Making sense of soil and herbage test inconsistencies

Fert Options
with Robin Boom
Agronomic Advisory Services

I have often been asked over the years by farmers to help resolve and interpret soil test data taken by different fertiliser company representatives. Sometimes the laboratory results come back with glaring differences from historical data, and explaining such differences is not always straight-forward. Soil and herbage tests are the best guides we can use to assess which nutrients are affecting production, and where a farmer should target their fertiliser spend. Soil testing is a ‘ball park’ science, meaning the levels found will be in the approximate ball park of reality. But the data produced by the lab should not be considered absolutely accurate because there are many variables that can influence the results.

Sampling technique

Normally the laboratory procedure should be fairly repeatable, if the testing procedures are followed by the book by the lab technicians, but this may not always be the case. A much greater cause for variation is the sampling technique. Sampling depth is a major contender for variation. Here in New Zealand, the standard soil testing depth on pasture is 75mm and for crops it is 150mm, whereas in Europe and North America both pasture and crops are 150mm. In Australia, and South American countries like Chile, Uruguay and Argentina, the standard depth on pasture is 100mm, and I too prefer this 100mm depth which I think more accurately represents the root zone from which pasture plants extract most of their nutrients. On volcanic soils in particular, testing at greater depths will dilute phosphorus which mostly sits in the top 10mm, so consequently I aim for lower P levels when I have taken the samples. Testing deeper can also give lower pH readings and exacerbate aluminium toxicity, which may not show up in 75mm samples, and is an important consideration for applying lime.

Another cause of inconsistency is the sampling lines taken and the different paddock histories. To avoid recent urine contamination which will elevate potassium, I like to use as indicator paddocks, those which have been spelled from cattle grazing for at least a couple of weeks if possible, and avoid any paddocks which have not been recently cropped or turned over. On steeper hill country, avoiding sheep tracks which receive more dung and urine as well as stock camp areas is important. Conversely it is not good to sample extremely steep sidelings, but to try and find average sloping microsites for pushing the probe into the soil.

Herbage testing

For herbage testing, it is important to avoid stalks and stem tissue, or weed species, as these can give spurious results. Taking herbage tests out of paddocks where stock are grazing can result in sampling pasture which the animals don’t like, so again can throw spurious results; so I again prefer to sample paddocks which have had at least two weeks of no grazing.

When it comes to interpreting results, this can become confusing as different advisors will emphasise different elements, and even graphical interpretations by laboratories can vary widely from lab to lab, particularly in herbage test data. With herbage testing there are also wild seasonal variations for critical elements such as molybdenum and boron, and even the major elements can vary significantly at various stages of growth.

Having been at the coalface of interpreting soil and herbage tests for more than 30 years I think I have got all of my levels pretty well perfect, but there will be other advisors with similar legacies who will disagree with me on some elements. When it comes to deciding which company can provide the best value for the nutrients required, and the form of those nutrients which will work best, there can be further pitfalls. Many times I have been able to save farmers tens of thousands of dollars on the quotes they’ve been given by a particular fertiliser company, by getting those same nutrients in the same form from a different supplier.

Being independent

Being independent with a broad knowledge of soil chemistry and plant and animal nutrition, means clients don’t waste their hard-earned money on snake oils or expensive sources of essential elements that can be purchased cheaper elsewhere.

We may not be able to control the weather yet, but we can alter the soil chemistry with fertiliser inputs to improve productivity by considering all 16 essential elements plants require, and do this in a holistic way which builds soil fertility and soil organic matter through sequestering CO2, the molecule of life, from the atmosphere. Correcting soil deficiencies cost effectively is a critical part of a sustainable future.

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


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