RBSE SS Agricultural Chemistry cheat sheet

Cation exchange capacity (CEC) = sum of exchangeable cations (cmol(+) kg⁻¹); base saturation (%) = (exchangeable bases ÷ CEC) × 100.

Typical CEC ranges: kaolinite 3–15, illite 10–40, montmorillonite 80–150, humus 150–300 cmol(+) kg⁻¹.

Soil pH buffering is strongest where CEC and organic matter are high; lime requirement rises with clay and organic matter content.

Al³⁺ dominates exchange sites below pH ~5.5; as pH rises, Al hydrolyses and precipitates, releasing exchange sites and raising CEC.

Nutrient availability windows: N, K, S maximal near pH 6.0–7.5; P maximal near pH 6.5; micronutrients (Fe, Mn, Zn, Cu) decline as pH rises above 7.

Law of the minimum: yield is limited by the most deficient essential nutrient, not by total nutrient supply.

Liebig's law applies to sufficiency; Mitscherlich/Baule response curves describe diminishing returns per added nutrient unit.

Available N estimate: alkaline KMnO₄–N (Subbiah & Asija) and organic carbon by Walkley–Black (OC% ≈ available N kg/ha ÷ 20 heuristic).

17 essential elements: C, H, O (non-mineral), N, P, K, Ca, Mg, S (macronutrients), Fe, Mn, Zn, Cu, B, Mo, Cl, Ni (micronutrients).

Criteria of essentiality (Arnon & Stout): indispensable for completion of life cycle, specific (not replaceable), and directly involved in plant metabolism.

Mobile nutrients (N, P, K, Mg) show deficiency first in older leaves; immobile ones (Ca, Fe, B, S, Cu, Mn) show it in young leaves.

Law of minimum, law of diminishing returns and factor interaction govern nutrient response; luxury consumption occurs above sufficiency for K and N.

N is a constituent of proteins, chlorophyll and nucleic acids; P of ATP, phospholipids and nucleic acids; K regulates stomata, enzyme activation and osmoregulation.

Ca is structural (calcium pectate in middle lamella); Mg is the central atom of chlorophyll; S is in cysteine, methionine and coenzyme A.

Fe and Mo are enzyme cofactors (cytochromes, catalase; nitrate reductase and nitrogenase respectively); B is involved in cell wall and pollen tube growth.

Critical concentration is the tissue nutrient level below which yield declines; DRIS uses nutrient ratios to diagnose imbalances.

Fertilizer grade N–P₂O₅–K₂O: %P × 2.29 = %P₂O₅; %K × 1.21 = %K₂O.

Urea (46-0-0) hydrolyses via urease to NH₄⁺ and CO₂; surface application can lose N as NH₃, especially on high-pH soils.

Ammonium sulphate (21-0-0-24S) is acid-forming; nitrification of NH₄⁺ releases H⁺ and lowers pH (about 2 kg CaCO₃ equivalent per kg N for AS).

Single superphosphate (16% P₂O₅, 11% S, 19–20% Ca) contains gypsum; triple superphosphate is ~46% P₂O₅ with little S.

Water-soluble P fertilizers form less-soluble Ca/Mg/Fe/Al phosphates in soil; P fixation is highest in acidic (Fe/Al) and calcareous (Ca) soils.

DAP (18-46-0) and MAP (11-52-0) are acidic in reaction; DAP temporarily raises pH near the granule due to ammonia release.

Muriate of potash (KCl, 60% K₂O) has high salt index; sulphate of potash (50% K₂O) is preferred for chloride-sensitive crops.

Fertilizer salt index ranks injury risk: NaNO₃ and NH₄NO₃ lower, KCl and urea higher relative to equal nutrient weight.

FYM composition (typical): 0.5% N, 0.2% P₂O₅, 0.5% K₂O; only 20–30% of its N is available in the first season.

C:N ratio controls mineralization: <20 immobilizes little and releases N; 20–30 near equilibrium; >30 net immobilization of N.

Compost maturity indicators: C:N <20, stable temperature, low NH₄⁺/NO₃⁻ ratio, germination test >80%, absence of phytotoxicity.

Vermicompost typical analysis: 1.5–2.5% N, 1.0–1.5% P₂O₅, 1.0–2.0% K₂O with high humic content and microbial load.

Green manures (e.g., sunhemp, dhaincha) fix N via symbiosis and add 40–60 kg N/ha; dhaincha tolerates salinity and waterlogging.

Organic farming standards prohibit synthetic fertilizers and pesticides; permitted inputs include compost, biofertilizers, botanicals and mineral rocks.

Humus improves CEC, water-holding capacity and aggregate stability; soil organic carbon is the key indicator of soil health.

Bulky organic manures supply micronutrients and improve physical properties but have low nutrient concentration per unit weight.

Rhizobium forms root nodules with legumes; Azotobacter is free-living aerobic N-fixer; Azospirillum associates with cereals and grasses.

Blue-green algae (Anabaena, Nostoc) and Azolla (Anabaena azollae symbiosis) fix N in flooded rice paddies.

Phosphate-solubilizing bacteria (Bacillus, Pseudomonas) release organic acids that solubilize Ca-phosphates; mycorrhizae (VAM/AMF) extend P uptake.

Carrier-based inoculants should carry ≥10⁷ viable cells/g at manufacture; avoid mixing with chemical fertilizers or fungicides at sowing.

Lime raises pH of acidic soils; gypsum (CaSO₄·2H₂O) reclaims sodic soils by replacing exchangeable Na⁺ with Ca²⁺.

Gypsum requirement (t/ha) = (initial ESP − desired ESP) × CEC (cmol/kg) × 1.7 (approximately, for 15 cm depth).

Biochar and press mud are carbon-rich amendments that increase CEC and water retention; biochar is alkaline and suits acidic soils.

Elemental S and gypsum are acidifying amendments; lime and basic slag are alkaline amendments used to correct soil acidity.

Organophosphates (malathion, chlorpyrifos) inhibit acetylcholinesterase; carbamates act similarly but are reversible; both are neurotoxic to mammals.

Pyrethroids (cypermethrin, deltamethrin) act on sodium channels; low mammalian toxicity but highly toxic to fish and bees.

Neonicotinoids (imidacloprid) act on nicotinic acetylcholine receptors; systemic and highly toxic to pollinators.

Herbicide modes: glyphosate inhibits EPSP synthase; 2,4-D and atrazine are synthetic auxins/photosystem II inhibitors; paraquat generates superoxide radicals.

WHO hazard classes: Ia extremely hazardous, Ib highly hazardous, II moderately hazardous, III slightly hazardous, U unlikely to present acute hazard.

Pre-harvest interval (PHI) is the minimum days between last application and harvest to keep residues below MRL.

Half-life (DT₅₀) determines persistence; repeated use of the same mode of action selects resistant biotypes.

Personal protective equipment and label directions govern safe handling; banned persistent organochlorines (DDT, endosulfan) remain in soil for decades.

Eutrophication is driven by excess N and P in water bodies; nitrate leaching and P runoff from fertilized fields are major contributors.

Nitrate above 10 mg N/L (45 mg NO₃⁻/L) in drinking water is the WHO limit; methemoglobinemia in infants is the health concern.

Heavy metals of concern: Cd, Pb, Cr, Ni, Hg, As; sources include sewage sludge, phosphatic fertilizers and industrial effluent.

Bioaccumulation and biomagnification increase persistent lipophilic pollutants (e.g., organochlorines) up the food chain.

Maximum residue limits (MRLs) are set by authorities such as Codex and national regulators; MRLs are enforcement standards, not safety thresholds.

Persistence, mobility and toxicity together determine environmental risk; leaching potential depends on soil texture, organic matter and water solubility.

Bioremediation uses microbes/plants to degrade contaminants; phytoremediation uses hyperaccumulators to extract heavy metals.

Greenhouse gas links: nitrous oxide from denitrification/nitrification is a potent GHG; methane from flooded paddy soils is mitigated by drainage.

Kjeldahl N: digest with H₂SO₄ + catalyst, distill NH₃ into boric acid, titrate with standard acid; %N × 6.25 = %protein (for feeds).

Walkley–Black organic carbon uses K₂Cr₂O₇ + H₂SO₄ oxidation; colorimetric or titration with ferrous ammonium sulphate.

Available P by Olsen (0.5 M NaHCO₃, pH 8.5) suits neutral-alkaline soils; Bray-1 (0.03 M NH₄F + 0.025 M HCl) suits acidic soils.

Available K by neutral 1 N NH₄OAc extraction and flame photometry; exchangeable cations by the same extractant with AAS.

Soil pH measured in 1:2.5 soil:water suspension; EC in 1:5 or saturated paste extract.

Quality control requires blanks, duplicates, certified reference materials and calibration curves; results reported with units and detection limits.

Atomic absorption/ICP-OES quantify metals; UV-Vis spectrophotometry quantifies P, N and colorimetric analytes.

Safety: fume hoods for acid digestion, acid-resistant gloves, and proper disposal of chromium and heavy-metal wastes.

Soil survey scales: detailed (1:5,000–1:15,000), semi-detailed (1:25,000–1:50,000), reconnaissance (1:100,000–1:250,000).

Soil taxonomy orders: Entisols, Inceptisols, Vertisols, Alfisols, Mollisols, Aridisols, Ultisols, Oxisols, Andisols, Histosols, Spodosols, Gelisols.

Diagnostic horizons include mollic (dark, base-rich surface), ochric, argillic (clay accumulation), and calcic (CaCO₃ accumulation).

Land capability classes I–VIII: I–IV arable with increasing limitations; V–VIII non-arable (V wetness, VI steepness, VII severe, VIII unsuitable).

Land suitability classes S1 (highly suitable), S2 (moderately), S3 (marginally), N (not suitable) follow FAO guidelines.

Soil erosion risk increases with slope, low cover and high rainfall erosivity; contour bunding, terracing and cover crops reduce it.

Saline soils have high EC and low ESP; sodic soils have high ESP (>15) and poor structure; saline-sodic combines both.

Remote sensing and GIS are used for soil mapping, land use planning and monitoring of land degradation.