Application of struvite to replace monoammonium phosphate in modern alfalfa systems

Erin M. Mackey

doi:10.7273/000003989

Most inorganic phosphorus (P) fertilizers are derived from depleting phosphate rock reserves. Producers of alfalfa (Medicago sativa L.) have experienced declining soil test P. Struvite is a slow-release P fertilizer recovered from wastewater and animal manure. Struvite's low solubility could require supplementation with a soluble fertilizer like monoammonium phosphate (MAP) to meet early growth requirements. Three experiments were conducted for this thesis: 1) an agronomic field plot trial with different ratios of MAP:struvite (0:0, 100:0, 75:25, 50:50, 37.5:62.5, 25:75, 12.5:87.5, 0:100) applied to fall-planted and spring-planted alfalfa; 2) a 63-day soil incubation that evaluated release of Olsen P (OP) from the same MAP:struvite ratios as temperature decreased (26 to 15°C) and increased (12.4 to 22.6°C) to mimic fall and spring weather, respectively; and 3) an agronomic demonstration in two commercial alfalfa fields in central Washington that compared MAP (100:0) to MAP:struvite 30:70 on an established alfalfa stand (Field 1) and struvite (0:100) on a new seeding of alfalfa (Field 2). Treatments 100:0 and 0:100 were statistically similar (P>0.05) in total DM yield, total P uptake, and average quality across the season and in the first cutting for fall- and spring-planted alfalfa. These results support completely replacing MAP with struvite in modern alfalfa systems where the soil applied struvite is incorporated. The 0:100 soils were statistically similar or greater in OP concentration compared to 100:0 for the first 14 days of fall incubation, but ended with significantly less OP. However, 100:0 and 0:100 soils had similar OP the entire spring incubation. Causes of fluctuation in OP as soils warmed was unclear, thus future research should focus on the effect of warming soil temperatures on sorption reactions and P-solubilizing microbial populations in relation to struvite dissolution. The MAP-fertilized alfalfa produced 0.6 Mg ha-1 DM yield and 1 kg ha-1 P uptake more than 30:70 across 4 cuttings in Field 1, whereas struvite produced 0.3 Mg ha-1 DM yield and 1.4 kg ha-1 P uptake more than MAP across 2 cuttings in Field 2. These results support the hypothesis that struvite can provide sufficient P to alfalfa during early development without MAP supplementation.

Application of struvite to replace monoammonium phosphate in modern alfalfa systems

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