Saturday, March 30, 2024

Soil test results are only a guide

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Last month I made the point that a soil test was a far-from-infallible means of arriving at the optimum fertiliser programme for any farm. To highlight some of the weaknesses in the soil-testing regime I used some anecdotes from my past experiences in this area, and hinted I would provide a few more.
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So here goes.
My first anecdote goes back to my return to my advisory job with MAF after spending a year on exchange with the Advisory Service of the United Kingdom Ministry of Agriculture. It wasn’t long after picking up where I left off that I discovered some significant changes in procedures had been introduced, in the guise of progress, into the Advisory Division. Principle among these was the increased employment and widening of the duties of advisory assistants – designed to relieve the more highly qualified and paid professional advisors from straight-forward technical tasks such as seed certification. Among these duties was soil sampling. Many of my colleagues welcomed the introduction of this policy and adopted it with alacrity, on the basis it relieved them of mundane and boring work. I wasn’t so keen, bearing in mind the strictures of my old mentor Jim Graham that soil test interpretations and recommendations should only be done by the person taking the soil samples. Apart from that, valuable discussion could take place while going round the farm with the farmer to collect the samples.
My prejudices were confirmed when discussing fertiliser policy with a farmer I was catching up with after my year away. I was quite surprised when he told me he had cut back on fertiliser inputs from what I had previously recommended. When being asked why, he said he’d had a soil test taken and the amount he was using was what it showed was required – and that a colleague who had been covering the district in my absence had provided the interpretation. This didn’t seem right to me, and I said so. To prove me wrong he produced the test results, with the accompanying fertiliser recommendations.
Sure enough the soil test results gave figures significantly above what my observation, experience and local knowledge suggested they should be. On quizzing the farmer on the procedures the reason for the anomaly quickly became apparent. This was hill country, and not easily sampled. When asked to describe how the advisory assistant did the sampling, the reply was “Easy, he sat in the Land Rover with me and when we came to a reasonably flat bit where I could easily stop it, he’d hop out and take a few samples”. That was mistake number one – obviously the sampling was heavily biased to higher fertility areas and stock camps. Mistake number two was made by my colleague in writing up the recommendations without eyeballing the pasture concerned. If he had done so, he would have realised that overall things did not equate.
My second anecdote is from more recent times, when I was employed by the-then MoaNui Dairy as farm production manager. One of my major tasks was to convert the old New Plymouth airport at Bell Block into a fully functional dairy farm, starting April 1 and to be completed by the start of calving on August 1. It was a reasonably challenging task on first impressions, but made much more so by the existing nature of the property. Ownership was with the city council which had basically abandoned it on the completion of a totally new airport more than 20 years earlier. It had been leased for a peppercorn rental and, in effect, mined. Pastures were totally sod-bound, browntop dominant, and as far as I could tell had had next-to-no fertiliser for a very long time.
The first step was to take a soil test and the results were sobering but not unexpected. Olsen P results were about 2 or 3, with one exception peaking at the giddy heights of 4. All pasture had that brown, dead-looking aspect, with any clover or ryegrass noticeably absent – and this was pasture that had to be supporting a milking herd from August 1.
There was a further aspect that on first sight appeared another big negative. The lessee had enjoyed his peppercorn rental for years and was somewhat miffed he had lost it. He was determined to extract the last possible benefit from his possession and so put a considerable effort into ensuring there was not a scrap of drymatter present on the farm at take-over. Consequently, by June 1, all paddocks had been comprehensively trodden and chewed out, which of course is the ideal starting point for rehabilitating sod-bound browntop pasture.
Drastic problems require drastic solutions.
In the first week of June all paddocks got the equivalent of 1250kg/ha of superphosphate, then late July 3750kg/ha Osflo (broiler chicken litter 3-2-1) to provide organic matter and nitrogen. In early September, with the spring grass coming away, there was another dressing of equivalent 625kg/ha superphosphate. Unfortunately I have no record of the amount of potash used, hence the use of the superphosphate equivalents.
The results were nothing short of spectacular.
By November, when we had an open day on the farm for suppliers, all paddocks were a picture of deep green, vigorous and healthy ryegrass white clover.
This was ideal dairy pasture and one would expect a reflection of ideal soil fertility, which would in turn be reflected in soil test levels. The rule of thumb for ash soils is to raise Olsen P by one unit an average of 11 kg/ha of phosphorus above maintenance is required, so we would have expected the Olsen P to lift to about 15-17 from the 2-3 it was before topdressing. Instead it had only lifted to 6-7. This, if taken at face value, would indicate a chronically infertile soil. Manifestly that wasn’t the case.
A characteristic of the Olsen P test is it will undervalue when heavy rates of phosphorus are applied to soils with low levels, and that certainly applied.
The soil test result gave a totally erroneous assessment of soil fertility status and would be completely useless, if not dangerous, for formulating an ongoing fertiliser policy. Needless to say, in this instance scant attention was given to those results.

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