Soil conditioners, such as biochar and zeolites, may play an important role in agriculture if they increase nutrient use efficiency, in particular, that of phosphorus (P), due to the depletion of phosphate rocks from which P-fertilizers are manufactured. This study report results from a field trial, where maize was grown in summer and oats in winter, and from a pot experiment with maize. In the field, the use of biochar and zeolites, along with an untreated control in combination with various P rates (0, 50, 100 and 200 kg P2O5 ha−1), was tested. In the pot experiment, six treatments were included, biochar, zeolites and a non-amended control with and without P application. Soil conditioners did not significantly influence tissue elemental composition or the dry matter (DM) yield of maize and oats in the field trial, nor the DM yield of maize in the pots. In the field trial, average maize DM yields varied from 14.3 to 15.6 t ha−1 and 11.8 to 13.7 t ha−1 and average oats DM yields from 2.1 to 2.4 t ha−1 and 3.0 to 3.2 t ha−1, respectively, in 2018 and 2019. Biochar only significantly increased total organic carbon (C); and zeolites the levels of potassium (K) in the soil, a result of their initial composition. P application increased DM yield of maize in the second year and oats in the two years of the field trial and also the DM yield of potted maize. In the pots, P application also influenced the concentration of P and several other nutrients in plant tissues and increased the labile soil P fraction. This study showed that the biochar and zeolite soil conditioners applied may not be suitable for short-term increase of crop production in soil with high agricultural potential and under intensive irrigated farming systems.
