Plants can utilize NH 3 by absorbing it through the roots and assimilating it into nitrogenous organic compounds. Nitrate is also a form of nitrogen that can be absorbed by plant roots. Animals feeding on plants acquire nitrogen and when plants and animals die and decay nitrogen is returned to the soil as ammonia through the decomposing activities of soil microbes. This process is called ammonicifation . Nitrate to ammonia is also a form of ammonification.
Animals also eliminate waste nitrogen as urea in the urea cycle.
It's interesting to note that plants can use both ammonia and nitrate. But in the soil the ammonium ion is positively charged and nitrate in negatively charged. Since soil particles are negatively charged it follows that the nitrates are readily removed from soil by rain while ammonia is retained. Microbes solubilize the phosphate salts making phosphorus available to the biological world. When microbes metabolize soil organics with the production of acid byprobucts it is the acid that actually solubilizes the phosphate salt in the mineral.