Soil Degradation: Another Carbon Story
Soil degradation can be a depressing topic. It is generally accepted that the decline in soil quality across Australia following European settlement has been extensive, and extreme in some areas.
Soil degradation involves a reduction in soil quality, and this can refer to a decline in physical, chemical or biological properties, or to the actual loss of soil through erosion and transport away from the land. And these are commonly inter-related through positive feedbacks which amplify the effect of changes in one factor by limiting or generating changes in other factors.
Soil erosion degrades soil in a literal sense – the height of the soil is actually lowered. I often think of gully erosion as a giant network of agricultural drains. Similar in effect to someone who wanted to drain excess water from their backyard, and so dug trenches to get the water away. This, of course, is not a great result in grazing or cropping lands within the driest continent on earth.
I want to focus on the degradation of soil properties, rather than erosion, as they are of critical importance to anyone who is growing things, and they can often involve similar approaches for remediation or improvement.
Soil degradation is often thought of as a ‘spiral’ of degradation. Commonly, the loss of a protective layer of plants leads to a loss of soil carbon (organic matter) through burning, clearing, ploughing or over-grazing. This loss of plants reduces the layer of litter on the ground and can expose the soil surface to raindrops which cause a crust to form, reduces the amount of plant roots within the soil which are necessary for good soil structure (peds and crumbs) and provide organic matter to fuel the soil biota (such as bacteria, fungi and protozoa) and their foodweb.
As a positive feedback, these changes reduce the factors needed for plants to grow: air and water through soil structure, and the availability of nutrients through nutrient exchanges with soil biota. Physical, chemical and biological factors with the soil decline in concert since they are all inter-dependent.
Organic matter (carbon) in soil can be seen as the key to soil health. Equally, the level of organic matter in soil can be seen as an indicator of soil degradation. A level of soil organic carbon of between 1 and 1.5% is often considered the critical level required to avoid the risk of soil degradation. This level is equivalent to 10-15 grams of carbon per kilogram of soil. However, this level is generally too low to say that a soil is healthy.
Soil carbon is often said to be naturally low in Australian soils. However, as Dr Christine Jones has found when she looked at historical accounts from the 1840s, that soils were often described as “soft, spongy and absorbent”. In one series of soil tests, the worst (i.e. least productive) 10 samples of 40 soils tested had organic matter levels of between 2 and 5%, while the best 10 samples had between 11 and 37%. Since 2% soil organic carbon is currently considered to be a healthy level for production, this suggests that in general our agricultural soils have experienced a massive decline in soil health.
The cheerier side to the story is that degraded soil can be regenerated, and unsurprisingly, since removal of plants was the initial cause of degradation, getting them back is a big part of the solution. Within intensive systems such as gardens and orchards a common remedy is to add organic matter in the form of mulch or a pioneering cover crop. Much has been written about techniques for this, and there is plenty of information within the Permaculture books and literature.
Similarly, there is a developing literature on techniques for regeneration of soil health within large scale, commercial farming enterprises – often termed “regenerative agriculture” and involving the maximizing of groundcover and plant growth through controlled grazing or pasture cropping. Christine Jones recently (March 2015) described the principles which underpin these approaches in an interview in Acres Magazine titled Save Our Soils.
Many Permaculture folk have land and farming enterprises which lie between these intensive gardens and extensive commercial operations. But the principles still apply on a few acres. A good first step is to assess the current condition of your land, and there are a couple of good tools to help with this.
Landscape Function Analysis was developed by CSIRO (Ludwig and Tongway) and is a monitoring tool which is particularly useful for before and after monitoring of land restoration or rehabilitation. It looks at the way in which physical (e.g. water) and biological (e.g. plants) elements move and/or are cycled.
The Quick and Dirty small farm health check is also a useful tool. As the title suggests, it is very quick and includes some “testing questions” for indicators of soil, pastures, water and native vegetation condition. While developed for people who have cattle, it is relevant for anyone with a few acres. It involves 20 simple questions that can be answered with a spade, a screwdriver and a bucket. You can download a copy from here.