Residual effect of rock phosphate and superphosphate

Authors

• Armi Kaila University of Helsinki, Department of Agricultural Chemistry

DOI: https://doi.org/10.23986/afsci.71728

Abstract

Results are reported of the residual effect of North African finely ground rock phosphate and superphosphate in two field trials on acid fine sand and humus soils. During the first seven years of these trials 166 kg P/ha was applied either as rock phosphate or as superphosphate to cereals and mixed leys. Then crops were grown for three years without any application of phosphorus fertilizers. The phosphorus analyses of the soil at the end of the seventh experimental year proved that the largest part of rock phosphate occurred in the acid soluble fraction, probably mainly as unweathered apatite. The superphosphate phosphorus seemed to be accumulated as fluoride-soluble and alkali-soluble forms. In the trial on the humus soil of a fairly high productivity, the residual effect of both fertilizers on three oat crops was insignificant. In the trial on the poorer fine sand soil, the residues of both fertilizers produced significant and mutually equal increases in the dry matter yields of the oat crop in the first year and of the hay crops in the second and third years. The tendency to a higher phosphorus content in the graminaceous plants from the superphosphate plots resulted in a higher uptake of phosphorus from these plots as compared with the phosphorus yield from the rock phosphate plots; only in the third year the difference was no more significant. Attention was paid to the good capacity of red clover to use phosphorus of rock phosphate.