How Does Humidity Affect A Fertilizer’s Ability To Be Blended?

High humidity can translate into caking for some fertilizers. How can Smart Nutrition™ MAP+MST® help with fertilizer blending?

Let’s start with the fact that most fertilizers are salts and salts absorb moisture. Each fertilizer has a critical relative humidity or value at which the fertilizer begins to absorb moisture from the air. Fertilizer materials that have a higher critical relative humidity can be advantageous in humid environments because they experience less caking.

Some of the most common fertilizers have the following critical relative humidity (at a temperature of 30° C or 86° F):

  • Urea: 73
  • Monoammonium phosphate (MAP): 92
  • Potassium chloride: 84
  • Ammonium sulfate (AMS): 79

Fertilizers that are stored individually have higher critical relative humidity. However, once blended, the critical relative humidity decreases, thus increasing the potential for higher moisture absorption at lower air humidity. Ammonium sulfate (79) and urea (73) both have relatively low critical relative humidity values before blended, and even lower (56) after blending. This blend will likely not store well or have ease of application under humid conditions. For this reason, fertilizers are usually applied shortly after blending instead of being stored. When stored fertilizers absorb moisture, the consequence is pile set and caking. Caking can cause safety concerns during storage and lead to issues with uniform application.

There are exceptions to fertilizer and moisture absorption. For example, fertilizers with sulfur present as elemental have lower moisture absorption, as elemental sulfur is not a salt.

Smart Nutrition MAP+MST has a guaranteed analysis of 15 percent elemental sulfur as micronize sulfur technology (MST). The critical relative humidity of MAP+MST is similar to that of MAP. Since the sulfur is mainly present as elemental, studies have shown less moisture absorption and less caking compared to other sulfur sources like AMS. This translates into safety in storage, ease of blending with other fertilizers like urea, and improved uniformity of application.

Other considerations when bulk blending Smart Nutrition MAP+MST include SGN, or the size of the fertilizer granule. The SGN of MAP+MST is similar to that of MAP and therefore, can be blended with other fertilizers comparable to MAP.

Consider Smart Nutrition MAP+MST as part of your fertilizer program to maximize nutrients in fertilizer blends. Learn more about how Smart Nutrition MAP+MST works or contact your local sales rep.