Understanding soil nutrition

April 2021

Tom Reid - Boorowa/ Yass Natural Resource Management Team.

Soils represent one of the biggest potential stores of organic carbon in the world, second only to the oceans. Huge potential exists for farmers to build carbon stores back up/draw down carbon from the atmosphere. Although, effectively sequestering carbon is more complex than just incorporating organic matter into the ground.

Soil nutrition (fertility) and soil carbon cycling are inherently interwoven concepts. In this article we will attempt to provide a basic overview of soil fertility management and building soil carbon in regenerative pasture based farming enterprises.  We argue that soil fertility should be considered holistically and that there are various interactions between plants, soil biology and chemical fertility that factor into building soil carbon.

Rule Green. Growing Plants with deep Roots is Important

At the most basic level, the more a plant is able to photosynthesise the more carbon it can pump into the soil. Aside from the organic matter that ruminants digest and turn into meat, organic matter will return to the soil. While dead plant material (mulch and extinguished roots) on the soil surface is an important addition of carbon to the soil (and protector of the soil), alive roots are equally important. They are the “pipes of life” for the soil biological community. Through photosynthesis, plants provide energy-rich organic compounds into the soil as energy for the soil ecosystem. The soil ecosystem in turn drives the cycling of organic and inorganic matter (carbon and nutrients) in the ground. It mineralises organic materials, it converts minerals into plant usable forms and it structures the soil particles so that water, air and roots can get down deep. Therefore, the more photosynthesis that can occur, the more carbon will be cycling through this soil ecosystem some of which will build in stable forms. Therefore, apply the principle: As much green as possible for as long as possible.

So should I dust off the spreader? - Nitrogen - the lollipop of the soil biome.

Logically it follows that if you want to increase the amount of green plants pumping sugars into the soil, you need to increase the productivity of your pastures. Conventional higher input systems are designed to fertilise using nitrogen, phosphorus & potassium (NPK) to artificially enhance growth in order to sustain high levels of animal and/or crop production per unit of rainfall. Whereas regenerative systems aim to utilise natural processes in order to support production and minimise input costs.

This article in no way argues that conventional production system won’t build soil carbon, but building carbon in these system does require significant attention to detail as over fertilisation, especially from nitrogen fertilisers can have the opposite effect to sequestering carbon. For example, two recent papers have shown that using synthetic nitrogen causes a boom in the population of bacteria in the soil. For these bacteria to process the nitrogen, they also need carbon (carbohydrate). These bacteria can consume carbon faster than it is added and the carbon will be respired as Co2. As the soil carbon store is depleted by this, the soil becomes less able to store nitrogen (carbon helps hold onto nitrogen in organic forms). This loop leads to a greater reliance on synthetic nitrogen to sustain plant production and so the cycle continues and soil carbon becomes depleted.

The role of legumes

It is well established that legumes play a vital role in the provision of nitrogen in pasture systems. There are also other ways to bring nitrogen into your pasture system than growing legumes. Legumes will always be a part of a pasture system, either as a minor component (if you wish not to use phosphorus, or as a more dominant component if you use phosphorus). Focusing on producing high volumes of legumes in your pastures, especially focusing on transient species such as clover, can leave you with bare patches once a season turns dry leading to declining soil health and higher risk of weed invasion. Focusing on balanced mixed species pastures, preferably perennial, will mitigate these risks.

So what should I be focusing on to build nitrogen?

The decision to focus on any one aspect of your pasture’s composition will almost always lead to a simplification or adverse reaction somewhere else. For example, too much clover can acidify the soil and cause animal health issues such as bloat, fertility problems (estrogenic clover) and nitrate poisoning. The good news is that soil ecosystems are more diverse than we can comprehend and within them exists bacteria that are able to convert nitrogen from the atmosphere into plant available forms without the explicit association with legumes. These are called free living nitrogen fixing bacteria. Along with legumes, they play a critical role in nitrogen accumulation in the soil so long as the conditions for them to thrive are met. Luckily, what they like is green growing plants, preferably diverse perennial plant communities exuding sugars deep into the soil. In turn they provide nitrogen in plant usable forms - FOR FREE!

Phosphorus.

Most Australian soils tend to be low in readily plant available phosphorus and most agronomists and much of the scientific literature will tell you that having sufficient phosphorus in your soils is important in growing deep rooted perennial pastures and therefore maximising carbon sequestration into the soil. There is unlikely to be a significant downside to applying phosphorus fertiliser in a regenerative grazing system, although, it may be worth seeing how much more growth you can achieve by allowing plant recovery before you start throwing fertiliser at it.

Does phosphorus kill the soil ecosystem?

While a potential link exists between some phosphorus fertilisers and decreased fungal phosphorus cycling, it is not well understood and is unlikely to be a dramatic issue. From a regenerative agriculture and carbon building perspective, if phosphorus is the main limiting factor to having more growing, green plants more of the time then there are a variety of ways to address this. Local Land Services’ team of agriculture advisors run a series of workshops on decision making for phosphorus fertilisation that we would recommend. Applying phosphorus fertiliser should be an informed decision linked to your environmental and production goals. Without arming yourself with a rigorous soil testing regime and some basic understanding of phosphorus cycling in the soil, you may waste time and money on phosphorus for very little gain.

Potassium

Very little information exists on the role of potassium in building soil carbon other than the obvious fact that like nitrogen and phosphorus, it is an important macro nutrient in plant growth. Potassium is unlikely to be limiting to plant growth in our soils other than in haymaking situations where large amounts of dry matter are removed regularly.

So should I be using fertiliser to help me build soil carbon if I am a using planned rotational grazing?

This question comes up often as many people moving to regenerative agricultural practices are often looking to cut their input costs. However, there is never a simple answer to this question. In many pastoral landscapes phosphorus is severely lacking in immediately plant available forms. By undertaking a rigorous soil sampling regimes landholder can build an understanding of their phosphorus limitations and make decisions.

Should I quit fertilisers cold turkey?

In many instances a continued fertiliser program will be beneficial to your business and building soil carbon. However, in some situations, fertiliser may not be necessary given your soil type, environmental and production goals. If you decide to cut fertilisation out of your business then it is not recommended to do it cold turkey, as it may take time for your soil ecosystem to adapt or you may experience a period of reduced pasture production. The first step in going into regenerative pasture systems should be to define what level of production you need in order to make money. The second step should be concentrating on building your ability to allow plants to recover from grazing. Addressing any chemical, biological and physical constraints will become important.

The broader view

If fertiliser application decisions are to be considered in a holistic way, then the impacts of fertiliser need to be considered in a context broader than just your farm. Aside from leaching and waterway pollution, there is a broader carbon impact that is rarely considered. For example, on average, single super spread at 50kg/ha uses up 14.5kg of Co2 equivalent per hectare in order to manufacture and transport it. Therefore, for each 50kg of fertiliser you use, you will need to be returning at least that amount of carbon above what you would be adding without fertiliser in order to have net zero emissions. Whereas if you use Mono-Ammonium Phosphate (MAP) at similar rates, figures are at least double that of the example above.

Conclusion

Building soil carbon can be achieved through various management practices. This article has attempted to give a view from a low synthetic input perspective. In conclusion, there are many different tools that can be used to build carbon, especially utilising planned grazing to optimise your pastures response. Feeding the soil ecosystem with more green plants more of the time is a good way to assess your decisions in conjunction with your business and broader environmental goals.

The South East Local Land Services Natural Resource Management team are working with landholders in the Boorowa, Upper Lachlan and Yass areas under our Local Land Stewards program to sub-divide paddocks to implement regenerative grazing principles.

The Local Land Stewards project is supported by South East Local Land Services, through funding from the Australian Government’s National Landcare Program.