How Soil Works
By Neville Passmore
The key to understanding how soil works is to understand the role that soil borne organisms play. This is the last great frontier of science and we have a long way to go to complete comprehension, however we do have a picture of what's going on based mainly on observing results.
Biodiversity of underground living creatures is an overwhelming fact. Two handfuls of humus rich soil contain more organisms than there are human beings on the planet. We have microbes, fungi, nematodes, earthworms, protozoa and the list goes on and on.
Organic carbon derived from plants is the fuel that all these bio-diverse life forms depend on. Plants supply this fuel, which can come from dead plants that have decomposed or perhaps more importantly from the carbohydrates (think of sugars) that living plant roots exude through tiny root hairs, directly into soil. Plants convert carbon dioxide and water into carbohydrates via photosynthesis. Photosynthesis is a process that occurs in green plants and is nature’s way of trapping the energy of the sun and passing this stored energy into the food chain.
The soil living organisms return the favour by providing their host plant with nutrients such as nitrogen, phosphorus and potassium as well as minor and trace elements. This supply quickly invigorates the plant. In addition insoluble nutrients are gradually unlocked by the soil biology supplying nutrients over an extended period.
There is another important benefit and this is where the natural power of biology comes in. The beneficial microbes and bugs in a healthy soil also help protect the plant against pests, diseases and even weed competition. This can be partly through competition and partly by actively destroying the bad bugs.
There are no pesticides in nature so soil biology provides the necessary plant protection. It’s quite simple really; a healthy soil will grow a healthy plant. (It’s a bit like our own health. When we are run down we are more likely to succumb to a bug.) The trick is to maintain a healthy soil and this is where the complex carbon helps.
Complex carbon is what’s left when plant matter is broken down by the soil organisms. The organisms get the benefit of the easily available energy and nutrients. What’s left is a complex ‘sponge’ of carbon that can hold onto water, store nutrients and act as shelter for the microorganisms. You may have heard of the term ‘humus’ to describe this complex carbon.
When we recycle organic waste through the composting process we copy nature by breaking down organic carbon through biological activity and end up with complex carbon.
Not all composts are the same when it comes to their long term effects on soil health. As you can imagine, one of the key differences is how well composted the products are and how much complex carbon or humus has been produced. How can you tell the difference?
The recently revised Australian Standard for composts and mulch described 3 maturity index levels. Maturity index level 3 composted products (mature composts) are the top of the list. These are highly concentrated, meaning that a little goes a long way. They're also very stable so they remain in the soil delivering benefits for years.
Composted products can be added to our extremely old Western Australian soils to give them a fertility and health boost that's fundamentally different to the form of nutrients in fertiliser.
Fertilisers tend to stimulate soil borne organisms to get into top gear. If used to excess these nutrients overstimulate the underground life so that it consumes soil carbon. Over time this can further deplete soil carbon and that is when we start to see problems. A soil with low carbon levels cannot hold onto nutrients or water and this is when we see nutrient run off that wastes money and causes off site environmental pollution. Our old sandy soils have little enough carbon to start with. Anything we can do to preserve and improve soil carbon is good for our gardens (and farms) and the environment.
The less mature a compost is the less stable the nutrients are. If you like, immature compost is a bit like a fertiliser in an organic base. The nutrients are more easily available and will stimulate the soil microorganisms. This is not necessarily a bad thing but we need to remember that the ‘green flush’ caused by the more easily available nutrients will also increase biological activity that will consume the carbon that we just added.
A more mature compost will have much longer lasting beneficial effects for our soils. And with any product that has been through a proper composting process you will know that it has been pasteurised to kill off weeds, pests and diseases so that it is safe to use.
Knowing what sort of carbon you are adding is useful information and this is where the new Australian Standard can start to help.
Of course the extra water holding capacity provided by the complex carbon in the soil also helps the plants avoid water stress during dry or hot spells. This also improves plant health and is becoming so important with even greater water restrictions forecast for the future.
So, if you want to know how to find concentrated carbon to boost your soil health then keep a lookout for the maturity index. You’ll start to see manufacturers including this in their product information over the next few years.