Plant availability of phosphorus accumulated from long‐term applications of superphosphate and effluent to irrigated pastures

Abstract The plant availability of soil phosphorus (P) which accumulated in two Winchmore soils receiving annual topdressings of sup'rphosphate at two rates (188 and 376 kg superphosphate/ha) and in Fairton soil receiving one rate (65 kg P/ha) of effluent over the last 30 and 80 years respectively, was determined. The experiment involved three successive croppings of perennial ryegrass plants ( Lolium perenne L.) grown under growth chamber and glasshouse conditions. The plants were fed with complete nutrients, except P, to favour growth. Each soil was subjected to three treatments consisting of Control (no nutrients added), and two treatments with complete nutrients, except P, added at two different rates of nitrogen (N) (100 and 200 kg N/ha on an area basis). A sequential P fractionation scheme (NaHC0 3 ⪰ NaOH ⪰ HCI ⪰ NaOH) was used on soils before and after cropping to detect changes in soil P fractions. Results obtained showed that dry matter yield increased with applied N but decreased with increased cropping and varied according to the extractable P status of the soils studied. Only a small proportion (2‐13%) of the total soil P was removed by plants after three successive croppings. Plants showed P deficiency in the fertilised Winchmore soils receiving N. Overall, decreases in NaHC0 3 and NaOH‐I inorganic Pfractions were 34 and 16%,respectively. Decreases in HCI and NaOH‐II inorganic P fractions in Winchmore soils were small <10%) and insignificant. A significant decrease in HCI inorganic P (16%), which occurred in Fairton soil, was attributed to the decrease in soil pH after cropping (from 6.2 to 5.1). Amounts of P in soil organic P fractions were unaffected by cropping showing that organic P was stable and did not contribute significantly to plant P uptake in the short term, although the possibility of organic P turnover under steady state conditions without causing changes in amounts of organic P cannot be discounted.