By Jeff Morton, MortonAg

While the high prices of inputs including fertiliser are beyond our control, there are ways to ensure you are getting the maximum return for investment.

When it comes to fertiliser, information is king. Many of you will have an effective soil testing programme that includes sampling fixed transects in monitoring paddocks at the same time each year. 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.

Interpreting soil test information

Determining the optimal soil test range for each nutrient across your farm is crucial. Using software, your agri manager can calculate your production levels and current returns to tell 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 whole farm (all-paddock) soil testing, then it is worth doing on flat and rolling land. An Olsen P test will tell you which paddocks can have P deferred and reallocated, giving a better return.

Compare costs and review the evidence

Traditional fertilisers have remained the most cost-effective per kg of nutrient applied, and despite their increase in price, this is still true. If you are considering a change from traditional fertilisers, be sure to:

1. Compare the cost per nutrient between options
2. Ask for evidence of any advertised benefit of the alternative product in the form of credible science.

All traditional products are supported by adequate peer-reviewed science. If the price is not competitive and/or there is a lack of evidence, then stick to conventional fertilisers.

Choosing the most cost-effective supply of phosphorus is also important. Currently it is cheaper to apply P and nitrogen (N) as coated urea mixed with superphosphate rather than DAP. But thanks to better flow, DAP may be better used as a cropping fertiliser when applied down the drill.

Understand soil characteristics

P is the most expensive nutrient and should be thoroughly reviewed. If you are cutting back on P applications then it can be done more rationally on ash than non-volcanic, sedimentary soils.

Ash soils hold P readily and require almost twice the amount to increase the soil Olsen P 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 (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 if your soil sulphate levels are greater than 6ppm on sedimentary soils. Ash soils better retain S, so S can be withheld for the same period as P.

For potassium (K), the tables are turned for soil supply. 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, so annual K is often required, preferably after calving in spring.

Economics are important for K on sheep and beef farms, with the optimal soil quick test K levels about 4–5. K application provides benefit under cattle grazing with clover-rich pastures and higher rainfall. K should ideally be applied after silage or hay has been removed to replace losses, but if soil quick test K levels are greater than 7 then this can be left out as a short-term cost saving.

The supply of magnesium and calcium (Ca) for pasture growth is adequate on nearly all soil types and Ca is 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.

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.

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).

Takeaways

• Ensure you have a soil testing programme that produces long-term soil testing trends
• Establish the optimal soil test ranges per nutrient for each management unit on your farm
• Compare alternative fertilisers on a $/kg nutrient basis
• Check the claimed benefits are supported by credible science
• P and S can be withheld for longer periods on ash soils than sedimentary soils
• If S is applied, elemental S should be included where rainfall is high and where fertiliser is applied in the autumn
• On sheep and beef farms, the economic benefit of K application should be carefully considered and only applied when quick test K levels are below optimum
• Only apply copper and cobalt in fertiliser after testing animals and only if it’s the most effective option
• In hill country, P and S generally give a better return than lime, except where P levels are more than adequate and soil pH low
• Seek professional advice based on robust science.

This advice is general and each farm is different, so make use of your agri manager or farm consultant to ensure you get the best bang for your buck.