The Living Soil
Understanding Healthy Soil & Plant Nutrition
- The Living Earth
- Banked & Invested Nutrients
- Beneficial Micro-organisms
- Amazing Earthworms
- This Wonderful C0-operation
We could plunge strait away into covering all the practical methods of growing food sustainably, but I think it is vitally important to first get an insight into what constitutes a healthy living soil and how plants in natural conditions obtain their nutrients. Once we gain an understanding of what’s involved, we will then have a better idea of how to encourage and enhance these natural processes, which will automatically lead to healthy crops and increasingly nutrient rich food, as well as a continuing improvement in the fertility of our soil – and that’s definitely a legacy worth leaving!
Rich healthy soil has a beautiful healthy smell – to quote from Frank Delaney’s book ‘Tipperary’, when a son asked his father, who was a farmer, about the smell of the land:
“My father said that you can only get this smell if you understand land, if you understand all the little roots and stones and worms and other works that are part of any piece of ground that you open up under your feet. He told me to watch out for the way the clay, the earth, allows little creatures to travel in it as we travel our fields. And then he pointed out to me the greater wonder that lay ahead – that we planted things in this substance they grew and became large enough to eat and to keep us alive.”
1. THE LIVING EARTH
All plants from the smallest to the largest trees are boundary creatures that connect the Earth and Sky. Plants have their roots in the good earth and their branches and leaves stretch to the sky. Plants get nutrition from both above and below – from both the air and the soil.
Through the miracle of photosynthesis:
- Plants absorb Carbon Dioxide (CO2) from the air through their leaves
- Also water (H2O) from rain via the soil, which provides them with Hydrogen (H2).
- They also absorb light from the sun to provide energy to transform Carbon, Oxygen and Hydrogen to make Sugars and spare Oxygen, through the process of photosynthesis.
- The Sugars are used as direct fuel for the plant, sugar is also turned into Starch to be stored, and also transformed into cellulous, which becomes the building blocks to make their cells and give the plants their structure.
- These three elements, Carbon, Hydrogen and Oxygen comprise about 95% of the total dry matter of most plants, and only the remaining 5% is made up of 14 essential mineral elements that come from the soil itself.
6CO2 + 6 H2O + LIGHT = C6 H12 O6 + 6O2
Carbon Dioxide + Water Sugar + Oxygen
However, in order for plants to photosynthesise at all, a plant can only do so properly if it obtains the 14 essential mineral nutrients it needs from the soil itself. A thriving healthy living soil is therefore essential for the health of plants, so that they can absorb the nutrients from the soil that they need to photosynthesise effectively. It is our job as gardeners and farmers to make sure the soil, and the life in it, is healthy and active. To do this we need to understand the processes of plant nutrition.
The driving force of Nature was seen in the past as competition, but more recently the more we have understood the processes of plant nutrition and the role that soil micro-organisms play in providing soil nutrients to plants, the more we see processes of co-operation, and interdependence – nothing is independent, nothing is separate, all are dancing together in a 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.
Let’s look at the physical structure of soils. There are several basic constituents that make up soil, but in different proportions in different areas. The three main mineral constituents are:
Sand is made up of a loose granular substance, typically pale yellowish brown, resulting from the erosion of siliceous and other rocks.
Silt is a grainy mineral, with the grain size between sand and fine clay. It is created by broken down rock, through weathering, or the grinding action of glaciers and is usually dark in colour.
Clay is made of very fine particles of hydrous aluminium silicates from the chemical weathering of rocks, 4 thousandth of a millimetre in size!
A sandy soil will have:
- 0 – 10% clay
- 0 – 10% silt
- 80 – 100% sand
A loamy soil will have
- 10 – 30% clay
- 30 – 50% silt
- 25 – 50% sand
A clayey soil will have:
- 50 – 100% clay
- 0 – 45% silt
- 0 – 45% sand
To work out the proportions of these three constituents in your own soil, you can do the simple water test described in the section: ‘Planning & Starting Your Food Garden’ – Soil Types & Testing.
But, the soil is not just clay, silt and sand. Any healthy soil also has a reasonable amount of organic matter in it. Add to these basic minerals, organic matter and you have soil. Mother Earth is alive. The soil is alive! In a balanced living soil, plants grow in an active and vibrant environment, enjoying very ancient partnerships with soil micro-organisms. Sure the sand, clay, ground down rocks and minerals provide the soil’s physical structure, but it is the life in the soil that powers its cycles and provides its fertility.
A healthy soil includes 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 (micro-flora) and the larger plants growing in the soil, plus worms of many different types, from the microscopic to the larger earthworms; and arthropods and a whole range of other animals, 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 allows soil organisms to feed and reproduce, and is often described as the “life-force” 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.
Humus is the most important constituent of a healthy living soil. It is the product of dead plants and animals in different stages of decomposition. The strict definition of humus, in soil science, refers to any organic matter that has reached a point of stability, where it will break down no further and might, if conditions do not change, remain as it is for centuries, if not millennia. This type of humus significantly influences the texture of soil and contributes to its moisture and nutrient retention; it is uniform in appearance – a dark, amorphous, spongy, sticky substance
However, humus also refers to organic matter that is in the process of becoming stable humus, which is still in the process of being broken down by the action of micro-organisms. This humus, which is readily capable of further decomposition, is sometimes referred to as ‘active’. ‘Active’ humus is organic matter that has already broken down from its original rough-looking material, into a more amorphous dark coloured material, but still has more decomposition to come.
Humus is essential, because it is like a storehouse or larder of food and energy. Or as Sir Albert Howard (1873–1947) an English botanist and soil scientist, and a principal figure in the early organic movement, so eloquently puts it in his book ‘An Agricultural Testament’:
“Humus is not in a static, but rather in a dynamic, condition, since it is constantly 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, and during decomposition gives off a continuous stream of carbon dioxide and ammonia. Humus is characterized by a high capacity of base exchange, of combining with other soil constituents, of absorbing water, and of swelling……To this list of properties must be added the role of humus as a cement in creating and maintaining the compound soil particles so important in the maintenance of tilth”.
The Carbon content of humus is usually 55 to 58 per cent and the Nitrogen content between 3 to 6 per cent. This is one of the most interesting things about humus, that whatever proportion of Carbon and Nitrogen in the original plant or animal remains, when it has decomposed it will end up in a stable proportion of approximately 10 to 1, and no surprise, this is the ideal balance for soil and plant life. 5% of the total organic matter in a soil is present as Nitrogen in various compounds. That means that a soil with an ideal 5% humus content would have 5,604kg of Nitrogen reserves per hectare (5,000lb per acre), releasing 89-133 kg (79-119lb) of Nitrogen per year for plants!
To Summarize the Benefits of Soil Organic Matter and Humus
- The process that converts raw organic matter into humus feeds the soil population of micro-organisms and other creatures, thus maintaining high and healthy levels of soil life.
- Both, ‘active’ and ‘stable’ humus are further sources of nutrients to microbes. The ‘active’ form provides a readily available supply, and the ‘stable’ form acts as a longer-term storage reservoir.
- Decomposition of dead plant material causes complex organic compounds (lignin-like humus), to be slowly oxidized or to break down into simpler forms (sugars and amino sugars, aliphatic, and phenolic organic acids), which are further transformed into the bodies of microbes, or are reorganized, and further oxidized, into fulvic and humic acids, which bind to clay minerals and metal hydroxides. There has been a long debate about the ability of plants to uptake humic substances from their root systems and to metabolize them. There is now a consensus about how humus plays a hormonal role rather than simply a nutritional role in plant physiology.
- Humus is a colloidal substance, and increases the soil’s ability to hold onto and store nutrients. These nutrients are held in the soil safe from being leached (washed out) by rain or irrigation, until the plants need them.
- As rainwater enters the soil the humus swells up like a sponge, with 1 part of humus holding 4 parts of water. 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, and the plants’ capacity to withstand drought conditions. With 0.5% to 1% organic matter in typical conventional non-organic farming soils, 80,000-160,000 litres of water per hectare (8,553-17,105 gallons per acre) can be stored in the top 30cm (12in) of soil.
- On the other hand – on organic and biological farms where the organic matter content of the soil is 4-5%, 640,000-800,000 litres can be stored in the top 30cm (12in) of soil per hectare (68,420-85,526 gallons per acre)! That is between 5-8 times the amount of water stored on conventional farms, just by increasing the humus content of the soil! Or to put it in gardening terms, a 1 square metre (1 square yard) patch of topsoil, 15cm (6in) deep with a 4%-5% content of humus, will hold between 273-323 litres (60-70 gallons) of water, or the equivalent of 10-15cm (4-6in) of rain!
- The biochemical structure of humus also enables it to moderate – or buffer – excessive acid or alkaline soil conditions.
- During the breakdown of humus, microbes secrete sticky gum-like mucilages; these contribute to the crumb structure (tilth) of the soil by holding particles together, and allowing greater aeration of the soil. Toxic substances such as heavy metals, as well as excess nutrients, can be chelated (that is, bound to the complex organic molecules of humus) and therefore prevented from entering the wider ecosystem.
- The dark colour of humus in soils (usually black or dark brown) helps to warm up cold soils in the spring.
Held Soil Nutrients
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 example, clay particles, that are around four 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 (although some have a positive charge).
Opposites attract, and 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. On the other hand 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, like Iodine. However, this positive attraction process, and the capture of anions onto the few positively charged humus colloids is less effective than negatively charged colloids. As a result, anions, like Phosphorus, Sulphur and Chlorine quite often remain mobile in solution and are more easily washed out of the topsoil.
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, plants exude Carbon Dioxide (CO2) through their roots, which when combined with water, produces Carbonic Acid (H2CO3) and the Hydrogen atoms in the Carbonic Acid replace the nutrients attached to the humic or clay colloids which then releases the nutrients, making them available for plants. However, there is a much more important and more efficient way in which plants obtain nutrients, and this is due to the presence of beneficial soil micro-organisms in the soil, which release nutrients for the plants, in exchange for food that the plants provide them.
As we have already seen, plants get their energy and their building blocks (cellulous) from Carbon Dioxide in the air, plus sugars that provide energy, through photosynthesis. Apart from using the Carbon to produce cellulous and sugars for themselves, plants also produce sugars, starches and proteins, but they produce more than they need. Plants exude the excess through their roots as a mixture of substances known as mucigel which contain simple sugars, simple proteins, carbohydrates, 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 that plants excrete through their roots encourages billions of micro-organisms to lap them up.
There are around 600 million bacteria in a gram of fertile soil, but in the film of micro-organisms around the plant’s roots there are a staggering million x 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, which the plant can then feed on. So the plants feed the bacteria and the plants in turn are provided with food, which the bacteria have made available for them. Once we begin to understand this wonderful co-operative relationship, we can learn how to encourage these natural processes.
3. BANKED & INVESTED SOIL NUTRIENTS
Apart from nutrients being held on clay and humus colloids, there are other more long term mechanisms for holding onto nutrients and releasing them in a controlled way – step by step. Unlike Nitrogen and Carbon, many plant nutrients come from the clay and minerals 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 originally many 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 nutrients like the indefinite supply of Nitrogen that comes from the air. In a lot of soil types, such as clay-based soils, there is an almost inexhaustible supply of many nutrients including trace elements. From the plants point of view it is the lack of available nutrients that is the problem.
So what are the mechanisms for making chemicals like 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:
- Potassium in a free form immediately ready for plant use, after being released from the cation sites by the activity of micro-organisms.
- Potassium in an ‘exchangeable’ form, which is released as and when the readily available potassium becomes depleted. This is the beauty of the system: the release of potassium from its exchangeable form by the action of micro-organisms 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.
- Potassium in a ‘fixed’ form, which can only be released when the levels of ‘exchangeable’ Potassium are critically low.
- Potassium in the ‘mineral’ form, which is only transformed into the ‘fixed’ form very slowly by weathering and through the action of Carbonic Acid (as already described) 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 themselves 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 of Australia and 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 essential minerals over time.
4. BENEFICIAL MICRO-ORGANISMS
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 bread crumbs. 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. On the other hand, 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 would deprive aerobic bacteria of the air that they require
- A slightly acid soil pH (6.5 is ideal)
- And where there is plenty of food (carbohydrates and micronutrients from organic matter) available.
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. Even though Protozoa don’t fix Nitrogen, as they eat the Nitrogen rich bacteria, they release excess Nitrogen that can then be used by plants and other members of the food web.
Protozoa therefore play an important role in making Nitrogen available for plants and other soil organisms. Bacteria eaten by protozoa contain too much Nitrogen for the amount of Carbon protozoa need. They therefore 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. Believe it or not, when protozoa eat bacteria, they actually stimulate the growth of the bacterial populations. They also help to suppress disease by feeding on disease organisms. Protozoa are also an important food source for earthworms.
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 chapter 2: ‘How to Build Soil Fertility – Practical Methods’ in the section on ‘LIQUID MANURES’ – Lucerne Hay Tea.
Most of the algae found in soils live on, or near the surface. 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 soil’s nutrients. 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 white 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 the tougher 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. Most vegetables, grasses, flowers, shrubs, fruit trees and ornamentals benefit from associations with mycorrhizae fungi, with the exception of brassicas, the beet family and lupins.
Recent research with mycorrhizal plants in temperate forests has indicated that mycorrhizal fungi and plants have a relationship that may be more complex than simply mutually beneficial. Researchers argue that some mycorrhizae start to distribute nutrients with surrounding plants and with other mycorrhizae when there are changes in the environment. This relationship was noted when mycorrhizal fungi were unexpectedly found hoarding Nitrogen from plant roots in times of Nitrogen scarcity. So when there is a scarcity of Nitrogen, plants will switch from a mixture of obtaining nutrients from both their surroundings and mycorrhizae to relying almost exclusively on mycorrhizae for this nutrient.
Endo mycorrhizae 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. Later the plant roots absorb the mycelium into its own tissues.
Mycorrhizae 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. Mycorrhizae fungi increase nutrient uptake of plants, not only by increasing the surface absorbing area of the roots, but by also releasing powerful enzymes into the soil that dissolve hard-to-capture nutrients, such as organic Nitrogen, Phosphorus, Iron and other ‘tightly bound’ plant nutrients. This extraction process is particularly important in plant nutrition and explains why those plants (brassicas & beets) that do not make mycorrhizal associations, require higher 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 mycorrhizae fungi obtain sugars that they require from the plants roots to obtain energy and make their cellulous. In return they transport nutrients and water back to the plant. It has been discovered that the mycorrhizae threads are much more efficient at absorbing water and plant nutrients, than plant roots on their own.
And very interestingly, mycorrhizae, along with bacteria, form a barrier web of filaments around the root hairs, protecting the plant against disease causing fungi and bacteria.
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.
Soil animals such as centipedes, millipedes, spiders, mites, springtails, larvae of various sorts, wireworms, ants, snails, slugs and earthworms, 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. Most of the beneficial soil 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, in other words that most miraculous of materials, humus.
The Living 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. A vast living community, exchanging, converting, eating, killing, reproducing, absorbing, transforming, unlocking, dying, releasing, oxidizing, trading, co-operating, competing, and forming mutual alliances. The vast majority of life on the earth lives within the top few centimetres of the soil. This thin living crust supports all the life on earth; all the plants, forests, animals, birds and insects as well as us humans, and we interfere and destroy this life-support system at our peril.
5. AMAZING EARTHWORMS
One of Nature’s greatest helpers is the earthworm. They are invaluable for the production of topsoil, soil crumb structure and plant nutrition. For gardeners and horticulturists they are invaluable allies and at the same time a real test of the health of our soils – the more healthy the soil the more earthworms – the more earthworms the more healthy the soil.
There are two types of earthworms that are important. There is the manure, or tiger worm Eisenia foetida, that ingests fresh animal manures and fresh plant waste and then there are earthworms, which live in and eat soil, which are next in the food line.
Manure or tiger worms are at the start of the humus food chain and must not be overlooked for the valuable role they play. These are found in cowpats, compost and muck heaps and in the leaf litter of woodlands, in fact wherever there are high levels of reasonably fresh organic waste. These worms are the first in line when it comes to the process of breaking down organic waste into plant food.
They will appear miraculously into a cooling compost heap, to help apply the finishing touches to the composting process. They will crawl considerable distances to find cow pats and other fresh organic waste.
Tiger worms are the ones used in worm farms for producing worm compost and liquid worm juice fertiliser.
The most useful earthworms in intensive farming and horticulture are the Lumbricidae species, which are native to Europe, but have been spread around the world, where European farmers settled. They were generally brought by accident, among plants, or in the soil used as ships’ ballast. This was offloaded at the ports, and the worms gradually spread outwards. Some farmers, after seeing the benefits, imported and introduced earthworms to their land.
The Grey Earthworm (Aporrectodea caliginosa) is one of the most common earthworms, which lives in the topsoil.
The grey earthworm has a body length of 10-15cm (4-6ins). Their colour, in spite of its name, can range from brown to greenish. As they need moist soil, during dry summer periods, they tend to go dormant, staying 30cm (12ins) below ground in tunnels. The grey and other earthworms are most active during the wetter spring and autumn. They feed in depths from 20cm (8ins) up to the surface. Leaf litter and other decomposing organic material mixed in the soil, plus protozoa and bacteria provide most of their food. The grey earthworm is one of the most widely distributed earthworms. It thrives in pastures, gardens and forest as well. It can be found in every type of substrate even in the poorest sandy soil. It is adapted to live in a disturbed environment, which is one of the reasons it is so useful for our purposes.
One of the other most useful earthworm is Lumbricus terrestris (the common earthworm) typically reaching 20-25cm (8-10ins) in length, which tends to pull leaves and other plant material down into its burrow, helping to increase soil organic matter.
The earthworm is basically made of rings of muscles with a mouth at one end and an anus at the other. In between it has a digestive system with a few interesting additions, such as a gizzard containing grit for grinding up food like a bird, and a special gland that secretes lime. The earthworm as it burrows through the soil ingests soil, which contains many forms of organic matter including decaying plant parts, decomposing remains of animals, and living organisms such as nematodes, protozoa, rotifers, bacteria and fungi. One of their favourite foods is protozoa, and one of the ways we can encourage an increase in our earthworm populations is to encourage protozoa, as already described, by making aerated lucerne hay ‘tea’. By watering this ‘tea’ on your soil, the protozoa population will increase and so will the worm population.
A unique range of microbes are incubated within the earthworm’s digestive system and are excreted amongst the castings to introduce these organisms to the soil. That is why growers have achieved such good results from earthworm juice (water that has passed through the worm beds and accumulated these micro-organisms). In the process earthworms intimately mix the soil constituents, at the same time turning nutrients that were unavailable to plants into available minerals, and turning plant remains into more valuable humus.
And do they shift some soil! Various estimates suggest that a healthy worm population will swallow, process and excrete up to 300 tonnes of humus rich castings per hectare (122 tons per acre) every year where there is a healthy soil population, which is not surprising because the number of worms in a healthy pasture can be up to 1,250,000 per hectare (506,073 per acre), weighing up to 650-1100 kilograms in every hectare (580-981 pounds per acre). Investigations in the US show that fresh earthworm casts are 1.5 times richer in available Calcium than surrounding soil, 5 times richer in available Nitrogen, 7 times richer in available Phosphorus, 11 times richer in available Potassium and 3 times richer in available Magnesium than in the surrounding upper 15cm (6ins) of soil.
Some like the grey earthworm, tend to feed on organic matter below ground, depositing their casts on the surface, thus building up a fine topsoil and carrying nutrients that have washed out of the topsoil back to the surface.The fine topsoil builds up over many years and the larger stones find their way to the bottom of the topsoil just above the subsoil, thus helping to provide a drainage layer at the base of the topsoil. Other worms deposit their casts within the soil. Both types are essential in creating a stable crumb structure. One can say without fear of contradiction – as Darwin observed – “that every morsel of topsoil has passed through a worm’s digestive system numerous times and as a result have been one of the major creators of top soil over millions of years”.
Although true earthworms will not survive in acid soils (anything below pH5), they have the ability to maintain a neutral soil themselves by excreting calcium surplus to their requirements from a special calcium gland in their gut. Some of the calcium they ingest is in an unavailable form, but during digestion it is converted into an available form, therefore increasing soil pH, and helping to maintain the essential crumb structure of a healthy soil. This is because calcium irons have a positive electrical charge that attracts the negatively charged particles of clay, which coagulate around the calcium particle forming a crumb and good soil tilth.
Calcium is removed with every crop, and by volume, is the most important nutrient, therefore earthworms perform a great function in making this nutrient available for plants. They also burrow deep in the soil bringing up calcium and other minerals into the root zone. In other words the earthworm by its activities – whilst continuously helping to create and maintain the environment in which it likes to live and prosper – also benefits a wide range of other soil organisms and plants, many of which also provide food for the worms.
Not only that but the worm adds to this mix a gluey muco-protein which glues the soil particles together forming crumbs which are able to resist the eroding action of rain that either hits it directly, or as it flows through the soil. This ensures the crumbs retain their stability for a considerable period of time.
Once more we see a self-supporting co-operative process at work – a “give and you shall receive” deal in nature. The earthworm is seeking as much plant matter and beneficial micro-organisms as possible because that is what it eats. The bacteria it excretes into the soil sponsors the production of more biomass, which means more food for the earthworm. These bacteria are also a food source for protozoa, which in turn are the favourite food of earthworms. In this manner, the system becomes self-supporting, as is the case with most natural systems.
To sum up:
- The worm casts are much richer in available plant nutrients
- They have some easily available Calcium
- They are richer in more valuable humus
- They have a much higher bacteria content than the original ingested soil
We now begin to see why the earthworm is one of the most fantastic of all soil creatures, with its importance in the creation of soil structure, and in its ability to make food available for plants, but there is more!
To reinforce the walls of their burrows they line them with slime and organic material that attracts other organisms which serve as a food source for the worms. As they move through their burrows they vacuum this supplementary food en route. And don’t plant roots also enjoy the results! In the plant root’s search through the soil, the roots find an aerated tube with easy access and plenty of available food. The worms create perfect passageways to improve gas exchange and improve water infiltration, so that when it rains the water runs conveniently down the tunnel to water the plant’s roots – what more could a plant want?
Soil Crumb Size
Well it is even better than that! Dr. Stewart, a past professor at Aberystwyth Agricultural College in Wales in the UK, a renowned soil scientist and expert on earthworms, did research into the ideal soil crumb size. He considered that there were two main factors that would determine the size of the perfect soil crumb. The first was determined by the ideal air to water ratio in the soil. This in turn is determined by the surface area of the crumbs in relation to the air spaces between them, and the nature of water’s surface tension. After rain and the excess water has drained out of the sponge like soil, some water remains as a film on the surface of the crumbs. If the crumbs are large there is too much air and not enough water. If they are too small there is too much water to the amount of air, indeed if too small, there is no room for the air at all, as in heavy clay.
The other factor Dr. Stuart considered important was that if the soil crumb was too big, oxygen could not penetrate to the centre of the crumb and the middle would become stagnant and anaerobic, which is not ideal for plant roots and most beneficial micro-organisms. Having used these criteria Dr. Stuart calculated the perfect crumb size and to his amazement, found that the size of the crumbs created by worms were exactly the same as his ideal one!
6. THIS WONDERFUL CO-OPERATION
Just think about all this for a moment – the more one discovers about the wonderful co-operation between plants and soil life, the more we begin to see that plants are not separate from the soil, from other plants, from water, micro-organisms, air, Carbon Dioxide or indeed 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 these natural processes of soil life and plant nutrition and the subtle forces of Nature.