Permaculture Designers Manual
CHAPTER 11 –
Section 11.3 –
Dryland Soils in Permaculture
We normally expect to find dominantly alkaline soils in the waterways of deserts, with areas of surface salts and carbonates. It is in these alkaline areas that we usually locate our settlements, to take advantage of water run-off. pH levels of 8.5 and 9.0 are not atypical and drying waterholes can reach pH values of 10-11.
Soils may have high nutrient potential if we adjust the pH and if water for irrigation is available.
Acidic sandy soils form around areas of deeply weathered granites and may dominate large areas of these landscapes.
High and low, pH areas have consequently low available mineral trace elements and in high pH, zinc, iron copper and manganese deficiencies are common, indicated in crops and fruit trees by such factors (in citrus) as inter-venal color loss and leaf thinning, with up curl in severe cases (manganese).
Zinc deficiency causes more severe leaf yellowing and is often associated with manganese deficiency.
Copper deficiency causes giant leaves, gum pockets in citrus branches and multiple budding or trunking in trees or citrus.
Foliar sprays, elemental sulfur and oxides or sulfates of the deficient elements can be used to correct these deficiencies.
Special problems may be caused by non-wetting sands (a fungus is responsible) and high salt levels in water or soils.
Bentonite clay or humus will ameliorate the “non-wetting” problem, as do swales and raised beds with enough edges to permit water run-off.
Salt problems, can sometimes be solved by flushing beds with fresh water, but if the water source is itself salty, salinity needs the combined solutions of humus production, perhaps a ponding period of water with algae production and water crop, ionic or distillation treatments and a choice of salt-tolerant crop.
Free draining sands, can be irrigated with salt levels in water much higher than will be tolerated as spray irrigation by plants (to 1500ppm).
Despite all of problems, we can usually establish home gardens and adapted tree crop systems and many selected areas (especially near scarps, rivers, hills or ranges) will grow excellent fruit, vegetable and tree crop with appropriate water run-off harvesting.
Perhaps the most important thing to remember about any activity in desert is that the natural systems are fragile. Good management and constant appraisal is essential.
Small systems may be called for, especially where we harvest runoff water.
We should carefully assess bore waters.
We should avoid broad scale or grand trials until we assess the capacity of the total system, especially of water resources.
Excessive mineral content in soil or water can also be toxic, to both plants and animals, and in particular, we must test bore waters for fluorine, sodium and radio-actives.
Nitrates are to be rigorously tested for, where children are consuming water and garden leaf products, and nitrate fertilizer used at minimal levels or not at all in the absence of mulch or high organic soil content (20% or more humus).
Aluminum, boron, sodium, and manganese can be in over-supply at very low or high pH values and only humus can buffer the uptake of these excess minerals.
Aluminum in acid soil solutions damages roots. and high levels of manganese causes stunting and yellowing in plants. A level of 0.5ppm copper or 10ppm lead or zinc can stop root growth completely (Bradshaw and Chadwick. I980).
A complex soil nutrient, essential to all plants for growth and enzymes, is phosphorus.
It readily becomes insoluble in acid soils, combining there with iron or aluminum. In calcareous soils, it forms insoluble calcium compounds. In arid areas, phosphates can be deficient except on humus, in forests and on the sill of ponds.
Manure from seed-eating birds has high phosphate levels. Correct inoculation of introduced plant species with phosphate-mobilizing mycorrhiza may be essential to their growth.
Acacias, eucalypts, legumes generally, pines, Casuarinas and even garden crops can benefit from the root associates that enable their roots to accumulate phosphorus from the surrounding soil.
Phosphates, must always be applied in small quantities, close to the crop.
Trace elements (zinc, copper) can be applied to soil (as sulfates) and 7 kg/ha of both zinc and copper
Sulfates added to alkaline South Australian soils enables pasture growth (and the carrying capacity of sheep) to increase by a factor of 40 times.
50 g/ha of molybdenum in more acid soils enables clover production. However, It is also possible to add micronutrients as foliar sprays, to infuse them into irrigation water at root level, to include them in slow-release pellets at root level or to mix them in bulk soil amendments such as dolomite.
Over-supply of these same nutrients will result in toxicity and health problems.
No amount of guesswork can supplant careful and skilled soil analyses.
Plant tissue from healthy garden plants should analyses about as in Table 11.1.
MINERAL SOLUBILITY AND pH; SOIL AMELIORATION
A general solubility diagram, given above, for soil minerals; this is, in fact, a measure of the availability of specific minerals to plants.
Important soil constituents (iron, cadmium and silica) show a range of solubility’s with pH as follows:
- IRON: very soluble at pH 3-3.5 as limonite (yellow) Fe2O3, and at pH 7-8 as iron oxide (FeO), which reddens alkaline deserts;
- ALUMINUM: very soluble al pH 4-4.5 and pH 9.5-10 as Al2O3; relatively insoluble over normal garden ranges;
- SILICA: slightly but increasingly soluble over pH 0-8, but then rapidly becoming more soluble up to pH 10 as SiO2.
Iron and manganese (Ferro-magnesium minerals) are closely associated in soils and show the same spectrum of deficiency with varying pH levels.
We can often therefore ameliorate soils at the higher pH levels with the addition of simple sulfur and restore iron, magnesium and phosphate levels to plants of these are “locked up” (and test as being present).
We seldom garden at pH 3-4, but if we do, we will need a massive calcium input as crushed or burnt lime, or crushed shells, to increase the availability of nutrients.
In the deserts, the soluble aluminum and silica may be evaporated to the clay minerals allied to illite or montmorillorite soils.
With all minerals, higher temperatures in water greatly increase solubility.
Aluminum ores, are usually formed at the low pH values and then hydrated to form gibbsite or bauxitic ores.
The carbonic and naturally present in rain (and very much increased in rainwater as it infiltrates soils} removes potash from orthoclase on granites.
From the plagioclase of white granites, calcium and sodium bicarbonates are derived in the same basic process. Potassium and magnesium carbonates are derived from biotite rocks (biotite, dacite).
FERTLIZERS IN ARID AREAS
Fertilizers, apart from humus and limited animal manures, should be used sparingly.
Excessive green growth can subject trees, in particular, to drought stress. Magg suggests a novel method of burying two plastic bags in the planting hole for valuable trees, provided with pinholes.
These contain 0.5 kg of phosphate and slow-release pelleted fertilizer. These last the life of the tree (Magg, D. H., The Potential for Horticulture in Central Australia, CSIRO Horticultural Science).
Gardeners generally have had good results from shredded bark, manure and leaf nutrients as mulch, with compost below this and some sulfur added if pH is high.
Others have developed pit composting with continuous mulch layers above.
An excellent system where salt in water is not a major problem is to combine a permanent compost pit with drip irrigation from a small sprinkler or a smaller humus pit with a jar for watering (Figure 11.69).
In dunes, phosphate, ammonia or guano and zinc give good results (for trees). Ammonium sulfate is also used .
Sewage waters have everywhere given good results with fuel wood trees (salt content must be checked for, as water gains in salts 300ppm on Its way through towns).
Sewage lagoon walls are excellent tree sites, as raised islands in the lagoons. Shallows grow lotus and water lily.
One feature, of such lagoons, is the large number of trees that are bird-carried; as it is of waterholes generally, (many seeds are defecated or regurgitated near water).
Most people do not add potash, as these salts and sodium, chlorine, carbonates, calcium, boron and magnesium may be plentiful in many arid soils, but the best advice is to get thorough analyses of both soils and leaf before and after growing.
THE EFFECTS OF POISONS IN DESERTS
All persistent biocides should be banned, from dryland, as at is the (missing) aquatic plants that break these down most effectively.
Herbicides used on lawns will seep slowly to sandy watercourses and down them for miles, travel up and down on groundwater and kill thousands of plants over time.
Water movement and soils as slow and deep sands do little to offset poisons. Thus, natural remedies are a high priority for deserts and all fragile ecosystems.
We should out-think, not poison out, any species that promise to trouble us, as most weeds provide biomass or mulch.
California is in very deep trouble with its ground water supplies after a 40-year chemical orgy and many Californians die of these poisons in their water.
The few nearby swamps and lagoons, are being wiped out, as are the trees near Alice Springs (Australia) where lawns are developed along with their poisons.
The position of settlement and the disposal of sewage, must be carefully assessed, in arid lands (including arid islands).
In Israel (New Scientist, 13 Oct., 1977) sewage and agricultural pollution rises through the sands in summer and nitrates can be included in bore water pumped for domestic use.
Winter rains again carry pollutants down and they must then be kept clear of the water pumped to reservoirs.
As many desert waters already have high levels of dissolved salts, any additional stress from nitrates may cause kidney malfunction.
Some 47% of Aboriginal outstation people, using bore water, do in fact; suffer kidney damage, consequent high blood pressure and excessive intercellular water (a form of dropsy).
It is easy enough to go wrong in desert and to put whole populations at risk; water in particular, must be subject to frequent analysis for pollution and salts and deep bore or well waters rigorously tested for excessive mineral and radioactive pollutants.