Soil Fertility and Organic Matter

Soil quality is the capacity of a soil to accept, store and recycle water, minerals and energy for optimum growth and it's ability to preserve a healthy environment.

The capacity of each soil differs depending on the processes which formed the soil, the weathering etc it has been subjected to since and of course any agricultural practices etc that have taken place on it.  The texture, structure, chemical properties and organic matter content of a soil all of an affect on it's quality.


The average top-soil, which is generally considered to be a silty loam, is around 46% mineral, 50% water and air (1:1 ratio) and only 4% organic matter.  Yet, this organic matter content is incredibly important.

In fact, the main reason for decline of soil quality, or fertility, is the loss of organic matter.

Working the soil, removing organic matter (e.g. plant stubble), continual cropping, leaving soil bare etc all contribute to the loss of organic matter and organic carbon from the soil.

Of the 4% organic matter mentioned above, less than 5% is living organisms, up to 10% is what is fresh (i.e not begun decomposing) and the rest is split fairly equally between organic matter that is actively decomposing, and that which is stabilised - meaning it will not decompose further.

Where does organic matter come from?  Well, organic matter forms in the soil from the breakdown of plant and animal matter, otherwise known as organic wastes.  It takes place as part of the wider Nitrogen and Carbon Cycles, for which there are a million and one helpful diagrams to be found if you do a google image search.  The breakdown of the organic matter itself however can be simplified using this nifty diagram (copyright, me :) ):

Humification (click to view in Google Drive)

Humus is a dark substance, due to the high levels of organic carbon contained in it, and is effectively the final stage of the breakdown of organic matter.  It will either not degrade further, or will do so at such a rate that it will persist in the soil for hundreds, if not thousands, of years.

It is a bit of a tricky substance and cannot be chemically described.  Effectively, it is a complex of many, many, carbon compounds and is generally separated from the non-humic substances (carbohydrates, proteins, fats, waxes, peptides etc) by the fact that they do have distinct chemical formulae.

Some may described garden compost as humus, but in fact this compost will still have some further decomposition to go through once added to the soil.  I know of some gardeners who insist on referring to compost as "active humus", meaning that it still needs to break down further.  Personally, I like to stick to calling it compost :)

The presence of humus in a soil increases it's fertility in a myriad of ways - here are just some of them:

• it provides food and therefore energy for the microorganisms that live in the soil;
• these microorganisms exude substances which them help to hold soil particles together into aggregates, improving structure and porosity.  Humus also forms colloids with silt and clay - therefore humus can help to improve penetration of air and water;
• humus has a large surface area and a negative electrical charge, which allows it to act as a sponge, soaking up water and improving it's retention and availability to plants;
• these electrical charges also attract nutrients and prevent leaching of essential nutrients out of the reach of plants, and facilitates the dissolving of these nutrients into a more readily available form;
• this also allows humus to hold onto toxic substances and heavy metals in soils, to such a degree that humic substances are used in the clean up of waste sites;
• humus also acts as a pH buffer, neutralising both acidic and alkaline soils;
• humus can also neutralise attack by pathogens, by holding onto enzymes that are released in order to lower plant defences;
• the uptake of humic and fulvic acids by plants allows enhanced uptake of the major nutrients: Nitrogen, Potassium and Phosphorus.  It also increases the uptake of other essential nutrients, such as Calcium and Magnesium;
• humic and fulvic acids are absorbed into seed cases resulting in higher germination rates and better seedling survival rates;
• humic and fulvic acids stimulate root development and growth;
• humic substances also stimulate metabolic reactions within plant cells, including those required for healthy growth.

In summary, a soil with a high humus / organic carbon content is a soil that is high in fertility and therefore optimised for production.

So, how do you build up organic matter in your soil?

The obvious answer is to remove as little as possible and return as much as possible!

This means proper management of your soil: ploughing in stubble; planting green manures and legumes, then turning these in; applying copious amounts of compost; using organic mulches; practicing crop rotation and avoiding monocultures; using minimum tilling; avoiding the use of chemicals that will harm the soil microorganisms; using humate based fertilisers instead of superphosphates.

Humate based fertilisers are available in most countries and are often made from substances such as brown coal.  Here in Victoria, a range of these products are made by VicMill and are pretty cost effective - around $40 for 25kg of their ready made products, though they also do prescription blends.

There's also been increasing interest in recent years in the use of man made biocarbon, or biochar.  Much of this was spurred by the investigations into Terra Preta, the black soils in areas of the Amazon basin.  This soil is very dark in colour, having an organic carbon content of well over 10%, and incredibly fertile.  Some of the local population also claim that it regenerates itself at a rate of about 1cm of depth per year, but I am a bit sceptical about this and suspect it may be their justification for it's removal and sale!

One of the reasons for the huge interest in biochar is it's potential for increasing storage of terrestrial carbon and consequently reducing the effects of climate change.  The jury is still out on whether it can be produced on a large enough scale, without itself causing further pollution.

In any case, biochar testing is continuing around the world and products are starting to filter their way onto the shelves.  If you're interested in this, then a quick search on YouTube of biochar gave these results for your perusal :)

For those at home, making use of your organic wastes and turning these into compost is probably the most effective and cost efficient method of improving your soils fertility.

So, what are you waiting for? :)