Can plant tissue tests be used to predict the critical soil test value?
Various experiments placed the target fertility for subterranean clover-based pasture grown on this soil at about 20-25 mg P/kg soil. Large increases in pasture yield were observed when P was applied in spring to paddocks with 7-10 mg P/kg soil (Colwell; 0-10 cm), a small increase on a paddock with 20 mg P/kg soil (significant at P<0.06), and no yield response in paddocks with 25-33 mg P/kg soil (unpublished data). In addition, the critical P concentration of soil (0-7.5 cm depth) that corresponded with 90% of maximum yield in subterranean clover pastures across 20 sites on the Southern Tablelands, NSW is reported to be 22 mg P/kg (Colwell test) (5). For a 0-10 cm depth soil sample, this would equate to a Colwell value of about 19 mg P/kg soil. Figure 2. Relationship between the P concentration in YOL and WS from adjacent subterranean clover plants in paddocks that covered a wide range of soil P fertility levels. Plants were sampled in mid spring of 1999 (circles), 2000 (squares) and 2
