improved release profiles which can , for the first time , be customized to match requirements of systems including crop nutrient uptake are realized by providing a plurality of heterogeneous particles comprised of water - soluble constituents set in a matrix of water - insoluble materials , and further providing a continuous homogeneous barrier of same , or a different water - insoluble material , juxtaposed the peripheral surface of each such heterogeneous particle . a first alternate embodiment includes forming in situ reaction products to reduce the specific gravity of such heterogeneous particles . in a second alternate embodiment , there is substituted for all , or a portion of such water - soluble material , organic media comprised primarily of composted poultry litter wherein soluble iron values remain available for substantial periods of time without incorporating or utilizing expensive conventional synthetic chelating agents .