What is the Best Source of Phosphorus Fertilizer?

Phosphorus (P), a macro nutrient, is a critical nutrient for crop production because it plays a role in early season development, photosynthesis, energy transfer, grain production, and helps fortify stress tolerance. Making sure your soils have adequate available P during the growing season is critical for helping to maximize yield potential. Phosphorus is a relatively easy nutrient to manage because there are readily available P fertilizers and how they interact with most agricultural soils is predictable. Commonly used sources of P fertilizer are very soluble in water and once they are applied, the P is available and is relatively non-mobile, unless a precipitation event physically moves soil.

Regardless of the P source used, under normal growing conditions it converts to plant available ortho-phosphate very rapidly. It may take up to a week or two for the inorganic P in the fertilizer to be available, then it remains in the soil until it is either taken up by a plant or tied up with soil cations. Organic P sources like manure take more time to be plant available while they are broken down by microbes (mineralization). Manure contains organic and inorganic P and like commercial fertilizers, the inorganic P becomes plant available shortly after application. Depending on manure source, there is a difference in analysis and ratios of organic and inorganic P (Table 1). Generally, a manure test analysis provides total P which combines organic and inorganic P. A general factor of 80% of the total P can be expected to be plant available the first year.

Table 1. Variability of Phosphorus by Animal*

Corn and soybean P uptake mirrors dry matter accumulation closely. When plants are small uptake is small and when the plants start to grow vigorously and accumulate grain dry matter, P requirements increase exponentially for a period and then maintain a much higher level of uptake until near physiological maturity. Recall that P is very important in early season development, thus in lower P testing soils and soils following sugar beets or fallow, application of a 2x2 P source can help prevent early season P deficiencies.

In summary, corn and soybean plants need and utilize P in large quantities throughout the growing season. Soil testing can help provide guidance on how crops may respond to P fertilization. Inorganic forms of P fertilizer like 10-34-0, MAP, or DAP can provide plant available P relatively quickly. Depending on soil characteristics and pH, inorganic P can be tied up and become unavailable to the crop. In soils that are more prone to tying up P, banding P can be a practice that reduces the impact of P tie up. Organic P takes time to be released through mineralization and can become an important source later in the season. The best potential P source for a farming operation is the source that provides plant available P for the least amount of money/pound. A pound of P is a pound of P.

Channel Agronomist
Tony Weis

Sources

Kaiser, D.E. and Pagliari, P. 2018. Understanding phosphorus fertilizers. University of Minnesota Extension. https://extension.umn.edu/phosphorus-and-potassium/understanding-phosphorus-fertilizers#corn-624160

Wilson, M. 2021. Manure characteristics. University of Minnesota Extension. https://extension.umn.edu/manure-management/manure-characteristics

Web sources verified 1/15/25. 1110_508410