Factors affecting growth and response of white clover in pots to applied phosphorus

Abstract

The response to phosphorus by white clover in pots was determined on soils of low, medium, and very high phosphate retention. Soil was obtained from permanent pasture paddocks and was collected to depths of 0–25 mm and 0–75 mm. The latter samples were used either mixed or layered as 0–25 mm soil over 25–75 mm soil. On the soils of medium and very high retention the responses on 0-75 mm (layered) were less than those on 0–75 mm (mixed), but were similar to those on the 0–25 mm samples.

Extraction of the 0–25 mm and 25–75 mm samples, alone and mixed, by Truog and Olsen reagents and by calcium chloride indicated that when the ratio of P (0-25 mm) : P (25–75 mm) was three or less there was no sorption of phosphorus from solution by the soil of lower P status. It is concluded that when this ratio is low, the greater response on 0–75 mm (mixed) samples is due to physical dilution of the layer of soil of high P status rather than to sorption of phosphorus from this layer on to the soil of lower P status below it.

When response to phosphorus was determined in pots, using 0–38 mm samples from the sites of seven field experiments on a soil of very high phosphate retention. the levels of response were similar to those obtained in the field. In contrast, use of 0–75 mm samples in the pot experiments resulted in responses which were exaggerated when compared with the field experiments.