Studies on temporal variation of Colwell soil‐test phosphorus measured on soil samples collected from November to March in south‐western Australia

Abstract

In south‐western Australia, farmers collect soil samples to measure soil‐test phosphorus (P) (Colwell procedure) from November to March when the soils are usually dry. When soil samples are collected from the same area at different times during the year to measure soil‐test P, the values can vary, often substantially. This is called seasonal or temporal variation of soil‐test P. This paper reports studies undertaken in south‐western Australia to determine whether temporal variation affects soil‐test P when soil samples are collected at different times during November to March. There were three studies. In one study, samples of different soils were collected from paddocks near West Dale. The samples were thoroughly mixed and placed in boxes in the weather from July to October before being placed in a glasshouse and allowed to air dry. Soil samples were collected on five occasions from early November to early February to measure soil‐test P. In a second study, soil samples were collected at weekly intervals from late December to mid March from 5 m by 5 m uniform areas in paddocks on fourteen different soil types. In a third study, soil samples were collected at fortnightly or monthly intervals from November to April from plots of two long‐term field experiments that were treated with fertilizer P eight or nine years previously. In all three studies, temporal variation was found to be small. It is concluded that soil‐test P is unlikely to be greatly affected by temporal variation in soil‐test P during the November to March period in south‐western Australia.