Print related Information Sheets — Soil Chemistry and Nitrogen Conversions

Soil chemistry controls the forms in which nutrients occur and how readily roots can obtain them. Nitrogen is especially mobile because microbes repeatedly convert it among organic matter, ammonium, nitrate, and gases. A fertilizer dose is therefore only the beginning of its fate.

Original schematic of major soil nitrogen pathways. Arrows show transformations or losses, not equal flow rates. Educational image created for the Plant Education Project; retained from the original Information Sheet.

Separate acidity nutrient storage and salinity

Property	Meaning	Practical limit
pH	A measure of acidity related to hydrogen-ion activity.	It affects availability; it does not measure the quantity of fertilizer.
Cation exchange capacity	Capacity to retain exchangeable positively charged ions.	It does not directly retain nitrate, which is negatively charged.
Electrical conductivity	A measure of the solution’s combined ion conductivity.	High EC can signal salinity; it cannot identify which nutrient is excessive.
Buffering	Resistance to pH change.	Equal pH values can require different amendment rates.
Redox conditions	The electron-transfer environment associated with oxygen supply and biological activity.	Waterlogging changes chemical forms and microbial pathways.

Nitrogen changes form through biological and chemical steps

Process	Conversion	Conditions and consequence
Fixation	Atmospheric N₂ to biologically usable nitrogen	Specialized microorganisms perform the reduction; legumes rely on compatible symbionts.
Mineralization	Organic nitrogen to NH₄⁺	Decomposition releases ammonium; temperature and moisture affect the rate.
Immobilization	Inorganic nitrogen to microbial organic nitrogen	Microbes can temporarily compete with roots, especially while decomposing carbon-rich residues.
Nitrification	NH₄⁺ to NO₂⁻ to NO₃⁻	Usually favored by oxygen; generates acidity and produces mobile nitrate.
Denitrification	NO₃⁻ to gaseous nitrogen products	Low oxygen and available carbon favor losses as N₂ and sometimes N₂O.
Volatilization	Ammoniacal nitrogen to NH₃ gas	Surface conditions, high pH, heat, and urea hydrolysis can increase loss.
Leaching	Dissolved nitrogen moves below roots	Nitrate is especially vulnerable where drainage water carries it downward.

Water oxygen and carbon govern the balance

Warm, moist, aerated soil can mineralize and nitrify nitrogen rapidly. Saturated soil restricts oxygen and may favor denitrification. Very dry or cold soil slows many processes. A sudden change in moisture can therefore change nutrient supply without any fertilizer application.

Plants commonly absorb both nitrate and ammonium, in proportions that vary by species and environment. Nitrate uptake is not proof that ammonium is useless. More nitrogen can increase growth only when some other resource or stress is not the controlling limit.

Use evidence before amending

Submit a representative sample to a laboratory using the appropriate soil or container-medium method. Specify the crop, site, and intended use. Follow the laboratory’s amendment recommendation rather than choosing a lime, sulfur, or fertilizer dose from pH alone.

Split nitrogen applications when appropriate for the crop and avoid applying before runoff-producing rain. Keep fertilizer out of drains and surface water. For a container, account for fertilizer already present in the starting mix and measure salts when symptoms suggest accumulation.

Yellow leaves can result from root damage, waterlogging, pH-related availability, disease, or insufficient light. A larger fertilizer dose can worsen several of those conditions.