Is your soil alive? Or barely breathing?

By Angus Deans

Air in the soil – what are you nuts? Aren’t we meant to pack the dirt tightly around the roots of the trees  and plants we plant?

So – what is the go?

A healthy living soil consists of minerals, organic matter, micro-organisms, air and water. Our modern, chemical agriculture has focused almost exclusively on minerals and is becoming rapidly more aware of the role that organic matter and micro-organisms play. We have all understood the importance of water but few have imagined that they have any control over how much falls on their land – unless they are irrigators; or have imagined that they have any influence over how much is absorbed by their land.

As I write this I have become aware of the fact that these 4 elements that have largely been the focus of modern agriculture are all something that someone has been able to ‘manufacture’ a dollar value into the provision of!!!!

So this leaves air – as yet we do not pay for air so it seems few people are pushing the barrow of air as a vital component of soil function! So if air has no commercial value what is the value of air? And to whom?

Try holding your breath … and see how long you last!

Let us imagine a soil without air, a horrifying vision of concrete comes to mind.   Unfortunately, this is the case in massive tracts of what was once deemed prime agricultural land in Australia. These are the black cracking soils that once supported vast grasslands before their destruction by overgrazing, cultivation, and year-on-year cropping with bare fallow in between. In these soils we see limited gaseous exchange to support soil life and its respiration. We also see poor water infiltration, as water and air both use the same pathways into the soil. When these soils are wet, they are very tight and basically anaerobic. When they are dry, they crack wide open enabling massive amounts of air into the soil which then is very harmful to living plant roots by virtue of air pruning. Too much air can have negative impacts on soil carbon and the microbial community, as anyone who loves to cultivate can witness.

So why is air so important in our soils? The pore spaces that air moves through our soil are also the same pathways for water to enter. Without the ability for air to enter we also have limited ability to absorb water, reducing rainfall use efficiency and profits. Air in our soils is vital for the respiration of aerobic soil life. Two of the by-products of this respiration are water and carbon dioxide, both essential for plant production.

Air is 78% nitrogen, yet another vital ingredient for plant growth and possibly the most applied plant nutrient in conventional agriculture. Anytime air movement is restricted in soils, our nitrogen cycle is also inhibited. Bringing air back into our soils rebuilds the pathways by which nitrogen can be fixed and supplied to plants for free by soil dwelling nitrogen fixing bacteria such as Azotobacter, in addition to the well-known nitrogen supplied by rhizobia on legume roots. With many agricultural lands showing significant compaction, requiring more inputs and more nitrogen; addressing air movement is key to healthy production.  It is not only heavy clay soils that require aeration. A fine silt soil lacking in organic matter will also benefit from aeration. As golf course managers discover, even sandy soils can compact and turn into sandstone with poor management.

There are many methods we can use to get air into soil:

One way is by opening the soil mechanically with  cultivation, or ‘ripping’ with a tined implement to open the soil but not invert it. Both are quick methods to introduce air into the soil profile.  However, if the reasons why soil became compacted are not addressed, using machines to address a biological problem are at best a temporary fix as the soil will quickly revert to its previous state. In cases which call for mechanical aeration, dripping biological stimulants into the soil at the same time helps to restore soil biological function and ensure air movement continues.

Another method is to use soil ameliorants to ‘open up’ the soil and allow more air to enter. Calcium helps soil flocculate, with its large particle size creating bigger pore spaces in the soil. Gypsum is well known for improving soil structure when chemical imbalances create tight soils, such as high sodium or magnesium ions.  Gypsum is high in sulphur, which reacts with sodium or magnesium ions and helps to leach them from the soil profile. While the calcium in the gypsum will help the aggregation of the soil. In some clay/sodic soils with adequate calcium levels and low sulphur levels, elemental sulphur can also be used to help flush magnesium or sodium ions from the profile. This method can be slower than cultivating or ripping, however it provides necessary minerals for improved plant health, as well as longer-lasting effects on soil porosity.

Biology create passages and airways through soil, such as the protozoa, nematodes and earthworms. Larger soil aggregates are constructed by beneficial fungi allowing air and water movement. Promoting the growth of mycorrhizal fungi on their host plants has many benefits including increasing a plants access to water and nutrients. Beneficial fungi are promoted by having adequate food in the soil, provided by trampling materials, root shedding, stubble, biological stimulants and by increasing plant brix. The benefit of aerating soil via means of fungal activity though is that the effects are long lasting – the opposite of aeration achieved by steel alone.

Cover crops are an increasingly popular tool to break through hardpans and compaction layers. Through supporting healthy plant growth, roots can begin to pump sugars around their root zone (rhizosheath) and begin to build aggregate structures.

Soils low in organic matter are more prone to compaction. Adding organic matter in the form of composts, vermicast, improving grazing and lifting brix all work to reduce compaction over time.

Lack of good airflow and gas diffusion is likely to be your number one limit to health, wealth and production.

Is your soil alive? Or barely breathing