Thursday, March 28, 2024

Assessing your nutrient status

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When it comes to determining fertiliser requirements, soil testing is currently the only way to get accurate, reliable information about existing nutrient levels.  Micheal Keaney, science extension officer at Ballance Agri-Nutrients, says that a well-reasoned soil testing programme provides the ability to maximise fertiliser efficiency.
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“A regular, structured soil testing programme will provide valuable information about the nutrient status of a soil, so contributing to overall farm management and productivity.”

Soil test timing

Soil test timing is important and there were several factors that needed to be considered, Keaney said.

Ideally paddocks that are being grazed or that have recently had stock in them should not be tested. These paddocks will have a high level of fresh deposits of dung and urine, which increases the risk of inaccurate results.

Soil should not be sampled when it is waterlogged or during a drought. Both of these conditions affect the availability of nutrients, and will result in misleading results. 

Soil sampling should not be carried out within three months of applying fertiliser or lime.

It takes time for the nutrients in these products to be incorporated into the soil, meaning that sampling too soon after fertiliser application gives artificially high nutrient levels.

Sampling too soon after an application of lime will give a misleading soil pH result and Olsen P results. 

The ideal time to sample is either in spring or autumn when soil tends to be neither too wet nor too dry, meaning the results are representative of the nutrient levels available to actively growing plants.

Keaney said that winter soil testing in Canterbury is reasonably common, provided it is not too wet. He said that this fits in with the dairy calendar in particular, in terms of avoiding busy periods. 

Regardless of the time of year sampling takes place, it is best to repeat the timing for subsequent samplings.

Sampling from the same transect, at the same time of year, helps minimise the amount of variation between years. This aids in building a more accurate picture of the changing nutrient status of a soil over time.

Transects

Transects are lines along which samples are collected. In order to establish transects for a property, it should first be divided into different zones. These zones can be based on soil type, topography, and management. 

A transect might only cross one paddock or could cover a number of paddocks if they are similar land, managed consistently. Transects should never run through different soil types.

There are different options for marking transects for future use. Global positioning systems (GPS), which are available through fertiliser representatives and also used by a number of farmers carrying out their own sampling, helps ensure accuracy.

Alternatively, flags or marker pegs can be used to ensure identification of sampling sites. However, these need to be sturdy enough to be permanent.

Keaney said there was no reason people could not carry out their own soil testing, provided they knew the procedure and had the correct tools. It was especially important that whoever is sampling took cores to the right depth, he said.

“At around $70 for a basic soil test, the cost of sampling can be easily covered in gained efficiencies in a fertiliser programme.”

Interpreting test results

Potassium (K) and sulphur (S) are covered by more than one test. When looking at results it is important to consider which tests were most relevant to the farm, Keaney said.

“There are low natural organic sulphur levels in the South Island, so no point spending money on that test annually. My advice is to monitor it every five years if desired and use the QT Sulphate test as the plant-available sulphur indicator.”

Keaney said that sulphur accumulates in soils at different rates, depending on the soil’s anion storage capacity (ASC), which determined the ability of the soil to retain sulphate, and the rate of accumulation of soil organic matter.

“Allophanic or ash soils have a high ASC and because of reasonable rainfall will grow pasture, which decays and is converted into organic matter relatively quickly – hence, sulphate and organic sulphur levels are higher in these soils.

"Sedimentary soils have a lower ASC though and in many cases lower rainfall, so organic sulphur levels are lower.

"Adding high rates of sulphur will not accelerate this process – only time, pasture growth, and animal grazing will increase these levels.”

Keaney said that in terms of potassium, Quick Test K is the default test.

“With high natural TBK (reserve K) levels in the South Island there is little point spending money on that test annually unless it is a cropping situation. Arable systems deplete the TBK fastest through grain and straw removal. 

“As with organic sulphur my advice is to monitor it through testing every five years if desired.” 

Testing times

How often should soil testing be done? 

Micheal Keaney of Ballance Agri-Nutrients said ideal sampling frequency depends on the type of farm. As a guideline, he recommended:

  • Sheep-and-beef farms – different paddocks annually and the same monitor paddocks every three to five years in order to keep track of trends
  • Arable farms – same as for sheep-and beef-farms depending on the farm system and rotations used, and
  • Dairy farms – same paddocks every year for five years to establish trends then the same paddocks every second year thereafter.
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