UPMSP Intermediate Agricultural Chemistry cheat sheet

Agricultural chemistry applies chemistry to soils, plants, fertilizers, and agro-chemicals; its core aim is sustaining crop yield without depleting soil capital.

Essential plant nutrients number 17: C, H, O (from air and water) plus macronutrients N, P, K, Ca, Mg, S and micronutrients Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.

Law of the minimum (Liebig): yield is limited by the nutrient in shortest supply relative to crop need, not by the total of all nutrients present.

Law of diminishing returns: each successive unit of a limiting nutrient adds less yield, so response curves plateau and can decline at excess.

Soil fertility = inherent capacity to supply nutrients; soil productivity = actual yield, which also depends on water, temperature, variety, and pest pressure.

Nutrient pools: soil solution (immediately available), exchangeable (rapidly available), and mineral/organic reserves (slowly available).

Cation exchange capacity (CEC) in cmol(+)/kg expresses the soil's capacity to hold exchangeable cations; higher CEC means greater nutrient-holding buffer.

Base saturation = (exchangeable Ca + Mg + K + Na) / CEC x 100; fertile agricultural soils typically exceed 60-80 percent base saturation.

Nitrogen is taken up mainly as nitrate (NO3-) and ammonium (NH4+); it drives vegetative growth and is a component of proteins, chlorophyll, and nucleic acids.

Phosphorus is absorbed as H2PO4- and HPO4(2-); it is mobile in plants, so deficiency first appears as stunted growth and purpling of older leaves.

Potassium is absorbed as K+; it regulates stomatal opening, enzyme activation, and water relations, and is highly mobile within the plant.

Mobile nutrients (N, P, K, Mg, S) show deficiency symptoms on older leaves first; immobile nutrients (Ca, Fe, B, Zn, Cu, Mn) show them on young leaves first.

Critical level is the soil-test value below which response to added nutrient is likely; below it, fertilizer is economic, above it response is unlikely.

Integrated nutrient management combines mineral fertilizer, organic manures, biofertilizers, and crop residues to match crop demand with all nutrient sources.

Split nitrogen application matches supply to crop uptake peaks and reduces leaching and volatilization losses compared with a single basal dose.

Site-specific nutrient management uses soil tests, yield targets, and nutrient balance (removal plus losses) to set rates rather than a blanket recommendation.

Fertilizer grade N-P2O5-K2O gives the percentage of nitrogen, phosphorus expressed as P2O5, and potassium expressed as K2O; 10-26-26 means 10 percent N, 26 percent P2O5, 26 percent K2O.

Element to oxide conversion: percent P = percent P2O5 x 0.436; percent K = percent K2O x 0.830; percent P2O5 = percent P x 2.29; percent K2O = percent K x 1.205.

Urea (46-0-0) is the most concentrated solid nitrogen fertilizer; it hydrolyses to ammonium carbonate and loses N as ammonia if surface-applied without incorporation.

Ammonium sulphate (21-0-0) supplies 24 percent S and is acid-forming; single superphosphate (16 percent P2O5) also supplies Ca and S; DAP is 18-46-0.

Fertilizer dose = (recommended nutrient rate / nutrient fraction of fertilizer) x area; for 120 kg N/ha using urea, 120 / 0.46 = 261 kg urea/ha.

Farmyard manure is bulky, low in nutrients (roughly 0.5 percent N), and improves structure and water-holding; green manure is a legume crop ploughed in for nitrogen.

Compost is decomposed organic matter; vermicompost is earthworm-processed; both supply nutrients slowly and improve biological activity and CEC.

Biofertilizers are live inoculants: Rhizobium for legume nodulation, Azotobacter and Azospirillum for free-living nitrogen fixation, and phosphate-solubilizing bacteria to release bound P.

Soil pH measures hydrogen ion activity in the soil solution; ranges are strongly acid below 5.0, acid 5.0-6.5, near neutral 6.5-7.5, alkaline 7.5-8.5, strongly alkaline above 8.5.

Most crops grow best at pH 6.0-7.5; at low pH, Al3+ and Mn2+ become soluble and toxic while P is fixed by Fe and Al oxides.

At high pH (calcareous soils), P is fixed as calcium phosphates and Fe, Mn, Zn, and B become less available, causing micronutrient deficiency.

Lime requirement is the amount of calcium carbonate needed to raise soil pH to a target; acid soils are corrected with lime or dolomite, alkaline soils with gypsum.

Cation exchange: clay and humus carry negative charges that hold Ca2+, Mg2+, K+, Na+, NH4+ and release them into solution for root uptake.

Anion exchange occurs on positive charges of Fe and Al oxides, retaining sulphate, chloride, and phosphate; phosphate is additionally fixed by precipitation.

Buffering capacity is resistance to pH change; soils high in clay and organic matter resist pH shifts more strongly than sandy soils.

Saline soils have high soluble salts (high EC) with pH usually below 8.5; sodic soils have high exchangeable sodium (ESP above 15) and disperse structure.

Classification by target: insecticides, fungicides, herbicides, nematicides, rodenticides, and plant growth regulators; by origin: inorganic, botanical, and synthetic organic.

Organochlorines (e.g., DDT, BHC) are persistent and lipophilic, accumulate in fatty tissue, and are largely banned for agricultural use because of biomagnification.

Organophosphates and carbamates inhibit acetylcholinesterase, causing nerve overstimulation; symptoms include salivation, miosis, and muscle twitching.

Pyrethroids are synthetic analogues of pyrethrins, act on sodium channels, and are effective at low doses but toxic to fish and bees.

Herbicide selectivity depends on dose, formulation, crop and weed morphology, and timing; 2,4-D is a synthetic auxin that kills broadleaf weeds in cereals.

Pesticide formulation terms: EC (emulsifiable concentrate), WP (wettable powder), GR (granule), SC (suspension concentrate); active ingredient is the toxic component.

Pre-harvest interval is the minimum days between last pesticide application and harvest; it protects against residue above the maximum residue limit (MRL).

Pesticide degradation is by microbial action, hydrolysis, photolysis, and volatilization; persistence is measured as half-life, the time for 50 percent loss.

Soil sampling uses a zig-zag pattern with a tube auger; 15-20 cores from 0-15 cm are mixed to form a composite sample of about 500 g.

Soil pH is measured in a 1:2.5 soil-to-water suspension with a calibrated glass electrode; a 1:2.5 soil-to-0.01 M CaCl2 suspension gives lower, more stable readings.

Electrical conductivity of a 1:5 soil-water extract estimates soluble salt content; readings above 4 dS/m at 25 degrees C indicate salinity problems.

Organic carbon by Walkley-Black: wet oxidation with potassium dichromate and sulphuric acid, back-titrated with ferrous ammonium sulphate; organic matter is approximately carbon x 1.724.

Available nitrogen by alkaline potassium permanganate method; available phosphorus by Olsen (0.5 M sodium bicarbonate, pH 8.5) for neutral-alkaline soils or Bray-1 for acid soils.

Available potassium is extracted with neutral 1 N ammonium acetate and measured by flame photometer; exchangeable sodium is used to compute ESP.

Safety and accuracy rules: calibrate glassware and instruments, use distilled water and analytical-grade reagents, label all solutions, and record raw readings in a laboratory notebook.

Fertilizer analysis: nitrogen by Kjeldahl digestion and distillation, phosphorus by spectrophotometry, potassium by flame photometry.

Soil pollution sources: industrial effluents, mine spoil, sewage sludge, agro-chemicals, excessive fertilizer, and atmospheric deposition of acids and heavy metals.

Heavy metals of concern include Cd, Pb, Hg, Cr, Ni, and As; they are non-biodegradable, persist in soil, and enter the food chain through crop uptake.

Cadmium enters soil mainly through phosphate fertilizers and sewage sludge; it accumulates in leafy vegetables and kidneys and is a chronic health risk.

Nitrate leaching from excess nitrogen fertilizer contaminates groundwater; nitrate above 10 mg/L as N in drinking water causes methaemoglobinaemia in infants.

Eutrophication results from phosphorus and nitrogen runoff into water bodies, causing algal blooms, oxygen depletion, and fish kills.

Biomagnification is the increase in concentration of a persistent pollutant at each higher trophic level; DDT and methylmercury are classic examples.

Greenhouse gases from agriculture: methane from flooded paddy and ruminants, nitrous oxide from fertilizer and manure, and carbon dioxide from residue burning and soil carbon loss.

Remediation options include bioremediation with microbes and plants, phytoremediation, soil washing, and liming to reduce heavy-metal mobility.

Organic farming excludes synthetic fertilizers and pesticides and relies on organic manures, compost, green manure, crop rotation, and biological pest control.

Certification requires a conversion period, typically 2-3 years, during which synthetic inputs are phased out and organic practices are followed before harvest can be sold as organic.

Crop rotation with legumes breaks pest cycles and adds biologically fixed nitrogen; intercropping and cover crops reduce erosion and suppress weeds.

Integrated pest management uses economic threshold levels, monitoring, resistant varieties, and biological control before chemical intervention.

Conservation agriculture rests on three principles: minimum soil disturbance, permanent soil cover, and diversified crop rotation.

Sustainable agriculture balances productivity, profitability, and environmental protection so that soil, water, and biodiversity resources are maintained for future use.

Soil organic matter is the key indicator of sustainability; it improves structure, water-holding capacity, CEC, and nutrient supply, and sequesters carbon.

Vermicompost and green manure are permitted organic nutrient sources; rock phosphate, bone meal, and neem cake are accepted organic amendments.