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Alfalfa Checkoff: Developing practical phosphorus and potassium tissue test recommendations and utilizing struvite in modern alfalfa systems

Steve Norberg, E. Mackey, S. Fransen, J. Harrison, D. Llewellyn and L. Whitefield for Progressive Forage Published on 16 July 2019

Fertilizer is the largest single expense for Western irrigated alfalfa producers. Even at modest rates, fertilizer can easily exceed over $200 per acre annually, with phosphorus (P) and potassium (K) contributing the largest component.

Alfalfa can remove 8 pounds of P2O5 and 54 pounds K2O per ton of alfalfa produced. With yields of 10 tons per acre attainable by excellent producers in the Pacific Northwest, this results in 80 pounds P2O5 and 540 pounds K2O removed per acre per year. More K is removed from the soil by alfalfa than any other nutrient.

Developing practical P and K tissue test recommendations would allow producers to use samples currently pulled for hay quality to also be used for P and K testing once they are calibrated. Alfalfa plants with deep penetrating taproots can remove K and other nutrients from much deeper depths, creating disproportional inaccuracies in crop response when related to standard 1-foot soil tests. Tissue testing provides opportunity to enhance accurate nutrient applications while determining in-season critical levels that would establish recommendations for applications between cuttings or through fertigation.

We selected first cutting to be of primary interest since it is the most desired by the dairy industry and likely the cutting to be nutrient-limited due to cold soils. However, in this National Alfalfa and Forage Alliance grant-funded study, all alfalfa cuttings monitor P and K nutrients in and out of the ecosystem. Maturity of hay affects the P and K levels in the plant. Most producers harvest their hay at mid-bud stage, so that is the target stage for developing information. This research is also investigating how differing fertilizer rates affect hay quality.

Dairies often need to move P off-farm, while alfalfa and other hay producers need P fertilizer. One source of P dairies can provide is struvite. Struvite is a sand-like granular fertilizer (5-29-0, NPK analysis) made from dairy liquid manure or municipalities’ waste treatment that is either granular or prilled. The low moisture and like consistency allows for easy handling and long-distance hauling. Struvite is pathogen-free and low in water content. Struvite provides a slow-release option for growers, but more information is needed in crop response on high-pH and very P-deficient soils. The goal is to determine if struvite will provide enough nutrient release to maximize yield and be cost-effective.

To accomplish this, three studies were initiated in spring 2017 at Washington State University Irrigated Research and Extension Center in Prosser on an irrigated soil that was depleted in the nutrients of interest. For the P study, initial P was 9 ppm and 8.1 for the struvite study. For the K study, K was at 100 ppm and P at 8.1 ppm for the struvite study within the first soil foot. In the struvite study, a common rate of P2O5 was applied at 144 pounds per acre but in differing ratios of mono-ammonium phosphate (MAP) and struvite to determine yield and quality effects.


The P study showed that 140 and 165 pounds per acre P2O5 maximized first-year gross revenue, after fertilizer costs, when soil was 9 ppm Olson P method and price of alfalfa hay valued at $150 and $200 per ton, respectively. Optimum alfalfa tissue P concentration was 0.24 to 0.25 for first cut, 0.28 to 0.29 for second cut and 0.26 to 0.27 for third cut for alfalfa hay priced at $150 and $200 per ton, respectively. Interestingly, our first-year data suggests applications of P2O5 decreased hay quality, increasing the importance of accurate rates to maximize profit (data not shown).

The potassium experiment started with soil levels of 101, 73 and 79 ppm K (ammonium acetate method) at depths of 0 to 12, 12 to 24 and 24 to 36 inches. Even at these low potassium levels, alfalfa yields were not affected with yield of the control plots of 6.5 tons per acre in three cuttings in this spring-planted field. The 240-pound K2O treatment pulled 308 pounds of K from the soil, and the no-K application alfalfa still pulled 198 pounds of K from the soil.

Current Pacific Northwest alfalfa soil testing recommendation would suggest 100 pounds per acre of K2O should have been applied. The addition of 240 pounds per acre K2O decreased relative feed value (RFV) and relative feed quality (RFQ) and increased lignin, again suggesting overapplication may hurt the pocketbook not only in fertilizer cost but in hay quality. Continued research is necessary to monitor consistency over years and after alfalfa plants are mature with more extensive taproot systems than in the seedling year.

To understand Figures 1 and 2, the horizontal axis has changing ratios of MAP and struvite nutrients, so as one increases, the other decreases.

In 2018, first-cutting dry matter yeild and phosphorus content as influenced by phospharus source

Dry matter yields (left vertical axis) with no additional MAP or struvite (red square) are lower in first cutting and total yields than when nutrients were applied (blue line), a yield response occurred with nutrient applications. Phosphorus concentration (right vertical axis) without additional MAP or struvite (green square) was also lower than treated alfalfa plants (purple line) for both first-cutting and total season yield.

First-year dry matter yield and overaged phosphorus contnet in the three cuttings harvested

This first-year data suggests struvite can replace MAP on a P2O5 basis when incorporated even on a high-pH soil at a low level. The second-year data is being collected this year where the same treatments were applied on the surface to this 1-year-old stand to see if we get similar results.

In conclusion, based on the seedling year of the experiment, optimum P alfalfa tissue phosphorus content should be between 0.24 to 0.28 and 0.25 to 0.29% when the alfalfa hay price is $150 and $200 per ton, respectively. Lower soil test levels are needed before a yield response to potassium sulfate will occur. First-year data also shows struvite can be used alone or in combination with MAP when put on prior to planting and incorporated without a yield loss even on a soil averaging 8.1 ppm (Olson method).

Our first-year data suggests it is important not to overapply P or K, as it can have a negative influence on hay quality and can affect aNDF (amylase-treated neutral detergent fiber), lignin, RFV, RFQ and nutrient value of hay ($ per ton) (data not shown). It is important to continue following best management practices in nutrient applications on irrigated alfalfa while being ready to change and adapt as new information is learned through field and laboratory studies.  end mark

Steve Norberg is the regional forage specialist at Washington State University – Pasco. Email Steve Norberg.