Thinking of increasing fertiliser and lime efficiency? Here are some key steps and considerations to ensure that you’re getting maximum return on investment.
When it comes to fertiliser, information is key. Many professionals have in place a soil testing programme. What can be learned from the long-term trends in soil nutrient and pH levels on individual farms is priceless when it comes to efficient fertiliser use.
Takeaways
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Ensure you have a soil testing programme in place that produces long-term soil testing trends for you to analyse.
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Establish the optimal soil test ranges per nutrient for each management unit on your farm.
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Compare alternative fertilisers on a $/kg nutrient basis.
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Check any claimed fertiliser benefits are supported by credible science.
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Phosphorus and Sulphur can be withheld for longer periods on ash soils than sedimentary soils.
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If Sulphur is applied, elemental Sulphur should be included wherever rainfall is high and when fertiliser is applied in the autumn.
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On sheep and beef farms, the economic benefit of potassium application should be carefully considered and only applied when a quick test shows that potassium levels are below optimum.
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Only apply copper and cobalt in fertiliser after testing animals.
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In hill country, Phosphorus and Sulphur generally give a better return than lime, except where Phosphorus levels are more than adequate, and the soil pH is low.
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Consider seeking professional advice based on robust science.
Interpreting soil test information
Determining the optimal soil test range for each nutrient across your farm is crucial. Using specialised software, it’s possible to calculate your production levels and current returns to tell you where your phosphorus (P) and potassium (K) levels and pH should be. Then, using your long-term soil testing trends, you can see where you stand in relation to these projections.
Ideally, the levels should be constant over time, which indicates you are replacing the nutrients removed from the farm and maintaining the soil levels.
If you have not already carried out soil testing across your farm, then it’s an exercise that’s worth doing on flat and rolling land. A Pasture Soil Test will tell you which paddocks can have P deferred and reallocated, giving you a better return for your money.
Compare costs and review the evidence
Despite price increases, traditional fertilisers have remained the most cost-effective per kg of nutrient applied. If you are considering changing fertilisers, be sure to check you’re getting the results you need by:
- Comparing the cost per nutrient per kg of the options you are assessing.
- Asking for evidence of any advertised benefit of the alternative product in the form of credible science.
All good products are supported by adequate peer-reviewed science. If the price is not competitive and/or there is a lack of evidence, be wary of efficacy claims that cannot be backed up.
Choosing the most cost-effective supply of phosphorus (P) is also important. Currently, it is cheaper to apply P and nitrogen (N) as coated urea mixed with superphosphate rather than di-ammonium phosphate (DAP). But thanks to better flow, DAP may be a good option as a cropping fertiliser when applied down the drill hole.
Understand soil characteristics
P is the most expensive nutrient and should be thoroughly reviewed before purchasing. If you are forced to cut back on P applications, that works better on ash than on non-volcanic, sedimentary soils: Ash soils hold P readily and require almost twice the amount to increase the Olsen P (soil test for phosphorus) by one unit. But, just like a long-term savings account, there is more phosphorus potentially available to carry you through for up to five years, if you decide to reduce or stop application for a time.
A sedimentary soil is more like a cheque account. There is not as much reserve P present and failure to maintain the soil levels can reduce production within one to two years. Pumice soils are intermediate between ash and sedimentary soils.
Good form
Sulphur and phosphorus
Sulphur (S) is required on most soils, so selecting an effective form is important. Where annual average rainfall is less than 1,000mm and fertiliser is applied in spring, then the 20–50kg/ha of S required (depending on soil type) can be in the sulphate form contained in superphosphate.
Autumn fertiliser application often requires slow-release elemental S so that there’ll be S available next spring. As annual rainfall increases above 1,000mm, elemental and sulphate-S should be applied as sulphur superphosphate in both spring and autumn.
If you wish to withhold P for one year, then S can also be withheld so long as your soil sulphate levels are greater than 6 ppm on sedimentary soils. Ash soils will retain S better, therefore, S can be withheld for the same period as P.
Potassium
For potassium (K), sedimentary soils, especially recent and pallic soils, can supply more K than ash or pumice soils. K losses are greater on dairy than sheep and beef farms, therefore, annual K is often required and should ideally be applied after calving in spring.
Economics are important for K on sheep and beef farms, with the optimal soil quick test levels for K being about 4–5. Application of K provides benefits under cattle grazing with clover-rich pastures and higher rainfall. It should ideally be applied after silage or hay has been removed to replace losses. However, if soil quick test K levels are greater than 7, then this can be left out as a short-term cost saving.
Magnesium and Calcium
The supply of magnesium and calcium (Ca) for pasture growth is adequate on nearly all soil types. Ca is usually applied with lime and superphosphate. Apart from selenium (which should be applied in fertiliser for animal health), copper or cobalt should only be applied if the need is supported by herbage and animal testing. Fertiliser application is the most effective option.
Nitrogen
Pasture assessment identifies potential deficits in feed supply, so N can be most profitably used to fill them. Research has shown that on ryegrass-dominant pastures, a better response to N is gained if S is included in the quickly available sulphate form at about 10kg/ha when applied in early spring.
Lime
Where lime is aerially applied on hill country, then P and S application should be prioritised over lime unless soil Olsen P levels are high (greater than 15 on hill country) and soil pH levels are low (less than 5.5).
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