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Solution concentrations of total A1 and of mononuclear A1 ions were determined as a function of pH for two acid soils. Polynuclear hydroxy-A1 was taken as the difference between total and mononuclear Al. Salt concentration at I : 5 soi1:solution ratio was varied from 0.008 to 1-0 equiv/l, for CaCI,, KCl, and NH,OAc at pH 4.8. Polynuclear hydroxy-A1 concentrations increased with electrolyte concentration and the valency of the desorbing cation, and rose to 0.2 mM (equivalent to 3 meq/kg soil) in unbuffered salt suspensions, and to I *o mM in NH,OAc at pH 4.8. Polynuclear hydroxy-A1 thus behaves like a normal exchangeable cation. I'OM NH,OAc released much more total A1 from some of the more acid samples than might have been expected from its pH of 4.8.

Sulphur deficiency reduced the yield and sulphur content of ryegrass at the fourth cut, while increasing the nitrogen content and N:S ratio. The distribution of the forms of nitrogen was also altered, with a decrease in the proportion of nitrogen recovered as amino acids and an increase in the concentration of asparagine. Amino acid analysis showed that the concentrations of the sulphur-containing amino acids cyst(e)ine and methionine were depressed by sulphur deficiency as well as those of arginine, histidine, lysine, glycine, leucine, serine and threonine. Sulphur deficiency, therefore, decreases the quality of crude protein found in grass, as well as reducing the yield.

The changeover from fertilizers containing calcium or ammonium sulphate to those containing little or no sulphur has led to the occurrence of sulphur deficiency in many areas of the world (McNaught & Christoffels, 1961). In the United Kingdom, it was thought that even in the absence of fertilizer sulphur, atmospheric inputs were sufficient to ensure an adequate supply for crops. Recent work in the north of Scotland has shown that this is not so, that many soils in the area are low in sulphur, and yield responses have been obtained following application of sulphur to grass (Soott et al. 1983; Scott, 1984). The purpose of the present experiments was to extend the work to cereals, in particular to examine the response to added sulphur of the grain yield of winter barley, and to measure the effect of sulphur on the chemical composition of plants.

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