The relationship in acid soils between base saturation and phosphorus uptake by grasses

Extract

Analyses of grass samples from seventeen fertilizer trials on acid soil showed that both the percentage phosphate in the grass and the amount of phosphate removed by the crop were significantly and directly related to the percentage saturation of the b.e.c. of the soil. The amount of acid-soluble phosphate by difference (Table 1, A — B) was also significantly and directly related to the percentage phosphate in the grass but not to the amount taken up by the crop. The amounts of exchangeable bases, exchangeable calcium and acid-soluble, adsorbed and watersoluble forms of phosphate showed no significant relationships with phosphate uptake and percentage phosphate in the grass.

The percentage saturation of the b.e.c. was found to be, on the average, about five times as effective a contributory factor to the percentage phosphate in the grass as the amount of acid-soluble phosphate by difference. The results confirm previous work, which indicated that in acid soils the main form of available phosphate is that associated with the exchangeable bases the effective presence of which is governed by the percentage saturation of the b.e.c. It is considered that phosphate availability and response in acid soils are best assessed by considering the percentage saturation of the b.e.c. rather than the amounts of some form or forms of phosphate in the soil.

The grass analyses also showed that when the amount of phosphorus (P) hi the dry grass exceeds 0·33% no response to phosphate is to be expected. When the percentage in the grass is less than 0·23% responses of 10% or more are likely.