Permaculture Designers Manual
CHAPTER 8 – WATER IN PERMACULTURE
Section 8.6 –
Primary Nutrients for Plants in Permaculture
Soils are often analyzed as deficient in both PHOSPHATES and POTASH in heavily leached areas.
Phosphates are supplied either by guano (bird manures) from dry islands, or from older deposits found on sedimentary rock.
Potash occurs in the mineral kainite, formed in areas or evaporated waters. Desert salts usually contains 20-25% potash.
Phosphatic rock is restricted in distribution, and contains 8-15% phosphorus in various combinations with oxygen or water (hydrated). There are large reserves of potash in common minerals like orthoclase (a major constituent of granite).
NITROGEN can be supplied by water or land plants inoculated with rhizobia, or fixed by algae and water plants such as Scirpus or Azolla .
We can create the conditions for fixing nitrogen by growing these nitrogen-fixing plants, inoculated with the appropriate rhizobia.
Much higher levels of nitrogen than were previously thought to be available are fixed by land plants, in a series of zones extending from the roots. Even after nitrogenous plant’s such as Aracin, Albizzin, and Eleagnus are cut, the root, zone will continue to release nitrogen for up to 6 years, so that pioneer legumes or nitrogenous trees serve as cover crop for trees, and release nitrogen during their lifetimes and for some years after.
Legumes may not be needed in older forests, and typically die out under canopy. Only a few, larger leguminous trees (Samanea, Acacia melanoxylon) persist as forest trees in a mixed forest.
Both phosphate (concentrated by seed-eating birds) and potash (from burnt and rotted plants or compost) can be locally produced if birds are plentiful and their manures are used.
The phosphates mined from marine guano, however, may contain concentrated levels of cadmium and uranium, either or both of which (and other heavy metals) can be taken up by the oceanic fish and shellfish used by marine bird colonies.
Continual heavy use of such resources is likely to become polluting to soils.
Our only ethical strategy is to use just enough of these resources, and to conserve them locally.
SOURCES OF MINERALS IN SOILS
It makes sense to assume that as soils are leached, and so made mineral-poor, these minerals later become more concentrated in the sea, in marine organisms, or in inland saltpans.
Seaweeds, sea grasses, and fish residues have always been part of agricultural fertilizers, and have maintained their place even in modem times.
As seawater evaporates, first calcite and dolomite, then gypsum and anhydrite separate out; all are used for soil conditioning, pH adjustment, or to restore soil crumb structure.
Next, rock salts crystallize out, but only wet tropical uplands may actually lack this common nutrient, although even there specific plants (often aquatic) concentrate salt which can be gathered or leached from their ashes.
Lastly, potash, magnesium salts, and a host of minor elements remain; the evaporites (those already deposited) being the most soluble and therefore earliest deposited. The liquid that remains after the common salt content deposition is a rich source of minor minerals and trace elements.
It is, in fact, sold as “bitterns” (bitter oily fluids) for dilution and incorporation in crop soils, or in low concentrations (diluted 100-500:1) used directly as foliar sprays in strengths varying from 1-20 1/ha.
Very corrosive, bitterns (which include bromine and many of the early elements of the periodic table, plus some rare minerals), are safely held and distributed only via non-corrosive vessels and pipes (today, polyethylene pipes and drums).
Bitterns are cheap and easily transported to leached areas, but their effects must be established by local trials on specific crop. As these evaporites are so easily dissolved, they are also those most likely to be carried to sea in rains.
Rocks and rock dusts
Granites contain felspars yielding potash or sodium salts. Limestone and dolomite yield calcium and magnesium, and mineral deposits or their ores give traces of the basic minor elements.
Of these, calcium (in all but highly calcareous areas) is most needed, dolomite (except where magnesium is already in high ratio) is next; phosphates and felspars follow, along with trace elements in small quantities (as low as 5-7 kg/100 ha for zinc, copper, cobalt, and molybdenum).
Field trials have established that cheap ores, finely ground, are as effective as more refined sulphates or oxides (Leeper 1982). Sometimes such minerals are given to animals as salt licks, in molasses, in water, or as injections or “bullets” of slow-release elements (cobalt) in a pellet which lodges in the rumen.
Some mineral elements also reach plants via urine, but foliar sprays are more rapid-acting and effective.
Fine rock dusts of a specific rock suited to local needs are often cheaply available from quarries or gravel pits.
Basalt dusts are helpful, for example, on leached tropical soils. Rock phosphate contains 8-15% phosphorus, but is very slow to release nutrient, and may in fact be absorbed completely on to leached days and clay-loams.
Super-acid phosphate added to compost, or to plants used in compost, may be necessary under such conditions.
Rock dust as an unselective category can do as much harm as good on soils, adding excessive or poisonous nutrients in some cases, or excessive micronutrients.