Soil serves as a primary source of nutrients for crop production, expected to supply nutrients to plants in the right amounts and at the appropriate times.
However, abundant soil nutrients may not always be available for crop use due to extraneous factors that govern their availability, beyond mere abundance or deficiency.
This article explores the factors affecting the soil’s ability to supply nutrients in forms available to plants.
Nutrients essential for plant growth are categorized into structural nutrients, major nutrients (primary and secondary), and micronutrients. Each group is discussed with reference to the factors influencing nutrient availability.
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Micronutrient Cations
Micronutrient cations (Fe, Mn, Zn, Cu, Al, and Co) are influenced similarly within the soil environment. Key factors affecting their availability include:
1. Soil pH: Micronutrients are most soluble and available under acidic conditions. In highly acidic soils, ions of Fe, Mn, Zn, Cu, and Ni are relatively abundant, with Mn often reaching toxic levels for plants. As pH increases, these cations transform into hydroxyl ions and eventually insoluble hydroxides or oxides.
The exact pH at which precipitation occurs varies by element and oxidation state. Over-liming acidic soils can lead to deficiencies of Fe, Mn, Zn, Cu, and sometimes B, particularly in high-pH calcareous soils.
A slightly acidic soil (pH 6–7) is generally desirable, as it maintains sufficient solubility of micronutrient cations to meet plant needs without toxicity.
2. Inorganic Reactions: Micronutrient cations interact with silicate clays in two ways: (a) through cation exchange, similar to Ca and Al, and (b) by being tightly bound or fixed to certain silicate clays, particularly 2:1 types. Zn, Mn, Co, and Fe may occur within the crystal structure of these clays, where they can be released or fixed depending on conditions.
Fixation may cause deficiencies, particularly for Co and Zn. Excessive application of phosphate fertilizers can reduce the uptake of Fe and Zn.
3. Organic Matter: Organic matter residues and manure applications influence the immediate and potential availability of micronutrient cations. Certain organic compounds form water-insoluble complexes with these cations, protecting them from binding with mineral particles in more insoluble forms.
Deficiencies of Cu, and to a lesser extent Mn, often occur in poorly drained, high-organic-matter soils like peats and marshes. Zn is also retained by organic matter.
Microbial decomposition of organic residues and manures releases micronutrients, but temporary deficiencies may arise due to assimilation by active microorganisms.
4. Role of Mycorrhizae: The symbiotic relationship between most higher plants and certain soil fungi forms mycorrhizae (fungus roots), which are more efficient than regular roots.
Mycorrhizae enhance plant uptake of P and micronutrients and protect plants from excessive uptake of micronutrients and trace elements when present in potentially toxic concentrations.
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Availability of Molybdenum (Mo)
Soil pH is the primary factor affecting Mo availability and plant uptake. At low pH, Mo is adsorbed by silicate clays, particularly Fe and Al oxides, through ligand exchange with hydroxide ions on colloidal particle surfaces.
Liming acidic soils increases Mo availability. Phosphate anions enhance Mo availability by competing for sorption sites on soil surfaces.
Soils may contain abundant nutrients that remain unavailable for crop use due to factors beyond mere presence or absence.
Nutrient availability depends on both internal and external soil factors, including pH, inorganic reactions, organic matter, microbial activity, and mycorrhizal interactions.
Understanding these factors is critical for optimizing nutrient supply and enhancing crop production.
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