RBSE SS Agricultural Chemistry flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

Cation exchange capacity

Total quantity of exchangeable cations a soil can hold per unit mass, expressed in cmol(+) kg⁻¹; higher in clay-rich and organic soils.

Base saturation

Percentage of the cation exchange capacity occupied by exchangeable bases (Ca, Mg, K, Na); higher values indicate more fertile neutral soils.

Law of the minimum

Yield is limited by the essential nutrient in shortest supply relative to plant need, regardless of abundance of other nutrients.

Alkaline KMnO4 nitrogen

Subbiah and Asija method estimating available soil nitrogen by oxidizing organic matter with alkaline permanganate and trapping released ammonia.

Essential element criteria

Arnon and Stout criteria: the element is indispensable for completing the life cycle, cannot be replaced, and directly participates in plant metabolism.

Mobile versus immobile nutrients

Mobile nutrients (N, P, K, Mg) relocate to young tissues, so deficiency appears in older leaves; immobile ones (Ca, Fe, B) show in young leaves.

Critical nutrient concentration

Tissue nutrient level below which yield or quality declines; used to interpret plant analysis and guide fertilizer recommendations.

Micronutrient list

Fe, Mn, Zn, Cu, B, Mo, Cl and Ni are required in small amounts; deficiency or excess both reduce growth and yield.

Fertilizer grade conversion

Percent P × 2.29 gives percent P₂O₅; percent K × 1.21 gives percent K₂O; N is reported as elemental nitrogen.

Urea hydrolysis

Urease converts urea to ammonium and carbon dioxide; ammonia may volatilize from surface application on high-pH soils.

Phosphate fixation

Soluble phosphorus reacts with Ca in calcareous soils and Fe or Al in acidic soils to form less soluble compounds, reducing availability.

Salt index

Relative measure of osmotic injury a fertilizer causes per unit of nutrient compared with sodium nitrate as the reference.

Farmyard manure composition

Typical FYM contains about 0.5% N, 0.2% P₂O₅ and 0.5% K₂O; only a fraction of its nitrogen is available in the first season.

Carbon to nitrogen ratio

Ratio of organic carbon to nitrogen in residues; above about 30, microbes immobilize nitrogen, while below 20 net mineralization occurs.

Compost maturity indicators

Stable temperature, C:N below 20, low ammonium to nitrate ratio and a germination test above 80% indicate mature, non-phytotoxic compost.

Green manuring

Growing a legume such as sunhemp or dhaincha and incorporating it before flowering to add nitrogen and organic matter to soil.

Rhizobium

Symbiotic bacterium that forms root nodules on legumes and fixes atmospheric nitrogen, supplying a substantial share of the crop's nitrogen.

Azolla-Anabaena symbiosis

Floating fern Azolla harbours nitrogen-fixing cyanobacterium Anabaena azollae; used as green manure in flooded rice paddies.

Phosphate solubilizing bacteria

Microbes such as Bacillus and Pseudomonas that release organic acids and solubilize insoluble calcium phosphates, increasing available phosphorus.

Gypsum for sodic soils

Calcium sulphate replaces exchangeable sodium on clay surfaces, improving structure and infiltration; requirement depends on ESP and CEC.

Organophosphate mode of action

Inhibit acetylcholinesterase, causing acetylcholine accumulation and nerve overstimulation; examples include malathion and chlorpyrifos.

Pyrethroid characteristics

Synthetic insecticides acting on sodium channels; low mammalian toxicity but highly toxic to fish and bees; examples cypermethrin and deltamethrin.

Pre-harvest interval

Minimum number of days between the last pesticide application and harvest, set so residues remain below the maximum residue limit.

Herbicide mode of action

Glyphosate inhibits EPSP synthase; 2,4-D acts as a synthetic auxin; atrazine blocks photosystem II; paraquat generates superoxide radicals.

Nitrate drinking water limit

WHO limit is 10 mg nitrate-nitrogen per litre (45 mg nitrate per litre); excess causes methemoglobinemia in infants.

Eutrophication

Nutrient enrichment of water bodies, mainly by phosphorus and nitrogen, causing algal blooms, oxygen depletion and fish kills.

Biomagnification

Increase in concentration of persistent lipophilic pollutants such as organochlorines at successively higher trophic levels of a food chain.

Maximum residue limit

Highest legally permitted pesticide residue in or on food and feed, established by regulators as an enforcement standard rather than a safety threshold.

Kjeldahl nitrogen determination

Acid digestion converts organic nitrogen to ammonium, which is distilled into boric acid and titrated; result multiplied by 6.25 gives crude protein.

Walkley-Black method

Estimates soil organic carbon by oxidizing it with potassium dichromate and sulphuric acid, then titrating excess dichromate with ferrous ammonium sulphate.

Olsen phosphorus test

Available phosphorus extracted with 0.5 M sodium bicarbonate at pH 8.5, suitable for neutral to alkaline and calcareous soils.

Land capability classification

Eight classes from I to VIII; classes I to IV are arable with increasing limitations, while V to VIII are unsuitable for cultivation.

Soil taxonomy orders

Twelve orders including Entisols, Inceptisols, Vertisols, Alfisols, Mollisols, Aridisols, Ultisols, Oxisols and Histosols, distinguished by diagnostic horizons.

Sodic soil ESP threshold

Soils with exchangeable sodium percentage above 15 are classed sodic, showing poor structure and low infiltration; gypsum is the common amendment.