Soil.  What is it; and what makes it 'Healthy' or 'Unhealthy'?

Understanding what soil is could be the key ingredient to staving off disaster in modern civilisation.  Soil is not merely dirt.

So, what is soil?  Simply put, soil is the living interface between inorganic parent material and subsoil, and the plant and animal life above.  Sand or clay are not soils, but subsoils.  When a soil ecosystem begins to extract key minerals and nutrients from those subsoils, the result is soil.

It is valuable to understand the common 'layers' of a soil structure.  

Humus - Where there is a functioning plant ecosystem on the surface, you'll have the 'humus' covering the surface.  The humous is a layer of purely organic material consisting of dead organic matter that is in the process of breaking down through biological action.

Topsoil - Directly below the humus is a layer of topsoil. The topsoil consists of a layer in which the organic material from the humus is being mixed with the mineral compounds of the subsoil below.  

Subsoil - Below the topsoil is the subsoil layer.  Consists primarily of the loose mineral material that has weathered from the parent material below.  However, it also contains some organic compounds which leach down into the subsoil from the topsoil.

Parent material - The partially weathered mineral/rock layer below the subsoil.

Bedrock - Where the soil structure meets solid rock below.


The layers of soil mentioned above are important to know, but understanding soil health requires one to carefully consider the soil 'Spheres'.  Soil is created by, maintained by and it's health depends on the strength of the soil ecosystem.  It is estimated that between one and two billion microorganisms live in each cup (250ml) of healthy soil.  Therefore, an assessment of soil health must focus on the soil ecosystem and it's 'spheres'.

Detritusphere - this is the soil surface layer, consisting of a layer of decaying plant residue.  This residue is made up of the leafy top plant material, not the roots.  A healthy soil detritusphere protects the soil beneath, providing the soil ecosystem's 'armour', helping to regulate temperature and protecting the soil from rain impact.  A healthy soil with a functioning detritusphere will begin to aggregate just beneath the residue, preventing crusting.  This keeps the soil surface porous, allowing the soil to breathe (healthy soil ecology needs oxygen) and absorb rainwater with no run-off.  Finally, but most importantly, the continued processing of decaying plant residue through the detritusphere feeds the biology in the soil ecosystem below.

The protection of the soil surface is essential for soil health.  An effective water cycle exists when rainwater penetrates the soil and only leaves through transpiration out of plant leaves, or through percolation into subterranean aquifers etc.  The detritusphere helps regulate temperature, insulating the soil biology from the diurnal extremes of temperature above ground.  A well armoured soil warms up earlier in the spring, cools off later in the autumn and doesn't overheat in mid-summer.  Heat is retained at night, enabling biological action to continue, and kept cool during the day to prevent evaporation and heat stress on the biology.

Tillage destroys the detritusphere, negating it's ability to protect and feed the natural soil ecosystem.

Aggregatusphere - below the detritusphere one finds dark 'cottage cheese' like formations.  These are the soil aggregates bound together by a organic glue secreted by soil biology.  These aggregates form the basic soil structure.  Tillage and over-grazing both stress the soil ecosystem and reduce aggregation.

Porosphere - in the aggregatusphere one finds pores.  Because the soil is bound up in small lumps or aggregates, pore spaces appear between them.  This pore space allows water to penetrate and be held in the soil and allows air to percolate through, providing oxygen to the soil biology.

Rhizosphere - This is the presence of rhizomes, or plant roots, in the soil.  It's through these roots that the plant transfers liquid carbon resulting from photosynthesis, in the form of sugars, down into the soil.  It's this carbon that the plant uses to feed the soil biology, receiving minerals and nutrients from the biology in exchange.

Drilosphere - No, I'm not making that up, it's the drilosphere where the larger soil biology, like works and dung beetles, leave tunnels through the soil.  These tunnels retain a large population of bacteria on their walls and see the movement of large amounts of potassium and nitrogen, both essential for plants.  A healthy soil will reveal 5-8 earthworms in single spade of soil, giving 850,000 to 1,500,000 earthworms per acre.  That number of earthworms will circulate 18 tonnes of soil per year, without a single drop of diesel being burned, or a single blade of steel entering the soil.  Nature's natural tillage.  Plant roots find these tunnels and follow them down into the subsoil where they can retrieve trace nutrients and move organic carbons deeper into the soil profile.

Monoculture crops cannot support a healthy soil ecosystem.  A healthy soil requires a broad mix of plant types, grasses, legumes, sedges and forbes make up a natural grassland ecosystem.  Farmers experimenting with mixed 'cocktail' crops have noticed the biggest improvements in soil health as they move toward 8+ species mixes, with significant gains to be found with 12-25 species mixes.

The soil ecosystem is healthiest when there is a living plant root system in the soil for the longest possible time in each growing season.  Therefore, a mix that includes cool season grasses and broadleaves, as well as warm season grasses and broadleaves, ensures that the soil biology is being fed by a diverse plant culture throughout the longest growing season possible.

All of which creates and builds healthy soil.  Since 1993, the Brown Ranch in North Dakota has taken land with an organic carbon content of only 1.7% and improved it, in some fields, to over 11%.  This is significant, not only because it frees us from reliance on synthetic fertilisers to grow our forage crops and vegetables, but because it represents the largest carbon sink known to science.

A 2% increase in soil organic carbon, over the world's agricultural lands, would use up all the CO2 emitted by mankind since the beginning of the industrial revolution.  All while building a healthier ecosystem.

Soil heath, it's in everyone's best interests to care about it.