Comparison of Struvite to Mono-Ammonium-Phosphate as a Phosphorus Source on Commercial Alfalfa Fields

The purpose of this project was to demonstrate a regional nutrient (phosphorus (P)) recycling relationship between the dairy industry and alfalfa forage growers. Dairies often have excess P in manure in relation to the need for crop production on-farm. Easily mineable reserves of phosphorus (P) worldwide are limited, with a majority residing in Morocco (USGS 2013). One approach to recycling P is to capture excess P from dairy manure in the form of struvite for off-farm export for use as a nutrient source of crop production. Washington State produces a significant amount of alfalfa for domestic and international sales.

What did we do

Struvite (Magnesium Ammonium Phosphate – NH4MgPO4· 6H2O) and Mono Ammonium Phosphate (MAP) were applied to 33 and 30 acres (control and treatment, Farm 1); and 60 and 55 acres (control and treatment, Farm 2) sections of alfalfa fields at two commercial forage producers in Eastern Washington. Fertilizer (struvite or MAP) was applied on an equivalent P2O5 basis in August 2017 and September 2018 (Farm 1 – existing stand) and September 2017 and September 2018 (Farm 2 – new seeding).

What have we learned

Accumulative yield of alfalfa in 2018 for Farm 1 was struvite = 7.14 tons, MAP = 7.51 tons. Accumulative yield (2 of 3 cuttings) of alfalfa in 2018 for Farm 2 was struvite = 3.08 tons, MAP = 2.95 tons. Average P concentration of alfalfa in 2018 for Farm 1 was struvite = 0.27, MAP = 0.27 (% DM).  Average P concentration in alfalfa in 2017 for Farm 1 was struvite = 0.31, MAP = 0.32 (% DM). Average P concentration of alfalfa in 2018 for Farm 2 for struvite and MAP was 0.27 and 0.28 % DM, respectively. Average accumulative P uptake of alfalfa in 2018 for Farm 1 was 38 and 39 lbs P/acre for struvite and MAP, respectively. Average accumulative P uptake (2 of 3 cuttings) of alfalfa in 2018 for Farm 2 was struvite = 15 lbs, MAP = 16 lbs P/acre. Results indicate that struvite is equivalent to MAP as a P source for commercial production of alfalfa.

Future Plans

The nutrient recycling project will continue through 2019. In addition, companion replicated plots studies are underway to evaluate the effects of ratio of MAP:Struvite and amount of P application for yield and quality of alfalfa.


Joe Harrison1, Steve Norberg1, Kevin Fullerton1, Elizabeth Whitefield1, Erin Mackey1, and Keith Bowers2.

1Washington State University,

2Multiform Harvest

Citations and video links

U.S. Geological Survey, Mineral Commodity Summaries, January 2013.

    The Mobile Struvite Project Overview Video: Capturing Phosphorus from Liquid Dairy Manure and Cost Efficient Nutrient Transport

    Dairy Struvite Video: Capturing Phosphorus from Dairy Manure in the Form of Struvite on 30 Dairy Farms in WA state

    Alfalfa Struvite Video: Struvite, a Recycled Form of Phosphorus from Dairy Manure, used as Fertilizer for Alfalfa Production


This project funded by the USDA NRCS CIG program and the Dairy Farmers of Washington.



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