Struvite is a phosphorus (P)-rich by-product of wastewater treatment facilities that can be recycled as a P source in agriculture. Because struvite is not water soluble, it is solubilized gradually by organic acids released by soil microorganisms and from growing plant roots when used as fertilizer. To speed up the solubilization process, struvite can be combined with biostimulants such as P-solubilizing bacteria (PSB) or earthworm casts (WC). The objective of this greenhouse study was to compare the fertilizer value of struvite, with and without PSB (Bacillus megaterium) or WC, with that of triple superphosphate in two contrasting soils (a low-P soil and a high-P soil). Oat (Avena sativa L.) was grown for 8 wk under a controlled environment, and dry matter yield at harvest, total N and total P uptake, the soil residual Mehlich-3 P, and phosphomonoesterase activity were measured. The high-P soil was unresponsive to P application, but the low-P soil was responsive. In the low-P soil, there was more Mehlich-3-extracted P when struvite was combined with PSB or WC compared with struvite alone, resulting in greater oat dry matter and more total N and more total P uptake. Combining struvite with biostimulants increased total dry matter and total P uptake by an average of 39% and 33%, respectively. We conclude that greater P release from struvite occurs when it is combined with PSB or WC, particularly in low-P soil, but this needs to be confirmed in field-scale studies.

Soil chemical properties can be improved by incorporating crop residues in soil and letting it decompose. This study explored the use of incorporating residues of cover crops for improvements in soil chemical properties including soil organic matter (SOM), soil pH, and the selected soil macro- and micronutrients in greenhouse and field trials. Factors of interest included (i) cover crops and their combinations and (ii) methods of crop termination and incorporation in soil (disc, mow + disc, glyphosate, roller crimper). The greenhouse trial showed up to a 20% higher amount of SOM accumulated in soils incorporated with crop residues. Buckwheat (3.12%) and phacelia (3.12%) produced significantly different and larger SOM than that of the control treatment that received no crop residues (p ≤ 0.05). The soil pH of the brown mustard treatment was also significantly affected by the experimental treatments (p ≤ 0.05). The incorporation of crop residues did not affect soil phosphorous (P) or potassium (K) concentrations, except for brown mustard, with significantly higher values of P and K than the control treatment. Calcium (Ca) was significantly higher in the soil of phacelia + pea treatment (p ≤ 0.05). Buckwheat + pea produced a higher concentration of Ca (1028 mg/kg) followed by buckwheat alone (1006 mg/kg). Analysis of variance (ANOVA) calculated on the results of the field trial showed that the mix treatment that had a mixture of four cover crops significantly increased the SOM content. Buckwheat produced the highest (2.95%) SOM, then brown mustard and timothy. This study concludes that, irrespective of the tillage incorporation methods, the residues from cover crops are a potential source of improvement in soil health, and this practice may promote sustainable agriculture in conditions similar to those in this study.