Effects of soil- and foliar-applied phosphorus fertilizers on the potato (Solanum tuberosum) crop

Twenty-two field experiments in England, done between 1986 and 2000, tested the effects of phosphorus (P) fertilizers on number of tubers and tuber yield in Solanum tuberosum. Applying P fertilizer resulted in statistically significant increases in tuber yield in six experiments and the optimal P application rate ranged from c. 90 to 180 kg P/ha. Statistically significant increases in yield in response to application of P fertilizers were found only in soils that contained <26 mg Olsen-P/l (<Index 3) and appeared to be associated with increases in ground cover. Statistically significant increases in the number of tubers in response to application of P fertilizer were found only in soils that contained <16 mg Olsen-P/l (<Index 2) and appeared to be associated with an increase in ground cover by the time of tuber initiation (c. 5-6 week after planting). Each tonne of tuber fresh-weight yield was, on average, associated with removal of 0.39 kg P but regression analysis showed that this value increased as soil Olsen-P increased. Re-analysis of published data showed that whilst the probability of a response to P fertilizer and the optimum P application rate may have been overestimated, some statistically significant responses to P fertilizer did occur when Olsen-P was >26 mg/l. The absence of yield responses on P Index 3 soils found in the current experiments was attributed to increased use of irrigation that may have increased the availability of soil P. Reinterpretation of data from long-term experiments showed that the agronomic benefits of increasing soil P status by applying more P than is removed by harvested crop parts, are small. Since large P residues, estimated by Olsen-P or degree of soil P saturation, are associated with desorption of P and consequent loss to drainage water it is inadvisable to increase soil P above Index 3. For these reasons, no P fertilizer is recommended for Index 4 soils, an amount equivalent to replacement is recommended for Index 3 soils but up to 110-130 kg P/ha should be applied to Index 0 soils. Applications of foliar P had no effect on number of tubers or tuber yield and this practice cannot be recommended.