In Texas and other regions with calcareous soils, ammonium sulfate (AMS) is often used not only as a source of nitrogen (N) and sulfur (S), but also to increase phosphorus and micronutrients availability to the roots.  In such cases, mature trees may get as much as 150 to 200 lb N/acre/year in the form of AMS, or about 750 to 1,000 lb AMS/acre/year.  In the case of non-calcareous soils, AMS may be blended with urea to a N:S ratio closer to 9:1 (instead of the 1:1 ratio in AMS).  Given that AMS is a very common fertilizer in Texas, you should have no trouble finding it at your local fertilizer or home and garden store.