Ammonium sulphate is often used where the soil pH is on the high side. In this form, the ammonia will need to be converted to nitrate before it is released from the attachment to the soil colloids into the soil solution where the plants can take it up.
This process will be greatly slowed down in cold conditions, as the bacteria that are responsible for this conversion are rendered inactive in the cold.
We also have to consider that all nitrogenous fertilisers have an acidifying effect on the soil. For instance, 1kg of ammonia requires 6kg of calcium to neutralise it. This needs to be taken into account when using ammonium sulphate on soils that are not alkaline.
Urea is often chosen as it has a very high nitrogen content and works out cheaper. What one needs to remember is that urea is converted to ammonium carbonate soon after entering the soil and, as with ammonium sulphate, needs to be converted to nitrate by soil bacteria before it becomes usable to the plants.
There are also other hazards associated with urea. One can lose 5% of the nitrogen in two days with soil of a pH of 7,5. Acid soils are safe.
Nitrogen losses can be up to 50% depending on factors that include temperature, timing, moisture and alkalinity. The urea should be incorporated as soon as possible with irrigation of the field.
Potassium nitrate is a more expensive form of nitrogen and contains two-thirds potassium; it’s often used for valuable crops for fast reaction where potassium is required as well as nitrogen.
It is often used as a top dressing for tomatoes and peppers, and can be useful when nitrogen and potassium are required in cold conditions.
Calcium nitrate is also used when growing a valuable crop to rectify a calcium deficiency urgently, and it contains about 50% of each element. It is often used for tomatoes if blossom end rot starts to develop.
Lan
A favourite fertiliser of mine LAN; it stands for limestone ammonium nitrate. The reason for the inclusion of the limestone is, to some extent, to cancel the acidifying effect of the nitrogen.
The big advantage of this fertiliser is that the nitrate fraction is immediately available to the plant. However, it can also be immediately leached in the case of heavy rain or over-irrigation.
This is where the ammonia fraction becomes a benefit, as it is not immediately available and cannot be leached from the soil. This is because it is magnetically attached to the soil colloids until converted by certain soil bacteria into its nitrate form.
This is a safety benefit so that one cannot lose all the nitrogen in one go.
Urea formaldehyde is a slow-release form of nitrogen that is gradually reduced to an available form of nitrogen. It is expensive and used to provide a gradual amount of nitrogen to the plants without the danger of leaching.
Urea formaldehyde is usually used for specific purposes where a gradual release is required on a reliable basis. I used it very successfully in a growing medium for seedlings. It still has a disadvantage in cold conditions as bacteria are required to release the nitrogen.
