Fertilizers and Plant “Availability”

Over the last few months, the ALGL agronomy staff have received many questions regarding different fertilizer products and whether the nutrients are plant available or how long it takes them to “release” the nutrients. The simple answer is that most fertilizer products are highly water soluble, and once they dissolve, the nutrients are in a plant-available form. However, this does not necessarily mean that the nutrients will be taken up by the plants right away. Below we will discuss the potential fate of the nutrients in a few common fertilizer materials if it is not taken up by the plant.

Most questions regarding nutrient availability are concerning phosphorus (P), MAP and DAP. Both products are more than 90% water soluble and are immediately in a plant available if there is adequate soil moisture. However, the P must be near an actively growing plant root to be taken up. So, what happens to the rest of the P if it is not taken up immediately? Much of the applied P will be loosely bound to the clay minerals through a process called adsorption. This fraction of the P can be released back to the soil solution as P concentrations are reduced through plant uptake. However different soil types can bind the P more tightly than others. Soils that are likely to bind P rendering it unavailable are soils with high clay content, high pH, and low soil test P.

Potassium (K) fertilizers such as potassium chloride and potassium sulfate also dissolve rapidly into the soil solution and are immediately in a plant available form that can be utilized by actively growing plants. A portion of the K will be held by the cation exchange capacity (CEC) of the soil and will be released as the K dissolved in the soil solution becomes depleted. The potential exists for the K to be lost to leaching or runoff if the K was applied in excess of the soils capacity to hold it or if there is no actively growing crop to utilize it. Soils most prone to K losses are high sand/low clay content soils, and soils with high organic matter.

The two most common dry nitrogen (N) fertilizers are ammonium sulfate and urea. Ammonium sulfate delivers nitrogen in a form that is immediately plant available. Since ammonium has a positive charge and can be held by the soils CEC, just like K, ammonium is generally considered the more stable form of plant available N. Urea, though it dissolves rapidly, is not in a plant available form initially. Urea requires an enzymatic reaction with urease to become ammonia, which quickly converts to ammonium to become plant available. If the N stays in the ammonium form, losses of N are minimized. Nitrogen losses occur when soils are warm enough for microbial activity to start converting the ammonium to nitrate. While nitrate is also plant available, it is a negatively charged so it is prone to leaching as it is repelled by the soil CEC.

The bottom line is that the best way to ensure adequate nutrient availability for your crops is to maintain good soil test nutrient levels and a desirable pH for and follow the 4R’s of nutrient management. Application of nutrients just prior to crop uptake reduces the potential for nutrient tie up and possible loss. If you have any questions about your specific crop and fertilizer situation, contact your ALGL agronomist.