Continuous maize on an acid sandy soil that no longer responds to fertilizer

The test describes a strongly acid, sandy, poorly buffered soil.

• pH 4.5 is well below the 5.5 threshold for maize. At this pH aluminium and manganese become soluble and toxic, root growth is stunted, phosphorus is fixed by iron and aluminium oxides, and molybdenum becomes unavailable.
• Base saturation 38% confirms low calcium and magnesium on the exchange complex; the exchangeable K of 0.15 cmol/kg is low for maize.
• CEC 5.8 cmol/kg with 12% clay means very little capacity to hold applied nutrients. Ammonium and potassium from his bags leach below the root zone in heavy rain, so the crop sees only part of what he pays for.
• Available P of 4 mg/kg is far below the roughly 10–15 mg/kg needed by maize; the 21 kg P2O5/ha he applies is largely locked up before the roots reach it.
• Organic matter 0.9% means little nitrogen is released from the soil, poor structure and low water holding capacity, so the crop also suffers in short dry spells.

The fertilizer is not 'wrong'; it is being applied to a soil where roots cannot exploit it. His neighbours' manure works because it raises pH slightly, supplies calcium and magnesium, complexes aluminium, adds organic matter and releases phosphorus slowly - it treats the cause rather than adding more of a nutrient the soil cannot deliver.

Buying four more bags of urea repeats an experiment that has already failed for eight seasons. Without correcting the acidity, extra ammonium nitrogen is poorly used, acidifies the soil further, and the marginal yield is small.

Spend it on lime plus a small amount of nitrogen:

• 1 t of dolomitic lime, MK 180,000.
• One bag of urea, MK 55,000, for a starter dose only.

Do not spread 1 t thinly over the whole hectare. One tonne per hectare is probably a quarter to a third of what this soil needs. Lime does not move in the soil, so spreading it thinly treats a large volume at a rate too low to change root-zone pH. Apply the tonne at about 2 t/ha on the worst 0.5 ha - the most acid area, or the block that will carry next season's most valuable crop - and lime the other half in a later season.

Application:

• Apply and incorporate into the top 15 cm two to three months before planting; lime broadcast on the surface without mixing acts very slowly.
• Do not apply lime and urea or ammonium-based fertilizer in the same operation or on the same day. Ammonium plus lime on the surface encourages ammonia loss.
• Use the remaining small budget on manure or compost on the same half-hectare, and band phosphorus fertilizer near the seed rather than broadcasting it.

A single season will not fix eight years of acidification; plan the correction in stages that the farm can finance.

Season 1: lime 0.5 ha at 2 t/ha, incorporated well before planting; 2–3 t/ha of cattle manure on the same half; band NPK near the seed at planting and top-dress with a modest amount of urea. Keep the other half under maize as before so the effect can be compared side by side.

Season 2: lime the second 0.5 ha, using the same rate and timing; continue manure, residues and banded P; introduce a legume (soya or groundnut) into the rotation to add nitrogen and break the maize pest and disease cycle.

Season 3: a second, smaller lime top-up if a repeat soil test shows pH still below 5.2; rotate maize with the legume; retain crop residues rather than burning, and do not remove all stover.

Monitor: repeat soil sampling at 0–20 cm every two years on the same spots, tracking pH, base saturation, available P and organic matter; record yield per half-hectare so the comparison is real. Judge the lime on the second season, not the first - a liming effect on a sandy soil builds slowly. Continue to band phosphorus; fixing pH increases P availability but does not replace it.