Over 98% of all nitrogen present in the soil is in an organic form (ie contains carbon in its molecular structure). Organic nitrogen cannot be taken up by the plant until it is mineralised by microbes into inorganic forms (principally nitrate (NO3-) and ammonia (NH4+). Mineralisation occurs throughout the growing season providing a steady, continuous supply of nitrogen to the crop. This process can be stimulated significantly by cultivation.
Ammonium is immobile in the soil, due to its positive electric charge. This causes it to be held on the exchange complex (lattices of clays and organic matter). Ammonium is an intermediate product in the conversion of organic matter to nitrate. It is usually found in lower concentrations in the soil compared to nitrate.
Nitrate is negatively charged which gives it the ability to move through the soil profile reasonably quickly (i.e. leaching). The advantage of a nutrient prone to leaching is that it can move through the soil profile to the crop roots. However, this means it can also leach deep into the soil profile below the crop root zone. Nitrate is the dominant form of soil mineral N in the soil. All nitrates taken up by the plant are converted to ammonium inside the plant and then into amino acids that make up the plant proteins.