Choosing the right fertiliser

Choosing the right fertiliser is of course critical to ensure your soil and crops are safe, and your potential yields are maximised.

Most individual fertilisers contain only a specified limited number of essential nutrients and are known as ‘straights’. As an example, Urea contains nitrogen with an analysis of 46 per cent and Muriate of Potash contains potassium with an analysis of 50 per cent.

There are only a select number of straights available. They must typically have several characteristics, including water solubility and plant availability as well as their cost effectiveness when compared to other fertilisers. 

For instance, to meet a combined nutrient requirement like that of nitrogen, phosphorus, potassium, sulphur, magnesium and a trace element requirement, fertiliser ‘straights’ have to be 'mixed' or 'blended' to match these multiple plant nutritional demands in one fertiliser product often referred to as “one-shot” blends.

This allows for the efficiency, cost benefits, timeliness, reduced trafficking and convenience of a single-pass application.

To apply a dollar value to such blend scenarios, let’s look at a hypothetical situation for a crop by assuming a soil test shows a requirement for 160 kg/ha of nitrogen, 15 kg/ha of phosphorus and 100 kg/ha of potassium.

If only a limited number of fertiliser blends were available in a supplier’s range, a blend with an analysis of 25 per cent nitrogen, 3 per cent phosphorus and 18 per cent potassium applied at 640 kg/ha would most closely match the nitrogen requirement, as mentioned above. However it would over apply both phosphorus and potassium. 

Using fertiliser costings* this equates to, on a per unit of nutrient basis:

$1.50 per unit of nitrogen as urea
$4.70 per unit of phosphorus as DAP
$2.40 per unit of potassium as muriate of potash
Applying the above fertiliser blend at 640 kg/ha, would lead to an over-application of phosphorus and potassium by 4 and 15 kg/ha respectively This alone equates to an additional fertiliser cost of over $54/ha, compared to using a custom blend to precisely match the soil test determined nutrient requirement of 160 kg/ha nitrogen, 15 kg/ha phosphorus and 100 kg/ha potassium.

It is easy to focus on the price of a tonne bag of fertiliser but as this example shows significant financial gains are to be made by applying only the required nutrients and hence making better fertiliser decisions.

Speciality products like compound fertilisers can also be used. Compound fertilisers differ from blends as there are multiple nutrients contained within every granule. Examples of compound fertilisers include Nitrophoska® Special fertiliser and the range of Incitec Pivot's Granulock® fertilisers. Compound fertilisers also give the potential for a more uniform nutrient application. 

Custom blends and compound fertilisers can be sourced through an Incitec Pivot Fertilisers distributor and delivered on-farm, generally within 24 hours of ordering.