Back in the day there was only a few fertilizers that were coated with control products. Today there are many different types of control products that are coated on many different types of fertilizers, synthetic and organic. The coatings on the fertilizers make the material sometimes cakey especially when humid. When using the permagreen agitator you always get a consistent even flow of materials 99% of the time and when you open up the holes the material flows right away. Organic fertilizers are not perfectly round size Prill also and don't flow evenly unless you have the right agitator.