Abstract
Field experiments were carried out using different amounts of ^32^P‐labelled superphosphate in combination with two rates of application of ammonium sulphate on soils high and low in “available” phosphate. The crop was sampled at three stages of growth and analysed for dry matter and for total and fertiliser‐phosphorus.
The rate of application of nitrogen had little effect on phosphorus uptake or crop growth but the uptake of soil‐phosphorus from both soils was depressed by added fertiliser‐phosphorus, the depression being very marked on the high‐phosphate soil. Higher uptakes of fertiliser‐phosphorus were not effective in increasing the final yield of tubers and the yield of dry matter was depressed on the high‐phosphate soil by the application of 1.0 and 2.0 cwt. of P^2^O^5^ per acre (as superphosphate) compared with application of 0.25 and 0.5 cwt. The optimum yields were reached with 0.25 and 1.0 cwt. of P^2^O^5^ per acre on the high‐ and low‐phosphate soils respectively.
Onset of tuber development was earlier on the low‐phosphate soil. Lower rates of application of phosphorus hastened the start of tuber development and the time of maturing of the plant, giving an earlier crop, but higher rates delayed both early tuber development and the attainment of the final yield. Such effects at levels of superphosphate normally applied to the potato crop in Scotland may lead to serious reductions in yield—particularly with early potatoes.