Ammoniacal nitrogen: Ammoniacal nitrogen includes ammonium, ammonia, and urea, all of which quickly become ammonium in the soil. Ammonium is not readily leached from the soil. It binds to the surfaces of the soil particles, which keeps it from washing away. Although many plants use ammonium, the availability of ammoniacal nitrogen is low to most plants, because it must be changed to nitrate by soil microorganisms before it can be washed to the plant roots. The activity of these microbes depends on soil moisture, aeration, and temperature. In warm, well-aerated soils, the nitrogen is changed to nitrate and is available to plants for a couple of weeks. The process is much slower in cold, wet soils, and will not take place at all when the soil temperature is below 30° to 40°F. If a plant growing in cold soil is in immediate need of nitrogen, a fertilizer should be applied that also contains nitrate nitrogen (see below). Ammoniacal nitrogen fertilizers have an acidifying effect on the soil. In areas of high rainfall, where soils are already acidic, it may be necessary to add lime.

Nitrate nitrogen: Fertilizers containing nitrate nitrogen are fast acting. They are very useful for plants in immediate need of nitrogen. The nitrogen is in a form that is readily available to plants. Microorganisms are not needed to break down the nitrogen, so the fertilizers are effective in cold or warm soils. Nitrate nitrogen does not become attached to soil particles, but floats freely in the soil water. With heavy rains or deep watering, the nitrogen may be washed below the root zone. Nitrate fertilizers are most effective if applied in frequent, light feedings, or in combination with a long-lasting ammoniacal or organic nitrogen.

Organic nitrogen: Organic nitrogen is in the form of protein or other insoluble compounds. It is found in plant and animal derivatives such as blood meal, manure, and sewage sludge. In uncomposted organic material, the nitrogen is not available to plants. The protein must be decomposed by soil microorganisms to ammonium, and then to nitrate, before the plant can use it. As with ammoniacal nitrogen, the rate of decomposition depends on soil temperature and moisture. Organic nitrogen usually lasts for a few months in the soil.