UNDERSTANDING PLANT NUTRITION
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- Soil Micro-Organisms
- Banked & Invested Nutrients
- Miraculous Co-Operation
All plants from the smallest to the largest trees are boundary creatures that connect the Earth and the Sky – Papatuanuku and Ranginui. Plants have their roots in the good earth and their branches and leaves stretched to the sky. Plants get nutrition from both above and below. Through the miracle of photosynthesis they receive carbon dioxide from the air plus light from the sun to provide energy to form of sugars and starches and their building blocks in the form of cellulous to make their cells and give them structure, as well as receiving essential life giving water from the heavens. From the soil they obtain a huge range of nutrients, including Nitrogen, which they combine with Carbon and Oxygen from Carbon Dioxide and Hydrogen from water, to make their proteins. Plants also provide us and all the animals on earth, with life giving Oxygen.
Before all else, we need to understand plant nutrition. Because Darwin’s and other related theories were born in the era and culture of Victorian England, where competition was seen as the driving force of societies, economics and politics, it was inevitable that theories of evolution were understood to be driven by the same forces. However, the more we understand the processes of plant nutrition and the intricate relationships of plants and soil micro-organisms, the more we see processes of co-operation, of interdependence – nothing is independent, nothing is separate, all are dancing together in mutual relationship. Once we begin to understand this, the miracle of soil and plant life begins to reveal itself to us. Having embarked on this journey of understanding, we will discover even wider relationships between plants and all other life, the air, water, our planet Earth, the Sun, Moon and beyond, but more of this later; let’s start with the plant and its roots at home in the soil.
Mother Earth is alive. The soil is alive! In a balanced soil, plants grow in an active and vibrant environment. Sure the sand, clay, ground down rocks and minerals provide the soils physical structure, but it is the life in the earth that powers its cycles and provides its fertility. The soil is a vibrant living ecosystem comprised of a huge range of micro-organisms, everything from different types of bacteria, actinomycetes, protozoa, fungi and algae, microscopic plants (microflora) and the larger plants growing in the soil, plus worms of many different types, from the microscopic to the larger earthworms; also arthropods and a whole range of animals, again from the smallest to the largest.
Most of these creatures live on a variety of organic residues made up of decaying plant and animal material from all the organisms that have lived and died within and on top of the soil; this broken down organic matter eventually becomes that most miraculous of materials, humus.
- Humus is not static, but dynamic, since it is constantly being formed from plant and animal residues and is continuously decomposed further by micro-organisms.
- Humus serves as a source of energy for the development of various groups of micro-organisms.
- Humus has the capacity to hold onto plant nutrients until they are needed by the plants.
- It also combines with other soil constituents.
- It swells as it absorbs water and can hold the equivalent of 80–90% of its weight in moisture, releasing it gently to the plants. This greatly increases the soil’s capacity to withstand drought conditions.
- The biochemical structure of humus enables it to moderate – or buffer – excessive acid or alkaline soil conditions.
- The dark colour of humus (usually black or dark brown) helps to warm up cold soils in the spring.
- Humus also acts as cement. During the humification process, microbes secrete sticky gum-like mucilages; and these help to form a good soil crumb structure (tilth) by holding particles together and allowing greater aeration of the soil.
Add to this mix, life-giving air and water and you have a wonderfully complex system that is in a state of continuous and dynamic interaction and change.
Here’s a good question, why is it that plant nutrients have not been washed out of the soils of the Earth into the oceans millions of years ago? Answer, because of static electricity. For instance, clay particles which are around one to two thousandths of a millimetre in diameter have a negative electrical charge and the finest particles of organic humus – humic colloids – which are even smaller, also have negative charge and some have a positive charge. As a result positively charged plant nutrients (cations pronounced cat-eye-ons) are attracted and held by the negatively charged colloids of clay and humus. These include Ammonia, Calcium, Magnesium, Potassium and Sodium along with some of the essential trace elements, and the positively charged colloids of humus attract and hold on to negatively charged nutrients (anions pronounced an-eye-ons) like Phosphorus, Sulphur and Chlorine and many other trace elements.
Now, the problem is, how do the plants manage to get hold of all these important nutrients if they are being held fast by clay and humus particles? There are two mechanisms involved. First the plants exude Carbon Dioxide through their roots which when combined with water produces carbonic acid and the Hydrogen atoms in the carbonic acid replace the nutrients attached to the humic or clay colloids and the nutrients are then available for the plant, but more importantly, soil micro-organisms play an even greater part in this process – but they don’t do it for free! As we have already seen, plants get their energy and their building blocks (cellulous) from carbon dioxide in the air, plus energy, in the form of light from the sun in the process known as photosynthesis. Apart from using the Carbon to produce cellulous to build their cells and structure, plants also produce sugars and starches, but they produce more than they need. Plant roots exude a mixture of substances known as mucigel which contain simple sugars, protein, carbohydrate, growth factors, organic acids and enzymes; in fact an ideal high energy food for the bacteria living in the rhizosphere (the area immediately around the root hairs). The excess they excrete through their roots encourages billions of micro-organisms to lap them up. There are around 600 million bacteria per gram of fertile soil, but in the film of micro-organisms around the plant’s roots there are a staggering million million per gram because of the highly nutritious food that the plant exudes. The bacteria in exchange release plant nutrients from the humus and clay particles that the plant can then feed on. So the bacteria get fed by the plants and the plants in turn are provided with food which the bacteria have made available for them.
A soil rich in humus is home to an almost unbelievable population of bacteria of many different types inhabiting many different niches and varying habitats even within a small area of soil. Soil populations vary hugely in soils, due to different soil types and soil temperature, even over a twenty four hour period. However, as already stated, a good average figure for a normal healthy soil is about 600 million bacteria per gram of soil! A healthy soil consists of soil crumbs, made up of smaller particles that have collected together similar to a coarse pastry mix before adding the water. This makes for a very varied environment – from films and pockets of water, to air spaces comparatively rich in oxygen, to areas where there is little or no oxygen at the centre of larger soil crumbs. There are enough types of bacteria to utilize every type of condition, some specializing in one type of environment, others able to adapt their metabolism from surviving in oxygen rich environments to those where oxygen is scarce. The population of some types of bacteria remain fairly constant, whilst others grow rapidly in numbers to suit the changing circumstances only to die back and hibernate in the form of spores waiting for the next time the conditions are ideal when they can come to life again. The bacteria have many different roles. Some specialist bacteria busy themselves by breaking down organic proteins into nitrites, others converting nitrites into ammonium, while again others convert ammonium into nitrates, thus helping to release and make available vital nitrogen for the growth of plants in this fascinating chain of events.
Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of flagella. The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed aerobic bacteria), whilst those that do not require air are referred to as anaerobic, and tend to cause putrefaction of dead organic matter. Aerobic bacteria are most active in a soil that is:
- moist (but not saturated, as this will deprive aerobic bacteria of the air that they require),
- a neutral soil pH (6.4 is ideal)
- And where there is plenty of food (carbohydrates and micronutrients from organic matter) available.
From the biological or organic gardener’s point of view, the important roles that bacteria play are:
Nitrification is a vital part of the nitrogen cycle induced by certain bacteria which are able to transform Nitrogen in the form of Ammonia NH4 (produced by the decomposition of proteins) into nitrates, which are then available to growing plants to form their own proteins.
In another part of the cycle, the process of nitrogen fixation constantly puts additional nitrogen into biological circulation. This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as Azotobacter, or by those that live in close symbiosis with leguminous plants, such as rhizobia, which form colonies in nodules they induce on the roots of peas, beans, and related species. These are able to convert nitrogen from the atmosphere into nitrogen-containing organic substances.
While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. This is done by denitrifying bacteria.
Actinobacteria are critical in the decomposition of organic matter and in humus formation, and their presence is responsible for the sweet “earthy” aroma associated with a good healthy soil. They require plenty of air and a pH between 6.0 and 7.5, but are more tolerant of dry conditions than most other bacteria and fungi.
As the soil warms up in the spring and the days lengthen, plants become more active, requiring more food. This increase in activity produces more root excretions from the plants, and along with the increase in soil temperature the bacteria become more active – which provides more food for the plants just when they need it. And there is more: the dense film of both the million million bacteria per gram, in combination with kilometres of fungal threads (hyphae), form a shield around the roots of the plant, protecting the plant roots against disease-causing organisms that try and attack them. And now the story gets even more fascinating, in the last few years it has been discovered that plants communicate chemically with the micro-organisms to tell them what nutrients they need at any particular time and the micro-organisms respond, in exchange of course for that tasty package of goodies that the plants exude.
Then there are protozoa. Protozoa are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. Protozoa play an important role in nutrient cycling by feeding intensively on bacteria. As they eat bacteria, protozoa release excess nitrogen that can then be used by plants and other members of the food web.
Protozoa therefore play an important role in mineralizing nutrients, making them available for use by plants and other soil organisms. Bacteria eaten by protozoa contain too much nitrogen for the amount of carbon protozoa need. They release the excess nitrogen in the form of ammonium (NH4+). This usually occurs near the root system of a plant where bacteria are most numerous. Bacteria and other organisms rapidly take up most of the ammonium, but the plants still get what they need.
Another role that protozoa play is in regulating bacteria populations. When they graze on bacteria, protozoa actually stimulate the growth of the bacterial populations (and, in turn, decomposition rates and soil aggregation.) Exactly why this happens is under some debate, but grazing can be thought of like pruning a tree – a small amount enhances growth, too much reduces growth or will modify the mix of species in the bacterial community. Protozoa are also an important food source for other soil organisms, especially earthworms, and help to suppress disease by competing with or feeding on pathogens.
Protozoa can be stimulated and increased in the soil by making an aerated tea using spray-free, or organically grown, lucerne hay in rain water – see the section on ‘LIQUID MANURES’ – ‘Compost Tea’.
Most of the algae found in soils live on, or near the surface. Like the higher plants, blue-green algae contain chlorophyll and are therefore able to photosynthesize. The blue-green algae can also fix atmospheric nitrogen, thereby continuously adding to the total nutrient status of the soil. They also have a binding effect on any exposed soil surface protecting it to some extent from erosion and also, presumably, from nutrient loss.
And what about beneficial fungi? Fungi spread underground by sending long thin threads known as mycelium throughout the soil; these threads can be observed throughout many soils and compost heaps. In terms of soil and humus creation, the most important fungi tend to be saprotrophic, that is, they live on dead or decaying organic matter, thus breaking it down and converting it into forms that are available to the higher plants. A succession of fungi species will colonise the dead matter, beginning with those that use sugars and starches, which are succeeded by those that are able to break down cellulose and lignins.
There is one type of fungus that is both essential for the majority of plant families and at the same time an indication of a healthy soil – Mycorrhizae (from myco meaning fungal and rhiza meaning root). Mycorrhizae are fungi that are able to live symbiotically with living plants, creating a relationship that is beneficial to both, similar to a plants relationship with micro-organisms, only in their own unique way.
The earliest fossil records of the roots of early land plants contain evidence of the fossil remains of mycorrhizal fungi over 400 million years ago. It is estimated that 95 per cent of all plant types make associations with mycorrhizal fungi – that is they receive energy in the form of simple carbohydrates from the roots of plants and in exchange provide nutrients and water to the plant and interestingly are able to access otherwise unavailable Phosphorus, making it available to the plant.
Having said all this, it is important to remind oneself here that plants obtain their nutrients not only by the activities of mycorrhizae, but many other soil organisms such as bacteria, protozoa, worms, algae, etc. All these play a vital part in the soil nutrient cycle and nutrient availability. However mycorrhizae are often overlooked in conventional agriculture and horticulture, simply because they are gravely diminished by modern practices and are not seen in any great numbers on farms that use water soluble chemical fertilisers and pesticides. On the other hand, where they are actively encouraged:
- they have an essential role in building soil carbon and humus
- improving soil structure
- increasing water availability
- increasing nutrient availability
- improving plant health
- increasing plant nutrient density
I attended a very interesting meeting of the local Nelson Permaculture Group in December 2013, because there was a local guest speaker who is an expert on Mycorrhizae Fungi and their association with plants. Don Graves is well known as an expert and promoter of encouraging Mycorrhizae in soil for many reasons here in New Zealand. I have written about the importance of mycorrhizae fungi in plant nutrition and healthy soils, but Don Graves provided a lot more information about mycorrhizae than I was aware of. As a result I will pass on what I have learnt. One of the great pleasures in life is that you never stop learning!
The most common Mycorrhizae, Endo-Mycorrhizal fungi fungal threads actually invade the root hairs of the plant and spread out their filaments, sometimes for many metres into the soil, effectively extending the plants root system. The plant roots often absorb the mycelium into its own tissues.
Mycorrhizae fungi , along with beneficial bacteria, form a protective barrier web of filaments around the root hairs, protecting the plant against destructive disease causing fungi and bacteria. Obviously the beneficial bacteria and mycorrhizae have a vested interest in maintaining their partners in good health.
Mycorrhizal fungi also connect most of the plants in an area together with their vast underground web of mycelia threads. In a forest, all the trees roots will be interconnected by Ecto-Mycorrhiza threads. The fungal mycelium forms an extensive network within the soil and leaf litter and nutrients have been shown to move between different plants through the fungal network.
Mycorrhizal fungi increase the surface absorbing area of roots 100 to a 1,000 times, thereby greatly improving the ability of the plant to access soil resources. Several miles of fungal filaments can be present in less than a thimbleful of soil. Mycorrhizal fungi increase nutrient uptake not only by increasing the surface absorbing area of the roots, but they also release powerful enzymes into the soil that dissolve hard-to-capture nutrients, such as organic nitrogen, phosphorus, iron and other “tightly bound” soil nutrients. This extraction process is particularly important in plant nutrition and explains why non-mycorrhizal plants require high levels of fertility to maintain their health. Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, conserving the nutrient capital in soils.
The mycorrhiza obtains carbohydrates (sugars) that it requires from the plants roots, in return it transports nutrients, including nitrogen, phosphorus and water back to the plant. It has been discovered that the mycorrhizae threads are much more efficient at absorbing water and plants nutrients than plant roots on their own.
There is increasing evidence that plants living with a healthy mycorrhizal population are richer in minerals, such as phosphate, and are healthier and more able to resist disease. Probably more important than the role of micro-organisms in phosphate availability, is the part that mycorrhizae fungi play in phosphate uptake by plants. The mycorrhizae are very efficient at converting phosphate into a soluble form and making it available to the plants. Ironically, super-phosphate fertiliser kills micorrhizae fungi, the very organism that in a healthy soil will make phosphorus available to plants. In comparison to plants without mycorrhizae root associations, plants with mycorrhizae have shown to have three to ten times more nutrition in their leaves, fruit and grain.
Plants are able to access nutrients by excreting Carbon dioxide from their roots, which turns into Carbonic acid when mixed with water. The acid releases nutrients for the plants, but this process is limited and the beneficial relationships that plants have with beneficial soil bacteria and mycorrhizal fungi are much more important and productive. The plant excretes simple sugars, such as glucose and sucrose, and simple proteins from its roots that feed the bacteria and simple sugars that feed the mycorrhizal fungi. In exchange the bacteria and mycorrhizae release and make available nutrients for the plants. Depending on the environment in which they are growing, plants may divert up to 80% or more of the net energy fixed by photosynthesis to below-ground processes. Some of this energy goes into root growth, but a high proportion may be used to feed mycorrhizal fungi and other soil organisms.
90 per cent of plants form an association with Endo-mycorrhizal fungi. In healthy soil conditions, most edible plants make associations with Endo-mycorrhizal fungi. The hyphae (fine threads) of these mycorrhizae fungi actually penetrate the root hairs of the plant, receiving sugars and providing nutrients and water directly into the plant roots.
Here is a list of edible plants that associate with Endo-mycorrhizal fungi:
Alfalfa, Almond, Apple, Apricot, Artichoke, Asparagus, Avocado Banana, Barley, Basil, Beans, Blackberry, BlackLocust, Cocoa, Carrot, Cassava, Celery, Cherry, Citrus, Clover, Coconut, Coffee, Corn, Cucu-mber, Currant, Eggplant, Fig, Flax, Flowers most, Garlic, Grapes, Grasses perennials, Herbs, Kiwi fruit, Leek, Lettuce, Locust, Lychee, Mango, Millet, Mulberry, Okra, Olive, Onion, Palms, Passion Fruit, Papaya, PawPaw, Peas, Peach, Peanut, Pear, Peppers, Pistachio, Persimmon, Plum, Potato, Pumpkin, Raspberry, Rice, Shallot, Sorghum, Soybean, Squash, Star Fruit, Strawberry, Sugar Cane, Sunflower, Sweet Potato, Tea, Tomato, Wheat, Yam, Yucca, Walnut.
Ecto-mycorrhizal fungi, on the other hand, do not usually penetrate the roots, but consist of a sheath, or mantle, covering the root tip and also a net of hyphae (fine fungal threads) surrounding the plant cells within the root cortex itself. 5 per cent of plants form an association with Ecto-mycorrhizal fungi. Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter. Usually trees and shrubs such as nut trees and fir trees, such as Sweet Chestnut, Filbert, Hazelnut and Pecan nuts make associations with Ecto-mycorrhizal fungi, as well as oaks and fir trees. Ecto-mycorrhizal fungi absorb sugars from the plants and deliver nutrients to them in the way that Endo-mycorrhizal fungi do.
Mycorrhizal fungi along with beneficial bacteria also form a protective web around the roots and root hairs, protecting the roots from destructive fungi and other micro-organisms. Obviously the beneficial bacteria and mycorrhizae have a vested interest in maintaining their partner in good health. Mycorrhizal fungi also connect most of the plants in an area together with their vast underground web of mycelia threads. In a forest, all the trees roots will be interconnected by mycorrhiza threads. The fungal mycelium forms an extensive network within the soil and leaf litter and nutrients have been shown to move between different plants through the fungal network.
High rates of fertilizers, especially phosphorus, inhibit the formation of mycorrhizae; organic forms of fertilizers seems to have less inhibitory effect on mycorrhizae than inorganic, soluble fertilizers. It is ironic that in trying to supply plants with Phosphorus, one is interfering with the very mechanism that will supply plants with the Phosphorus they can obtain naturally!
Because plants in a healthy soil are interconnected via the mycorrhizal hyphae (threads), older seedlings and more mature plants help to nurse newly germinated seedlings which have not yet started to photosynthesise by supplying nutrients via the mycorrhizal web.
I have been aware that on hill-land pasture farms, where it is impossible to make enough compost to increase soil carbon and humus levels, amazingly soil carbon and humus does increase considerably when the application of chemical fertilisers and fungicides are ended and organic and/or biological methods are adopted; but I could not understand the mechanism(s) that would bring this about. Water soluble Nitrogen applications actually depletes soil carbon, because soil micro-organisms feed on Nitrogen and excess soluble supplies of Nitrogen send them into a frenzy of activity. That activity is focussed on breaking down organic matter (carbon rich humus). Regular dousings of water-soluble nitrogen fertiliser (and yes, that also includes concentrated chicken litter and blood meal) destroys soil carbon/humus. So, stopping nitrogen applications will stop the destruction of soil carbon, but how does it get built up again on upland pasture farms?
Mycorrhizae fungi have a vital part to play in building up soil carbon levels and helping to increase levels of soil humus – how? Plants photosynthesise sugars and excrete simple sugars out through their roots to feed the mycorrhizae which use the sugars (largely Carbon and Hydrogen) to build their bodies and also produce a gluco-protein called Glomalin, also rich in Carbon – and this is how the Carbon content of soils can be greatly increased when mycorrhizal fungi are encouraged. A study done by Mike Amaranthus Ph.D, Dave Perry Ph.D Jeff Anderson, & Zack Amaranthus called ‘Building Soil Organic Matter Biologically A powerful sink for the greenhouse gas CO2’ (see: http://www.mycorrhizae.com) showed that fungi nearly doubled soil carbon percentage in just one year when tall fescue grasses were inoculated with mycorrhizal spores, while no increase in soil carbon was seen where the fescue grass were not inoculated. They said that “Glomalin has been investigated for its carbon and nitrogen storing properties, including as a potential method of carbon sequestration”.
Glomalin is also a protein ‘glue’ that glues soil constituents into aggregates, improving soil structure, aeration and the water holding capacity of soils. This is a very important attribute of mycorrhizae fungi along with the muco-proteins that beneficial soil bacteria and earthworms produce. Earthworm’s casts which are bound by muco-proteins are the perfect size for optimum aeration and water holding capacity in the correct proportions.
Plant Families that Do not Make Mycorrhizae Associations
5 per cent of plant families do not make associations with mycorrhizae fungi and these include Brassicas and the Beet family. This means if you grow brassicas and beets and spinach in one bed as part of a six course rotation, as I do, there are no plants to feed the endo-mycorrhizae fungi and keep them alive; so it is necessary to allow a few weeds to grow and/or sow some red clover seed around the Brassicas and Beets, or companion plant with lettuce. This will assure the survival of the fungi till the next year’s crop which does make associations.
How to encourage healthy mycorrhizal growth:
- Stop using water soluble chemical fertilisers and pesticides, especially fungicides.
- Mycorrhizae prefer a pH range of 5.5-7.5 – so aiming at 6.4, which is ideal for most food crops, will also be beneficial for the fungi.
- Reduce soil cultivation to a minimum, because the fine mycelial threads (hyphae) are easily broken. Hoeing and other cultivations near the surface are OK, and when you are planting out seedlings take out only as much soil as is needed to plant them. A bulb planter is a useful tool to use for transplanting.
- Thick mulching over a wide area will reduce the feed plants and the mycorrhizal fungi. Living mulches of red clover, deep rooting herbal orchard mixes and grasses under fruit and nut trees will keep the fungi and trees healthy.
- Sow red clover in beds where brassicas and beets are grown exclusively. Clear away a small patch of clover when planting out the brassicas to avoid competition from the clover. I have had good results using this technique.
- When planting out seedlings, or bigger plants, add the roots of non-invasive weeds from the same plot or area to the planting hole, as the roots should have mycorrhizae fungi and spores in them.
- If the soil has been greatly disturbed by building activities, conventional farming using chemical fertilisers and pesticides, regular deep ploughing or digging etc – then using a mixture of both endo and ecto-mycorrhizal spores should be very helpful.
- If there are sites in the local area which have not been conventionally farmed (preferably mixed pasture), then taking soil samples from these areas and applying them to the affected site should re-colonise the local strains of mycorrhizae.
You can also buy spores from reputable companies. I use Environmental Fertiliser’s Bio-Vam Bio-Vam is a dry blend of endo- and ecto-mycorrhizal fungi, with the addition of other beneficial soil microbes that is a constituent in a composted balanced fertiliser that I use.
As part of the process of building soil carbon levels, creating vibrant, healthy, living soil and nutrient dense healthy food – encouraging a thriving population of soil mycorrhizal fungi, is an essential part of the process, along with all the other organic and biological methods that encourage the whole range of essential soil micro-organisms.
These are not micro-organisms, but I have included them as part of the life in the soil. Soil animals include centipedes, millipedes, spiders, mites, springtails, larvae of various sorts, wireworms, ants, snails, slugs and earthworms, etc; all have their part to play. Some eat plants and their remains, others like centipedes, spiders and beetles eat the plant eaters, keeping them in check and most of the larger ones help to aerate the soil with their burrowing.
BANKED & INVESTED NUTRIENTS
Unlike Nitrogen and Carbon, many plant nutrients come from the earth; it is there to a greater or lesser degree in the soil, clay and original mineral from the underlying rock, such as Phosphorus and Potassium. Of course these minerals are returned time and time again in the form of decaying plant and animal material but this only came from the rocks in the first place. In a natural system it can’t be added to, so the mechanisms for stopping its loss and the control of its release have to be much tighter than those for Nitrogen. In a lot of soil types, such as clay based soils, there is an almost inexhaustible supply. From the plants point of view it is the lack of available nutrients that is the problem. So what are the mechanisms for making Potassium and Phosphorus available to plants? It is a four step process as one form of less available minerals changes into the next more accessible form. As an example let us look at the four forms of Potassium:
1. Potassium in soluble form immediately ready for plant use.
2. Potassium in an ‘exchangeable’ form which is released as and when the potassium in solution becomes depleted. This is the beauty of the system: the release of potassium from its exchangeable form is triggered by the reduction of available potassium which is triggered by the plants roots. In other words, it is released when the plants need it. This means there is the minimum amount of potassium at any one time that could be washed away by the soil water. There are different levels of availability, as in soil nitrogen, but the mechanisms of potassium release are a much more tightly controlled system.
3. Potassium in the ‘fixed’ form which can only be released when the levels of ‘exchangeable’ potassium are critically low.
4. Potassium in the ‘mineral’ form which is only transformed into the ‘fixed’ form very slowly by weathering and soil microbial activity.
This then is such a tightly controlled mechanism, that often when soil tests are done on organic soils, or in natural conditions, there is little or no water soluble potassium detected, although the plants show no signs of potassium deficiency because they are receiving potassium in solution only as and when they need it.
The plants trigger the release of just enough potassium ions from the ‘exchangeable’ form when they need it, by encouraging bacterial activity in their root environment as described above. This in turn depletes the ‘exchangeable’ stocks, causing the release of ‘fixed’ potassium, which is eventually re-stocked from the ‘mineral’ form. Thus after millions of years we still have an almost inexhaustible supply, trebly protected against loss.
However, there are areas of the earth where Potassium supplies have run low due to geological stability over billions of years, as in parts ofAustraliaand the Amazon basin, leading to supplies slowly being leached away by the action of rain over the millennia. For most of the earth’s surface however, where volcanic activity has been more recent, or where old sedimentary rocks have been forced to the surface by geological folding, nutrients such as potassium are renewed on a comparatively regular basis. In other words volcanic activity and plate tectonics renew the ‘mineral’ form over time.
Just think about all this for a moment – the more one discovers about this miraculous co-operation between plants and soil life, the more we begin to see that plants are not separate from the soil and the micro-organisms, the air, carbon dioxide and the energy from the Sun. What we see is a single process, a synergy of all the parts and influences.
For someone who wants to grow plants, this is vital knowledge. The role of the gardener or horticulturist becomes one of increasing humility and one who wants to encourage and nurture the natural processes of soil life and plant nutrition and the subtle forces of Nature. Each section of this work starts with some revelation and then the practical application of the knowledge gained.
Further Reading: ‘Teaming with Microbes’ by Jeff Lowenfels & Wayne Lewis - ISBN 13: 9781604691139 or ISBN 10: 1604691131