Rills, low pH and a stream below: sequencing erosion control and fertility on a sloping field

Erosion evidence

• Rills 10–15 cm deep running downslope are the fingerprint of concentrated flow. Low pH or low phosphorus does not cut channels in a field; running water does.
• Sand deposits along the lower fence and exposed pale subsoil in patches show that topsoil has moved from the upper slope and accumulated at the foot of the field. That produces a yield pattern - poor on the eroded upper slope, better in the deposition zone - which a simple fertility decline does not produce.
• Eight years of continuous maize with residues grazed off leaves almost no cover at the start of the rains, exactly when a 60 mm storm arrives on a 7 % slope.

Fertility evidence, real but secondary

• pH 4.9, Bray-1 P of 4 mg/kg, CEC 4 cmol/kg and organic carbon 0.5 % are all poor. Years of nitrogen fertilizer and removal of grain and stover have acidified the soil and stripped nutrients.
• The two problems interact: a low CEC means the soil cannot hold nutrients, and erosion removes the very clay and organic matter that would hold them. Fertilizer applied to an eroding field is partly lost downslope.

How to prove it Compare yields in upper, middle and lower slope strips this season, and dig a 30 cm pit on the upper slope. If the dark topsoil above the lighter subsoil is much shallower upslope than downslope, erosion is a major driver and must be treated first.

Order of priority: stop the loss, correct the worst chemical constraint, then build organic matter.

Before the next planting - cheap and high return

1. Stop residue grazing and retain at least 30 % cover. This costs nothing and is the single most effective measure on this field; if cattle must be fed, cut and carry from elsewhere.
2. Make contour ridges or tied ridges across the slope, about 15–20 m apart, with cross-ties. These shorten the flow path and trap water and soil in the rills. The farmer's own plough and oxen can do it; labour is the main cost.
3. Plant one-row grass strips (vetiver, elephant grass or a local bunch grass) on the contour every 20–25 m and along the lower boundary. Planting material is cheap and the strips are permanent.
4. Repair the existing rills by hand - fill with residue and soil or small check dams - before the rains return.

Then the soil chemistry 5. Lime is the biggest single yield lever at pH 4.9 (target 5.8–6.0). Get a lime requirement from the laboratory, incorporate it, and apply 2–3 months before planting; at 1.5 ha the haulage cost matters, so check the rate before ordering. 6. Apply phosphorus in a band at planting rather than broadcasting. With CEC 4, broadcast P is fixed or washed away; banding puts it where roots can reach it. Do this after liming, because P is less available at low pH.

Where the money goes If cash allows only one chemical input, buy lime before fertilizer. Do the cover and contour work regardless, because buying fertilizer for a field that will lose its topsoil in the first storm is money thrown away.

What leaves the field Sediment, and the nutrients attached to it or dissolved in the water: nitrogen, phosphorus, potassium and lime. The stream 60 m below receives them. Phosphorus bound to soil particles and nitrogen in sediment and solution encourage algal growth, which lowers oxygen and affects the households that use the water. Nitrate can also move down through the sandy soil into groundwater, where boiling does not remove it. The farmer loses the most fertile part of his soil and the fertilizer he paid for, so the on-farm cost is at least as serious as the off-site one.

Cheap measures

1. Leave an uncultivated, vegetated buffer strip 5–10 m wide along the lower boundary, never heavily fertilized or grazed, to trap sediment and nutrients before they reach the stream.
2. Keep fertilizer and manure applications at least 10 m back from the stream, never spread just before heavy rain, and split nitrogen dressings so less remains in the soil when a storm comes.
3. Maintain the rills and contour structures and keep a grassed waterway or small silt trap at the field outlet. Store lime, manure and fertilizer on a flat, covered site away from the waterway.

Longer term Record what is applied and when. If soil losses continue despite the contours, the cheapest long-term fix for the steepest strip is to take it out of annual cropping and put it into permanent grass or trees, rather than chasing yield with more fertilizer.