RBSE SS Agricultural Chemistry key terms

The total quantity of exchangeable cations a soil can hold on clay and humus surfaces, measured in centimoles of charge per kilogram. Higher values mean the soil retains nutrients like calcium and potassium instead of losing them to leaching.

The proportion of the soil's cation exchange capacity occupied by basic cations such as calcium, magnesium, potassium and sodium rather than acidic ions like hydrogen and aluminium. It is expressed as a percentage and rises as soils become less acidic.

The share of the cation exchange capacity occupied by sodium ions, expressed as a percentage. Values above roughly fifteen indicate sodic soils, which disperse, lose structure and restrict water movement and root growth.

The negative logarithm of hydrogen ion activity in the soil solution, indicating acidity or alkalinity on a scale from zero to fourteen. It governs nutrient availability, microbial activity and the solubility of aluminium and micronutrients.

The amount of agricultural limestone needed to raise a soil's pH to a target value for a given crop, determined by buffer-pH or titration tests. It depends on soil texture, organic matter and the initial acidity.

The concentration of dissolved salts in the soil solution, usually assessed through electrical conductivity of a saturated extract. Excess salinity raises osmotic pressure, making water harder for roots to absorb and reducing yields.

A ratio comparing sodium concentration with the square root of calcium plus magnesium concentrations in a water or soil extract. It predicts whether irrigation water will raise soil sodicity and damage soil structure.

The carbon held in soil organic matter, including living roots, microbes and decomposed residues. It is a key indicator of fertility, structure and water-holding capacity, and is reported as a percentage of dry soil mass.

The dark, relatively stable fraction of soil organic matter remaining after microbial decomposition of plant and animal residues. It improves aggregation, water retention and cation exchange and slowly releases nutrients.

The mass ratio of carbon to nitrogen in an organic material or in soil organic matter. Materials with wide ratios, such as cereal straw, immobilise nitrogen during decomposition, while narrow ratios release it.

An element required by plants that satisfies criteria of necessity: a plant cannot complete its life cycle without it, and it has a specific physiological role that no other element can fully replace.

An essential element needed by plants in relatively large amounts, conventionally nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Concentrations in plant tissue are usually measured in grams per kilogram of dry matter.

An essential element required by plants in very small quantities, including iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel. Deficiency and toxicity can occur within a narrow concentration range.

The soil test value below which a crop response to added nutrient is likely and above which fertiliser is unlikely to pay. It is established by calibrating test results against field response trials.

The proportion of applied nutrient taken up by the crop, or the yield produced per unit of nutrient applied. It reflects losses through leaching, volatilisation, fixation and runoff as well as crop uptake capacity.

A fertilizer material that supplies essentially one nutrient, such as urea for nitrogen or muriate of potash for potassium. Its nutrient concentration is stated on the bag as a percentage of the product mass.

A fertilizer in which two or more nutrients are chemically combined in the same granule, such as ammonium phosphate. Each granule contains the stated nutrients in fixed proportions.

The guaranteed nutrient content of a fertilizer expressed as percentages of nitrogen, available phosphorus as phosphorus pentoxide and soluble potassium as potassium oxide. It is printed as three numbers in that order.

Loss of nitrogen to the atmosphere as ammonia gas, mainly when ammonium or urea fertilizers are applied to warm, moist, alkaline or high-lime soils. Incorporation into the soil reduces such losses.

The enzyme-driven conversion of urea to ammonium carbonate in soil, which temporarily raises local pH and can cause ammonia loss. The reaction is catalysed by the enzyme urease produced by soil microorganisms and plant residues.

The microbial oxidation of ammonium to nitrite and then nitrate in aerobic soil, carried out by specialised bacteria. Nitrate is mobile and readily leached or denitrified, so nitrification influences nitrogen efficiency.

Removal of soluble nutrients, particularly nitrate and sulphate, from the root zone by water moving downward through the soil profile. Sandy soils and heavy rainfall increase the risk.

A mixture of animal dung, urine and bedding material that has been stored and partly decomposed before field application. It supplies organic matter and small amounts of all essential nutrients.

A crop grown specifically to be incorporated into the soil while still fresh, to add organic matter and nitrogen. Legume green manures fix atmospheric nitrogen through root nodule bacteria.

The degree to which organic material has stabilised after decomposition, judged by reduced respiration, lower carbon to nitrogen ratio, absence of phytotoxic compounds and acceptable smell and temperature.

Organic material produced by earthworms feeding on partly decomposed residues, resulting in a fine, nutrient-rich cast with high microbial activity. It is used as a soil amendment and potting ingredient.

Conversion of atmospheric nitrogen gas into ammonia by microorganisms, either free-living or in symbiosis with legumes. The enzyme nitrogenase catalyses the reaction and requires molybdenum and iron cofactors.

A carrier-based preparation of selected nitrogen-fixing bacteria applied to legume seed so that nodules form and fix nitrogen. Inoculants must be stored cool and used before expiry to remain viable.

A bacterium or fungus that releases organic acids or phosphatases, making sparingly soluble soil phosphorus more available to plants. Such organisms are formulated as biofertilizers for seed or soil application.

A symbiotic association between a fungus and plant roots in which fungal hyphae extend into soil and improve uptake of phosphorus, zinc and water in exchange for carbohydrates from the plant.

Testing of carrier-based microbial products for viable cell count, contamination, pH and moisture so that the labelled strain is present at an effective population when the farmer uses it.

Calcium sulphate applied to sodic soils to replace exchangeable sodium with calcium, after which sodium salts can be leached away. It is also used to improve structure in dispersive clay soils.

The physical form in which an active ingredient is delivered, such as emulsifiable concentrate, wettable powder or granule, together with solvents, surfactants and carriers. Formulation determines handling, mixing and residue behaviour.

The chemical component of a pesticide product that is responsible for the toxic effect on the target pest. Its concentration is declared on the label and is the basis for calculating application rate.

The minimum number of days that must pass between the last pesticide application and harvest so that residues fall below the permitted limit. It is specified on the product label for each crop.

The highest concentration of a pesticide residue legally permitted in or on a food or feed commodity, expressed in milligrams per kilogram. It is set from residue trials and dietary risk assessment.

Laboratory determination of pesticide residues in food, soil or water, typically using solvent extraction, cleanup and chromatographic separation with selective detection or mass spectrometry.

Nutrient enrichment of water bodies, especially by nitrogen and phosphorus, that stimulates excessive algal growth and later oxygen depletion. Agricultural runoff and leaching are major contributors.

A measure of a solution's ability to carry current, used to estimate dissolved salt concentration in soil extracts or irrigation water. It is reported in decisiemens per metre.

An analytical separation technique in which volatile compounds are carried by a gas through a coated column and detected individually. It is widely used for pesticide residues and fatty acid analysis.

A plot of detector response against known concentrations of a standard, used to convert instrument readings into sample concentrations. Linearity, blank correction and quality control samples support reliable quantification.

The process of relating laboratory soil test values to crop response in field trials, so that fertilizer recommendations can be derived. It links measured nutrient availability with economic optimum rates.

Systematic field examination, mapping and classification of soils at a defined scale, producing maps and descriptions of soil units. Survey intensity determines the detail of land-use interpretation.

The lowest widely used category in soil classification, comprising soils with similar horizons and properties formed from similar parent material under similar climate and vegetation.

A grouping of land into classes according to its suitability for sustained agricultural use and the severity of limitations such as slope, erosion risk, drainage or salinity.