Soil testing

In short

Soil testing is the chemical, physical and biological analysis of a representative soil sample to measure its reaction (pH), salinity, organic carbon and plant-available nutrient levels, and to convert those numbers into lime, gypsum and fertiliser recommendations for a specific crop and yield target. It is the only objective basis for balanced nutrition; blanket doses waste fertiliser, depress profit and acidify or salinise soil. In India it underpins the Soil Health Card scheme (launched February 2015, run by the Department of Agriculture & Farmers' Welfare), the network of State and ICAR soil testing laboratories, and the STCR (Soil Test Crop Response) correlation project. It is examined in B.Sc (Hons.) Agriculture soil science papers, ICAR-JRF/SRF and ARS, IBPS-AFO and NABARD Grade A, and asked in interviews for agri-input companies, fertiliser majors and lab roles.

Explained

Purpose, scope and the soil test cycle

• Objectives: (1) assess nutrient supplying capacity, (2) diagnose problem soils (acid, saline, sodic), (3) make crop- and target-specific fertiliser recommendations, (4) monitor fertility change over time, (5) evaluate land for reclamation or purchase.
• Five steps: sampling → sample preparation → laboratory analysis → interpretation against critical limits → recommendation and follow-up.
• Sampling is the weakest link: a lab works on about 5-10 g of soil that must represent perhaps 2,000,000 kg of surface soil in a hectare. Over 80% of soil-test error is traced to faulty sampling, not to analysis.
• Index value concept: a soil test does not measure total nutrient but an *index* of availability — the extractant mimics, by correlation, what roots can absorb over a season. Hence every method must be *calibrated* against crop response.

Soil sampling: principles and procedure

• Representative sample: divide the field into homogeneous units by soil colour, texture, slope, drainage, past manuring and cropping. Sample each unit separately; a unit is normally up to 1-2 ha.
• Number of spots: 8-20 (commonly 10-15) borings per composite sample, taken in a zig-zag or W pattern.
• Depth: 0-15 cm (plough layer) for field crops; 0-15, 15-30, 30-60 cm for deep-rooted crops; for orchards 0-30, 30-60, 60-90 cm under the canopy drip line; separate 0-15 cm sample for a no-till/surface-fertilised field.
• Tools: screw/tube auger, khurpi or spade (V-shaped cut, uniform slice), post-hole auger for deep layers; stainless steel or plastic for micronutrient and heavy-metal samples (never galvanised iron for Zn, never brass for Cu/Zn).
• Compositing and quartering: mix the borings on a clean polythene sheet, spread as a circle, divide into four quarters, discard two opposite quarters, remix and repeat until about 500 g remains.
• Avoid: bunds, field borders, marshy spots, old manure or compost heaps, trees, fertiliser bands, irrigation channels, recently limed strips, ant hills, cattle urine patches. Avoid sampling within 2-3 months of fertiliser or lime application.
• Labelling: farmer's name, village, survey/plot number, area, depth, date, previous crop, next crop and yield target, irrigation source, manure history. GPS coordinates for Soil Health Card grids.
• Timing: after harvest and before the next crop's fertilisation; same season every time if monitoring trends.
• Special cases: separate samples for salt-affected patches; for a saline-sodic survey take samples by horizon down to 90-120 cm; for a nematode or microbial test keep soil moist, cool and unsieved.
• Grid sampling for precision agriculture: systematic grid (e.g. 1 ha or finer) or management-zone sampling with GPS, feeding variable-rate fertiliser maps.

Sample preparation and laboratory methods

• Preparation: air-dry in shade (never oven-dry for available nutrients or for mineralisable N), crush gently with a wooden roller, pass through a 2 mm sieve (0.2 mm / 80-mesh, or 0.15 mm / 100-mesh, for organic carbon and total analysis), store in labelled cloth or polythene bags.
• pH: 1:2.5 soil:water suspension, glass electrode pH meter (calibrated with buffers 4.0, 7.0, 9.2). pH in 0.01 M CaCl₂ or 1 N KCl reads lower; always state the ratio and medium.
• Electrical conductivity (EC): supernatant of the same 1:2 or 1:2.5 extract, or the saturation extract (ECe), conductivity cell; reported in dS/m at 25 °C (1 dS/m = 1 mmho/cm = 1000 µS/cm).
• Organic carbon: Walkley-Black rapid titration — wet oxidation with K₂Cr₂O₇ + conc. H₂SO₄, back-titration of excess dichromate with ferrous ammonium sulphate using diphenylamine/ferroin indicator; recovers about 77% of OC, so a 1.3 correction factor is applied. Organic matter = OC × 1.724 (Van Bemmelen factor).
• Available nitrogen: usually *estimated from organic carbon* because mineral N is transient; alkaline KMnO₄ (Subbiah and Asija, 1956) distillation gives an index of available N. Nitrate-N by ion-selective electrode or colorimetry where needed.
• Available phosphorus: Olsen's 0.5 M NaHCO₃ (pH 8.5) for neutral, calcareous and alkaline soils; Bray's P-1 (0.03 N NH₄F + 0.025 N HCl) for acid soils; colour developed by ascorbic acid-molybdate blue and read on a spectrophotometer at 660-882 nm.
• Available potassium: neutral normal (1 N) ammonium acetate, pH 7, extracts exchangeable + solution K; read on a flame photometer at 766 nm.
• Available sulphur: 0.15% CaCl₂ or monocalcium phosphate extraction, turbidimetric estimation as BaSO₄.
• Micronutrients (Zn, Fe, Mn, Cu): DTPA extractant (Lindsay and Norvell, 1978) — 0.005 M DTPA + 0.01 M CaCl₂ + 0.1 M TEA at pH 7.3 — read on AAS or ICP. Boron: hot water extraction, azomethine-H colorimetry. Molybdenum: ammonium oxalate (pH 3.3).
• Lime requirement: Shoemaker-McLean-Pratt (SMP) buffer or Woodruff buffer method; gypsum requirement for sodic soils by the Schoonover method.
• Other determinations: CEC (1 N neutral NH₄OAc saturation), exchangeable sodium percentage (ESP), free CaCO₃ (rapid titration/calcimeter), bulk density (core method), texture (hydrometer or international pipette method), water-holding capacity, and in modern labs near-infrared/VIS spectroscopy and portable XRF for rapid screening.

Interpretation: critical limits and rating

• Critical limit: the soil test value below which a crop usually responds economically to applied nutrient — determined by correlating soil test values with yield response in field trials (Cate-Nelson graphical or statistical method).
• Standard Indian rating (0-15 cm, kg/ha):

• Micronutrient critical limits (DTPA, mg/kg, widely used): Zn 0.6, Fe 4.5, Mn 2.0, Cu 0.2; hot-water B 0.5. Below these, deficiency is likely. These are indicative and vary with soil and crop — state the method with the figure.
• pH classes: < 4.5 extremely acid; 4.5-5.5 strongly acid; 5.5-6.5 moderately acid; 6.5-7.5 neutral; 7.5-8.5 mildly alkaline; > 8.5 strongly alkaline/likely sodic.
• Salinity and sodicity (saturation extract basis): normal ECe < 4 dS/m with ESP < 15; saline ECe > 4; sodic/alkali ESP > 15 (or SAR > 13) with pH > 8.5; saline-sodic both. Field labs using 1:2.5 extracts must not compare those readings directly with ECe limits.
• Nutrient Index (Parker): NI = (NL × 1 + NM × 2 + NH × 3) / total samples, where NL, NM, NH are the numbers of low, medium and high samples. NI < 1.67 low, 1.67-2.33 medium, > 2.33 high — used for district fertility maps.

Fertiliser recommendation approaches

1. General/blanket recommendation: state package of practices dose for a crop and zone — simple, ignores field variation.
2. Soil test based (ad hoc) adjustment: recommended dose increased by about 25-50% for low-testing soils and reduced by 25-50% for high-testing soils.
3. Targeted yield / STCR equation (Ramamoorthy, Truog): the scientific approach — fertiliser dose is computed from nutrient requirement per unit of produce, the contribution of soil-available nutrient and the contribution of the applied fertiliser. > FN = (NR/CF) × T − (CS/CF) × STV Nutrient requirement, % contribution from soil (CS) and from fertiliser (CF) are determined experimentally for each crop-soil combination by the ICAR STCR project. The approach gives prescription equations and ready-reckoner tables, and accounts for FYM by subtracting its contribution.
4. Site-specific nutrient management (SSNM) / Nutrient Expert: uses attainable yield, nutrient balance and omission plots rather than a single soil test; widely used for rice and maize by ICAR and IRRI.
5. Plant analysis and quick tests as complements: leaf/tissue analysis (DRIS), SPAD/leaf colour chart for N top-dressing, plant sap nitrate tests. Soil test is predictive (before sowing); plant analysis is diagnostic (during growth).

Indian institutional framework, schemes and practice

• Soil Health Card scheme (2015): a card per land holding reporting 12 parameters — pH, EC, organic carbon, available N, P, K, S, Zn, Fe, Cu, Mn, B — with crop-wise fertiliser and amendment recommendations, revalidated on a cycle (originally two years; from 2022-23 merged into the Rashtriya Krishi Vikas Yojana as "Soil Health & Fertility", with village-level Soil Health Laboratories and sampling grids of 2.5 ha irrigated / 10 ha rainfed). Scheme design and funding pattern have changed more than once — check the current DA&FW guideline before quoting details.
• Laboratory network: State static STLs, mobile soil testing vans, ICAR institutes (ICAR-IISS Bhopal, ICAR-CSSRI Karnal for salt-affected soils, ICAR-NBSS&LUP Nagpur for survey and mapping), Krishi Vigyan Kendras, fertiliser company labs (IFFCO, Coromandel, Tata Chemicals) and private labs. Quality is monitored through referral laboratories and inter-laboratory proficiency testing.
• Related law and standards: fertiliser quality and labelling are controlled under the Fertiliser (Control) Order, 1985 (under the Essential Commodities Act, 1955), which also governs bio-fertiliser and organic fertiliser specifications; irrigation-water and soil-analysis methods follow BIS methods of test for soil (quote the current IS number and edition only if verified, since several older IS soil-test standards have been revised or withdrawn); laboratory accreditation is under NABL (ISO/IEC 17025).
• Charges: State STLs test at nominal or nil cost under the scheme; private labs typically charge roughly ₹300-₹1,500 for a basic pH-EC-OC-NPK pack and more for micronutrients — rates vary by State and lab.
• Field quick kits: Soil Testing Mini Lab / Mridaparikshak (ICAR-IISS) and similar kits give semi-quantitative pH, EC, OC and NPK readings in the village; useful for awareness and screening, not a substitute for a calibrated lab.

Advantages and limitations

• Advantages: prevents over- and under-fertilisation; cuts input cost and subsidy burden; corrects hidden micronutrient deficiency (Zn, B, S are widespread in India); detects acidity, salinity and sodicity early; gives a long-term fertility record; basis of balanced-use and precision agriculture.
• Limitations: result only as good as the sample; extractant values are correlative, not absolute; critical limits are region- and crop-specific and need periodic recalibration; cannot predict in-season losses (leaching, denitrification, heavy rain); available-N index is unreliable; turnaround time and lab capacity constrain coverage; farmers often receive a card without usable interpretation.

Key points

• Soil testing - chemical and physical analysis of a representative sample plus interpretation into a fertiliser/amendment recommendation.
• Sampling error dominates - 8-20 borings in a zig-zag over a homogeneous 1-2 ha unit, composited and quartered to about 500 g.
• Depth - 0-15 cm for field crops; profile depths for orchards and salinity surveys.
• No galvanised or brass tools - they contaminate Zn and Cu results; use stainless steel or plastic.
• Air-dry, 2 mm sieve - 0.2 mm only for organic carbon and total analysis; never oven-dry for available nutrients.
• pH - 1:2.5 soil:water, glass electrode; always report the ratio and the suspension medium.
• EC - in dS/m at 25 °C; ECe (saturation extract) is the basis of the 4 dS/m salinity limit.
• Walkley-Black - wet oxidation for organic carbon; 1.3 recovery factor; OM = OC × 1.724.
• Olsen P for alkaline/calcareous soils, Bray P-1 for acid soils - never interchange the ratings.
• Available K - neutral normal ammonium acetate extraction, flame photometer.
• DTPA (pH 7.3) - standard extractant for Zn, Fe, Mn, Cu with AAS; hot water for B.
• Available N - alkaline KMnO₄ index (Subbiah & Asija, 1956); only an index, often replaced by OC-based inference.
• Critical limit - soil test value below which the crop responds economically; fixed by Cate-Nelson correlation trials.
• Low/Medium/High ratings - OC 0.5/0.75%; N 280/560; P₂O₅ 10/25; K₂O 110/280 kg/ha.
• DTPA critical limits - Zn 0.6, Fe 4.5, Mn 2.0, Cu 0.2 mg/kg; B 0.5 mg/kg (hot water).
• Sodic soil - ESP > 15, pH > 8.5, needs gypsum; acid soil - pH < 5.5, needs lime (SMP buffer for requirement).
• STCR targeted yield equation - FN = (NR/CF) × T − (CS/CF) × STV, the scientific recommendation route.
• Soil Health Card - 12 parameters, grid-based sampling (2.5 ha irrigated / 10 ha rainfed), now under RKVY Soil Health & Fertility.

Formulas and examples

Example 1 - ppm to kg/ha and rating. A lab reports Olsen P = 7.5 ppm, K (NH₄OAc) = 120 ppm, 0-15 cm, BD 1.5 (furrow slice 2.24 million kg/ha).

• Available P = 7.5 × 2.24 = 16.8 kg P/ha → P₂O₅ = 16.8 × 2.29 = 38.5 kg P₂O₅/ha... but ratings are built on the Olsen P₂O₅ scale from the same ppm basis: P₂O₅ = 7.5 × 2.29 = 17.2 ppm → 17.2 × 2.24 = 38.5 kg/ha. Against the rating (10/25 kg P₂O₅/ha) this is High.
• Available K = 120 × 2.24 = 268.8 kg K/ha → K₂O = 268.8 × 1.205 = 324 kg K₂O/ha → High.
• Note the trap: 7.5 ppm Olsen P is "medium-high" only after conversion to the kg P₂O₅/ha basis used in the Indian rating table; comparing 7.5 directly with "10" would wrongly call it Low.

Example 2 - organic carbon from Walkley-Black titration. 1.0 g soil, 10 mL of 1 N K₂Cr₂O₇, blank titre 20.0 mL and sample titre 14.0 mL of 0.5 N ferrous ammonium sulphate.

• Dichromate consumed by soil ≡ (20.0 − 14.0) × 0.5 = 3.0 meq.
• 1 meq of carbon = 3 mg C → C oxidised = 3.0 × 3 = 9.0 mg = 0.0090 g.
• OC (uncorrected) = 0.0090/1.0 × 100 = 0.90%; corrected OC = 0.90 × 1.3... the 1.3 factor applies when the standard 77% recovery is assumed: OC = 0.90 × 1.3 = 1.17%? Recovery correction is applied as OC = measured × (100/77) = 0.90 × 1.3 = 1.17%.
• Organic matter = 1.17 × 1.724 = 2.02% → rating High (> 0.75% OC).

Example 3 - STCR targeted yield for wheat. Target 50 q/ha. For N: NR = 2.4 kg N per quintal, CS = 23.4%, CF = 61.8%, soil test N = 250 kg/ha.

• FN = (2.4/0.618) × 50 − (0.234/0.618) × 250
• = 3.883 × 50 − 0.3786 × 250 = 194.17 − 94.66 = 99.5 ≈ 100 kg N/ha.
• If FYM at 10 t/ha supplying 0.5% N with 30% contribution is applied: N from FYM credited = 10,000 × 0.005 × 0.30 = 15 kg → adjusted dose = (99.5 − 15 × (100/61.8))? The standard STCR form subtracts the FYM contribution on the same CF basis: 15/0.618 ≈ 24.3 → FN ≈ 75 kg N/ha.
• Urea (46% N) needed = 75 × 100/46 = 163 kg/ha.

Example 4 - gypsum requirement for a sodic soil. CEC = 12 cmol(p⁺)/kg, exchangeable Na = 3.6 cmol(p⁺)/kg, target ESP 10%, depth 0-15 cm, BD 1.33, gypsum 80% pure.

• Present ESP = (3.6/12) × 100 = 30%; target Na = 10% of 12 = 1.2 cmol/kg → Na to be replaced = 3.6 − 1.2 = 2.4 meq/100 g.
• At 1.72 t/ha of pure gypsum per meq/100 g for 0-15 cm: 2.4 × 1.72 = 4.13 t/ha pure gypsum.
• Adjusted for 80% purity = 4.13/0.80 = 5.16 t/ha of the available gypsum.

Example 5 - fertiliser blending from a Soil Health Card. Card: available N 240 (Low), P₂O₅ 18 (Medium), K₂O 300 kg/ha (High); crop maize, general dose 120:60:40 kg N:P₂O₅:K₂O/ha.

• Adjust: N low → +25% = 150 kg; P medium → no change = 60 kg; K high → −50% = 20 kg.
• DAP (18% N, 46% P₂O₅) for P: 60 × 100/46 = 130 kg/ha, supplying 130 × 0.18 = 23.4 kg N.
• Remaining N = 150 − 23.4 = 126.6 kg → urea (46% N) = 126.6 × 100/46 = 275 kg/ha.
• MOP (60% K₂O) = 20 × 100/60 = 33 kg/ha.
• At indicative subsidised prices (urea ₹266/45 kg bag, DAP ₹1,350/50 kg bag, MOP ₹1,700/50 kg bag — prices change, verify current rates): urea ≈ 275/45 × 266 ≈ ₹1,626; DAP ≈ 130/50 × 1,350 = ₹3,510; MOP ≈ 33/50 × 1,700 ≈ ₹1,122 → about ₹6,260/ha.

Where it is used

• State Department of Agriculture STLs and mobile vans: sample receipt, analysis and Soil Health Card generation; roles of Soil Chemist, Agriculture Officer and Lab Technician recruited through State PSCs and IBPS-AFO.
• Fertiliser and agri-input companies: IFFCO, KRIBHCO, Coromandel International, Tata Rallis, Chambal and Zuari run soil-testing labs and village campaigns as the entry point for selling customised blends, water-soluble fertilisers, Zn and B products and bio-stimulants.
• Sugar, tea, coffee and plantation companies: cane development departments and Tata Coffee/tea estates test soils plantation-wide for lime, gypsum and K scheduling.
• Precision agriculture and agritech: grid soil sampling with GPS, VRT spreaders and advisory apps (Soil Health Card portal, Kisan Suvidha, DeHaat, Dhanuka, AgroStar); NIR/XRF-based rapid soil scanners in startups.
• Reclamation and watershed projects: ICAR-CSSRI gypsum and lime recommendations for sodic and acid soils; land capability classification by NBSS&LUP; soil survey for irrigation command areas and NABARD-financed projects.
• Research, education and accreditation: SAU soil science departments, ICAR AICRP-STCR centres, NABL-accredited commercial labs, export-oriented residue and heavy-metal testing for organic certification (NPOP) and contract farming.

Interview questions

• Q: Why is sampling considered the most critical step in soil testing? A: The lab analyses a few grams meant to represent about 2,000 tonnes of surface soil per hectare, so an unrepresentative sample gives a precise but meaningless result — most soil-test error originates here, not in the laboratory.
• Q: Which extractant would you use for available P in a calcareous black soil of Maharashtra, and why? A: Olsen's 0.5 M sodium bicarbonate at pH 8.5, because Bray's acid-fluoride extractant is neutralised by free CaCO₃ and over- or under-reads P in calcareous and alkaline soils.
• Q: Why is available nitrogen rarely trusted from a soil test? A: Mineral N is highly mobile and changes with moisture and temperature within days; the alkaline KMnO₄ figure is only an index, so recommendations usually lean on organic carbon and crop response data.
• Q: What is a critical limit and how is it established? A: The soil test value below which a crop gives an economic response to the applied nutrient; it is fixed by plotting relative yield against soil test value from many field trials and finding the break point (Cate-Nelson method).
• Q: How do you convert a DTPA-Zn reading of 0.45 mg/kg into a recommendation? A: It is below the 0.6 mg/kg critical limit, so the soil is Zn-deficient; apply about 25 kg/ha zinc sulphate heptahydrate (or roughly 15-16 kg/ha of the monohydrate, which carries 33% Zn against 21% in the heptahydrate) to the soil, or spray 0.5% ZnSO₄ with lime for a standing crop.
• Q: A soil reads pH 8.8 and EC 1.2 dS/m. What is the problem and the amendment? A: High pH with low salinity points to a sodic (alkali) soil — confirm with ESP or SAR and apply gypsum based on the gypsum requirement, with organic matter and leaching, not lime.
• Q: Difference between soil testing and plant analysis? A: Soil testing is predictive, done before sowing to plan fertiliser; plant/tissue analysis is diagnostic, done on the standing crop to confirm what the plant actually absorbed. They complement each other.
• Q: Why must stainless steel or plastic tools be used for micronutrient samples? A: Galvanised iron augers shed zinc and brass sheds copper and zinc, contaminating the sample and producing falsely high DTPA readings.
• Q: What does the STCR targeted yield approach add over a blanket dose? A: It links fertiliser dose to a specific yield target, the measured soil test value and experimentally derived soil and fertiliser contributions, so the same field gets different doses for different targets — blanket doses ignore both.
• Q: Why should a soil sample be air-dried and not oven-dried? A: Oven heat alters organic matter, releases fixed ammonium and changes extractable K and P, so available-nutrient values become unreliable.
• Q: What 12 parameters does a Soil Health Card report? A: pH, EC, organic carbon, available N, P, K, S, and the micronutrients Zn, Fe, Cu, Mn and B, with crop-wise fertiliser and amendment advice.
• Q: Two labs report different pH for the same soil — give two legitimate reasons. A: Different soil:solution ratios (1:1, 1:2.5, 1:5 give progressively higher pH in acid soils) and different suspension media (water versus 0.01 M CaCl₂ or KCl), apart from calibration and CO₂ effects.

Common mistakes

• Taking one sample for a whole large, variable field, or including bunds, old manure heaps and fertiliser bands in the composite.
• Comparing a ppm result directly with the kg/ha rating table, or mixing the P and P₂O₅ (and K / K₂O) bases — always state which.
• Using Bray's P-1 ratings for an alkaline calcareous soil or Olsen's ratings for an acid soil.
• Quoting EC from a 1:2.5 extract against the 4 dS/m saturation-extract salinity limit, which overstates how safe the soil is.
• Forgetting the 1.3 recovery factor in Walkley-Black, or confusing organic carbon with organic matter (factor 1.724).
• Prescribing lime for a high-pH sodic soil or gypsum for an acid soil — the amendment follows the diagnosis, not just the pH number.
• Sampling soon after fertiliser, lime or manure application, or using galvanised/brass tools for a micronutrient test.
• Treating a village quick-kit reading as a calibrated lab value, and reporting a recommendation without the crop and yield target it was computed for.

Related topics

Soil fertility and nutrient management · Fertilisers and manures · Soil chemistry and ion exchange · Problem soils and their reclamation · Soil survey, classification and land use planning · Precision agriculture and site-specific nutrient management