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Acid Soil Is Associated with Reduced Yield, Root Growth and Nutrient Uptake in Black Pepper (Piper nigrum L.)


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Low pH is a major limiting factor for the production of black pepper (Piper nigrum L.) in Hainan province. Black pepper gardens often exhibit a decrease in soil pH (to 5.5 - 5.0) on orchards with a multi-year production history. An exploratory hydroponic experiment was conducted to examine the effects of increasingly acid nutrient solution pH (7.0, 5.5, 4.0, and 3.5) on seedling growth, tissue nutrient concentrations and root morphological traits. The results indicated that low pH may directly inhibit root development and function, limit K, Ca and Mg absorption and reduce seedling growth. At pH 5.5, black pepper attained maximum growth, while the minimum growth occurred at pH 3.5. It can be concluded that low pH reduces plant growth and is associated with low root nutrient concentrations of Ca and Mg, which may explain the decline of the yield in the seven pepper gardens of the Institute.
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Agricultural Sciences, 2014, 5, 466-473
Published Online April 2014 in SciRes.
How to cite this paper: Zu, C., et al. (2014) Acid Soil Is Associated with Reduced Yield, Root Growth and Nutrient Uptake in
Black Pepper (Piper nigrum L.). Agricultural Sciences, 5, 466-473.
Acid Soil Is Associated with Reduced Yield,
Root Growth and Nutrient Uptake in Black
Pepper (Piper nigrum L.)
Chao Zu1,2,3, Zhigang Li1,2,3, Jianfeng Yang1,2,3, Huan Yu1,2,3, Yan Sun1,2,3, Hongliang Tang4,5,
Russell Yost6*, Huasong Wu1,2,3*
1Spice and Beverage Research Institute, Chinese Academy of Tropical Agricultural Sciences, Wanning, China
2Key Laboratory of Genetic Resources Utilization of Spice and Beverage Crops, Ministry of Agriculture, Wanning,
3Hainan Provincial Key Laboratory of Genetic Improvement and Quality Regulation for Tropical Spice and
Beverage Crops, Wanning, China
4Key Laboratory of Plant-Soil Interaction, Ministry of Education, Beijing, China
5Center for Resources, Environment and Food Security, China Agricultural University, Beijing, China
6Department of Tropical Plant and Soil Science, University of Hawai’i at Manoa, Honolulu, USA
Email: *, *
Received 16 February 2014; revised 16 March 2014; accepted 25 March 2014
Copyright © 2014 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
Low pH is a major limiting factor for the production of black pepper (Piper nigrum L.) in Hainan
province. Black pepper gardens often exhibit a decrease in soil pH (to 5.5 - 5.0) on orchards with a
multi-year production history. An exploratory hydroponic experiment was conducted to examine
the effects of increasingly acid nutrient solution pH (7.0, 5.5, 4.0, and 3.5) on seedling growth, tis-
sue nutrient concentrations and root morphological traits. The results indicated that low pH may
directly inhibit root development and function, limit K, Ca and Mg absorption and reduce seedling
growth. At pH 5.5, black pepper attained maximum growth, while the minimum growth occurred
at pH 3.5. It can be concluded that low pH reduces plant growth and is associated with low root
nutrient concentrations of Ca and Mg, which may explain the decline of the yield in the seven pep-
per gardens of the Institute.
Nutrient Concentration, Pepper Seedling Growth, pH, Root Morphology, Soil Acidification
*Corresponding authors.
C. Zu et al.
1. Introduction
Acid soils occupy approximately 30% of the world’s land area and restrain global agricultural production [1].
Soil acidification is a growing problem in soils of Chinese agricultural systems in the tropical region [2]. Nu-
merous factors can result in soil acidification, such as large inputs of inorganic fertilizers, high rainfall, acid de-
position and greenhouse gas. As the concentration of H+ in the soil increases, it can inhibit root growth [3], dis-
rupt the functions of the plasma membrane [4], cell wall [5], or by increasing Al3+ toxic levels [6]. Deficient le-
vels of calcium (Ca), magnesium (Mg) and phosphorus (P) are also frequent under low pH conditions.
Black pepper (Piper nigrum L.) is one of the important cash crops in the tropical agricultural regions of China.
Hainan province, a major producer and exporter of black pepper in China, has 22,000 ha in cultivation and pro-
duces 36,000 Mg of pepper berry annually, comprising 90% of the pepper production in China. Production of
pepper provides income to approximately 1 million rural growers and has become an important tropical crop
industry with output value of more than 1 billion Chinese yuan.
The suitable soil pH for pepper growth is 5.5 - 7.0 [7]. However, soil pH below 5.5 occupies approximately
50% of the typical pepper gardens in Hainan province [7]. Soil pH values less than 5.5 suggest the possible
presence ot toxic levels of soil aluminum. Long term continuous planting of pepper may result in significant soil
acidification [8]. Many pepper gardens with low soil pH have produced pepper for more than 30 years [7] [9]. In
these gardens, pepper often showed poor growth, serious plant diseases and insect pests, nutrient deficiency, low
yield and poor quality [8]. These problems become detrimental for the pepper industry. Therefore, it is urgent to
investigate the effects of pH on pepper growth and nutrient absorption.
The objective of this study was to determine whether the decrease in pepper productivity of aging pepper
gardens was associated with the decreased soil pH and how soil acidification affected pepper growth and nu-
trient absorption.
2. Materials and Methods
2.1. Experiment 1: Survey of Institute Gardens with Pepper
The study was conducted in Spice and Beverage Research Institute (SBRI) at the southeast of Hainan Province
of China (18˚72'N - 18˚76'N, 110˚19'E - 110˚22'E). The area is characterized by a tropical monsoon climate with
an average temperature of 24.6˚C, while the absolute maximum may reach 38.7˚C and the minimum may be
11.6˚C. Average annual precipitation of the area is 2150 mm, and the annual mean relative humidity is 85%.
The study area was comprised of a selection of seven gardens, which were divided into five age groups (10,
15, 20, 25 and 30 years of production) of black pepper (Piper nigrum L. cv. Reyin No.1) (Table 1). Row spac-
ing was 2.5 m, while spacing in the row was 2 m. Within each garden five sampling locations were selected on
an S-shape sampling configuration. A series of measurements were taken at each sampling location such as soil
pH and pepper yield per plant. Four soil cores at each of the five sampling locations were composited for the soil
sample to be analyzed. The soils were sampled at the 0 - 20 cm soil depth. Soil pH was measured using a ratio of
10 g soil to 25 ml of water and stirred for 30 minutes.
Table 1. Survey of pepper gardens at the spice and beverage institute, Hainan Island, China.
Pepper garden identification Pepper garden age, yr Number of plants reaching fruiting age
No. 13 26 424
No. 14 28 415
No. 15 28 382
No. 19 21 85
No. 21 15 312
No. 22 15 310
No. 24 10 377
C. Zu et al.
2.2. Experiment 2: Solution Culture Experiment
A solution culture experiment of black pepper (Piper nigrum L. cv. Reyin No. 1) seedlings was carried out to
quantify the response of black pepper seedlings to increasing acidity. Seedlings were cultured in sand until the
extensive development of roots. Seedlings were weighed and transferred to 15 L plastic pots with nutrient solu-
tion consisting of K2SO4 (750 μM), MgSO4 (650 μM), KCl (100 μM), KH2PO4 (250 μM), Ca(NO3)2 (2000 μM),
Fe-EDTA (100 μM), H3BO3 (100 μM), MnSO4 (1 μM), ZnSO4 (1 μM), CuSO4 (0.1 μM), (NH4)6Mo7O24 (0.005
μM) [10]. The experimental design structure was a completely randomized combination of four solution pH
treatments (3.5, 4.0, 5.5, and 7.0) with 8 replications. The pH of the nutrient solution was adjusted once a day by
HCl. Nutrient solutions were renewed every 4 days and continuously aerated with a pump. The pots were re-
randomized every 20 days. Pepper seedlings were grown in a glasshouse at the SBRI with a photoperiod of 11
h·d1, average temperature and photosynthetic photon flux density were 23˚C and 234 μmol·m2·s1 at the shoot
level and a relative humidity of 85%.
After 65 d of growth, seedlings were harvested and separated into leaves, vines and roots. Fresh weight was
determined. Root morphological parameters, such as total root length, root surface area, number of lateral roots,
average diameter and volume of roots, were determined from scanned root samples with an Epson V700 scanner
and the software WinRHIZO according to the method described by Liu et al. [10]. Dry weight of the root (WR)
and shoot (WS) were measured after drying in an oven at 70˚C.
2.3. Statistics
Data were analyzed using regression and ANOVA using SAS (version 8.02) statistical software. Lines of best fit
for the curves and error bars for the bar chart were calculated using GraphPad Prism (Version 5, GraphPad
Software) [11].
3. Results
3.1. Variation in Soil pH with Pepper Garden Age
Across the seven gardens, soil pH significantly declined with increasing garden age (P = 0.018) (Figure 1). Ac-
cording to the regression equation initial pH was approximately 5.84, while soil in 10-year-old orchards aver-
aged 5.64, and 15-year-old gardens were characterized by a pH of 5.54. The 28-year-old garden averaged pH
3.2. Pepper Seedling Response to Reduced pH (Solution Culture Experiment)
3.2.1. Fresh Weight Growth
Fresh weight (shoot plus root) growth decreased significantly with decreasing solution pH. Pepper accumulated
the maximum fresh weight of both shoots and roots (average 90.1 g) at pH 5.5 (Figure 2), and a minimum fresh
510 15 20 25 30
8y = -0.02x+5.84
R= -0.398
Pepper garden age
Soil pH
Figure 1. Variation in soil pH with planting year
across seven pepper gardens at the SBRI, Hainan Isl-
and, China.
C. Zu et al.
weight (average 56.5 g) at pH 3.5. There was no significant difference in fresh weight between pH 7 and pH 5.5,
both of which were much higher than plant fresh weight at pH 3.5. At pH 4.0, the fresh weight was much lower
(68.3 g) compared with the weight in solutions of pH 5.0 (90.1 g). There was no significant difference of fresh
weight between pH 4.0 and pH 3.5 (Figure 2).
3.2.2. Black Pepper Shoot and Root Growth and Nutrient Concentrations in Response to Solution
The effects of solution pH on root and shoot fresh weights are also shown in Table 2. Low pH decreased root
Figure 2. Growth yield of Piper nigrum L. cultured
with four solution pHs (3.5, 4.0, 5.5, 7.0). Error bars
indicate the standard error of the mean (n = 3); columns
not sharing the same letter indicate significant differ-
ences according to the ANOVA in SAS (P = 0.05).
Table 2. Pepper (Piper nigrum, L.) seedling fresh weight and nutrient concentrations in the root and shoot of black pepper at
65 days after transplanting as affected by solution pH.
pH Fresh weight (g·plant1) Nutrient concentrations
N (mg·g1) P (mg·g1) K (mg·g1) Ca (mg·g1) Mg (mg·g1)
8.17 ± 0.73
33.77 ± 1.54
2.41 ± 0.10
10.24 ± 0.87
12.63 ± 0.29
3.69 ± 0.13
5.50 7.77 ± 0.20 42.70 ± 1.81 3.05 ± 0.09 9.68 ± 0.67 8.90 ± 0.27 2.83 ± 0.12
6.70 ± 0.40
43.89 ± 1.06
4.45 ± 0.38
3.80 ± 1.20
8.00 ± 0.48
1.70 ± 0.32
5.77 ± 0.62
53.00 ± 2.06
3.71 ± 0.37
3.63 ± 1.33
5.54 ± 0.33
1.09 ± 0.06
Coefficients of regression equationb
y0 3.86 66.33 5.85 3.87 0.17 1.31
78.50 ± 11.29
27.70 ± 0.99
1.30 ± 0.09
21.78 ± 0.27
10.68 ± 0.45
4.09 ± 0.57
82.33 ± 0.83
34.89 ± 2.42
1.66 ± 0.27
20.86 ± 0.89
10.51 ± 0.40
4.52 ± 0.30
61.57 ± 3.80
37.58 ± 2.83
1.37 ± 0.12
25.61 ± 1.07
11.09 ± 0.85
3.86 ± 0.07
50.73 ± 3.35
35.67 ± 0.71
1.62 ± 0.19
21.98 ± 2.16
8.22 ± 0.23
3.80 ± 0.65
Coefficients of regression equation
0.05 ns
0.61 ns
0.43 ns
0.12 ns
Each value represents the mean of three replicates ± s.e.m. ans, no significance; ** and ***, significance at P < 0.01 and P < 0.001 level, respectively.
bThe general regression equation is y = y0 + ax, where y is the molar ratio of nutrient and x is level of nutrient solution pH.
C. Zu et al.
and shoot fresh weights of pepper seedlings (P < 0.01).The extremely low pH caused a greater decrease in shoot
than in root weights. The smallest root and shoot fresh weights were observed at pH 3.5 with decreases by 29%
and 35% when compared with pH 7.0. However, little or no injurious effect was observed on root and shoot
fresh weights at pH 5.5.
Most of the variation of nutrient concentrations in both root and shoot came from different pH treatments
(Table 2). Root nutrient concentrations differed with the nutrient. Compared to pH 7.0, N and P concentrations
increased by 146% and 54% at pH 3.5, respectively. In contrast, root potassium (K), calcium (Ca) and magne-
sium (Mg) decreased sharply with the decreasing pH. At pH 3.5, K, Ca and Mg decreased 65%, 56% and 70%,
3.3. Prediction of Pepper nigrum L. Growth from Root Nutrient Concentrations
The relationship between pepper growth and nutrient concentrations in roots varied with the nutrient (Figure 3).
All nutrients were significantly related with growth except for P. The relationship was strongest for Mg (R2 =
0.63), followed by Ca (R2 = 0.44) and K (R2 = 0.44). However, growth was significantly negatively related with
N concentration in root.
3.4. Effects of Solution pH on Root Morphology
Pepper root growth decreased consistently with decreased solution pH (Figure 4). The root morphology of pep-
per was sensitive to the change in solution pH, with root length, surface area and number of laterals apparently
Figure 3. The prediction of Piper nigrum L. growth from root nutrient concentrations after
65 d growth.
=0.3080 P
30 40 50 60
150 y=-1.375x+135
=0.3568* N
01 2 34 5
150 y=1263x+45.99
=0.6254** Mg
4812 16
=0.4431* Ca
0510 15
150 y=3.164x+53.75
=0.4367* K
Fresh weight growth (g·plant
Nutrient concentrations of root (mg·g
Nutrient concentrations of root (mg·g
C. Zu et al.
Root surface area (cm2)
Average diameter (mm)
Root volume (cm
Total root length (cm)
Specific root length
( cm· g
root DW)
Number of laterals
Figure 4. The relationship between solution pH with root morphological traits of Piper nigrum L.
grown for 65 d.
more sensitive to acidity than were root diameter and volume (data not shown). No significant difference was
detected in terms of specific root length.
The increasing acidity was apparently toxic, reducing total root length by 15%, 26% and 57% at pH 5.5, 4.0
and 3.5, respectively (Figure 4). Pepper root surface area was reduced by 10%, 26% and 57%, respectively. The
surface area of root showed strongest negatively correlation with pH (r2 = 0.53). The number of laterals was re-
duced by 19%, 25% and 71% in lower pH solutions. Low pH also appeared to be inhibitory for root diameter
and volume of roots but less so than for root length, surface area and laterals. The specific root length was not
inhibited greatly under low pH (Figure 4).
4. Discussion
In the present study, soil pH exhibited a significant decrease with increasing pepper garden age. The results from
a hydroponic experiment suggested that low pH considerably inhibited seedling fresh weight and root morpho-
logical parameters. Root nutrient concentrations of potassium, calcium and magnesium decreased, while nitro-
gen and phosphorus increased under low solution pH. Indeed, a decrease in favorable root morphology does not
mean a weak ability to acquire N and P from solution, as illustrated by the results. These differences are likely
related to differences in K, Ca and Mg absorption and fresh weight growth. We further explore this in the fol-
lowing sections to provide insights into the variation that we have obtained.
4.1. The Soil pH Changed over Time
In Hainan province, soil pH of common pepper garden appeared to decrease after many years of production.
C. Zu et al.
Large inputs of chemical fertilizers, particularly ammoniacal fertilizers, may be one of the key factors reducing
pH. White pepper production and fertilizer N applications reached 1 and 0.58 kg·plant1 nationally in recent
years, respectively, reducing 50% and increases 154% as compared with year 1990 [12]. These applications re-
sult in large amounts of N fertiliser (1,167 kg·ha1 each year). Furthermore, forms of applied N are usually
urea-N, synthetic fertilizer-N (15-15-15) and manure-N, and, the percentage of inorganic-N is 77%. High rates
of N fertilization can cause soil acidification both directly and indirectly [13] [14]. Consequently the decreasing
soil pH may have affected pepper growth.
4.2. Fresh Weight Growth Inhibition under Low Solution pH
The solution culture showed that fresh weight growth of pepper seedlings was significantly inhibited at pH 4.0
and 3.5. Potassium, calcium and magnesium concentrations of roots were also significantly inhibited under low
solution pH. A regression analysis was performed to examine the effect of root nutrient concentrations on the
seedling fresh weight. The inhibition of K, Ca and Mg uptake was associated with poor fresh weight growth.
4.3. Root Growth and Nutrient Absorption Limitation in Response to Low pH Conditions
Among the various plant parts, the roots are directly or indirectly affected by the pH of the growth medium. Low
pH injury or H+ injury is one of the factors responsible for growth retardation in acid conditions [5]. In the
present study, decreasing growth medium pH significantly reduced root growth and nutrient content. Root
growth was obviously inhibited by low pH, especially surface area. Root surface area decreased linearly with
declining solution pH (P = 0.0071). Number of laterals and total root length increased significantly, but less than
surface area with increasing pH (P = 0.0080). Root diameter and volume showed significant positive correlation
with pH (P < 0.05). Excess H+ in the growth medium affects plant growth by two processes: a) H+ may cause
injury to the root tissue, therefore root elongation, lateral branching, surface area and volume stretching were in-
hibited [5], and b) Specific effects on root ion fluxes via H+ competition with base cations for uptake and H+
damage to the ion-selective carrier in root membranes. For example H+ decreases the function of the plasma
membrane and promotes K loss or the inhibition of K uptake, and consequently brings about poor root growth
A decrease in the K, Ca and Mg concentrations in seedling roots was observed under low pH conditions. A
previous study showed that an increase in the hydrogen-ion concentration of the medium generally caused a de-
crease in the rate of cation absorption, probably as a result of competition between the similarly charged ions for
binding and carrier sites [5]. The abnormal morphology caused by acidic conditions may be another reason for
the reduced absorption of cations. Root N concentration decreased with increasing pH in the range of 3.5 - 7.0,
however, plant N content did not appear to decrease.
5. Conclusions
The decline in soil pH with years under pepper production may correlate with high levels of N fertilization,
which usually leads to reduced soil pH. Solution culture studies of the effect of low solution pH indicated inhi-
bition of Piper nigrum L. seedling growth and nutrient uptake in high H+ conditions. Pepper seedling fresh
weight was reduced 34% at pH 3.5 compared to pH 7.0. The root K, Ca and Mg absorption were restricted by
the low pH. The decrease in fresh weight was clearly affected by the K, Ca and Mg decline. The root morphol-
ogy was significantly modified, usually reduced at the low pH. Root elongation, lateral branching, surface area
and volume stretching usually enhance the ability to take up nutrients of K, Ca and Mg from acid solution. Al-
ternatively, excessive H+ decreases the function of the plasma membrane and promotes K, Ca and Mg loss or the
inhibition of K, Ca and Mg uptake, and consequently brings about poor root growth.
Nutrient solution pH significantly reduced growth of pepper seedlings with most reduction occurring in the
pH range from 4.0 to 3.5, according to minimal fresh weight, morphology of root and root K, Ca, Mg concentra-
tions. Outside this range, growth was progressively increased with suitable pH values. Determinations of root
morphology should be the priority in evaluations of pepper regarding growth yield in response to pH.
This work was supported by the Natural Science Foundation of China (grant nos. 31301857) and Natural
C. Zu et al.
Science Foundation of Hainan province (grant nos. 311084). We are grateful to Russell Yost for helpful com-
ments on the manuscript and Hongliang Tang for helping with part of the statistical analyses. We also thank two
anonymous reviewers for their constructive comments, which helped in improve the manuscript.
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... Thus it was observed that the root growth was significantly affected by the acidic soil pH condition. Zu et al. (2014) also indicated that low soil pH at 3.5 may directly inhibit root development and reduce seedling growth of black pepper (Piper nigrum L.). Furthermore, different soil texture of the marginal soils also influence the root growth as the proportion of sand, silt and clay has effect on soil bulk density which is related in root penetration capability. ...
... In the present study, pot experiments confirmed that long-term continuously cropped black pepper orchard soil showed significantly lower vanilla Fusarium wilt disease, implying that crop rotation is an effective management practice to reduce soil-borne plant disease in agro-systems ( Wang et al., 2015). In addition, it will also help us to take advantage of the large area of black pepper continuous cropping soil in tropical China ( Zu et al., 2014;Xiong et al., 2015a). ...
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Soil acidification is a major problem in soils of intensive Chinese agricultural systems. We used two nationwide surveys, paired comparisons in numerous individual sites, and several long-term monitoring-field data sets to evaluate changes in soil acidity. Soil pH declined significantly (P < 0.001) from the 1980s to the 2000s in the major Chinese crop-production areas. Processes related to nitrogen cycling released 20 to 221 kilomoles of hydrogen ion (H+) per hectare per year, and base cations uptake contributed a further 15 to 20 kilomoles of H+ per hectare per year to soil acidification in four widespread cropping systems. In comparison, acid deposition (0.4 to 2.0 kilomoles of H+ per hectare per year) made a small contribution to the acidification of agricultural soils across China.
Recently, there has been interest in shifting to carbon-neutral sources of energy, including bioenergy from short rotation woody crops. This study looked at the growth and yields of short rotation willow in an agroforestry intercropping system compared to a conventional single variety plot system used as a control. Three willow clones (Salix dasyclados SV1, Salix miyabeana SX67 and Salix purpurea 9882-41) were established in each field setup, where in the agroforestry field willow plots were located between 15m wide rows of 20-year-old mixed tree species. Differences in photosynthetic photon flux density (PPFD), soil temperature, soil moisture and soil/foliar nitrogen between the two field setups were investigated from 2006 to 2007. Willow yields were significantly higher in the agroforestry fields during both years of the study, with 0.8 and 0.5odtha−1 for the agroforestry and control fields, respectively, in 2006, and 3.0 and 1.1odtha−1 respectively, in 2007. There were opposite trends in clonal yields between the two field setups in 2006, but in 2007, clonal yields were in the same order across fields with averages of 2.8, 2.2 and 1.2odtha−1 for SV1, SX67 and 9882-41, respectively. Daily average photosynthetic photon flux density was 210μmolm−2s−1 (16%) lower in the agroforestry system, and PPFD was correlated with soil temperatures that were on average 0.4°C and 2.7°C lower in the agroforestry field in 2006 and 2007, respectively (r=0.82 and 0.93). Soil temperatures were negatively correlated with soil moisture levels that were on average 1.4% and 1.9% higher in the agroforestry field in 2006 and 2007, respectively (r=−0.54 and −0.41), and soil moisture content was positively correlated with willow yields (r=0.49 and 0.72). There was less soil available nitrogen in the agroforestry field, but no difference in foliar nitrogen between fields. An experiment excluding root competition in the top 1 m of soil between intercropped trees and willows in the agroforestry field found no significant competition for soil moisture or nitrogen in the first two years of growth. Results of this study suggest that moderate shading in an intercropping setup can result in a buffering effect on microclimate conditions, where there is less variation in soil moisture content and soil temperature across a range of weather conditions.
The acidification of allophanic Andosols by fertilizer application in relation to soil productivity was studied with special reference to the soil colloidal composition. Among the Japanese cultivated allophanic Andosols, in 95% of the samples, the exchange acidity y1 was < 6, while in 5% of the samples (30 soil samples) the exchange acidity y1 was ≧ 6. The strongly acidic allophanic Andosols (exchange acidity y1≧ 6) did not significantly differ from the weakly acidic allophanic Andosols in the contents of amorphous materials. However, the mean values of the strongly acidic allophanic Andosols for the soil pH and base saturation which were 4.5 ± 0.3 and 15 ± 9%, respectively, were relatively lower than those of weakly acidic allophanic Andosols. The content of KCl-extractable Si in the strongly acidic allophanic Andosols dominated by allophane (exchange acidity y1≧ 6 and allophane/clay content ratio > 0.7) ranged from 51 to 141 mg kg−1, and the values increased with increasing concentrations of KCl-extractable Al. KCl-extractable Al in the cultivated allophanic Andosols was considered to be derived from both the allophane fraction and Al-humus complexes. In the field studies of allophanic Andosols with successive application of fertilizers conducted at the Fujisaka Branch of Aomori Agricultural Experiment Station, the value of the exchange acidity y1 of the topsoil and subsoil exceeded 6 about 40 and 50 years after the start of the experiment, respectively. The yield of flint corn (Zea mays L. var. indurata Bailey. cv. Onoa) in the fertilizer plot decreased by the strong soil acidification. The amount of nitrogen mineralized in the soil of the fertilizer plot during a 280-d incubation was 0.114 g kg−1 in the topsoil and 0.041 g kg−1 in the subsoil. These values were relatively low compared with those in the lime + fertilizer plot. Thus, we confirmed that the subsoil fertility of the fertilizer plot decreased by heavy application of fertilizers.
Increased use of N fertilizer and more intensive cropping due to the rising food demand in the tropics requires design and evaluation of sustainable cropping systems with minimum soil acidification. The objectives of this study were to quantify acidification of an Oxic Kandiustalf with different types of N fertilizer in two cropping systems under no-tillage and its effect on crop performance. Chemical soil properties in continuous maize (Zea mays L.) and maize-cowpea (Vigna unguiculata (L.) Walp) rotation were determined with three N sources (urea (UA), ammonium sulfate (AS) and calcium ammonium nitrate (CAN)) in Nigeria, West Africa, during five years. Chemical soil properties were related to grain yield and diagnostic plant nutrient concentrations. For the three N sources, the rate of decline in soil pH in maize-cowpea rotation was 57±7.5% of that in continuous maize, where double the amount of N fertilizer was applied. The rate of soil acidification during the five years was greater for AS than for UA or CAN in continuous maize, and not different for UA and CAN in both cropping systems. With AS, soil pH decreased from 5.8 to 4.5 during five years of continuous maize cropping. Exchangeable acidity increased with N fertilization, but did not reach levels limiting maize or cowpea growth. Return of residues to the soil surface may have reduced soluble and exchangeable Al levels by providing a source of organic ligands. Soil solution Mn concentrations increased with N fertilization to levels likely detrimental for crop growth. Symptoms of Mn toxicity were observed on cowpea leaves where AS was applied to the preceding maize crop, but not on maize plants. Soil acidification caused significant reductions in exchangeable Ca and effective CEC. Main season maize yield with N fertilization was lower with AS than with UA or CAN, but not different between UA and CAN during the six years of cropping. The lower maize grain yield with AS than with the other N sources was attributed to lower pH and a greater extractable Mn concentration with AS. When kaolinitic Alfisols are used for continuous maize cropping, even under no-tillage with crop residues returned as mulch, the soil may become acidifed to pH values of 5.0 and below after a few years. The no-till cereal-legume rotation with judicial use of urea or CAN as N sources for the cereal crop is a more suitable system for these poorly buffered, kaolinitic soils than continuous maize cropping. The use of AS as N source should be avoided. H Marschner Section editor
Acid soils occupy approximately 30% or 3950 m ha of the world's ice free land area and occur mainly in two global belts where they have developed under udic or ustic moisture regimes. The northern belt (cold and temperate climate) is dominated by Spodosols, Alfisols, Inceptisols and Histosols and the southern tropical belt consists largely of Ultisols and Oxisols. Sixty-seven percent of the acid soils support forests and woodlands and approximately 18% are covered by savanna, prairie and steppe vegetation. Only 4.5% (179 m ha) of the acid soil area is used for arable crops. A further 33 m ha is utilized for perennial tropical crops. The value of the annual production in these areas is approximately US$ 129 billion. Value of products from forests, woodlands and permanent pastures on acid soils is difficult to evaluate. Forests of the tropics and wetlands have an invaluable role in global, regional and local ecosystem balance and a protective role for flora, fauna and water resources. While acid soils in the northern belt are increasingly protected and reafforested, the destructive exploitation of timber and abusive modern shifting cultivation have contributed to the loss of >250 million ha of tropical forest during the second half of this century leaving vast areas of anthropic savannas on heavily eroded and degraded acid soils. The authors believe that attempts to develop acid soils for agriculture and agroforestry in the tropics should concentrate on these deforested and abandoned areas of degraded acid soils. However, this will be difficult without significant initial investment and adequate technology. A three step development approach is suggested, which could help prevent or halt the annual destruction of >5 mill. ha tropical forests by “untraditional shifting cultivators’. It would help to protect the fragile natural ecosystems on tropical acid soils now considered to be indispensable for the future life on earth.
Exploring the genetic resource of crops is one alternative way to utilize the less available phosphorus (P) in soils, and copy with the incoming shortage of rock phosphate (Rock P). Genotypic differences in low-P tolerance exist in many crop species and result from various physiological and morphological mechanisms. In this study, low-P tolerance of two maize genotypes that had been identified to have contrasting P efficiency (grain yield) in a calcareous soil was investigated. Parameters measured were biomass accumulation, root growth and root exudation of organic acids, root acid phosphatase (APase) activity, and rhizosphere pH under P-deficient (−P) and P-sufficient (+P) conditions in solution culture. The results showed that −P treatment increased root biomass (from 6th to the 15th day), root to shoot ratio, lateral root length, and APase activity in roots and on the root surface, but reduced root exudation of organic acids and pH in rhizosphere for both genotypes. The P-efficient line 181 had a larger root system in terms of root weight and lateral root length than the P-inefficient line 197 in both P treatments, indicating root morphology of line 181 is an advantageous but non-specific trait in adaptation to low P stress. Genotype 181, when grown with −P markedly reduced the pH of the solution and rizhosphere and increased the APase activity in the roots and on the root surfaces. Surprisingly, root exudation of organic acids was reduced by −P in both genotypes. Exudation rate of organic acids in 181 was lower than that of 197 under both P treatments. It was concluded that efficient use of P in the calcareous soil by 181 is related to its large root system, greater ability to acidify the rhizosphere, and positive response of APase production and excretion to low P conditions.
Aluminum (Al) is the most abundant metal in the earth's crust, while its soluble ionic form (Al(3+)) shows phytotoxicity, which is characterized by a rapid inhibition of root elongation. Aluminum targets multiple cellular sites by binding, resulting in disrupted structure and/or functions of the cell wall, plasma membrane, signal transduction pathway, and Ca homeostasis. On the other hand, some plant species have evolved mechanisms to cope with Al toxicity both externally and internally. The well-documented mechanisms for external detoxification of Al include the release of organic acid anions from roots and alkalination of the rhizosphere. Genes encoding transporters for Al-induced secretion of organic acid anions have been identified and characterized. Recent studies show that ABC transporters are involved in Al resistance. The internal detoxification of Al in Al-accumulating plants is achieved by the formation of nontoxic Al complexes with organic acids or other chelators and sequestration of these complexes in the vacuoles. In some plant species, Al shows beneficial effects on plant growth under particular conditions, although the exact mechanisms for these effects are unknown.