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The roles of organic fertilizers in the management of nutrient deficiency, acidity, and
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toxicity in acid soils
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Patrick S. Michaelϕ
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Department of Agriculture
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PNG University of Technology, Lae, MP411, Papua New Guinea
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Telephone +6754734460 Facsimile +675 75667
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https://orcid.org/0000-0003-4068-7276
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Abstract
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The different sources of organic fertilizers when available in soils play significant roles in
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establishing appropriate microbial ecology, make available soil nutrients, moisture level, and
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change the biochemical and biophysical properties. In problem soils, in particular acid soils,
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where acidity, low nutrient availability, and toxicity are concerns for general soil use,
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organic fertilizers are important. The importance of organic fertilizers is not limited to their
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roles as a source of the reservoir of soil nutrients, soil moisture, and ameliorants to
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important soil properties that determine soil fertility status but the management of acidity
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and toxicity as two important factors that affect soil productivity. Soil acidity and its
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associated problem, toxicity, is not only a problem for plant productivity, hence livestock
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production but soil microbial ecology which are the major sources of organic fertilizers. This
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paper has synthesized relevant literature and point outs that death microbial, animal and
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plant biomass as the main sources of organic fertilizers. It is further emphasized that organic
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availability of organic fertilizers improves low availability of soil nutrients, ameliorates
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acidity and detoxify toxicity of toxic cations (Al, Mn, and Fe), and enhances microbial
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ecology of problem soils, globally. The underlying mechanisms for these processes,
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improving soil nutrients, amelioration of soil acidity and detoxification of toxicity have been
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discussed.
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Keywords: OFs, soil infertility, acidity, toxicity, management
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1. Introduction
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Organic fertilizers (henceforth OFs) are materials of plant, animal, or microbial origin, partly
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or completely decomposed that add plant nutrients (N, P, K, Ca, S, Mg, etc.), compared to
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organic amendment (e.g. legume-based green manure or crop residue) that adds nutrients as
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well as organic matter (Melero et al., 2007). The availability of OFs varies widely between
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ϕ Correspondence: patrick.michael@pnguot.ac.pg
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land use conditions and even among regions. The common animal origin OFs are manures,
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deadstock, and industrial processing wastes. The OFs of plant origin are green manure,
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residues (vermicompost), and industrial and municipal wastes (Bernal et al., 1998). Under
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natural soil conditions, OFs in root exudates and plant litter have bio-fertilizing effects on soil
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(e.g. Bauer and Black, 1994). In as much as animals and plants are being the major sources,
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OFs of microbial origin are important but their importance is not widely popular.
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There are countless microbes (bacteria, virus, and fungi) which either are free-living in
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the soil or in animals and plants hosts that accumulate in the surface environments and
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provide ecological services (Baumann et al., 2009), e.g. decomposition of organic matter.
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When these microbes die, as a result of depletion of resources or detrimental change in the
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soil ecosystems, the dead microbes become an important source of OFs (Albiach et al.,
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2000). This is the main reason pioneer microbes and microscopic plants are the fundamental
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sources of OFs in a barren soil, to begin with. The composition of the microbes varies
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spatially depending on the type of microbial ecology the types of OFs are able to establish.
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The quantity of OFs of microbial origin are affected by host soil types, soil conditions, and
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climatic variability (Belaeva and Haynes, 2009), similar to OFs of animals’ and plants’
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origin. For instance, there could be more organic fertilizer of microbe origin in an alkaline
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soil from bacteria than other microbes. Similarly, in an acid soil, there could be more from
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fungal biomass (matter).
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The different sources of OFs when available play significant roles in establish
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appropriate microbial ecology and that in turn affects nutrient composition and availability,
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moisture level, and biochemical and biophysical properties of soils (Larney et al., 2011). In
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problem soils, in particular acid soils, where acidity, low nutrient availability, and toxicity are
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concerns for general soil use and management (e.g. Michael et al., 2015), OFs are important
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(Materrechera and Mkhabela, 2002). The importance of OFs are not limited to their roles as
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reservoirs of soil nutrients but the management of acidity and toxicity as two important issues
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that affect soil productivity (Pichtel et al., 1994).
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Finally, OFs improves low nutrient availability, ameliorates soil acidity and detoxify
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toxicity of toxic soils, and enhances microbial ecology of problem soils. Although the
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importance of OFs addition to manage soil fertility and properties that determine them are
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widely available, the roles of OFs in the management of three common issues of acid soils
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(low nutrient availability, acidity, and toxicity) in some regions, such as in the humid tropics,
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need to be clearly pointed out. This paper, therefore, aims to clearly point out the importance
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of OFs application and their role in the management of acid soil infertility (low nutrient
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availability).
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2. Methodology
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Literature (journal, book, and online source) containing data and information on organic
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fertilizer published in the last 10 years was surveyed. These data were sorted out into three
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classes as microbial, animals, and plants to clearly point out the origin of the OFs. To clearly
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address the aim, the roles of each of the organic fertilizers in the management of infertility,
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acidity, and toxicity as three main issues of acid soils, and the different sources of organic
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fertilizer are presented. Based on the types of ecological functions provided, the soil microbes
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have been put together, e.g. bacteria and fungi because of their roles in decomposition. The
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roles of OFs in the management of acid soil infertility, acidity, and toxicity, have been
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discussed, separately. The conclusion draws out the major issues in acid soils and establishes
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the roles OFs play in management of the issues in relation to general land use and
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management, points out possible future research directions.
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3. Microbes as organic fertilizers
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The nutrients from OFs that are available in the soil and to plants depend on the initial
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microbial ecology that was established and the conditions, e.g. soil type and moisture content,
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aeration, and structure that affect the functions of microbes. The composition of the microbial
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ecology is important for OFs include bacteria, fungi, algae, protozoa, viruses, and
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actinomycetes (Fig. 1). Decomposition of organic fertilizer sources (Table 1) and the release
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of nutrients is carried out by microorganisms interacting with the prevailing biophysical and
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biochemical components of the soil environment (Hoyle and Murphy, 2006b). In all, soil
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microbes are producers and components of the mechanisms of soil fertility, and compared to
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the type and kind of biomass that can be found, their contribution is enormous. A number of
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key processes influenced by microbes that conditions the soil include carbon sequestration,
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moisture, water holding capacity, aeration, temperature, soil reaction, and mineralization.
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Some groups of bacteria, e.g. the Rhizobia, are able to trap atmospheric nitrogen and
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incorporate it in their tissues which later becomes part of the microbial mass or becomes
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available in the soil when the hostplants die and decompose. In almost any
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surface environment including soil, more than 80% of the microbes are found within the 10 –
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15 cm below the surface (Michael, 2020c), indicating that microbes need the resources all
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living things need to survive and provide ecological services.
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Figure 1. Soil microbes and their inter-linked ecological services to soil.
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3.1 Bacteria and Fungi
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Bacteria and fungi by no means are similar, biologically or taxonomically. Bacteria are
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single-celled and microscopic compared to fungi which are unusual organisms in which are
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not animals or plants. Bacteria are also more abundant than fungi in the soil. An estimated 10
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million to a billion bacteria are in a spoon of moist soil compared to 100 to 10 000 fungi in a
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gram of soil. The turnover of these types of populations and their contribution to organic soil
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fertility are important. Despite their differences, these organisms are considered together as
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far as their primary ecological service is concerned in the decomposition of dead matter is
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concerned. Decomposition of organic matter (Table 1) and release of nutrients accessed by
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plants is by bacteria, and to a certain extent by saprophytic fungi.
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The functions of bacteria and fungi in terms of soil fertility, however, are indirect
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and/or in biological or mechanical pathways conditioning soil fertility generation and
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sustainability processes. Saprophytes change organic matter into fungal biomass, carbon, and
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Algae
o Soil fertility
o
Carbon and oxygen
o Binding
o Water retention
oWeathering
o
Nitrate leaching
Fungi
o
Decomposers
oSaprophytic
oMutualists
oParasites
Bacteria
o
Decomposers
oSymbiosis
Microbes
Actinomycetes
oSoil odor
Virus
oSoil
ecology
Protozoa
o
Bacterial and
microbial
balance
Nematode
oPlant feeders
oBacteria feeders
oFungi feeders
oProtozoa feeders
oMineralizers
oSoil fertility
oNematode
feeders
oSoil ecology
oMicrobial
equilibrium
ecology
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organic molecules, e.g. organic acids. Apart from the saprophytes, there are parasitic and
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mutualistic fungi (Fig. 1). The parasitic fungi are pathogenic in nature and cause diseases in
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plants and animals. The mutualistic, in particular, the mycorrhizal fungi, colonize roots of
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trees (ectomycorrhizae) and grasses (endomycorrhizae) and make available nutrients from
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decomposition by bacteria and saprophytic fungi to plants. It’s the function of fungi that
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results in the decomposition of the recalcitrant components of organic fertilizer sources, such
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as lignin of plant and animal matter.
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Figure 2. Biofertilizers are microbes that solubilize phosphate, fix nitrogen, uptake potassium
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and promote plant growth (Kloepper et al. (2004)1, Gizaw et al. (2017)2, Iqbal et al. (2021)3).
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The surface soil is often frequently altered, affecting the microbial entity and its services.
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This is the prime reason loss of soil fertility is an issue in the surface soil, e.g. in soil under
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agroforestry systems. In surface environments where there is constant alteration, e.g. under
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tillage in farm soil, drought due to dry spell, inundation due to flooding, or following natural
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catastrophic events like a volcano, bacteria dominate fungi, even for a number of minutes,
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when the alteration becomes conducive, resulting in exponential growth. Turnover of the
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huge population of bacterial becomes a soil fertility source for the soil. For this reason,
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bacteria are often referred to as ‘bags of biofertilizer’.
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The fertilizing effect coming from the feeding actions of protozoa and the release of
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nutrients like nitrogen and phosphorus. Some bacteria are able to secrete enzymes that help
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digest the dead matter of microbes to release nutrients. Similarly, a number of bacteria
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Algae3
o Azola
o Cyanobacteria
o Azospirillium
oMycorrhizae
oPrototheca
oHelicosporidium
oMicrasterias
oAscophyllum
oRhodophyta
oCoelastrella
oChlorolobion
Fungi2
o Mycorrhizae
o
Ectomychorrhizae
o Endomycorrhizae
o Tricoderma spp.
o Penicillium spp.
o Aspergillus spp.
o Chaetomium spp.
Bacteria1
oAzotobacter
oFrankia
oBacillus
oEnterobacter
oKlebsiella
oPseudosomonas
oArthrobacter
oBurkholderia
oRhizobium
oAllorhizobium
oAzorhizobium
oBradyrhizobium
oMesorhizobium
o
Methylobacterium
oSinorhizobium
Microbial
biofertilizers
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produce polysaccharides or glycoproteins that cement soil particles (sand, silt, and clay) and
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help in the formation of micro-aggregates, important for soil structure. Most bacteria,
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e.g. Aerobacter, are aerobic and need oxygen to decompose organic matter, whereas others
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are anaerobic and strive under anaerobic, limited oxygen conditions (hydric soil). In terms of
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microbial interactions, bacteria and fungi (see below) seem to have common in symbiotic
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associations with plants (e.g. Rhizobium and Mycorrhizae) and nutrient cycling, partly
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because their major role is decomposition, acting on the original organic fertilizer sources as
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by bacteria or acting on the recalcitrant components, such as by fungi.
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In making reference to soil and organic fertilizers, fungi (there are about 70, 000) help
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decomposes hard to decompose components, e.g. lignin, and consumer the simpler ones, e.g.
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from organic matter of microbial origin. There are four major groups: Zygomycota (<1000
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species), the common bread molds; Ascomycota (30,000), mostly yeast used in baking;
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Deuteromycota include mushrooms, toadstools, and puffballs, and Deuteromycota are
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lichens and the mycorrhizal fungus (Lavelle and Spain, 2005). This role performed by
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fungi results in a release of simple organic compounds (residues) which are decomposed by
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other microbes and improve soil fertility.
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The roots of most higher plants including crops (30% of the rhizosphere) are colonized
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by symbiotic mycorrhizae fungi which form the network of hyphae and help plant roots to
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acquire nutrients (nitrogen, phosphorous, potassium, and micronutrients). The mycorrhizae
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network is considered to be responsible for cycling nutrients (reabsorbing and redistributing)
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between the soil and plant roots (Islam, 2008). In healthy soils, the biomass of fungi is
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estimated to be 499 to 4990 kg, equivalent to two to six cows (Smith et al., 1992). Turnover
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of this biomass as a source of organic fertilizer to the soil is a huge contribution from fungi
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alone. The fungal ecosystem tends to be undisturbed soil of perennial plant inhabitants,
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organic residues of high carbon to nitrogen content (C:N) compared to bacteria which
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dominate highly disturbed soil ecosystem, low C:N organic residues, and prefer annual
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plants. The fungal interactions with plants are that of bacteria and that is around the symbiotic
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relationship that exists and recycling of nutrients by the mycorrhizae network.
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3.2 Algae
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In a gram of soil, 100 to 10 000 algae are present. The roles of algae are diverse compared to
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bacteria and fungi (Fig. 1). The algae are photosynthetic and are classed into four groups as
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blue-green (e.g. Cyanobacteria), green (e.g. Prototheca and Helicosporidium), green-yellow
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(Micrasterias sp.) and diatoms (e.g. Centric, Araphids, and Raphids). In an aquatic system,
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algae are the major source of organic compounds and produce oxygen for other life forms
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(Lee et al., 1989). The contribution of algae to soil fertility is enormous (Fig. 2) in addition to
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bio-controlling of pests, soil reclamation, and treatment and cycling of wastewater (e.g.
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Uysal et al., 2015). The marine algae (large brown, Ascophyllum nodosum, and
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red, Rhodophyta spp.) which are rich in potassium and are widely used as organic fertilizer in
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farm soil.
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The Cyanobacteria, in addition to being photosynthetic, are responsible for producing
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biologically active organic compounds in the habitats they share, important as organic
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fertilizer for plants. Studies showed these fungi are involved in nitrogen fixation and reduce
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atmospheric N to ammonia. An earlier study showed a turnover of dead matter of algae
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contributes to organic matter and the mucilage of pamelloid green microalgae binds soil
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particles and forms texture, improving the humus content and making the soil habitat
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conducive for plants (Marathe and Chandhari, 1975). Cyanobacteria species, e.g. Tolypothrix
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tenuis, are important in paddy rice production and the results of studies (e.g. Saikia and
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Bordoloi, 1994) have shown they are important for agricultural sustainability and sustainable
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food production.
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3.3 Protozoa and Nematodes
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Protozoa are single-celled animal-like organisms bigger than bacteria and their population is
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estimated to be 10 000 to 100 000 per gram of soil. On the other hand, nematodes are not
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microorganisms and are worms, mostly found in the surface soils. Despite their differences,
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their commonality in feeding on other soil microbes (Fig. 2), thereby maintaining a microbial
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equilibrium, which makes a comparison of their roles in soil fertility much easier. Most
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protozoa and nematodes feed on other microorganisms and therefore control their
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distributions (Fig. 1). The feeding actions of protozoa and nematodes on microbes and the
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turnover of organic substances from them is the significant contribution made to soil.
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Considering the number of soil microbes that can be available in a given amount of soil, e.g.
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a gram of well moist and aerated soil can contain from one to several thousand, the
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contributions from microbes to soil fertility are huge.
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The feeding actions of protozoa and nematodes, and the turnover of organic substances
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are the primary source of OFs of microbial origin. The microbial interrelationships and the
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associations are essential to the fertility of the soil from organic sources. Initially, like organic
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matter from plants and animal origin (Table 1), bacteria and saprophytic fungi breakdown
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dead microorganisms that end up in the soil due to natural processes (e.g. infections caused
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by viruses and parasitic fungi and resultant death) or as a result of the feeding actions of
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protozoa and nematodes. The mineralization and cycling of the nutrients from decomposition
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and availability to plants and the sustainability mechanisms (reducing of leaching, binding,
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and moisture retention) within the soil-water-plant interface are functions of nematodes.
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These functions are shared responsibility as fungi are considered to perform the same
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functions, quite important because most often not all microbes are present together, any given
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time.
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3.4 Actinomycetes and Viruses
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Somewhere between the bacteria and fungi are actinomycetes which have no clear functions
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related to soil fertility. Genus such as Streptomyces (bacteria) is responsible for the
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characteristic smell from decomposition. Association of viruses is in plants’ and animals’
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diseases, and members of the microbial community. Viruses have no clear roles in relation to
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soil fertility except the that transfer of viral genes to the hosts' genomes results in mortality
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which contributes to soil organic matter and fertility.
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4. Plant and Animal Organic Fertilizers
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The main sources of OFs as pointed out are of plants, animals, and soil microbes (bacteria,
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fungi, and algae) origin. The organic fertilizer of animal origin includes manure, bone meal,
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blood meal, fish emulsion, milk, urea (urine), and manure tea (Table 1). The common sources
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of OFs of plant origin are cottonseed meal, molasses, legume cover crops, green manure
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cover crops, kelp weed, and compost tea (Table 1). The fertilizer or the nutrient content of all
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these sources of OFs may vary depending on the biological and nutritional status of the
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animals and plants and the environment in which the organic fertilizer sources were found.
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For instance, the nutrient content of well-fed and healthy animals could be high compared to
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a poorly fed and biologically sick one. Similarly, the types and production systems of plants
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affect their nutrient contents.
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The interactive performance of plants and animals with their local environments affects
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the type and the number of nutrients that could be sourced from them. The parent materials
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and the soil-forming factors of a soil on which plants were produced or pastures raised have
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significant effects on nutrients that are available to plants. Because OFs are solely for plant
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nutrition, the sources of the OFs, prevailing soil microbial ecology, and environmental
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conditions are important factors that determine the fate of the sources. Some plant and animal
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matter are quite significant sources of OFs compared to others. For instance, green manure
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crops or legumes to any plant material and animal dung compost (cattle, hog, chicken, sheep,
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goat, and duck dung) per se that initially does not contain sufficient plant nutrients (Table 1).
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Table 1. Some common sources of OFs and their major nutrients contents.
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Plant origin
Animal origin
Source
NPK ratio
Source
NPK ratio
Alfalfa meal
3:2:2
Cow manure
2.5:1:1.5
Cottonseed meal
6:1:1
Poultry manure
3.5:1.5:1.5
Corn gluten meal
0.5:0.5:1
Earthworm castings
2:1:1
Compost
2:1.5:15
Blood meal
12:1.5:0.5
Soybean meal
7:2:0
Bone meal
4:20:0
Sea weed
1.5:0.8:5
Feather meal
12:0:0
Grass clippings
1:0:1.2
Seabird Guano
10:10:2
Copra meal
na*
Bat Guano
10:10:2
Palm kernel
na*
Fish meal
5:2:2
Peanut meal
na*
Fish emulsion
5:1:1
Others
na*
Shell meal
5:2:5
*The nutrient contents of most plant source of organic fertilizers is not available (na). This is
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an issue that future research directions need to address.
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Table 1 shows selected sources of OFs containing any amount of N, P, and K. This does not
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necessarily mean, however, the nutrients are crop-available. Nitrogen, for example, the
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amount of ammonium determines its availability compared to N which is less available. That
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means N mineralization must occur. The availability of ammonium N is influenced by
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climate, e.g. weather conditions of the place of application. The nutrient content of a soil type
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from organic fertilizer is important. When the content of a particular nutrient is high, the
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availability of that nutrient from OFs is low, and vice versa. The release of a particular
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nutrient, e.g. N from OFs is slow compared to chemical fertilizers, meaning OFs are not
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quick-fix solutions. The release of nutrients from the OFs are slow and lasting dependent on
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decomposition which directly depends on microorganisms and the type of microbial ecology
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an organic fertilizer source is able to establish (Chang et al., 2007). Chang et al. (2008)
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compared the characteristics of some selected OFs and variations that exist in them.
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Some OFs, e.g. soya bean meal, contain more N than the animal sources, e.g. dairy, hog, or
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chicken dung. Compared to the total N content of the plant-based OFs, the total P content is
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high in the animal-based OFs. And, the total K content is variable, with some sources having
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more K than others (Table 1).
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In addition to nutrients, OFs are important for other soil characteristics (Michael et al.,
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2015). Organic fertilizers enhance micro- and macro-porosity and the associated increase in
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aeration and drainage (Apia and Michael, 2018; 2019), water holding capacity, and available
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water (the water held at field capacity less than that retained at the permanent wilting point)
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(e.g. Michael, 2019). Water movement infiltration into soil is important to plants and to
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manage surface runoff and erosion. Several studies have shown that OFs help improve
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infiltration which decreases over time as nutrients get used up. Organic fertilizers are free of
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potentially harmful substances that can either pollute or contaminate the natural environment
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(Larney et al., 2011). The content of the OFs are basically organic in nature, therefore, slow
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to release and are lasting, important for primary succession and for ecosystem restoration.
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5. Acid soils and management
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Soils with pH less than 5.5 are acid with acid sulfate soils (ASS) with pH less than 4.5 being
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the extreme types. In the humid tropics, Ultisols and Oxisols dominate, covering 22% and
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11% of the tropical land, respectively. On the other hand, ASS covers 17-24 million ha of the
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global land of which 6.5 million are in Asia, 4.5 million in Africa, 3 million in Australia, 3
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million in Latin America, 235 000 in Finland, and 100 000 in North America, respectively
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(e.g. Simpson and Pedini, 1985; Michael, 2013; Michael et al., 2015). The common
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characteristics of acid soils are extreme low pH (4.5), high availability of potentially toxic
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metals and metalloids (e.g. species of aluminium, iron, and manganese), and very low soil
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nutrients (e.g. nitrogen, phosphorus, potassium, calcium, and magnesium, etc.).
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The increase in toxicity and decrease in soil nutrients are caused by the dissociation
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reactions of a low pH (<5.5). An acidic soil condition leads to a dissociation reaction of the
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soil matrix in which both the source of contaminants and the soil nutrients are found. The
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resultant effects are the release of the contaminant sources into waterways or surface
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environments where they accumulate and contaminate them. On the other hand, the soil
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nutrients remain in solution and get leached deep into the soil depth, e.g. following the onset
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of rain, or get washed off and transported to other surface environments, enriching the
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nutrient contents of soils there. Nutrient enrichment in the receiving end surface environment
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has always been a problem. For instance, enrichment of phosphorus in lagoons or stagnant
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water sources as a consequence of surface runoff has resulted in algal blooms
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(eutrophication). Not only that, the acid soils face serious deficiencies in soil nutrients as well
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as extremely altered biophysical and biochemical soil fertility ecological services, e.g. slow
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decomposition due to the poor establishment of microbial ecology. In so far as soil fertility is
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concerned, soil acidity, toxicity, and low nutrient availability are the major concerns of acid
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soils. In the next two sections, the importance of soil microbes in management of these
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concerns and microbial biomass in addressing nutrient deficiency are discussed.
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5.1 Management of nutrient deficiency
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As pointed out above, the availability of most soil nutrients is pH-dependent (Table 2). The
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concentration of acid minerals, iron, manganese, zinc, copper, zinc, and aluminium increases
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as soil pH drops below 5 units (Fig. 3). At this pH, the availability of nitrogen, phosphorus,
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calcium, magnesium, potassium, sulphur, molybdenum, and boron decreases. It is true too
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that nitrogen, potassium, sulphur, and molybdenum availability are high beyond pH 8.5
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units, whereas at this pH the availability of phosphorus, calcium, magnesium, and boron
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becomes severely deficient in the soil. The lack of nutrients in acid soils is further
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complicated by the absence of organic matter due to the lack of higher plants or poor
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microbial ecology. Poor microbial ecology in part is due to lack of metabolic substrate and
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the harsh soil conditions for survival. These point out the management of acid soil is
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important for soil health and productivity through organic matter amendment, revegetation,
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and establishment of productive microbial ecology. As pointed out, acid soils are only able to
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support a number of plants in the form of shrubs and adopted tress because of poor soil
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conditions (a nutrient deficiency, acidity, and toxicity).
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Table 2. Optimum soil pH for availability of important nutrients (Foth, 1990).
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Nutrients:
N
P
K
S
Ca
Mg
Fe
Mn
B
Cu
Zn
Mo
pH range:
6 – 8
6.5 – 7.5
>6
7 – 8.5
<6
5 – 6.5
5 – 7
7
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Under natural conditions, it is impractical to manage a large area of problem soils, however,
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under general soil use and management conditions, such as in farm soil or under land
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management situations, e.g. revegetation, some approaches are practical. One of the primary
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approaches to recondition acid scalded soils is by organic fertilizer amendment. The various
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types of organic fertilizer sources that are freely available or at a reasonable cost are given in
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Table 1. Organic fertilizer amendment not only improves the soil nutrient conditions and
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enhances the growth of high plants but makes the soil conducive for microbial ecology to
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establish and improve the productivity of the soil (Bulgarelli et al., 2013). Under harsh soil
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conditions, the turnover of organic matter from live plants may be small but immediate,
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allowing a significant amount of moisture to build up and help more microbes to settle in. As
370
microbial population increases, more organic matter is turned into the soil as organic fertilizer
371
which not only builds the soil fertility but helps other important microbes, e.g. the
372
mycorrhizae to colonize the rhizosphere and make plants more productive (Hassan and
373
Mathesius, 2012).
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375
376
377
378
379
380
381
382
383
384
385
386
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388
389
390
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392
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397
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Figure 3. pH-dependent availability of essential soil nutrients (adapted from National Soil
399
Survey Manual, USDA, NRCS, 2017). The relative availability is indicated by the width of
400
the band. pH less than 5.5 and 7.5 units is over acidic and alkaline, respectively. Sulfuric soil
401
pH can be less than 4.0 units.
402
403
The turnover of organic matter, as well as microbes, are important sources of organic
404
fertilizer in acid soils. A combination of turnover of organic fertilizer from the microbes as
405
well as plants is important for the initial buildup of soil fertility in acid soil. In problem soils,
406
microbes need the energy to decompose organic matter to release organic fertility. Therefore,
407
the type of microbial ecology established is affected by the labile carbon source which are
408
sourced from residues of plant and animal origin or from the exudates of roots of plants
409
13
(Cookson et al., 2008). When predator microbes engulf microbes or decompose death ones,
410
the microbial biomass becomes a significant contributor to OFs. Depending on the soil types
411
and conditions, the microbial biomass nitrogen can be as high as 70% and the nitrogen
412
released can be as high as 80% (Murphy et al., 1998).
413
414
5.2 Management of acidity and toxicity
415
416
Two other most important issues in acid soil, apart from nutrient deficiency, is soil acidity
417
and toxicity. Soil acidity and toxicity are interconnected because acidity is not only caused by
418
H+ per se but high amounts of acid minerals (Al3+, Fe2+, and Mn4+) whose concentration
419
increases as pH decreases (Fig. 3). The conventional method of managing soil acidity is the
420
application of an alkaline material, such as a mineral lime (e.g. Michael et al., 2015). The
421
affordability of fine lime is an issue in some regions and the practicality of applying it in
422
large areas is impossible (e.g. Michael et al., 2015). Application of lime in protected areas,
423
such as in Ramsar Wetlands, is even not allowed. Under such conditions, the return of
424
organic matter and the constituents released from decomposition as sources of OFs to the
425
highly oxidized (>400 mV) acid soil is important. Organic compounds released from the
426
organic matter into the soil carry far more negative charges which are able to retain the acid
427
cations and reduce their availability. The detainment of the cations is sufficient to reduce
428
leaching, preventing accumulation and having toxic effects elsewhere. The reduction in
429
leaching activities further results in the accumulation of cationic nutrients (e.g. Ca2+),
430
essential for the management of acid soils.
431
432
433
434
435
436
437
438
439
440
Figure 4. Adsorption of acid cations by organic matter in a soil solution. The anions (negative
441
charges) in an organic matter are indicated by the broken ( ) lines.
442
443
Acid cations in
soil solution
(H+ and Fe, Al,
Mn species)
Organic
matter
H+
Fe2+
Al3+
Fe2+
Al3+
H+
H+
H+
Fe3+
Fe3+
Mn2+
+
H+
H+
14
Under stressed soil conditions, for instance, in acid soils, plants release root exudates,
444
purportedly as a survival mechanism. Ziegler et al. (2016) showed phosphate deficiency
445
resulted in an abundance of oligolignols and a lower coumarins abundance. The release of
446
exudates by plant genotypes and the microbe recruitment processes to act on them and
447
contribute to organic soil fertility are variable (Haney et al., 2015). The variation in the types
448
of exudates in turn affects the microbial ecology to be established. Hütsch et al. (2002)
449
showed pea plants released more sugars and oil radish released more organic acids as
450
exudates. The types of exudates releases are influenced by the stress types and soil conditions
451
(Neal and Ton, 2013). Bacteria are often abundant in slightly alkaline soil and fungi under
452
acidic conditions (Rousk et al., 2009). Under anaerobic conditions, wetland plants exuded
453
more oxalic acid (Wu et al., 2012). There are reports of plants under stress conditions
454
recruiting more beneficial microbes. Yan et al. (2011) and Lee et al. (2002) have shown
455
foliar feeding by insect pests (aphids) increased rhizosphere population of beneficial gram-
456
positive bacteria and fungi.
457
Soils that are strongly acid (pH<5.0) negatively affect plant productivity and only
458
tolerant plants dominate. The issue of soil acidity is not only a problem on agricultural soils
459
but an issue for revegetation and environmental sustainability. Figure 4 shows the liming and
460
ameliorative effects of organic matter as a source of organic soil fertility. The mechanisms
461
involved in amelioration of acidity and detoxification of toxicity by organic matter are further
462
shown. The transformation of organic carbon (decarboxylation of the organic anions)
463
consumes protons and ameliorates soil acidity (Yan et al., 1996). Transformation of nitrogen,
464
that is ammonification of residue-nitrogen, ameliorates soil acidity. When we added urea in
465
an extreme type of acid soil (i.e. acid sulfate soils) of pH<4 and incubated for 6 months, the
466
low pH was increased to near pH 8 units (Michael et al., 2016). We observed similar changes
467
in pH when organic matter of high nitrogen content was added in extremely acidic soils in
468
various studies (e.g. Michael et al., 2015; 2016; 2017). Mineralisation of the residue-N and
469
acidification of the soil has been pointed out by other researchers (e.g. Xu et al., 2006; Xu
470
and Coventry, 2003).
471
The association or dissociation reaction of organic compounds and residues of organic
472
material origin has been pointed out earlier (Fig. 4). We have pointed out too that the carbon
473
and the nitrogen transformation processes that manage acidity in soil are part of the microbial
474
ecology. The carbon and the nitrogen transformation and cycling processes meet the energy
475
and growth requirements of the microbial ecology. The demand for carbon and nitrogen
476
created by the microbial ecology affects availability and limitation may set in under cropped
477
15
and vegetated soils. One of our studies (Michael et al., 2016) has shown that establishment of
478
microbial ecology capable of inducing significant changes in soil chemistry occurs when both
479
carbon and nitrogen are present. There were no significant net changes in soil chemistry (pH,
480
redox and sulfate content) following addition of sucrose as a carbon source, compared to urea
481
containing both carbon and nitrogen.
482
The carbon and nitrogen needs of plants under crop or revegetation further limits their
483
availability. Generally, where these elements are engaged in either the soil-microbe or soil-
484
plant system, the net ameliorative change in acidity (i.e. increasing pH) is small. In toxic
485
soils, active uptake of nitrate (NO3-) by plants or microbes results in removal of cationic toxic
486
minerals (e.g. Al2+, Mn3+ and Fe2+), and that has a detoxifying effect on the soil. In soils with
487
high concentrations of toxic metals and metalloids, normal plant growth and development are
488
difficult to be established and important plant species such as legumes capable of enriching
489
the nitrogen and carbon content are not present. In such soils, e.g. in the humid tropics,
490
continue to remain acid and toxic for a longer period, unless managed.
491
The need for land uses, in particular, and the need for increase in food production and
492
more space for infrastructure development in general, has put an emphasis on acid soil and its
493
management using OFs. The main source of organic fertility following land conversion, e.g.
494
tropical rainforest to cropland, is decomposition of plant matter, however, the increase in
495
organic soil fertility is short-lived. Heavy cropping followed by torrential rainfall deplete the
496
nutrient input of organic matter and the acidity resurrects. Soil toxicity, which was buffered
497
by the frequent turnover of organic matter, as discussed above, may become more
498
pronounced in the absence of the tropical rainforest tress, worsened by climate change. Some
499
of these issues call for more advance research in management of soil acidity and toxicity in
500
relation to organic soil fertility.
501
502
6. Conclusion and Future Research
503
504
Biofertilizers are sources of soil nutrients, ameliorants, or detoxifiers of completely
505
decomposed matter of microbial, animal, or plant origin. The main sources of biofertilizers
506
are dead microbes, animals, and plants or their end products (e.g. death carcass and litter).
507
The bioavailability of the biofertilizers is entirely dependent on the type of microbial ecology
508
that is established in the presence of the sources. Decomposition is the function of bacterial
509
ecology in alkaline soils (pH>5) whereas in acid soils (pH<5) is a function of fungal ecology,
510
instead. The biofertilizers that become available in the soil as the function of microbial
511
16
ecology address the issues of nutrient deficiency, ameliorate acidity and detoxify toxicity
512
through the association and dissociation reactions which not only consume acidity but reduce
513
the concentrations of cations with uptake of anions and cation-exchange capacity. The
514
availability of nutrients to offset deficiency, amelioration of acidity, and detoxification of
515
toxic soils are three major importance of biofertilizers in the management of problems
516
(nutrient deficiency, acidity, and toxicity) soils.
517
In the light of climate change and the increase in human population need strategic plans
518
to increase production under the altered climate. In many regions, the concept of increasing
519
food production is not that easy because there is not enough land area or there is land but
520
highly depleted of soil nutrients and need serious management, e.g. application of inorganic
521
fertilizers. The nutrient deficiency issue is compounded by acidity and toxicity problems in
522
most soils which affect productivity. In the poor economies where malnutrition is an issue
523
associated with not producing and having the right kind of foods, soil nutrient deficiency,
524
acidity and toxicity need to be managed. Since having access to and affordability of inorganic
525
fertilizers are issues for farmers in poor economies, organic fertilizers of microbial, animal
526
and plant origin seem to be an inexpensive alternative to address problem soils.
527
In the light of all of these, the quest to increase food production on problem soils under
528
an altered climate to establish various importance of organic fertilizers need advance
529
research. One of these is that whilst the underlying mechanisms in addressing nutrient
530
deficiency, acidity, and toxicity are understood, there is a lack of data on how much organic
531
fertilizer is exactly contributed by microbes, animals and plants under different surface
532
environments. Researches need to establish the types and kinds of organic fertilizers that
533
different types of microbes contribute to organic soil fertility under acid, alkaline, saline,
534
drought, or flooded soil conditions. Future research direction with biofertilizers dealing with
535
live microbes and their interactions is important and needs to go together with organic
536
fertilizer research endeavours.
537
Under certain farming or cropping systems, such as the slash-and-burn agriculture
538
where the initial microbial ecology is lost or a new but complex one is formed because of
539
multiple crops being cultivated needs investigation, how much organic fertilizer is added
540
needs to be researched. Intercropping legumes with non-legume crops, for example, increase
541
crop production. The kind and types of organic fertilizers contributed by the specific type of
542
microbes, the legumes and other crop plants need to be established. These types of researches
543
though complicated conventionally, modern molecular and use of genome techniques make it
544
possible to ascertain what type of organic fertilizer was contributed by soil microbes and the
545
17
crop plants at different stages of growth and under different crop production management.
546
The full nutrient contents (macro- and micronutrients) of most plant and animal sources of
547
organic fertilizer are not widely available in the literature, this needs to be researched.
548
549
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