The initial leaching of hayed-off pasture plants in relation to the recycling of phosphorus

Abstract

Hayed-off phalaris and clover plants of various nitrogen and phosphorus contents were leached under a wide range of laboratory conditions designed to simulate various first rainfalls following haying off. Of the total phosphorus in ground plant material, 60–83% was water-soluble and most of it was inorganic. Continuous leaching equivalent to 24.4 cm rain was required to leach more than 90% of the water-soluble phosphorus from certain samples of whole plant material, but leaching rates equivalent to only 1.25 cm over 96 hr were quite effective and removed up to 62%. The percentage of phosphorus leached at low intensities over periods of up to 4 days varied with the plant sample. It depended largely on the rate and extent of microbial conversion of inorganic phosphorus to water-insoluble forms. The percentage of soluble phosphorus immobilized increased as the nitrogen/phosphorus ratio of the sample increased. Some of the inorganic phosphorus rendered insoluble by microbes reverted to soluble phosphorus following a mild drying treatment, but about half of the solubilized phosphorus was organic. The results are discussed in relation to the recycling of phosphorus in pastures.