Reviewing expensive fertiliser programmes

Fert Options
with Robin Boom
Agronomic Advisory Services

 In the past month I have been emailed soil tests taken by different fertiliser reps, plus their recommendations, by two farmers who have been concerned that they are spending too much money and wanted an independent opinion.

The first one was a 200ha dairy farm up north on sedimentary clay soils whose annual fertiliser bill has been averaging $90,000 ($450/ha) for the past five years. The eight soil tests showed pH levels varying from 5.7-6.6, with an average of 6.1.

The Olsen P levels ranged from 25-47, averaging 34; the MAF Quicktest potassium levels ranged from 8-22, averaging 15; the MAF Quicktest magnesium levels ranged from 33-73, averaging 53, and Sulphate Sulphur levels ranged from 10-16, averaging 13.

I could immediately tell this farmer had been taken for a ride, and was applying far more than he needed to.

Biological optimum

The biological optimum Olsen P level on sedimentary clay soils is 20.

Biological optimum is defined as being the level in which 97 per cent of the potential production can be achieved for that particular element assuming all other elements are at the optimum.

It has been argued that with extremely efficient dairy farms when all other possible nutrient limitations have been met, that you could aim for an Olsen P of 25 to get that last three per cent of potential production.

As all of this farm’s phosphorus levels are above the biological optimum, there would be no production lost if he did not apply any phosphatic fertiliser for a year, and on much of his farm, he could apply no P for several years before he needs to start applying maintenance P, which for 1000 kg MS/ha is around 40 units.

The biological optimum MAF Quicktest potassium level ranges from seven to 10 for sedimentary clay soils. Most of this farm did not need potassium at all, and again, like phosphorus, could have no potassium applied for several years without seeing any loss in production. Maintenance potassium on sedimentary soils for 1000 kg MS/ha is around 75 kg/ha.

The biological optimum MAF Quicktest magnesium levels range from eight to 10, although for animal health purposes it can be raised to 25. All of this farm’s magnesium levels are high and again no magnesium needs to be applied for a number of years.

The biological optimum sulphate sulphur level ranges from 10-15, so this farm needs to maintain sulphur, preferably elemental sulphur, as clay soils do not retain sulphur all that well.

My recommendation was to apply lime on the parts of the farm with pH levels below six, some maintenance potassium on the two blocks with MAF K levels of eight, and 125 kg/ha Sulphur Gain 30 over the whole farm.

Major savings

All up my recommendation would cost around $15,000 which is one sixth of what they have been needlessly spending the past five years.

The second farm I was asked to review is a dairy farm on a volcanic ash soil type.

The Biological Optimum Olsen P on volcanic ash soils is 22, but again on extremely efficient dairy farms aiming for 25-30 for that extra three per cent production could be warranted.

Recent soil tests on the milking platform showed pH levels range from 6.1-6.2, Olsen P levels range from 33-55, potassium MAF units range from 10-19, magnesium MAF units range from 39-44, and sulphate sulphur levels range from 11-20, so all elements are above the biological optimum and if no fertiliser was applied for a year or two, there would be no drop in production.

What the fertiliser rep however with their computer program using Overseer recommended was going to cost $400/ha as she wanted the farmer to maintain these high levels, which is not only a waste of money, but also not good for the environment.

This is bad advice and unnecessary spending, and it can also lead to metabolic problems in cows when there are imbalances between phosphorus and calcium, potassium and magnesium, and potassium and sodium.

Too often farmers are needlessly wasting their hard earned money on fertilisers which are unnecessary, or an expensive way of applying nutrients, having confidence their rep has their best interests at heart.

This may not be the case. With significant price increases the past few months, it pays to shop around, and also apply what it going to work on your property.

Alternatives waste products like chicken manure, goat manure, composts, sludges, whey etc can all be considered. Also be aware of price differences between different suppliers.

As I write, for some mainstream potassium and phosphorus products the differences are more than $200/tonne.

 

Robin Boom, member of the Institute of Professional Soil Scientists.

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