Guideline Index

Chapter 15: Nutrient Planning

15.10. Monitoring of soil nutrient status

An effective way to determine whether sufficient fertiliser is being applied to meet capital and maintenance requirements is to monitor soil fertility. Regular soil testing of Farm Management Zones, using correct sampling procedure, is vital to monitor your fertiliser program’s effect on soil fertility.

Two useful tools that can help to record results, monitor soil fertility trends, and make much more efficient fertiliser decisions are:

15.10.1 Fertiliser monitor charts

Recording soil test results in the form of a graph (referred to as a fertiliser monitor chart) allows farmers to more easily identify or monitor the trend of the soil fertility in nominated areas on their farm over time. If fertiliser applications exceed the maintenance requirements, then the soil nutrient levels will increase. If applications are below the maintenance requirement, then the soil nutrient levels will decline. A sharp rise or fall in nutrient level may be the result of incorrect soil sampling, and may be ignored if the soil fertility trend has been steady and cow numbers and fertiliser applications have not altered substantially in that soil test period.

Figures 15.10a, b, c, and d show some examples of how to interpret soil Olsen P monitor charts. When trends on the fertiliser monitor chart are not what you expect, consider whether this is the result of management or environmental factors, such as the effect of applying effluent, feeding out in a sacrifice paddock rather than over a larger portion of the farm, or the effect of a very wet year on K and S.

Figure 15.10   Example fertiliser monitoring charts recording phosphorus levels and showing possible reasons for the interpretations
Figure 15.10 Example fertiliser monitoring charts recording phosphorus levels and showing possible reasons for the interpretations

Figure 15.10a shows the soil Olsen P level rising steadily as capital applications above maintenance are applied and then levelling off when the target soil Olsen P level has been reached and maintenance application rates only are applied.

Figure 15.10b shows a sudden drop in the soil Olsen P level, even though the stocking rate and fertiliser application rates remained unchanged. It is unlikely that the soil Olsen P level would suddenly decline, and the drop may be due to a soil sampling or testing error. Sampling soon after a fodder crop has been removed and before the new season’s fertiliser is applied may result in a slight drop but not a drastic decrease in one year.

Figure 15.10c shows the soil Olsen P level steadily rising and then declining, even though the fertiliser application rates remained the same. The change in the soil fertility trend may be due to an increase in stocking rate, regular cutting of hay or silage, or nutrient transfer to elsewhere on the farm if the paddock is routinely used as a day paddock.

Figure 15.10d shows a steady rise in the soil Olsen P level towards the target P range and then a sharp rise. Again, this may be due to a soil sampling or testing error, paddock management changes, or application of an incorrect rate of fertiliser.

Make a graph for each nutrient or soil condition indicator you want to monitor, such as P, K, S, pH, and aluminium. You will need a set of these graphs for each area to be monitored on the farm.

15.10.2 Computer software

Many farm or herd management computer programs have sections which allow pasture and nutrient records to be kept. Alternatively fertiliser monitor charts can quickly be designed using spreadsheet software found on most computers.