As spring approaches and Montana farmers make final cropping and fertilizing decisions for 2024 — including how and how much phosphorus fertilizer to apply — Montana State University Extension specialists have recommendations to help growers make informed decisions when it comes to application timing and strategy.

Phosphorus is one of the three major plant nutrients. It’s essential for plant growth, energy and sugar synthesis, and, accordingly, crop yield, said Clain Jones, MSU Extension soil fertility specialist and a professor in MSU’s Department of Land Resources and Environmental Sciences.

Global supply chain issues and high natural gas prices have caused fluctuating fertilizer prices in recent years, Jones said. While nitrogen is sourced from the atmosphere and is inexhaustible, phosphorus is mined from more limited phosphate rock and guano deposits. As a result, phosphorus fertilizer costs are expected to continue to fluctuate with supply and demand.

Jones said growers should consider the “4Rs” — right source, right place, right time and right rate — for phosphorus fertilization that maximizes efficiency and economic yield.

According to Jones, runoff from phosphorus over-fertilization is the main cause of toxic algae blooms in lakes and rivers, so determining a proper fertilization rate can protect water quality and community and family health while also saving money. 

Phosphorus immobility

Immobility is a trait that affects how phosphorus is managed. Phosphorus binds to soil particles, so it doesn’t move to roots easily. Jones said phosphorus “might only move about a quarter inch in a growing season, far too slow to get to the root zone in one season if improperly placed.” Phosphorus levels can be highly variable across a field and even within a row because phosphorus does not move from where it is placed in narrow seeding bands.

“Over time, if rows are oriented at different angles, phosphorus levels across a field will likely somewhat even out,” said Jones.

Monoammonium phosphate is the most common form of phosphorus used in Montana. According to numerous studies, Jones said, there is not much evidence that liquid forms of phosphorus work better than granular products, because once soil moisture dissolves granular fertilizer, both forms behave similarly with the phosphorus binding to soil – especially in soils with high pH. Soil pH levels between 6 and 7 help make phosphorus more available to plants.

“Phosphorus sources that include elemental sulfur might lower pH and make phosphorus fertilizer granules somewhat more available, though this requires soil to be warm and moist, and Montana soils may be too cold early in the growing season for the combination to work well,” Jones said.

Fertilization timing and placement must also take immobility into account, with the best option being in-furrow fertilization while seeding to keep fertilizer close to where it is needed. If in-furrow fertilization is not an option, Jones said that fertilizing in the fall is better than in the spring to allow more time for phosphorus to dissolve and move into the root zone. If phosphorus is applied on the surface, Jones recommends doubling the in-furrow application rate to achieve similar results.

Phosphorus releases more slowly from manure, compost and phosphate rock than from treated or commercial fertilizers and provides a longer-term supply. Manure is an excellent organic phosphorus source.

Jed Eberly, associate professor in soil microbiology at MSU’s Central Agricultural Research Center near Moccasin, studies the role of bacteria that help phosphorous move in soil, referred to as phosphorus-solubilizing bacteria.

“These soil organisms make phosphorus soluble for themselves and crops by producing chemicals that release plant-available phosphorus from soil,” said Eberly.

Bacteria make phosphorus available at slower rates than commercial fertilizers, so it can be challenging to ensure sufficient phosphorus is available when plants need it. There is some evidence that addition of phosphate-solubilizing bacteria to soils can increase crop yield, and Eberly is studying their effectiveness in Montana soils.

Testing and fertilizer rates

Crop type and soil test results determine the recommended rates for phosphorus fertilizer. The Olsen P test works well for most Montana soils except those that are very acidic. In those soils, the test overestimates true availability, said Jones.

If the Olsen P test shows 12 parts per million (ppm), MSU guidelines recommend applying 30 pounds of phosphorous (P2O5) per acre for both spring wheat and canola, and 20 pounds P2O5 per acre for pulse crops like peas and lentils.

The “critical level” is the soil test level above which little or no fertilizer is recommended because the soil is not deficient in plant-available phosphorus. The Olsen P critical level is 16 ppm in Montana, said Jones.

“Keep in mind an average Olsen P result of 16 ppm could still mean a sizeable portion of the field is phosphorus deficient,” he said. “Mixing several soil samples together from a field with high phosphorus spatial variability can skew results if there are a few areas high in phosphorus.” He added that grid sampling allows producers to determine which areas need more or less phosphorus, and only needs to be done occasionally.

If Olsen P soil concentrations are sufficient to optimize yield, meaning near 16 ppm, producers can also estimate fertilizer rates from the amount of phosphorus removed from fields during harvest, which is called a ‘maintenance approach’. Recommended phosphorus fertilization rates based on both Olsen P test results and removal rates can be estimated using MSU’s Fertilizer Guidelines for Montana Crops, which can be found at landresources.montana.edu/soilfertility/fertilizers.html.

These are general Montana guidelines, and rates should be adjusted based on growing conditions and past responses to phosphorus fertilizer, Jones said. Applying different phosphorus rates in test strips and observing differences in color, plant height, maturity date and yield can help identify a deficiency and find how much phosphorus is needed on a specific farm or field. Visual signs of phosphorus deficiency include delayed plant growth, a spindly appearance and leaves turning dark green or purplish.

For questions about phosphorus or soil nutrients, contact Jones at [email protected] or 406-994-6076, or find MSU Extension resources at landresources.montana.edu/soilfertility/.

Source: Montana State University