All fertilizers have three numbers listed on their packages. These represent the major macronutrients including phosphorus, potassium and nitrogen. In South Texas nearly all required nutrients are available in the soil, with the exception of iron. But of the macronutrients, only nitrogen requires constant replenishing because it leaches from the soil with rain and irrigation water.
Nitrogen is possibly the most important nutrient for plants as it is required for photosynthesis, cell walls, metabolic processes, hormones, reproduction, etc. Sources include lightening, atmospheric fixation by bacteria and fungi, manures, manufactured fertilizers, and decomposed leaves and grass clippings.
All plants respond to applications of nitrogen regardless of the source, and the carbon from organic sources builds soil structure and provides energy to soil organisms. Since many packaged fertilizers use oil in their manufacturing, using leaves, twigs, and grass clippings as compost or to fertilize turf and beds, we reduce our use of oil. Turf requires approximately 2 to 2 ½ pounds of nitrogen annually per 1,000 square feet of landscape. Coincidently, depending on the grass type this is almost the amount of nitrogen supplied annually if the grass clippings are mulched directly onto the grass. So consider the natural way, let your yard fertilize itself and use leaves and grass clippings year round.