Most organic fertilizers are safe for horse pasture. If using manure (horse, cow or other species) let it sit in a pile and compost a bit to break down pest eggs, break down and generate most valuable particle size. If using compost or manure make sure they have no pine needles, heavy herbicide residue, heavy fertilizer residue, or weed seeds in the product. Lawn clippings are usually very high in nitrates which can be hard on horses. The first step is always to test the soil in the pasture and then find the cheapest and safest "organic" fertilizer. You can always have possible source3 fertilizers tested also. We usually just let cow and horse manure sit in a pile for 3-4 months and then spread on pastures.