Fertilizing effect of a recirculated calcium phosphate : does the addition of organic acids increase efficiency?

Sammanfattning

Finding new ways to utilize existing phosphorus resources and increasing the recycling of fertilizer products in the European Union is important, as phosphorus is an essential nutrient for agronomic production. Previous studies have shown that organic acids, such as those exuded by plant roots, can increase phosphorus availability in soil through ligand exchange or chelation mechanisms. This study aimed to investigate how the addition of organic acids, specifically citric acid and a phenolic acid, affects phosphorus availability and plant uptake from the recycled calcium phosphate fertilizer RevoCaP. A greenhouse experiment was conducted using Lolium perenne L. with nine treatments in total, including RevoCaP with or without added organic acids, soft rock phosphate (RP), superphosphate (SP) (with two reduced-rate references), and an unfertilized control. The experiment was arranged in a random complete block design with four replicates. Fertilizers were applied at 15 kg P/ha, under two soil pH levels (6 and 7), over nine weeks with two harvests. Dry matter yield, phosphorus concentrations, phosphorus yield, and mineral fertilizer equivalent (MFE) were assessed. At the first harvest, no significant differences were observed among RevoCaP treatments regardless of organic acid addition or pH level. MFE values were generally low across both pH levels. At the second harvest, RevoCaP treatments showed significantly higher dry matter yield, phosphorus concentration, phosphorus yield and MFE at pH 6 compared to pH 7. However, the addition of organic acids did not significantly affect phosphorus availability or uptake within RevoCaP treatments at the same pH. In conclusion, organic acid additives did not enhance phosphorus availability in RevoCaP under the conditions of this study. Observed differences were primarily related to soil pH and the inherent phosphorus availability of the fertilizer product, with RevoCaP performing better under acidic conditions.