The great mantra of Organic gardening has been ‘Feed the Soil, and the Soil will feed the Plants’. By feeding the soil they mean adding organic matter, such as compost. However, plants cannot eat organic matter, and they rely in microbes to convert it into a form that they can use. Understanding what these microbes are, how they work, and what they need, is essential to maximise the health of your plants.
In order for a plant to obtain nutrients, the nutrients need to be soluble, and in the immediate are of the roots, the Rhizosphere. This is a conundrum, because if the nutrients are soluble, they can be washed away easily, ending up in groundwater, or water courses. In a healthy soil, it is the soil microbes that act as sponges, using up nutrients to grow, and acting as a reserve. They are like little pockets of plant food. As plants cannot eat microbes either there has to be some sort of recycling of the nutrients, which makes small quantities of this ‘locked up’ food available to plants. Before I explain how this recycling takes place, it’s important to understand a bit about the microbes themselves.
Bacteria
Bacteria are second only to Fungi in the decomposition of organic matter. They reproduce mainly by division, and in ideal conditions, a single bacteria could become 5 billion bacteria within 12 hours. In order to achieve this sort of rate, they need moisture, and a constant supply of food. In the soil, and in compost, their numbers are limited by predators, chiefly protozoa. The bacteria break down organic matter, taking in Nitrogen, carbon, and other nutrients. They are better at the breakdown of the fresher, ‘green’ elements of a compost heap, or in the soil, than the woody bits. There are species that break down cellulose (Actinomycetes and Cellulomonas).
Bacteria are the principle drivers of the Nitrogen cycle.
A soil fed with compost from grass and straw will favour bacteria over fungi. In the soil, plants that prefer a bacterial dominate soil are annual weeds, and brassicas.
Fungi
Fungi are the principle agents for the breakdown of organic matter. They are particularly successful at utilising woody materials, producing enzymes that can dissolve lignin, and a hard tip to the hypha, which can penetrate hard surfaces. Fungal growth is comparatively rapid. Once a source of food has been located, and ingested, the nutrients are transported along the hypha, and can be used by the organism, elsewhere. As the hyphae are thinner than plant roots, they can penetrate smaller gaps between soil particles, and reach further for nutrients, particularly phosphorus. They also have the ability to chemically unlock the phosphorous from the compounds binding it, and move it around.
Fungi can form associations with the roots of plants for mutual benefit. On the whole, these mycorrhizal fungi associate with trees, shrubs, and other perennial plants. 10-15 disturbances, ploughing/digging, is enough to destroy fungi in the soil, as does soil compaction, and many chemicals, including fertiliser.
Nematodes
Most gardeners know about root feeding nematodes, which damage plants. Some will have heard about nematodes used to kill slugs, but there are also Nematodes that feed on bacteria, fungi, and other nematodes. These are essential to keep a healthy balance in the soil. The nematodes that feed on bacteria and fungi, release nutrients that can be used by plants, think nematode poop. The nematodes that feed on other nematodes, ensure that not all of the bacteria and fungi are eaten. If there are not enough of the other types of nematode, the cannibals cannot survive, and therefore cannot keep the root feeders in check. As nematodes are larger than many of the other microbes, if the soil is compacted, they cannot get to their food source, and will move away, stopping the recycling of nutrients.
Protozoa
Protozoa are massive consumers of bacteria, averaging 10,000 per day. This releases masses of nutrients, which can be used by plants, or by other bacteria, or fungi, to grow. Due to their relative sizes, many bacteria cannot be reached by the protozoa, which are too big to get into the spaces in which bacteria can live. This helps to prevent over grazing. If the bacterial population reduces, the larger protozoa will start to eat the smaller ones, reducing the pressure on the bacteria. Protozoa are important sources of food for worms, and other soil organisms.
Plant Nutrition using Microbes
Once a seed germinates, it sends roots down into the soil. These roots come into contact with soil microbes. As the plant develops leaves, and starts to photosynthesize, it uses the energy from sunlight to change CO2 from the atmosphere, into carbohydrates for energy. Up to 25% of this carbohydrate is secreted by the plant, mainly through the roots. At first this seems like a waste of valuable nutrients, but that’s not true. These sugars, in the Rhizosphere, feed bacteria, and fungi, which use the free energy to multiply, or grow, absorbing Nitrogen, and other nutrients. This doesn’t benefit the plant, but once nematodes and protozoa start to eat the smaller microbes, nutrients are released, which the plant can then use. All of this occurs right next to the roots. In effect, the plant is farming microbes, and benefiting from the process. Where beneficial relationships are formed with fungi, the fungi use the energy from the plant to help it ‘mine’ phosphorous, and other minerals, which it exchanges with the plant. What is interesting is that the same fungus, can associate with many plants, of differing species, each of which can have different surpluses and needs. The fungi act as a sort of sharing mechanism, able to move these between different plants. Truly amazing.
Gardening with Microbes
There are some simple things that you can do to help you to utilise microbes in the garden. Firstly stop, or reduce digging, to allow fungi to develop. Using beds, will help you to stop walking on soil, and reduce compaction, benefiting fungi and predators. For trees, shrubs, and perennials, use wood based compost, such as wood chip, leaf mould, hedge prunings etc. to favour fungal growth. Applying this to the soil surface also suits fungi. For Brassicas, and short lived annuals, use a grass/straw based compost, or animal manure. For grasses or row crops, a mixed fungal/bacterial compost is best. Stop applying chemicals. Most are harmful to microbes, and are therefore just leached away.
In pots, and for sowing seeds, apply compost tea to kickstart the recycling of microbes/nutrients before the plant is able to produce surplus sugars. This should result in better initial growth. Compost Tea can also be used to get a better mix of microbes into soils that have been depleted, providing that there sufficient organic matter has been added.
Working with microbes allows you to benefit from the normal behaviour of billions of tiny creatures, whilst doing less yourself. Working with nature, rather than constantly battling against it.
Good luck with your growing
BB
Anybody interested in learning more about soil microbes should read the book “Teaming with Microbes”, by Jeff Lowenfels and Wayne Lewis. Some of the figure quoted have come from the book.
