Sulphur in Northern Nigerian Soils 1. The effects of cultivation and fertilizers on total S and sulphate patterns in soil profiles

Organic-S and sulphate-S were measured in the Northern Guinea savannah zone down soil profiles under undisturbed natural vegetation, and in land cleared and cropped, with and without fertilizers and farmyard manure. The main soil type was a drained, red to red-brown, sandy clay to clay loam (pH 4·1–5·6 in 0·01 M-Cal2), which strongly sorbed sulphate.

Organic-S was most in the surface layers and decreased with depth. Root remains from the natural fallow vegetation, present up to 9 years after clearance, strongly influenced the distribution of organic-S. The roots disappeared after 19 years, when distribution of organic-S mainly reflected crop-root distribution; amounts of organic-S were related to crop yields. FYM was less effective in maintaining organic-S than fertilizers containing P and N.

Profiles under natural vegetation contained little sulphate-S but it accumulated in the sub-soil after clearance when S was not removed. The sulphate pattern of profiles under unfertilized crops resembled that under natural vegetation. All fertilized sites had a well-defined sulphate sorbtion pattern, the deepest maximum was at 50 cm on a plot where more than 800 kg S/ha had been applied in nineteen annual aounts. Sulphate-S ranged from > 1 ppm, in the deepest samples, to 52 ppm at the absorption maximum.

Almost all the sulphur applied was in the profiles or removed in harvested crops, showing that losses from erosion and leaching were small.
RESP-6564