Essential Plant Nutrients and Their Ionic Forms

Notes from ISA Certification study session.

Plant growth depends on mineral nutrients that occur in soil as dissolved ions or weakly bound to soil particles. While carbon is obtained from atmospheric carbon dioxide and hydrogen and oxygen from water, all remaining essential nutrients are absorbed through roots in specific chemical forms. Nitrogen is taken up primarily as nitrate (NO₃⁻), a highly mobile anion, and as ammonium (NH₄⁺), a cation that can be retained on soil exchange sites. Phosphorus is absorbed as the phosphate anions dihydrogen phosphate (H₂PO₄⁻) and hydrogen phosphate (HPO₄²⁻), which are strongly sorbed to soil minerals and therefore move very little in soil. Potassium enters plants as K⁺, while calcium and magnesium are taken up as Ca²⁺ and Mg²⁺, respectively. Sulfur is absorbed mainly as the sulfate anion (SO₄²⁻).

Micronutrients, though required in much smaller quantities, are equally critical to plant metabolism. Iron is absorbed primarily as Fe²⁺ or Fe³⁺, manganese as Mn²⁺, zinc as Zn²⁺, and copper as Cu²⁺. Boron is taken up as boric acid (H₃BO₃), which behaves differently from charged ions in soil, while chlorine is absorbed as chloride (Cl⁻). The chemical form of each nutrient governs its mobility, soil retention, and biological availability.

Macronutrients in Commercial Fertilizers and Fertilization Practices

Commercial fertilizers are formulated to supplement soil nutrients in known concentrations, most commonly expressed as nitrogen, phosphorus, and potassium. Fertilizer labels present these values in the N–P–K format, indicating total nitrogen, available phosphate expressed as P₂O₅, and soluble potash expressed as K₂O. While these numbers communicate nutrient quantity by weight, they do not describe the rate at which nutrients become available to plants.

Nitrogen availability is strongly influenced by fertilizer formulation. Controlled-release nitrogen refers to nitrogen sources that are coated or chemically modified to regulate dissolution and microbial breakdown. Water-insoluble nitrogen represents the fraction of nitrogen that is not immediately soluble and must be mineralized by soil microorganisms before uptake can occur. These nitrogen forms provide a sustained nutrient supply, reduce nitrate leaching and volatilization losses, and lower the risk of root injury from excessive soluble nitrogen. Their use aligns with best management practices for fertilization, which emphasize matching nutrient supply with plant demand, minimizing off-site movement, and protecting water quality.

Nutrient Location and Movement in Soil

The AI improved version of my notes.

Within the soil matrix, nutrients exist in distinct physical and chemical pools. Positively charged ions such as Ca²⁺, Mg²⁺, K⁺, NH₄⁺, Fe²⁺, and Zn²⁺ are electrostatically attracted to the negatively charged surfaces of clay minerals and organic matter. This reversible binding forms the basis of cation exchange capacity, which determines a soil’s ability to store and supply nutrients over time. Soils with higher clay content and organic matter typically have greater cation exchange capacity and improved nutrient retention.

Negatively charged ions such as nitrate (NO₃⁻) and sulfate (SO₄²⁻) are not retained by soil particles and remain dissolved in soil water. As a result, these nutrients move readily with water through the soil profile and are more susceptible to leaching losses, particularly in coarse-textured soils or under excessive irrigation or rainfall.

Soil structure further influences nutrient availability through the presence of micropores. These small pore spaces retain water and dissolved ions by capillary forces and act as nutrient reservoirs during periods of limited moisture. Although bulk roots cannot penetrate these pores, root hairs and microbial hyphae can access nutrients within them, linking soil physical structure to biological nutrient acquisition.

Mycorrhizal Associations and Nutrient Exchange

Mycorrhizal fungi form symbiotic relationships with plant roots that significantly enhance nutrient uptake. The fungal hyphae extend far beyond the root surface, increasing the effective absorptive area and accessing soil volumes unavailable to roots alone. Mycorrhizae are particularly effective at acquiring phosphorus in the form of phosphate ions, which are otherwise poorly mobile due to strong soil binding. They also contribute to the uptake of zinc as Zn²⁺, copper as Cu²⁺, iron as Fe²⁺ or Fe³⁺, nitrogen in both inorganic and organic forms, and water held within soil micropores.

In exchange for these resources, plants supply mycorrhizal fungi with carbohydrates produced through photosynthesis. This reciprocal exchange supports fungal metabolism and growth while improving plant nutrition, drought tolerance, and overall soil structure. The mycorrhizal relationship represents a critical biological pathway through which soil chemistry and plant physiology are integrated.