It all depends on what you mean by slow release. There are many forms of slow release N, including but not limited to, urea formaldehyde, SCU, PSCU, Methylene urea (MESA), stabilized urea(UMAXX, UFLEXX). I will assume you are asking about polymer coated, as I will go out on a limb and say this is the most widely used in our industry. I think a minimum of 50% of the product should be coated if you want any longevity. The reason I say this, is what the bag says and what's actually still coated by the time it lands on a lawn are sometimes a long ways apart. I would guesstimate 10-15% of a bag could be compromised. So if you start out with only 30%, you'll have a lot less coated product on the lawn than if you start out with 50%. The prills crack, which compromises the release length. If many prills are cracked, you'll have a fast release with very little slow release. How do they crack? Handling the bags is probably the biggest factor. How far are you from the plant? How many bumps did the truck go over? How many times did the bag/ pallet get moved? Did your guys throw them into the truck? Drop them on the ground to break up clumps? Then there's the manufacturing process itself. What's the quality of the coating? I'll stop there, because there are many others who could write circles around me about this, but that's a start.
Proud supporter of Green Care for Troops
Member Mich. Green Industry Assoc. (MGIA)
Member Mich. Turfgrass Foundation (MTF)