Wet seasons, big crops, high fertiliser prices make fertiliser decision more important than ever

High fertiliser prices particularly for Nitrogen (N) means fertiliser decisions for the coming winter crop will have a large impact on gross margins.

A large crop in 2021 will have resulted in removal of greater amounts of nutrient particularly N from the system. Adding to this wet conditions in 2021 will also have had an impact on N cycles. Growers will need to look carefully at crop budgets factoring in increased input prices. It must be remembered however that the main drivers of potential crop yield in cereals is water and N availability.

Paddock history can be used to predict likely N supply and therefore the additional N required. Records of previous yields and associated proteins allows N removal to be calculated. Records of N fertiliser applied enables a calculation of how much N was supplied from soil mineralization and therefore how we can expect the soil to supply in the future. This data collected over many years can give a useful estimation of likely mineralization. The key here is good paddock records.

Flooding can result in loss of N from the system through denitrification and loss as gaseous forms of N. Denitrifying bacteria use the oxygen in Nitrate (NO3- ) molecules as a source of oxygen when the soil is waterlogged and free oxygen is no longer available. The vertosols in northern NSW have limited drainage which can encourage the formation of waterlogged conditions. This process requires organic matter to be present as this provides a food source for the bacteria.  Denitrification is difficult to predict and can be variable across a paddock.

Soil testing is also useful tool however, it is time consuming and relatively expensive compared to using paddock history. For this reason it needs to be done properly to gain worthwhile information.

Soil nitrogen can be quite variable across a paddock and like all tests the result is only as good as the sample sent to the laboratory. To get a reasonable estimation of soil N at least 8-10 soil samples need to be taken to a depth that will cover the bulk of root activity usually 90-120 cm, although this may be less in shallow soils or those with subsoil constraints such as salinity or sodicity which prevent nutrient extraction.

Samples need to be taken from appropriate soil depths eg 0-10, 10-30, 30-60, 60-90, 90-120 cm and analysed as separate samples. Where heavy rain has occurred, it is likely at least some nitrogen (being very soluble and mobile) will have moved down the profile. This can have implications for crop management as crops will not find this N until later in the season, possibly being used more for protein production than yield.

North West growers with questions about fertiliser decisions can contact North West Local Land Services Cropping Officer Bill Manning via william.manning@lls.nsw.gov.au.

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