Fert Options with Robin Boom Agronomic Advisory Services

In last month’s column I discussed how soil tests should be taken, and soil sampling depths. From what I can ascertain, New Zealand is the only country worldwide where a 7.5cm depth sample on pasture is the standard.

Last year I was invited by a fertiliser company based in Chile to do a series of talks to pastoral farmers there on phosphorus fertilisation. In this particular part of Chile, the soils are volcanic, much like the better soils of the Waikato and Taranaki regions where dairying has been dominant; and they too have deep rich topsoils with high phosphate retention characteristics, so like our soils they are very P responsive in their natural state.

For more than 25 years I’ve advocated taking 10cm samples, which is what Australians recommend for their pastoral soils, and it was good to find the Chilean Agricultural Department also advocating 10cm sampling on pastoral farms, which I think better represents the zone where grass and clover feeder roots scavenge for nutrients rather than 7.5cm.

However, I met Professor Dante Pinochet, based at Valdavia University, who is a world expert of phosphorus and he advocates doing a 20cm sample. He maintains good pastures have feeder roots going down to this depth, so his students agronomists are doing 20cm samples on Chilean farms. An invigorating debate on what was the best depth to sample was had between myself, Professor Pinochet and a soil scientist from the Chilean Department of Agriculture.

Root zone

Over there, like in NZ, the standard phosphorus test is the Olsen P test, and for a 10 cm depth sample, the optimum level to aim for is 20, whereas for a 20cm sample Professor Pinochet aims for an Olsen P figure of 10. This is because when phosphorus is applied to the soil surface, on most soils it only moves about 1cm, so a deeper soil test will result in a lower phosphorus reading.

From historical NZ research, it was found the optimum Olsen P figure for most soils when doing 7.5cm samples is 25, which lines up with Chilean research once you account for the different soil depths. However, potassium and sulphur levels may increase with depth as these leach through the soil. And when considering things like aluminium toxicity and acidity, these increase with increasing sampling depth. This is why I believe, for pastures, a 10cm depth gives the best picture of what is going on in the root zone.

Although most people in the fertiliser industry only look at six elements in the soil, plants need 16 elements to grow and animals need 17. It’s important to also consider the organic matter or carbon levels in the soil, nitrogen, boron, copper, manganese, zinc, molybdenum and aluminium levels, and this is one reason why I’ve been sending my soil samples to Brookside Laboratories in the US, which has been doing these comprehensive tests for nearly 70 years.

When looking at micronutrients, it’s important your advisor understands all of these extra elements if they’re going to be tested for and what the optimum levels should be. Some laboratory graphs I’ve seen for these micronutrients are way out and shouldn’t be used as guidelines.

Soil testing

Most farmers and growers get fertiliser company reps to do their soil testing, but this can become problematic because the advice given will be slanted towards the particular flavour of products that company sells. And I find there are often marked differences between the cost of nutrients from various fertiliser companies for similar products. Much advice given to farmers unfortunately is unqualified, incompetent and costly; and I have many clients who’ve paid dearly by listening to salesmen when it comes to fertilisers. Soil testing is the cheap part.

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