Most pastoral farmers in New Zealand already know that white clover (Trifolium repens) fixes atmospheric nitrogen into plant available nitrogen via the symbiotic relationship with the Rhizobia bacteria in its nodules. However, what seems less recognised is just how productive this system can be.
Clover is a great source of nitrogen but needs fertile, healthy soil to grow well.
An ancient system
Symbiosis is an ancient system and it’s more common that you might think. For example, the human gut and its microbiota share this kind of relationship. While the gut provides the right kind of environment for the microbiota to live in, the microbiota develops our immune system and protects us against certain pathogenic bacteria and toxins.
In the case of white clover, the legume provides a stable environment and energy in the form of carbohydrates to the bacteria. In return, the bacteria provide nitrogen in the form of ammonia (NH3) for the host plant.
Companion pasture plants like grasses benefit when the nitrogen-rich clover roots are naturally sloughed off, or when the protein-rich clover leaves are eaten by animals, then recycled into soil organic matter through their waste.
How efficient?
For white clover, it seems the rhizobia bacteria may provide up to 90% of the ammonia needed to make the amino acids, proteins and nucleic acids essential for their healthy growth and heredity. That’s a very efficient system, and a very natural, enviro-friendly source of nitrogen.
Research shows the average amount of clover-fixed-N in NZ is 185kg per hectare per year (185kg/ha/yr), although it is claimed that the potential is greater, exceeding 250kg/ha/yr. So growing clover can certainly reduce the reliance on urea to provide the nitrogen needed for pasture growth.
We just have to provide the clover plant with what it needs to do its job.
Growing more clover
Observation tells us that clover enjoys fertile soil with good structure and healthy biology. Using ag-research validated science, we can identify optimum levels or ‘zones’ for each of these factors which make an excellent guide for working out how to maximise clover growth.
Soil fertility and herbage tests
Firstly, we can test soil fertility. For clover, phosphate, potassium, sulphur, calcium, and magnesium are particularly relevant. This is because clover isn’t a ‘pushy’ plant – it’s not great at competing with companions like ryegrass when these nutrients fall outside an ideal zone.
Herbage tests are valuable for detecting deficiencies of trace elements like iron, manganese, zinc, copper, boron, cobalt, and molybdenum, all important nutrients for clover. For example, if cobalt is outside the optimum zone, the Rhizobia bacteria can’t produce leghaemoglobin, crucial to maintaining the correct O2 level in the clover nodule. A shortage of molybdenum means the Rhizobia can’t make the enzyme reductase, used to fix nitrogen.
Of course, pH is important too. In the case of clover, a pH around 6.2 seems optimum because trace elements become unavailable to the clover if the soil is too acid or too alkaline.
Soil structure and soil biology
By conducting a Visual Soil Assessment (VSA), we can check other factors like soil structure and soil biology for their suitability for growing clover. Developed by Graham Shepherd, this assessment examines soil texture, porosity, colour and smell, as well as earthworm numbers, the status of plant roots and leaves, and so on.
Alongside VSA, soil biology can also be tested for its percentage of organic matter, the total organic carbon, and the carbon-nitrogen ratio to check that they fit into the ideal zone for growing clover.
The CloverZone® Programme
So, to support clover growth, it’s simply a matter of checking that the plant gets what it likes: fertile soil with good structure and healthy biology. The Fertco CloverZone® Programme focuses on supporting farmers to do exactly that by identifying the optimal zones for each of these factors, then designing a fertiliser programme to get the clover growing.
This article was published in the Coast & Country News.
