Microgranular fertilizer and biostimulants as alternatives to diammonium phosphate fertilizer in maize production on marshland soils in northwest Germany

The eutrophication of groundwater through widespread diammonium phosphate (DAP) fertilizationand excessive farm fertilizer is one of the major problems in European agriculture. Organomineralmicrogranular fertilizers that have a reduced phosphorus (P) content, alone or in combination withbiostimulants, offer promising alternatives to DAP fertilization. We conducted a field experiment withmaize (Zea mays) on a marshland soil site in order to compare the yield increase and the phosphorusbalance of DAP and microgranular fertilizer variants. P content of the soil on the study site is 3.9 g Pper 100 g soil. Treatments involved a combination of two fertilizers, namely DAP or a P-reducedmicrogranular slow-release organomineral fertilizer (Startec) and the biostimulants mycorrhiza, humicsubstances and soil bacteria, applied individually or along with two of the above biostimulants.Fertilizer variants were also tested individually without additional biostimulants. One in four plotswas used as a control, treated only with biogas slurry, to identify site-specific spatial variability and toimplement correction factors to process raw data using standardized methods. Startec performed aswell as DAP in terms of both the yield and corn cob ratio, while the P excess was lower in plotstreated with Startec (av. = 4.5 kg P2O5 ha-1) compared to DAP (av. = 43.7 kg P2O5 ha-1). The latterdifferences are of statistical significance. Individual biostimulants and a combination of multiplebiostimulants rarely resulted in significantly higher yields, with the exception of some combinationswith humic substances and mycorrhiza in individual years. The influence of the climatic conditions ineach of the years was higher than the influence of the biostimulants. However, average increases inyield over three years would be economically beneficial for farmers in the case of the applied humicsubstances product and mycorrhiza. An adequate alternative to DAP was found in the form of a Preduced microgranular fertilizer from Startec.