Permaculture Designers Manual
CHAPTER 8 – WATER IN PERMACULTURE
Section 8.11 –
Gaseous content and processes in soils in Permaculture
Solis are permeated, where not waterlogged, by the gaseous components of the atmosphere (80% nitrogren, 18% oxygen). This enters the soil via pore spaces, cracks and animal burrows, and diffuses via pore spaces to plant roots.
The exchange of gases, atmosphere to soil and soil to atmosphere, the breathing of earth, is achieved by a set of physical and biological processes, some of which are:
EARTH TIDES. The moon tides, much subdued on continental masses, nevertheless affect groundwaters in cobbles or boulders, the earth itself (about 25 cm rise and fall across the continental USA), and of course the soils of estuaries and mud flats (where air can be seen bubbling up for hours as the tide rises).
Everywhere, low or high pressure cells and turbulent wind flow creates air pressure differentials that draw out or inject air via crevices and fissures, or burrows. These same effects assist or retard the diffusion of water vapour across soil surfaces, and just as the wind dries out surface soils so it also, by lost flow, draws out other gases from soil.
The gases of soil respiration (carbon dioxide), oxidation , anaerobic and anaerobic metabolic processes, radon from radioactive decay, and simple or complex hydrocarbons from earth deposits or humus decay pass to air.
Much of the ammonia and carbon dioxide in the atmosphere, and at least 16% of the methane, is supplied by soil processes.
A single large broad leaf tree, actively transpiring, may increase the area of transpiration of one acre of soil by a factor of forty; a forest may do so by hundreds of times.
So oxygen, carbon dioxide, water vapour, metallic vapours, ammonia, and hydrogen or chlorine gases are transpired by algae, rushes, crops, trees, and herbs or grasses. Plant groups vary tremendously in the volume and composition of gases transpired.
Although we owe much of our atmospheric oxygen to trees, a great many non-woody plant species consume more oxygen than they produce.
Thus, the plant is a gaseous translator, trading both ways with air and soil. Some specifics of this trade are given in the section below on oxygen-ethylene processes.
Animals, too, are very active in opening up soils with small or large burrows; they act as pump pistons (like a train in a tunnel) to draw in and exhaust both their waste gases and atmospheric gaseous elements, which are then diffused to roots via soil pores.
Many burrowers (ants, crabs, termites, prairie dogs, worms, land crayfish) raise up mounds or chimneys which then act as Pitot tubes for air now or to create pressure differentials which draw air actively through their burrows.
Or they erect large surface structures of permeable sediments across which waste gases diffuse (e.g.. termite mounds).
By a great many such devices, animals contrive to live in aerated or air-conditioned undergrounds, and increase gaseous exchange in the soils.
GASES MANUFACTURED IN SOILS
By the way of titanium or rutile (TiFe), ammonia is manufactured in sands in the presence of sunlight. By way of ferrous iron, ethylene (C4h4) is manufactured in anaerobic soil microsites.
In anaerobic soils and waters, carbon dioxide, methane, sulphuretted hydrogen, ethylene, and sulphur dioxide are formed, and escape to air as biogas or marsh gas.
The same products are present in the mottled soils of hydrophobic days in winter, soils where crumb structure has been destroyed by misuse, where salt has deflocculated clays and caused soil collapse, or where water periodically floods the soil.
Many of these gases are found as a result of humus decay and thiobacillus (sulphur bacteria) action.
Ammonia is released from actively nodulating legumes (trees and herbs), and is usfel in unploughed soils as a plant nutrient.
Thus, gaseous compounds are continually made in the soil itself by process of metabolic growth and decay in the process of metallic catalysts and micro-organisms.
Moybdenum vanadium , and zinc all assist root bacteria in the creation of available soil nitrogen (as catalysts).
GASEOUS MOSAICS OVER TIME; SOIL MlCRO-SITES
Like pH, cation exchange, and structure, natural soil is always a mosaic of aerobic and anaerobic patches called micro-sites, where either oxygen (aerobic) or ethylene (anaerobic) sites develop.
Ethylene inhibits (in the sense of sense suspending), microbial activity, and like carbon dioxide, is present as 1-2 ppm in soils (Smith 1981).
As the ethylene at an oxygen-exhausted site diffuses out, oxygen floods back and re-activates the site. Under natural forests and grasslands, this cycle, or dance, of oxygen-ethylene is continuous, and most nitrogen there occurs as ammonia, useful to plants and plant roots.
When we cultivate and aerate, nitrogen becomes a nitrate or a nitrite (which then inhibit ethylene product ion), and ferric rather than ferrous iron forms, thus making ethylene formation difficult (the process from decaying leaf to ethylene production requires a ferrous iron catalyst).
Also, plant nutrients are tightly bound to ferric iron and become unavailable for root uptake. It follows that the production of ethylene is essential to plant health and the availability of nutrients.
It is Important to realize that the aerobic condition of soils such as we get from ploughing or digging not only creates a condition of “unavoidable” nutrients and ferric iron, but also oxidises humus, which goes to air as carbon dioxide.
Also, most plant root pathogens require the aerobic condition, as well, the nitrate form of nitrogen, which is highly mobile leaches out when bare soils (not plants) occupy the site.
In all, ploughing and earth turning create a net loss of nutrients in several ways, thus atmospheric pollution, stream pollution, and low soil nutrient states.
Smith (Ibid.) therefore recommends least soil disturbance, the use of surface mulch (not incorporated) as an ethylene precursor (old leaves are best for this), and very small but frequent ammonia fertilizer, until soil balances are recovered.
The ideal conditions would be:
- Permanent pasture;
- Orchards with permanent green crop as mulch;
- No-dig or mulched gardens;
- No- or low-cultivation of field crop, or field crop between strips of forest to provide leaves and nutrients; and
- The use of legumes in a similar proportion to that occurring in natural plant associations in the area, at all stages of the succession.
Under these conditions, soil mineral availability is made possible and soils do not lock up nutrients in oxides or produce pollutants.
When we achieve this balance, soil loss and mineral deficiency become yesterday’s problems.
And when plant leaf (not soil) deficiency can be adjusted with aqueous foliar sprays, nature then starts to function again to obtain nutrients from soils via microbes and root mycorrhiza at the microsite level. (Smith, A., 1981, “The Living Soil”, Permaculture Journal #7. July ’81.)