Getting the Dirt on Soil

by Josh Verhoek – Ballance Agri-Nutrients

What are nutrients?

Nutrients are mineral elements that plants and animals need to grow and function. Animals get their nutrients from the plants they eat. Plants get their nutrients from the soil. If the nutrient content of the soil is not right, then the plants growing in the soil and any animals feeding off those plants will not thrive.

Nutrients come from two main sources in the soil: the soil mineral content (breakdown of rocks) and soil organic matter (breakdown of plant and animal material). These natural processes don’t happen fast enough to replenish soil nutrients at the rate our farm systems use them, so this is why we use fertilisers.

In a farm system, growth or productivity will be limited by the nutrient in shortest supply. This is known as Leibig’s ‘Law of the Minimum’ (Figure 1).

Figure 1. Leibig’s Law of the Minimum.

Justus Von Leibig’s “The Law of the Minimum” published in 1873

“If one growth factor/nutrient is deficient, plant growth is limited, even if all other vital factors/nutrients are adequate…plant growth is improved by increasing the supply of the deficient factor/nutrient.”

Macro and Micronutrients

Plant and animal tissue is largely made up from carbon, hydrogen and oxygen plus 16 essential mineral elements (nutrients) (see Table 1).

Table 1. List of essential plant and animal mineral elements.

Soil tests and what they tell us

Soil tests support good fertiliser practice by telling you about the soil’s nutrient content. Macro-nutrients in particular are well calibrated to pasture production, that is, the relationship between the test results and the expected pasture production at that given level. Micro-nutrients however, with the exception of boron, are not well calibrated and should be evaluated only with herbage tests (how much the plant is actually taking up from the soil).

Most commercial laboratories provide tests for pH, phosphorus (P), potassium (K), magnesium (Mg), sulphur (S), and anion and cation storage capacity.

The individual tests are as follows:

  • pH – a measure of soil acidity, and thus an indication of the need for lime.
  • Olsen P – a measure of plant-available phosphorus.
  • Quick Test K – a measure of plant-available potassium.
  • Tetraphenyl boron K – a measure of K supply (useful in some sedimentary soils).
  • Quick Test Mg – a measure of plant-available magnesium.
  • Sulphate-S – a measure of sulphur immediately available to plants.
  • Organic-S – a measure of the long-term supply of sulphur.
  • Anion storage capacity – a measure of the soil’s capacity to store nutrients such as P and S; previously called phosphate retention.
  • Cation storage capacity – a measure of the soil’s capacity to store nutrients such as Ca, Mg, and K; also called cation exchange capacity.

Results from soil tests can be classified as low, medium or high. A low result for a nutrient element indicates that pasture will almost certainly respond to fertiliser. A medium figure indicates that only small responses may be obtained, and a high result shows that a response is unlikely. Soils with a high anion storage capacity (ASC) will need larger amounts of P to overcome a deficiency than will soils with a low ASC. The cation storage capacity (CSC) figure is used by some consultants to determine lime requirements. The higher the CSC of the soil, the more lime will be required to raise soil pH.

Table 2 shows high, medium and low ranges for the commonly used soil tests. These figures should be used as a guide only as some numbers will fluctuate based on other factors, e.g. soil type. Talk to your consultant before deciding your fertiliser strategy.

Test Low Medium High
pH <5.6 5.6 – 6.0 >6.0
Olsen P <15 20 – 35 >35
Quick Test K <4 4 – 8 >8
Quick Test Mg <6 6 – 8 >8
Sulphate-S <6 6 – 12 >12
Organic-S <10 10 – 20 >20
ASC (%) 0 – 40 41 – 75 76 – 100
CSC (me. %) 6 – 12 12 – 25 25 – 40

Table 2. Generalised interpretation of soil tests under low, medium and high levels.

Figure 2 below is an example showing two different soil sample results, which include the basic soil test and sulphate sulphur.

Figure 2. Example soil test result.

How often should I soil test?

Soil testing is much more valuable if it is carried out regularly over several years. This will enable the farmer to monitor the fertility trends on the farm, and will smooth out random variation between samplings. Samples should be taken at least every 2 to 3 years. Each sampling should be a few weeks before fertiliser application so that results are available before fertiliser is purchased.

Managing nitrogen on farm

Nitrogen (N) plays an essential role in NZ agriculture. As a growth multiplier, fertiliser nitrogen is used to increase the yield of pasture and crops. The nitrogen cycle is dynamic, with different forms of N either being taken up by plants or lost from the cycle to air and water. N is a very mobile nutrient, so it is important to understand how it reacts and behaves in the soil and on the soil surface. Below we describe the key factors affecting N loss (Figure 3).

Figure 3. The five key factors of nitrogen loss

The five factors of nitrogen loss include:

  1. Animals – animals are the primary driver of N loss, as loading of N from the urine patch is equivalent to 600-1000 kg N/ha. Therefore, any factors that can have an influence on urinary N output will help reduce N loss, e.g. reduced stocking rate, breed selection, reduced protein feed.
  2. Effluent – when applied not using best practice, N loss can be significant. You want to avoid wet pastures, application rates that are too high (>24mm), and effluent areas that are too small meaning the return time is quicker.
  3. Fertiliser – following the “four R’s” is the best way to minimise N loss. These include choosing the right product, the right rate, at the right time, and in the right place.
  4. Feed – high protein feeds typically lead to more urinary N output. Also, winter grazing on fodder or forage crops can lead to high loss zones, as well as strip grazing with high concentrations of urinary N and dung along the grazing strips.
  5. Drainage – soil structure and rainfall are the key factors that can influence the amount of N loss. For example, sandy soils are more porous and cause more drainage than sedimentary soils. We have the least amount of influence on the drainage factor, but how we treat the soil will have an impact on the soil characteristics, e.g. cultivation weakens soil structure and its water holding capacity.

The below diagrams are visual representations of the above points, highlighting the significance of each key N loss factor. The larger and more bold the word, the higher the impact it has on N loss.


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