Permaculture Designers Manual
CHAPTER 8 – WATER IN PERMACULTURE
Section 8.4 –
The Structure of Soils in Permaculture
Soil is a complex material, and if it has enough plasticity (usually clay), or glue or fibre from organic sources, it can be pressed or compacted into mudbricks, hard pise, or baked to clay or stoneware. In any of these forms, it is of little use to plants.
Uncompacted soils are open, crumbly, or soft unless concreted by chemical solutes or compacted by ploughs, hooves, or traffic.
Crumbly soils have nevertheless a definite structure. The soil particles are in nodules or clumps held together by roots, day minerals, and chemical bonds.
If we speed a plough or drag harrows through these fragile assemblies, they may powder up as they do in a potter’s ballmill, or on outback roads. (“Bulldust” is the term used in the Australian outback. A soil scientist might speak of “snuff”.) Dryland soils with a high sait content are particularly susceptible to loss of crumb structure, only partly relieved by application of gypsum.
The mantle of soil and subsoil that covers the earth is as thin as the shine on the skin of an orange, and this mantle extends as living mud below the waters of earth as well as on land.
It is composed of these:
MINERALS, mainly silica, oxides of iron and aluminium, and complex minerals.
SOIL WATERS, fresh, saline, with differing pH, and dissolved minerals and gases.
CASES, some from the atmosphere, others emitted by the breakdown of rocks and the earth’s interior.
LIFE FORMS, from fungal spores and bacteria to wombats and ground squirrels, from massive roots to minute motile algae.
ONCE-LiVING REMAINS; the humus of the earth; decayed, compressed, and fossil organic material.
Soils rarely extend much below 1-2 m, and are more often a living system 12 cm deep. Subsoils, lacking the life components, and buried soils or deep washed silts are rare and confined to valley floors and deltas, or glacial mounds.
To estimate the proportion of clay, sill, sand, and coarse particles in soil, a sufficient first test (for judging the suitability of soil for dam building, mud brick construction, and crop types), mix a sample of soil from a few typical sites, and pour a cup of soil in a tall jar, filled almost with water.
Shake vigorously and let the soil fractions settle out over a day or a week (clay can remain in suspension for up to a week). 40%, or so day is needed for dam walls, and less than that for good mud bricks (without lime or cement added).
Of these fractions, the coarse particles are inert, although useful in fine soils as a wind –erosion deterrent. Sands are 0.05-2 mm, silt 0.5-.02 mm, day particles less than 0.002 mm (I g of day has a surface area of up to 1000 times that of I g of sand).
Soil crumb structure, aided by lime (calcium) aids the bonding together of these fractions and creates 20 – 60% pore space.
The organic materials and gels hold the structure open in rain, and where plant nutrients can become soluble for absorption by roots.