Fertiliser arithmetic and ammonia loss on a calcareous zero-till field
A zero-till wheat grower with 1.0 ha has a recommendation of 150 kg N, 60 kg P2O5 and 40 kg K2O per hectare. His stock is urea (46-0-0), DAP (18-46-0) and MOP (0-0-60). Soil: sandy loam, pH 8.4, free CaCO3 14%, organic carbon 0.36%, EC 0.19 dS/m. He broadcasts all the DAP and urea on the undisturbed surface two days before sowing, sows with a zero-till seed drill that has no fertiliser box, and delays the first irrigation to 25 days after sowing because canal supply is rostered. Sowing is on 18 November; day temperatures are 28-30 °C and no rain is forecast for three weeks. He also plans to place about 40 kg/ha of the DAP with the seed in a few patches 'to give the seedlings a start'.
1. Calculate the quantities of urea, DAP and MOP needed, and state how the urea should be split.
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Step 1 - phosphate first (it fixes the DAP rate): DAP needed = 60 / 0.46 = 130 kg/ha. This also supplies 130 x 0.18 = 23.4 kg N/ha.
Step 2 - nitrogen balance: N still required = 150 - 23.4 = 126.6 kg N/ha. Urea needed = 126.6 / 0.46 = 275 kg/ha.
Step 3 - potash: MOP needed = 40 / 0.60 = 67 kg/ha.
Check: 130 + 275 + 67 = 472 kg of product per hectare, delivering 150-60-40 exactly.
Splitting the urea: apply one-third (about 92 kg) as basal, one-third at the first irrigation and one-third at the second. DAP and MOP go entirely as basal. The chemical reasoning is that a single 275 kg dose of urea on a sandy, low-CEC soil puts far more ammoniacal N in the soil than the young root system can capture, and the surplus is exposed to volatilisation and leaching. The basal urea should also be banded or drilled, not left on the surface.
2. Estimate the fate of the surface-broadcast urea over the three rainless weeks and explain the chemistry. Then say how the application should be changed.
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Chemistry of the loss: urea is hydrolysed by the enzyme urease (abundant in crop residues and soil) to ammonium carbonate within a few days when the soil is moist: (NH2)2CO + 2H2O -> (NH4)2CO3. The ammonium enters the NH4+/NH3 equilibrium, and that equilibrium is pH-dependent: at pH 8.4 and 28-30 °C a substantial share of ammoniacal N exists as NH3 gas. With no rain and no irrigation to wash the ammonium into the soil, that gas simply escapes. On a calcareous soil, surface-broadcast urea commonly loses 15-30% of its N this way, and surface DAP loses ammonia as well.
What should change:
- Place, don't broadcast. Band urea 5-7 cm deep so the ammonium is adsorbed on the exchange complex and the gas cannot escape.
- Irrigate soon after application. A light irrigation immediately after top-dressing moves urea and ammonium into the soil. A 25-day delay after sowing is also agronomically wrong, because crown-root-initiation irrigation is critical for wheat.
- Change part of the source. Ammonium sulphate or calcium ammonium nitrate is preferable to urea on a calcareous, high-pH soil: both are acidifying or neutral, and neither volatilises as readily as urea.
3. Evaluate the plan to place 40 kg/ha of DAP with the seed.
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The intention is right - banded phosphate is much better than broadcast on a calcareous soil - but seed placement of the whole DAP dose is not safe.
Chemistry and physics of the risk:
- DAP has a high salt index (about 30) and a strongly acidic granule. Dissolving in the moist zone around the seed raises osmotic potential and can delay or kill germination.
- DAP carries 18% N as NH4+. In a calcareous soil, the pH of the granule microenvironment rises during dissolution and hydrolysis, generating NH3 right next to the germinating embryo. Ammonia is extremely toxic to seedlings.
- On a zero-till drill without a fertiliser box, the 130 kg/ha dose would be concentrated in the seed row, magnifying both effects.
Safe practice: seed-placed DAP is normally limited to about 25-30 kg P2O5/ha, i.e. roughly 55-65 kg of DAP, and only with good soil moisture. For the full 60 kg P2O5, place DAP 2-3 cm below the seed or use a separate band. Banding also reduces P fixation by CaCO3, because it limits the volume of soil where calcium phosphate precipitation can occur.