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The importance of nutrition

Soil nutrient and fertiliser: fuel for pasture growth
Sunlight, water and nutrients are essential to pasture growth. Without them, there is no pasture. Sunlight provides the energy that drives photosynthesis, enabling plants to turn carbon, oxygen and hydrogen into carbohydrates, plant cells and the plants they build.

Water is the primary constituent of living cells, providing the soup that makes life work.

Nutrients are the food for the plant’s biochemistry. The food for pasture growth. While we can do many things to help pastures grow, providing an adequate supply of nutrients is one of the more important. In turn, fertiliser is one of the more important means we have of supplying the nutrient requirements of plants. While fertiliser can attract a range of opinions, in essence it’s simply a practical tool for supplying nutrients that are frequently lacking.
 
In that statement, ‘lacking’ is an important word. Nutrients are not often completely absent. There will be something to feed growth, but that growth will almost always be limited in some way by a restriction in the plant-available nutrients. It’s up to us to determine if that restriction is something to address or not, and by what means.

What we can always do is understand what’s required, identify what‘s lacking, recognise the effects of nutrient limitation, and understand the benefits of amendment, as we seek to identify what’s the most appropriate course of action.

Each paddock, pasture and farming situation will have an individual context for what’s appropriate. Quick and easy generalisation or average assumptions are convenient, but we’re not talking about fast food, we’re considering what will help a specific pasture and production system.

Key points
  • Soil nutrition is a primary driver of pasture growth, alongside improved pasture species and grazing management that allows plants to grow.
  • Deficiency in any of these reduces growth potential.
  • When sufficient nutrients are available to pasture plants, the effects can be dramatic, increasing plant health and vigour, shoot density, root depth, the amount of clover, the amount of green leaf, the length of the growing season, resilience to dry conditions and more megajoules of energy and feed for stock to graze.
  • Soil nutrients are frequently limiting.
  • There is a wide range of essential nutrients. Some nutrients are more likely limitations than others.
  • Nutrients work together, with the most limiting nutrient reducing the benefit from all.
  • More than one nutrient may need to be improved for any benefit to be realised.
  • Soil testing can identify the limits at play and priorities for nutrient application.
  • Observations of pasture condition, composition and performance can provide clues to these nutritional limits and stimulate enquiry. Slow growth responses in good times, undesirable species, lack of clover, pale pasture colour, thin shoot density, discoloured leaves, can all hint at limitations.
  • No single nutrient will fix everything, and nutrition alone is not everything. Productive pasture species, management of grazing and stocking rate, are all required to get the most out of good nutrition.
What nutrients are needed?
A wide range of nutrients are required by pasture plants and the animals that graze them. They are essential to the processes of plant growth. When the supply of these nutrients is limited, growth will be limited too.
  • Carbon, oxygen and hydrogen are available from air and water, while other nutrients must be extracted from soil or partnerships with soil-borne fungi and bacteria.
  • Major nutrients nitrogen, potassium, phoshorus, magnesium, sulphur and calcium are needed in greatest quantities.
  • Minor nutrients boron, copper, zinc, molybdenum, cobalt, manganese, iron, sodium, chlorine may only be required in small quantities, but play critical roles.
  • Some minor nutrients like selenium may be required by animals, but not by pasture plants. Others like copper and cobalt may be needed more by animals than plants, while molybdenum and cobalt are essential to nitrogen fixation in clovers and other legumes.
  • Deficiencies in nutrient supply limit total pasture growth to that possible from the most limiting nutrient.
  • Excess supply in one nutrient does not overcome deficiency in another. Sometimes excess may even induce deficiency in an antagonistic relationship.
Even in a brief introduction, it’s easy for nutrient discussions to quickly get complicated. In practice, however, there are a few nutrients to pay attention to first, before looking for more complex issues and solutions.
What nutrients are most important?
Where should we start looking? Major nutrients nitrogen, potassium and phoshorus command most attention as common and significant limitations to pasture growth. Deficiencies in magnesium and calcium are uncommon, and sulphur is often sufficiently available where single superphosphate has been used.

Of the minor nutrients, molybdenum and copper are important to consider as potential limits to nitrogen supply, as a result of their role in the nitrogen fixation process in legumes.

These nutrients can be common limitations to pasture growth, either singly or in combination.
Where do the nutrients come from?
Nutrients essential for plant growth must either be present in the soil material or added in the form of a fertiliser amendment, the addition of some form of organic material, or as with the case for nitrogen, potentially by a biological extraction or fixation process.

Nutrient limitations occur where there are either insufficient nutrient present in total, or if there is insufficient release of these nutrients into the pool of nutrients available to fuel plant growth. Soil tests attempt to assess this pool of plant available nutrient. Where limitations are apparent, fertiliser provides an accessible and practical means of boosting the immediately available nutrient supply.

Soil chemistry is complex and not all of the nutrient present in the soil will be available to growing plants, or be available when it’s needed. Nutrient inputs are used to improve or top up the available nutrient supply.

An alternative approach is to live with a lower equilibrium of nutrients and the lower production this can entail. Even in this situation, removing nutrient in product and through losses associated with livestock grazing will lead to declining nutrient availability.
How can we tell what's lacking?
Soil testing – identifying nutrient limits.

Soil tests are used to assess how much nutrient in the soil system is available to plants. When used correctly they provide great information on what’s likely to be limiting plant growth and what amendments are likely to improve growth. While observing pasture growth can provide good clues as to what nutrients are lacking, soil tests go back to the basics of how much of each element is there.

For some elements the relationships between soil tests and growth may be complicated. Nitrogen status, for example, can be difficult to interpret because the element is very mobile in soil and its availability is often related to biological processes that are affected by environmental conditions like soil moisture and temperature. Test results may thus be more reflective of the now and not the next few weeks of growing. For minor or trace elements where very little quantity is required, variable nutrient distribution and sampling technique may distort results.

In fact, regardless of the nutrients of interest, the process of collecting the soil samples for testing is important.
  • Sample to the requirements of the lab. Commonly this will involve collecting 10 cm deep core samples.
  • Avoid hotspots of nutrition (dung, urine spots, stock camps, burn sites, old fertiliser dumps).
  • Sample from areas most representative of the paddock.
  • Be systematic to sample across the area of interest and get enough soil for the test requirements.
  • Avoid averaging distinctly different areas. Different areas deserve different tests.
  • Identify the location of sampling to allow return visits that assess changes in soil nutrient availability, while limiting the differences that arise from sampling in different places.
Paddock history and soil type will obviously have a big impact on soil nutrient availability. Consider this carefully before assuming that one paddock can represent others. If there are differences in fertiliser history, crop or pasture type, hay or silage cutting, or even grazing and feeding, there may easily be significant differences in nutrient availability.
What are the targets?
Nutrient availability targets have an interesting history. For something so important, access to robust well-researched benchmarks has been surprisingly slow to develop. Initially targets were formed around what was observed in the accumulated results of soil tests, forming a census of frequency not effect.

For both phosphorus and potassium, however, a comprehensive consolidation of nationally available pasture trial data developed nutrient response curves that provide a sound basis for decision making. This information is well documented in Making better fertiliser decisions for grazed pastures in Australia.

In summary, critical value benchmarks were developed that suggest soil test nutrient levels that are capable of achieving 95% of the potential pasture growth.

For phosphorous that target is 15 mg/kg (ppm) from the Olsen test.
For potassium the targets in the Colwell test vary with soil texture, ranging from 126 mg/kg in sand to 161 mg/kg in clay-loam. Here the critical test value increases with increasing clay content.
 
Finally, the role of fertilisers in sustainable grazing systems is rightfully open to scrutiny. The importance of soil nutrient however should not be overlooked. Observations and contentions about the importance of nutrient should always consider the context. Stopping fertiliser application in a system of high nutrient availability may have no effect. For a while. That while will be longer if nutrient loss from the system is low. Adding one nutrient when more are limiting may have no effect. It isn’t that nutrient is not the issue, just that not all the issues have been addressed. Similarly, if nutrient has been properly addressed but the benefit is still not apparent, it may be that pasture species, pasture condition and grazing pressure are bigger limits.

As for examples of no fertiliser or nutrient addition but fantastic productivity, they are readily balanced by low nutrient, no pasture observations visible not far from almost any Tasmanian paddock. Testing to see what can make a difference in your paddock is the important consideration.

Time to get growing.