Urea is very soluble in water, so if applied as a fertilizer by itself only a very small amount can be used and it will quickly be carried away in drainage. The urea cannot be used directly by plants but it is naturally converted into ammonia on contact with water in the soil. This then dissolves in the water and can be absorbed by plants for growth. Nitrogen is needed by plants to make protein and also to make chlorophyll, the green pigment in plants used to trap light and make sugars to grow.

However, if there is too much urea or ammonium in the soil it will draw water out of the roots and cause ‘fertilizer burn’, with the leaves shriveling and dying, often also killing the plant. This means that pure urea must be applied in very small amounts very often to be effective as a fertilizer. This is also what happens when dogs or cats urinate on your lawn.