Continuing our Nutrient Availability series started in the January issue with pH and manure management, this month will focus on phosphorus (P) availability and function within the plant.

Continuing our Nutrient Availability series started in January 2023 with pH and 

; this article focuses on phosphorus nutrient availability in corn and soybeans - both in the soil and its function within the plant.

Phosphorus is often found in abundant levels in the soil in two forms: organic and inorganic phosphorus. Organic phosphorus refers to the portion of phosphorus that is not available for plant use. It is comprised of decayed plant and animal residues and phosphorus that is part of soil microorganisms.

Over time, organic phosphorus can be converted into inorganic (plant-available) phosphorus through mineralization (by microbes) and weathering. Inorganic phosphorus makes up almost 65% of the total phosphorus in soils.

Inorganic phosphorus is a primary focus for crop production and year-to-year fertility recommendations. Inorganic phosphorus can be further classified into three subcategories depending on its availability to the plant and the speed at which it is released from other compounds to be used by crops.

These "pools" of phosphorus are often referred to as:

Solution Pool (available for plant uptake)

These three pools of inorganic, plant-available phosphorus are often held in equal parts in most soils. When applying phosphorus fertilizers to the soil for crop production, either through commercial fertilizers or manure applications, only 20% to 25% will be available in that first year of application.

The remaining fertility needs of the crop are gained through the active and fixed pools of phosphorus in the soil profile. Understanding phosphorus interaction and its availability is critical to how we apply phosphorus fertility and manage our inputs for maximum success.

Evaluating phosphorus uptake patterns utilization in crop production is another critical component in managing nutrient availability. We often associate phosphorus in corn production as the nutrient that helps with early-season growth and development, especially since 

phosphorus availability in cold, wet conditions can be limited

Although this is true, when we look at the phosphorus uptake graphs of both corn (Figure 1) and soybeans (Figure 2), around 50% of the phosphorus is acquired by the plant during the reproductive stages! This is because phosphorus plays a very diverse role in corn and soybean growth and development.

Phosphorus is also a key nutrient in root and shoot development, seedling vigor, and energy. In addition, it helps with metabolic reactions (respiration in corn and photosynthesis) and plays a significant role in the reproductive processes and grain development.

A final characteristic of phosphorus that must be accounted for is where the phosphorus taken up by corn and soybeans resides at harvest.

Almost 80% of phosphorus taken up by corn ends up in the grain and is removed during harvest.

Around 70% of phosphorus taken up by soybeans resides in the grain.

This means we must be diligent with our phosphorus applications since only a low amount will be returned to the soil through residue degradation.

 and applications, we often refer to the 4Rs (right product, right place, right time, and right rate). Nutrients like phosphorus are often looked at as “deep placement applications” because of their immobility in the soil and because it's needed in comparatively large amounts.

Maximizing this “right” placement increases the opportunity for the plant roots to reach the phosphorus earlier in the growing season. In addition to quicker interception by roots, deeper placement of phosphorus improves the likelihood of soil moisture throughout the growing season, aiding in nutrient uptake.

Increased root proliferation is often seen in areas with a high concentration of phosphorus, resulting in increased root mass. Another benefit to placing nutrients, like phosphorus, below the soil surface is that it reduces the likelihood of displacing the phosphorus by surface run off and erosion.

Deep-banded nutrient applications have the potential to improve the crop's ability to recover certain nutrients quickly and efficiently. To maximize the effect, rows should be planted close (less than 6 in.) to the band of fertilizer left by the deep placement equipment.

Because phosphorus has many unique forms and compounds in the soil, it can be challenging to manage. In addition, substantial in-season applications or rescue treatments of phosphorus with the current products available can also be difficult and extremely costly.

The best management practice to ensure sufficient levels of phosphorus is to manage soil fertility levels and apply it as needed via commercial dry/liquid fertilizers or manure. Phosphorus is relatively stable within the soil, and the main forms of loss are through surface erosion, soil degradation, and runoff. You should always take these factors into account to protect your fertility investment when determining your application method(s).

The final thing to keep in mind when developing your nutrient plan is to stay current with the nutrient removal recommendations formulated by your local universities. Over the past five to seven years, many institutions have reduced the removal rate of phosphorus for both corn and soybeans between 8% to 15%!