Yield responses to P fertilisation of onion (Allium cepa L.) and cabbage (Brassica oleracea Capitata Group L.) in Finland
Finnish data on vegetable crops’ yield responses to phosphorus (P) applications are scarce, but P is usually applied in quantities that meet the crop demand with wide safety margins. We determined yield responses to P fertilisation of onion and cabbage at three sites in 3-year field trials. Only on a sandy loam with low P status did annual P applications give statistically significant yield increases, 7% and 20% over the P-unfertilised treatment for onion and cabbage, respectively. The maximum P rate allowed by national regulation for this soil is 80 kg ha-1, but P rates of 10–12 kg ha-1 were sufficient to produce 97% of the yield maxima. The results strongly suggest that the P demand of the studied vegetables is smaller than previously thought also in a boreal climate. Critical soil test P concentrations for vegetables should be established to avoid unnecessary build-up of soil P that may be uneconomical and elevate the risk for P losses to waterways. However, too few data exist for this at present.