Sizes of Urea Particles in Fertilizer

Distribution

• If you took all the particles in a batch of dry solid urea, measured them and graphed the results, you would expect your data to approximate a bell curve or "normal distribution." The narrower this distribution, the better. In other words, you want the size of the particles to be as uniform as possible. Uniformity ensures you don't get large granules that take a while to break down in some places and small granules in others.

Standards

• The UN's Food and Agriculture Association defines standards for fertilizers such as urea against which quality may be judged. For free-flowing urea, 90 percent or more of the granules should be able to pass through a 2.8 millimeter sieve yet not be able to pass through a 1 millimeter sieve, and no more than 5 percent overall should pass through the 1 millimeter sieve. In other words, the bulk of the granules should fall in the 1 to 2.8 millimeter range.

Granular

• So-called "free-flowing" solid urea consists of very finely divided particles. Granular urea has somewhat larger particles, so the standards here are a little different. The FAO suggests 90 percent or more should be able to pass through a 4 millimeter sieve but not through a 2 millimeter sieve, and no more than 5 percent should be able to pass through a 1 millimeter sieve. In other words, the vast bulk of the particles will fall in the 2 to 4 millimeter range.

Liquids

• If the particle size is not uniform or falls chiefly outside these ranges, companies will sometimes dissolve it and sell a liquid solution instead. This so-called "fluid urea" is more uniform than the granular preparations. If you are purchasing fertilizer in small quantities rather than bulk from a local home and garden store, these kinds of issues become a less important consideration, because the fertilizers you buy have typically been blended already.