Yield response of iceberg lettuce (Lactuca sativa Capitata group) to phosphorus fertilisation in a boreal soil

Cold soils and a short boreal growing season are assumed to necessitate high soil phosphorus (P) status and ample P applications to vegetable crops. Yet, a previous Finnish study indicated lower than anticipated yield responses of onion and cabbage. Here we report 2-year P trials with iceberg lettuce (cv. Skindel) transplanted in an open field in spring and midsummer on a clay soil with moderately low P status. During the spring plantings, P concentration in soil solution was followed using Plant Root Simulator (PRS) probes in P0 (0 kg P ha-1) and P60 (60 kg P ha-1) treatments. The PRS probes indicated initially 5 to 7-fold higher soil solution P concentration in the P60 treatment compared to P0 due to fertilisation, but thereafter P concentrations equalised. For spring plantings, P applications did not explain yield variation, giving statistically non-significant, maximum 14% higher yields over P0. In summer plantings, about 30% of the yield variation was explained by P applications, and the P60 rate gave 20–35% higher yields over P0. Mitscherlich type model integrating all data predicted a maximum 20% yield, 10% of the variation being accounted for. The model suggested that 32 kg P ha-1 brings 97% of the maximum yield, whilst the Finnish P fertilisation regulation allows 60 kg ha-1 for the given soil. No correlation between P applications and P concentrations of lettuce leaves was found. The results stress the need for empirical evaluation of P requirements of vegetable crops to avoid unnecessary P applications.