A high-phosphorus soil test and a low-zinc maize crop
A farmer has a 1.0 ha sandy loam field with pH 7.8, organic carbon 0.45%, available N 180 kg/ha, available P2O5 52 kg/ha (high), available K2O 210 kg/ha (medium) and DTPA-Zn 0.32 mg/kg (critical level 0.60). He plans maize with a state recommendation of 120 kg N, 60 kg P2O5 and 40 kg K2O per hectare. He has 100 kg DAP (18-46-0), 200 kg urea and 50 kg muriate of potash in the store. He intends to apply all the DAP and all the potash as basal, then split the urea into two equal doses. Three-week-old maize in patches shows pale stripes on young leaves and shortened internodes; the farmer thinks it is a nitrogen shortage and wants to add more urea. He has 25 kg zinc sulphate and asks whether to use it.
1. Interpret the soil test values and decide whether the planned DAP application is justified. Show the nutrient calculations.
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The soil test shows low available nitrogen, high available phosphorus, medium potassium and deficient zinc. The DAP plan is not justified. DAP 100 kg supplies 18 kg N and 46 kg P2O5. With 200 kg urea supplying 92 kg N, the total N is 110 kg, which is 10 kg short of the 120 kg recommendation, but the P2O5 total would rise to about 98 kg/ha against a recommendation of 60 kg/ha. Since the soil already tests high in P, adding 46 kg P2O5 is wasteful and may aggravate zinc deficiency. A better plan is to skip DAP, buy about 61 kg more urea so that total urea is about 261 kg for 120 kg N, and apply it in splits. For potassium, 40 kg K2O requires about 67 kg muriate of potash; he has 50 kg, so buy about 17 kg more. Apply 25 kg zinc sulphate per hectare as a basal soil treatment. If some starter phosphorus is wanted for early maize vigour, a small banded dose can be considered, but it should not be the full 100 kg DAP on a high-P soil.
2. Explain the young-leaf symptoms. Why does high soil phosphorus make zinc deficiency more likely, and how should zinc be applied?
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The pale stripes on young leaves and shortened internodes fit zinc deficiency in maize, sometimes called white bud when severe. Zinc is relatively immobile in the plant, so symptoms appear first on younger tissue. The soil test supports this: DTPA-Zn is 0.32 mg/kg, below the 0.60 critical level. High available phosphorus can induce or worsen zinc deficiency through an antagonistic effect on zinc uptake and internal distribution, and the alkaline pH of 7.8 plus the sandy, low-organic-matter texture reduce zinc availability further. Adding more urea will not correct the problem because the symptoms are not primarily nitrogen deficiency. Apply zinc sulphate at 25 kg/ha to the soil before or at sowing and incorporate it; a foliar spray of about 0.5% zinc sulphate can be used as a quick corrective, preferably with a little hydrated lime to reduce leaf burn. Do not mix zinc sulphate with phosphate fertilizer in the same band, because that can reduce its effectiveness.
3. How should the nitrogen and potassium be split and placed to reduce losses?
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On a sandy loam with low organic carbon, splitting nitrogen is important because nitrate can leach and urea can be lost if left on the surface. Apply about one-third of the nitrogen and all the potassium as a basal dose, banded and incorporated rather than broadcast. Apply the remaining nitrogen at the knee-high stage and again just before tasselling, preferably just before or with irrigation. Place potassium in a band rather than scattering it, because the sandy soil has a low capacity to hold it. Keep the zinc sulphate separate from the phosphate band. The farmer should also record the actual nutrient rates: about 261 kg urea, 67 kg muriate of potash and 25 kg zinc sulphate per hectare, rather than using the original DAP-based plan.