Top-dressing of grassland with phosphates and lime

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Abstract
Results of a trial on yellow-grey earth which compared the effects of eight annual top-dressings of superphosphate, basic slag, and Gafsa (rock) phosphate on limed and unlimed ground were summarised in this Journal by Doak (1942a) and Elliott and Lynch (1942). Residual effects on pasture production, botanical composition, herbage chemical analyses, and soil analyses were measured for 18 years subsequently. About four years after top-dressing ceased pasture production fell on all treatments to about two-thirds, of the original level. Retop-dressing after 10 years without fertiliser did not restore production to the original level. Responses to the three forms of phosphate differed on un limed compared with limed ground and their relative effectiveness in the “residual” period differed from that of the “top-dressing” period. Lime responses (in the presence of P) lasted for about eight years after regular liming ceased, but subsequently limed plots were inferior to unlimed on the superphosphate plots but not on basic slag and Gafsa phosphate plots. Perennial rye grass (Lolium perenne L.) responded most to phosphate and white clover (Trifolium repens L.) to lime. Lime and basic slag increased the calcium content of the herbage 18 years after their last application. They markedly depressed manganese levels. Differences in phosphorus contents of the herbage among trial treatments were small. Soil analytical levels consistently reflected the various trial treatments. The increase in soil pH from lime and basic slag was still evident 20 years after their last applications. Four to seven years after top-dressing ceased, total P levels slumped, the average loss being 129 lb/ac 10 in.

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