Soil test pH 8.4, Olsen P 9 and Rs 9,000 of fertilizer: computing and defending the dose
A farmer's 1 ha field tested before wheat sowing gave pH 8.4, EC 0.18 dS/m, organic carbon 0.38%, available N 168 kg/ha, Olsen P 9.0 kg/ha, ammonium-acetate K 145 kg/ha, DTPA-Zn 0.42 ppm and hot-water-soluble B 0.38 ppm. The soil is calcareous and is irrigated with tube-well water of RSC 1.2 meq/L. The recommended dose for a 5 t/ha wheat crop is 120 kg N, 60 kg P2O5 and 40 kg K2O per hectare. The farmer owns urea (46% N), DAP (18% N, 46% P2O5), muriate of potash (60% K2O) and zinc sulphate (21% Zn), and has Rs 9,000 for fertilizer. He wants to broadcast everything as one basal dose a week before sowing because 'more DAP means more yield'. He also asks whether he should add gypsum, since a neighbour did.
1. Calculate the fertilizer quantities he needs, show the nitrogen credit from DAP, and specify placement and timing.
Read the worked answer
Step 1 - fix P2O5 with DAP. 60 / 0.46 = 130 kg DAP per hectare.
Step 2 - nitrogen already supplied. 130 x 0.18 = 23.5 kg N.
Step 3 - balance nitrogen with urea. (120 - 23.5) / 0.46 = 209.8, so about 210 kg urea.
Step 4 - potash. 40 / 0.60 = 66.7, so about 67 kg muriate of potash.
Check: N = 23.5 + 96.6 = 120 kg; P2O5 = 60 kg; K2O = 40 kg.
Indicative cost: DAP 130 x Rs 27 = Rs 3,510; urea 210 x Rs 6 = Rs 1,260; MOP 67 x Rs 34 = Rs 2,280; zinc sulphate 25 kg x Rs 45 = Rs 1,125. Total about Rs 8,175, leaving roughly Rs 800 for two foliar zinc sulphate sprays.
Placement and timing - his plan is wrong here:
- All DAP and MOP and one-third of the nitrogen (about 70 kg urea) as a basal band drilled 5-7 cm below and beside the seed, not broadcast. Broadcast P in a calcareous soil is fixed near the surface and largely wasted.
- Remaining urea in two equal top-dressings at crown root initiation (about 21 days) and at 45-50 days.
- Do not band zinc sulphate with DAP; the two antagonise each other. Apply zinc separately.
2. Explain why the zinc result and the high pH change the plan, and what the foliar zinc schedule should look like.
Read the worked answer
Why zinc matters here: DTPA-Zn 0.42 ppm is below the 0.6 ppm critical level, so the field is zinc deficient. At pH 8.4 with free lime, zinc is strongly adsorbed as insoluble calcium zincate, and phosphate applied as DAP competes with zinc for uptake. A farmer who adds 'more DAP' therefore deepens the zinc problem: the crop will show the classic pattern of a phosphate response capped by zinc deficiency - tillering, then interveinal chlorosis on middle leaves and poor grain filling.
Plan:
- Soil application of 25 kg/ha zinc sulphate (21% Zn), giving about 5.25 kg Zn/ha, broadcast and incorporated before sowing, or drilled separately from the DAP bands. Soil zinc lasts 2-3 seasons; do not repeat every year.
- Two foliar sprays of 0.5% zinc sulphate, that is 5 g per litre, about 200 litres per hectare per spray, at tillering and at flag leaf. Add a sticker; do not mix with phosphatic or high-lime sprays.
- Keep the foliar sprays if cash is short - they are cheaper than soil application and correct the deficiency within the season.
Address the pH: 8.4 with free lime locks up both P and Zn, so placement plus foliar correction is the practical answer; acidifying a calcareous field is not economic. Low organic carbon (0.38%) aggravates the problem - FYM or a green-manure crop is a slower, cheaper fix than extra DAP.
3. Decide whether gypsum should be applied, and rank the field's constraints with the reasoning you would give the farmer.
Read the worked answer
Gypsum: not yet - the test does not answer the question. Gypsum is a sodic-soil amendment, and pH 8.4 alone does not establish sodicity; a calcareous soil can easily be pH 8.4 with normal exchangeable sodium. The missing test is exchangeable sodium percentage (or SAR). Advise gypsum only if ESP is above about 15. The irrigation water's RSC of 1.2 meq/L is marginal and should be monitored, but it is not on its own a gypsum trigger. Applying gypsum without an ESP value wastes his money and may worsen infiltration if the soil is already flocculated.
Rank the constraints:
- Zinc deficiency (0.42 ppm) - immediate, cheap to fix, and it caps the response to the DAP he wants to buy first.
- Very low organic carbon (0.38%) - the medium-term limiter of water-holding capacity and nutrient availability; FYM, residue retention and a legume in rotation.
- Phosphorus (Olsen 9 kg/ha) with high P-fixation -- do not cut P, but place it in bands rather than broadcast.
- Potassium 145 kg/ha - medium. In wheat, response is modest; if cash is short, 40 kg K2O can be reduced to 25-30 kg, but the P must not be touched.
- Boron 0.38 ppm - borderline; watch for hollow stem, do not apply borax blindly.
Message to the farmer: more DAP is not more yield on this field. Right placement, split nitrogen, zinc, and organic matter are the paying investments.