Targeted aerial fert applications

Fert Options
with Robin Boom
Agronomic Advisory Services

In a recent discussion with a Hawke’s Bay drystock farmer, who had suffered terrible stock losses from the drought earlier this year, he mentioned to me that for his steep hill country farm it was no longer worth fertilising. He believed his farm was more challenged by moisture rather than nutrients, and when the rain and climate were conducive to growing grass, it would grow anyway. He also emphasised the fact that if he came to sell his farm, it would probably go into trees for carbon sequestration as with current and pending environmental legislation, livestock farming on his property would be uneconomic.


On some of his northerly faces, most of the ryegrass and clovers had died out, except for patches of subterranean clover, and low fertility grasses and weeds were all there was left to fertilise. Whereas on his southerly faces, some better pasture species had survived. I suggested on these better areas he would still get good economic responses to fertiliser nutrients, and he could get the plane to target these parts of his farm.

In a phosphate trial on my own property in Taumarunui I recorded a 30 per cent increase in pasture production from a capital application of 500kg/ha of superphosphate, which was highly economic even in the first year, let alone the long-term benefits from improved fertility. I also observed from a four-year lime trial, much greater drought resilience from raising the soil pH from 5.0 to 6.0, almost doubling annual dry matter production for this drought-prone area.

Most planes these days have variable rate application technology using Tracmap or similar programs, so if parts of the farm are less fertile these can be uploaded onto the computer system so that fertiliser can be applied heavier to those areas. Also for those parts of a farm that are stocked higher and working harder, maintenance fertiliser on such areas is higher so could benefit from higher application rates.

Targeting known deficiencies

Another benefit from the likes of Tracmap, is different fertiliser blends can be applied to specific parts of a farm targeting known deficiencies, rather than the typical ‘one size fits all’ approach of blanketing the whole farm in one fertiliser mix at one rate. This may be acceptable if the whole farm is showing similar fertility trends, but where there are big differences paying a little more for application costs targeting certain fertiliser nutrients where they are needed can be a worthwhile exercise.

With some private fertiliser importers selling very competitively-priced high-analysis fertilisers like triple super, MAP, DAP and feed grade dicalcium phosphate, some farmers are now using helicopters to apply these. If a nitrogen boost is required at a particular time of year, then DAP and MAP can be an attractive option.

Research by Dr Greg Lambert a number of years ago showed for some hill country pastures, the nitrogen response was just as good on infertile pasture species as it was on improved pastures, so nitrogen-containing fertilisers can be a useful tool for feed budgeting purposes.

One detrimental side of using nitrogen on hill country pastures is it increases the rate of acidification, so extra lime needs to be applied and that can be very costly when flown from a plane or helicopter compared to ground-spreading. A reason for the very good nitrogen responses on poorer hill country pastures was attributed to a lack of nitrogen from poor clover performance and the resulting carbon to nitrogen imbalance in the thatch layer at the soil surface.

So for the Hawke’s Bay farmer whose pastures had suffered a huge setback from the drought, nitrogen-based fertilisers may be his best option going forward either going into winter or coming out of it.

Robin Boom, CPAg, member of the Institute of Professional Soil Scientists. Phone: 0274448764.


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