About 1 out of 5 bags of fertilze will have these clumps that clog the spreader on low setting. Last patch was 34Nitrogen. I have noticed this black residue in the bottom of my Lesco spreader that gets gu-e after setting in the sun. So I am wondering if the iron is melting and clumping in the bags. Local Lesco guy has pallets of fertilize setting in full sun. We used to have to screen it on the farm guess I will go back to that.