bàrberi, 2008). In this regard, AMF have proved quite useful. ...
... With the steady increase in world population, the need to supply food to a large community has become a challenge, specifically requiring the agriculture sector to adopt farming practices that ensure better yields and crop production. For the sustainability of agricultural land and better crop production management, efforts are required to comprehend the complex processes related to soil biological systems and agro-ecosystems (Pimentel et al., 2005;Moonen and Bàrberi, 2008). In this regard, AMF have proved quite useful. ...
... With the steady increase in world population, the need to supply food to a large community has become a challenge, specifically requiring the agriculture sector to adopt farming practices that ensure better yields and crop production. For the sustainability of agricultural land and better crop production management, efforts are required to comprehend the complex processes related to soil biological systems and agro-ecosystems (Pimentel et al., 2005;Moonen and Bàrberi, 2008). In this regard, AMF have proved quite useful. ...
... Undeniably, profits of organic crops/products have been established to be around 25% inferior to modern farming (Stolze and Lampkin, 2009), but OFS is environmentally friendly, could improve nutritional quality through incorporating diversified crop cultivars (Rigby and Cáceres, 2001). A study conducted over 22 years in Rodale Institute test farm (Pimentel et al., 2005;Steffanie et al., 2018). A 30 th -anniversary report was published by Rodale in 2012 (Rodale Institute, 2020). ...
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Azospirillum Bioinoculant Technology: Past to Current Knowledge and Future Prospects Palani Saranraj, Abdel Rahman Mohammad Al–Tawaha, Panneerselvam Sivasakthivelan, Abdel Razzaq M. Altawaha, Kangasalam Amala, Devarajan Thangadurai, and Jeyabalan Sangeetha
... Undeniably, profits of organic crops/products have been established to be around 25% inferior to modern farming (Stolze and Lampkin, 2009), but OFS is environmentally friendly, could improve nutritional quality through incorporating diversified crop cultivars (Rigby and Cáceres, 2001). A study conducted over 22 years in Rodale Institute test farm (Pimentel et al., 2005;Steffanie et al., 2018). A 30 th -anniversary report was published by Rodale in 2012 (Rodale Institute, 2020). ...
Chapter
PART III: ORGANIC AGRICULTURE FOR FOOD SAFETY 13. Organic Production Technology of Rice Shah Khalid, Amanullah, Nadia, Imranuddin, Mujeeb Ur Rahman, Abdel Rahman Al Tawaha, Devarajan Thangadurai, Jeyabalan Sangeetha, Samia Khanum, Munir Turk, Hiba Alatrash, Sameena Lone, Khursheed Hussain, Palani Saranraj, and Arun Karnwal
... Undeniably, profits of organic crops/products have been established to be around 25% inferior to modern farming (Stolze and Lampkin, 2009), but OFS is environmentally friendly, could improve nutritional quality through incorporating diversified crop cultivars (Rigby and Cáceres, 2001). A study conducted over 22 years in Rodale Institute test farm (Pimentel et al., 2005;Steffanie et al., 2018). A 30 th -anniversary report was published by Rodale in 2012 (Rodale Institute, 2020). ...
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This research presents damage causes of the pylons in the ancient Egyptian temples based on 3D finite elements analysis. The main purpose of the research determines the failure causes of the first pylon of the Ramessium temple, which is situated in Upper Egypt, at Luxor “Thebes” on the west bank of the Nile River. The first pylon of Ramessium temple subjected to seismic activity effects on long term, combined with several structural damage factors such as the defects resulting from the construction technique, where the builder used the poor quality of stones in foundations of the pylon, the building materials residue was used as filler for the core of the pylon walls, and it lacked vertical joints between the courses. In addition to it founded on alluvial soil that is vulnerable to contaminated water, it is still suffering damage factors and urban trespasses at the moment. All of the former factors helped the pylon to be affected by the earthquakes loads that occurred on it. The structural behavior of the pylon under self-weight and earthquakes loads were carried out by Numerical analysis to find out the loads and stresses which caused collapsing of the pylon. Results of the study indicated that the pylon subjected to a horizontal displacement due to old earthquakes force, led to collapse of the pylon. Finally, the study represents use of modern technique to study the structural behavior of the most important architectural units in ancient Egyptian temples to identify the causes of its collapse.
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A number of excellent textbooks on general ecology are currently available but‚ to date‚ none have been dedicated to the study of soil ecology. This is important because the soil‚ as the ‘epidermis’ of our planet‚ is the major component of the terrestrial biosphere. In the present age‚ it is difficult to understand how one could be interested in general ecology without having some knowledge of the soil and further‚ to study the soil without taking into account its biological components and ecological setting. It is this deficiency that the two authors‚ Patrick Lavelle and Alister Spain‚ have wished to address in writing their text. A reading of this work‚ entitled ‘Soil Ecology’‚ shows it to be very complete and extremely innovative in its conceptual plan. In addition‚ it follows straightforwardly through a development which unfolds over four substantial chapters. Firstly‚ the authors consider the soil as a porous and finely divided medium of b- organomineral origin‚ whose physical structure and organisation foster the development of a multitude of specifically adapted organisms (microbial communities‚ roots of higher plants‚ macro-invertebrates).
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Production costs have been analyzed in several studies using such normative approaches as budgeting and mathematical programming, and positive approaches as estimation of production, cost, or profit functions. This study used budgeting methods to analyze the costs and benefits of adopting integrated crop management (ICM) or organic methods versus conventional agriculture for tomatoes (Lycopersicon esculentum Mill.), sweet corn (Zea mays L. var. saccharada), and pumpkins (Cucurbita pepo L.). Data were collected using field studies conducted at the Rutgers University Snyder Research and Extension Farm, Pittstown, N.J. Time and motion study techniques were used to record machinery use and labor quantities. Records of production inputs and yields were also collected. These records were then converted to a 1.0-acre (0.4-ha) basis to constructed crop budgets. Results show that ICM systems are more profitable than conventional and organic systems. Organic systems had the lowest net returns. However, because of the organic price premium, the net returns were fairly close to those for conventional and ICM systems.
Pesticides cause serious public health problems and considerable damage to agricultural and natural ecosystems. We confirm previous reports that it is feasible to reduce pesticide use by 50 % or more. The Swedish Government achieved a 61 % reduction in pesticide use and the Indonesian Government achieved a 65 % reduction in pesticide use without a reduction in crop yields. In fact in Indonesia the result of the reduction in pesticide use was a 12 % increase in rice yield. © Springer Science+Business Media Dordrecht 2014. All rights are reserved.
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Arbuscular mycorrhizal (AM) symbioses can affect the water balance of amply watered and droughted plants. The mycorrhizal influence most often examined within the field of water relations has been alteration of stomatal behavior. This review summarizes possible biophysical and biochemical mechanisms, discussing how AM fungus-induced changes in tissue hydration, plant size, tissue elemental concentrations and nonhydraulic root-to-shoot signaling may affect stomatal regulation. The review also provides a brief catalog of published mycorrhizal effects on stomata behavior.
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The roots of most plants are colonized by symbiotic fungi to form mycorrhiza, which play a critical role in the capture of nutrients from the soil and therefore in plant nutrition. Mycorrhizal Symbiosis is recognized as the definitive work in this area. Since the last edition was published there have been major advances in the field, particularly in the area of molecular biology, and the new edition has been fully revised and updated to incorporate these exciting new developments. . Over 50% new material . Includes expanded color plate section . Covers all aspects of mycorrhiza . Presents new taxonomy . Discusses the impact of proteomics and genomics on research in this area.
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In the hierarchical model of soil aggregate formation proposed by Tisdall and Oades (1982), a major mechanism involved in the binding of microaggregates into macroaggregates is physical entanglement by roots and mycorrhizal fungus hyphae. Using data collected from soils of a chronosequence of tallgrass prairie restorations and an adjacent prairie soil cultivated to row crops for at least 100 yr. it was found that root lengths by diameter size class, the lengths of roots colonized by mycorrhizal fungi within each root-size class, and extraradical hyphal lengths of mycorrhizal fungi were all highly correlated with the geometric mean diameter (GMD) of water-stable soil aggregates. To better understand the relative contributions of roots and mycorrhizal fungi to water-stable aggregation, a conceptual model emphasizing the interrelationships between roots of differing size classes, mycorrhizal fungi and aggregate-size distribution was developed and evaluated using path analysis. From path analysis, it was found that extraradical hyphal length followed by fine (0.2–1 mm dia) root length had the strongest direct effects on GMD. It was expected that a physical entanglement mechanism would involve the very finest roots; however, the direct path between very-fine (<0.2 mm dia) root length and GMD was not significant. Although both root-size classes exhibited significant indirect effects on GMD via the relationships between their colonized lengths and xtraradical hyphal length, the overall effect of fine root length on GMD was much stronger than the effect of very-fine root length. To determine the relative influences of plant lifeforms on root morphology and estimate the effects of various lifeforms on GMD, data on aboveground standing crops associated with the root and fungus data were added to the path model. Prairie grasses were associated with both fine and very-line root lengths and exhibited the strongest effects on GMD; whereas, non-prairie grasses were most strongly associated with very-fine root length and only weakly affected GMD. Perennial species of Compositae were associated with line root length and also had a fairly strong influence on GMD. This study supports the conceptual model developed by Tisdall and Oades and suggests that a better understanding of soil aggregate development may be achieved by considering the interactions between roots and mycorrhizal fungi in relationship to plant community composition.