Robin Tennant Wood

Professional Services - Research & Writing

Part 2 of a series of posts on the joy of manure in soil ...

The dirt on soil

Back in the early years of my academic career I worked in a university department with a number of soil scientists. ‘Soilies’ are, generally speaking, affable types who wear sturdy boots and get inordinately excited about digging holes in paddocks. In their natural environment they tend not to get irritated by much, but nonetheless, I managed to annoy them by referring to their soil samples as ‘bags of dirt’. This invariably earned me a frown and a grumbled, “It’s soil. Soil. Dirt is what falls on the floor of the lab.” What, then, is soil?

Soil is the thin layer of friable earth that covers most of the landmasses on Earth. It is a living, breathing organism. The healthier the soil, the more life it supports. Soil consists, in varying proportions, of organic material, minerals, water and air. Each of these provides nutrients to enable the soil to continue to live and, in turn, support life.

Organic matter consists of humus, usually the decomposed or decomposing remains of animal or vegetable material. On average, around 58% of organic matter in soil is soil organic carbon – healthy soil is a carbon sink. The organic component in soil is the basis of its fertility. Healthy soil should contain about 5% healthy soil copyorganic matter, but this varies. In Australia, alpine soils may contain up to 10%, desert soils less than 1%. It is cause for alarm, however, that in some areas, agricultural soils have dropped below that 1% mark.

Minerals comprise the non-organic component of soil and the largest component at around 45%. These can range from silica through to clay and are usually the residue of the rock structures around and under the soil. Different regions obviously have different soil minerals depending upon the landscape and its underlying mineral structure.

The remaining 50% of soil components are water and air, each around 20-30% depending on the soil type and surrounding conditions.

Air provides the aerobic action necessary for the soil. Just like our skin needs to breathe, so too does the soil. Air is trapped around the particles of the soil and allows organic material to decompose, which in turn provide nutrients. Just like us, without air the soil cannot breathe and will die.

Water is held in the soil by the organic material and acts to dissolve solutes, maintain subsoil moisture levels and regulate temperature. Soils that are low in organic matter will also be low in water content and will not retain water. Such soils may become ‘hydrophobic’, which means that water will pool and run off the surface without sinking into the soil structure.

While a balance of all four components is necessary for soil function, for my purposes in this series of blog articles I will be focusing on the organic component, and more specifically, what we can do to enhance and maintain a healthy, productive soil.

Our soils are currently in decline. In Australia, our soil is old. This is the oldest landmass on Earth and as such, our mountains are eroded, our rocks ancient and our topsoil layer thin. While it is easy to blame spatial and temporal geography for having thin soils of poor quality, that is a flimsy excuse. Australian soils have been, to put it bluntly, flogged to death.

The most fertile soils, and thus the most productive agricultural regions,organicsinsoil in Australia occur along the east coast, and in the south-east and south-west corners. These areas are also, and not coincidentally, the areas with the highest annual rainfall and the highest human population density. This means that while nature took several billion years to produce a good, fertile layer of topsoil, we have taken only a couple of hundred to cover it with roads, houses, airports, factories, schools, shopping malls… Every day, on that relatively narrow strip of fertile land, urban development is encroaching on agricultural land.

Areas that were, until recently, large farms producing food for our cities, are rapidly being subdivided into ‘rural residential’ blocks, which are not large enough to sustain agriculture and draw on urban utilities, while farming is being marginalised into the less fertile areas.

nitrogeninsoilOne of the points of this series of blogs is to get fertility back into urban and peri-urban soils, so that food may once again be produced where the soil and rainfall is best. The other point is to provide a perspective on biowaste that views it not as something to be disposed of and which draws on resources for that disposal, but as a resource itself.

Your soil needs you.

Watch this space for the next post on The tao of poo

[Soil maps from CSIRO Soil and Landscape Grid: http://www.clw.csiro.au/aclep/soilandlandscapegrid/ViewData-Portal.html

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