Can crop yields be secured while reducing phosphorus accumulation in soil? Assessing decades of contrasting fertilizer strategies.

Abstract

ContextA balanced phosphorus (P) use in agriculture, where P fertilizer input matches crop offtake, is necessary to avoid accumulation of P in soil that is a chronic source for diffuse P loss. Moreover, mineral P fertilizers produced from rock phosphate is a limited resource, and identification of alternative P fertilizer sources such as animal manures that secure yields without P surpluses is therefore essential for sustainable P use. ObjectiveThe objective was to explore crop productivity and soil P accumulation after long-term input of different P fertilizer sources. Moreover, Olsen-P contents in soil were determined to evaluate if crop yields and P balances were reflected in measures of available P. MethodsOn a coarse sandy soil installed in lysimeters under field conditions, crop yields and P surpluses from 1992 to 2022 were quantified as affected by P fertilizer source (annual additions of solid pig manure, pig slurry and mineral fertilizer) since 1975. All lysimeters received mineral nitrogen (N) fertilizers according to the crop demand. Olsen-P contents in the upper soil layer (0–25 cm) were measured over three decades, and vertical distribution of P in soil down to 140 cm depth was studied at the end of the experimental period. ResultsFor oats, barley, pea and beet root, long-term fertilization with solid pig manure at a rate of 15 kg P ha-1 year-1 resulted in similar yields as after fertilization with 30 kg mineral P ha-1 year-1. After 30 years, the build-up of P in soil was much lower after solid pig manure inputs (accumulation of 8 kg P ha-1) than after mineral P fertilization (accumulation of 51 and 434 kg P ha-1 in soil receiving 15 and 30 kg mineral P ha-1 year-1, respectively). The long-term fertilizer treatments affected Olsen-P levels in the upper soil layer with highest Olsen-P contents (>40 mg P kg-1) after fertilization with 30 kg mineral P ha-1 year-1. Only maize benefitted from high Olsen-P levels and generous input of mineral P, whereas the yield potential for the other crop types could be reached at low to moderate Olsen-P levels after long-term additions of solid pig manure. More P was found in the deep soil layers (65–140 cm) after addition of pig slurry than solid pig manure. ConclusionsThe results showed that solid pig manure was a valuable P source that had the potential to replace mineral P fertilizers without compromising crop productivity, while ensuring low P surpluses. ImplicationsThis study suggests that the Olsen-P test could not stand alone in predicting P requirement but should preferably be supplemented with knowledge on fertilizer source, crop type and P offtakes to ensure a more balanced fertilizer P use.