Phosphorus and potassium availability from cattle manure ash in relation to their extractability and grass tetany hazard

Due to a decrease in phosphorus (P) and potassium (K) mining, manure is incinerated to concentrate P and K in ash. To understand the alternative use of manure-derived ash as P and K sources, laboratory and greenhouse experiments were conducted to determine the relationship between extractability and P and K uptake in cattle manure ash (CMA) and that between CMA application and a grass tetany hazard. The results showed that more P was extracted with 2% citric acid (90% of the total P) than with 2% formic acid (72–84% of the total P). Ninety-one percent of the total K was soluble in water. A greenhouse pot experiment was conducted to test P and K availability to Guinea grass (Megathyrsus maximus). Cattle manure ash or calcium dihydrogen phosphate (CF) was incorporated into sandy soil at 10, 20, and 50 g P₂O₅ m⁻². Two combinations of CMA and CF were tested at 20 g P₂O₅ m⁻². Potassium rates followed K content in CMA applied at different rates of P equivalent to 19, 38, or 96 g K₂O m⁻². In four harvests, there was no significant difference in the total yields between CMA and CF treatments. The total P uptake was significantly lower in the CMA treatment than in the CF treatment, while it was not in the combined CMA and CF treatments. The P uptake in response to different extraction methods indicated that the extraction of P by 2% formic acid without sonication is recommended to predict P availability in CMA. The potassium uptake from CMA application was comparable to that from the KCl application, and excessive K occurred at 38 and 96 g K₂O m⁻². The grass tetany hazard ratio higher than 2.2 was observed at the beginning period at the lowest application rates of CMA and CF. In conclusion, the combination use of CMA and CF was better than the single use of CMA. Moreover, CMA would be an available K source, but the grass tetany hazard still needs to be considered in application rates and pretreatments.