Phosphorus fertilization: A meta-analysis of 80 years of research in Finland

In order to integrate Finnish phosphorus (P) fertilization trials, we performed a meta-analysis on the relationship between P rates (6-100 kg ha(-1)) and the yield increase of agricultural crops, and studied the source of variation in yield responses (soil group, soil test P, soil pH, plant group, cereal species, productivity without P fertilization, and cultivation zone). Our database consisted of over 400 short- and long-term experiments conducted in Finland over 80 years, between 1927 and 2007. The crops studied were spring-sown barley, oats, spring and autumn-sown wheat and rye, potato, turnip, pea, and perennial grass mixtures. The experiments had been carried out all over the country, on clay, coarse-textured mineral and organic soils. The meta-analysis demonstrated that P fertilization significantly increased crop yields (by 11%) compared to the control (fertilized with nitrogen and potassium). However, the yield response to P fertilization was highly dependent on the soil group: on clay soils it was only 5%, while on coarse-textured mineral and organic soils it was 10% and 15%, respectively. On clay and organic soils, the lowest P rates (6-15 kg ha(-1), mean 13 kg ha(-1)) were enough to gain the maximum yield increases of cereals, while coarse-textured mineral soils needed higher P rates (16-30 kg ha-1, mean 25 kg ha(-1)). In the case of perennial grass mixtures, maximum yield increases were obtained in all soil groups with the lowest P rates (6-15 kg ha(-1), mean 13 kg ha(-1)). Yield responses to P fertilization were apparent on coarse-textured mineral and organic soils with low soil test P, measured by the acid ammonium acetate method (< 10 mg P-AC l(-1), coarse-textured mineral: <8 mg P-AC l(-1), organic), or on soils with low productivity without P fertilization (<2000 kg ha(-1)). The responses were negligible or low on soils with medium or high soil test P (>10 Mg P-AC l(-1) , coarse-textured mineral soils: >8 Mg P-AC l(-1), organic soils), or with medium (2000-3500 kg ha(-1)) or high (3500-5000 kg ha(-1)) productivity. On clay soils, P fertilization increased yields to the same extent regardless of soil test P level or productivity. Finally, neither soil pH (3.2-6.6) nor cultivation zone (with growth periods varying between 100 and 175 days) had any effect on yield responses in any soil group. For cereals and perennial grass mixtures, the P rates justified by the meta-analysis were only about half of the maximum values allowed by the third Finnish Agri-Environmental Program, or those applied in practice on Finnish livestock farms. Thus excessive P application can be further markedly reduced without measurable yield decrease. (c) 2008 Elsevier B.V. All rights reserved.