Potential phosphorus, potassium, and magnesium surpluses in an irrigated maize monoculture fertilized with pig slurry
Pig slurry (PS) is extensively applied to irrigated maize in nitrate-vulnerable zones of the Ebro Valley, Northeast Spain. Farmers base PS rates on the European Union (EU) Nitrates Directive (up to 170 kg N ha–1 derived from organic sources) or on maize-N requirements (about 300 kg N ha–1). The aim of the study was to assess whether these PS rates would lead to P, K, and Mg surpluses in excess of maize requirements. An irrigated-maize monoculture was fertilized with PS over 6 yr. Pig slurry treatments comprised a control without PS (PS0), 29 m3 PS ha–1 (PS29, based on the EU Nitrates Directive), and 51 m3 ha–1 (PS51, based on maize-N uptake). Mineral P and K fertilizers were added to all plots. On average, 199 kg N ha–1, 45 kg P ha–1, 128 kg K ha–1, and 25 kg Mg ha–1 were annually added to the soil with PS29, and 354 kg N ha–1 yr–1, 79 kg P ha–1, 232 kg K ha–1, and 43 kg Mg ha–1 were annually added with PS51. Maize exported 47 kg P ha–1, 202 kg K ha–1, and 33 kg Mg ha–1 in harvested grain and stover without differences between PS rates. The sum of mineral fertilizers plus PS produced P and K surpluses, which led to soil P and K accumulation at the end of the experiment. If mineral P and K fertilization was not considered, PS29 would have sustained soil P status or reduced soil K and Mg content. Fertilization of irrigated maize with PS rates based on the EU Nitrates Directive would not lead to soil P, K, and Mg accumulation if stover is removed. However, there would be a risk of P, K, and Mg surpluses without stover removal.