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Soil 3 - How do we know that a diverse and thriving soil ecosystem benefits us?

This is just the obvious way to conclude this series of blogs on soil. Agriculture, horticulture and amateur gardening are all thriving perfectly well using conventional practices, with mineral fertilisers, dairy slurry, pesticides, fungicides and so on. So how can we prove to ourselves that the whole soil biodiversity thing is not just a green and organic option? As a committed gardener for many years, I can personally verify that the only time my garden has produced good quality vegetables with minimal pest or disease issues, with minimal 'bolting' plant responses to climate stress, and fast growth with fully functioning fertilisation and fruiting and plant development, was when the soil organic matter was at a maximum and - this is crucial - the soil is not disturbed by extensive and deep digging.

The application of high humus content compost to the surface of the soil as a mulch, and the allowing of that compost to be assimilated into the soil by the soil biota is all highly conducive to encouraging and aiding a full, thriving and complete soil ecosystem.

The natural predators of slugs, and slug eggs and larvae, are beetles and beetle larvae. I see fewer slugs and slug damage when I see more ground beetles.

Drainage of the soil is best with minimal waterlogging in wet weather where a liberal humus content occurs. This is caused by active macro biota in the soil, like beetles, worms, slugs etc. making channels and burrows, creating pathways that aid drainage. But by allowing water to permeate throughout the soil structure, films are formed around all the many different sizes of particles, increasing the water holding ability of the soil but still allowing the passage of gases. Oxygen is as crucial to the survival of plant roots and soil biota as water is. With optimum amounts of organic matter roots develop profuse growth to access both the water films and the nutirents within the particles. The root systems of such plants are far more extensive and constitute an enormous capacity for retention between the land and the plant, with great resistance to direct and easy, and damaging, flow of water, which has to permeate, as well as presenting a more complex surface of resiustance to erosion and weathering.

Compaction is minimal in similar situations, with high organic matter in the soil, root systems grow to be more extensive, more stabilising, but also, paradoxically but true, better able to be plucked from the ground - the deep tap roots of pernicious weeds can be withdrawn relatively easily, despite their obvious size and health.

Mossy areas of lawn are eradicated by surface application of compost which changes the soil biota and structure, enhancing drainage, relieving compaction, encouraging drought resistance, improving aeration, and allowing extensive root growth. The grass thus outgrows the moss in a soil that is damp, aerated, and not waterlogged or compacted.

Not turning the soil allows biotic populations in the soil to establish themselves in a suitable and stable environment, in some cases forming complex but very delicate weblike structures - like fungal mycelial networks - within the right conditions of dampness, light, aeration. Microbiota are specific in their requirements of water, gases, light, and turning the spoil over buries those at the surface and exposes those from depth. a few inches may not seem much to us, but it can spell death to microbes.

The burial of weed seeds, as layers of compost are added to the soil, means that they are not exposed to light and air. This prevents them from germinating, thus minimising weed growth. But burial of surface living microbes is not complete and does not result in the same disruption as turning the soil does.

Here we come up against some of the very real obstacles to understanding how the soil behaves, and how it needs to be managed. The microbiota is so small we cannot watch it or monitor how changing conditions affect it. It is, of course, possible to take samples of soil from different depths or different conditions, but then the microbes, of many different types, of minute size, and enormous numbers, need to be found, identified, counted. It is an impossible task, especially for the smaller microbes, like bacteria. The only possibility, which is just now starting to be utilised, is DNA analysis.

In an approach called shotgun sequencing, which takes its name from the wide spread of target organisms, DNA can be extracted from samples of soil. The strands of DNA are recorded, as a sequence of bases, or base pairs, resulting in long chains of the four letters that represent the amino acids used in DNA encoding - A, G, T, and C. These lines of code can then be matched against known sequences that have previously been obtained from organisms and stored in a reference database. If the code sequence is in the reference database, the strands match, and the organism was present in the sample. If it doesn't match, then the likelihood is that the organisms DNA is not stored int he reference database. As time goes on, of course, the reference databases are actuively added to by researchers, and the percentage of successful matches increases. What the results of such analyses are, and will be, remains to be seen, but currently this is an exciting and rapidly developing revolution in our understanding of microbial ecosystems.

Coming back to the more everyday issues of management, the main problem that requires some careful management, is the production and creation of ample amounts of compost such that liberal applications can be made to wider areas of land - such as in horticulture and commercial vegetable production, and most particularly farms. The comments - findings - above are all well and good in a single household garden. The larger the farm, in land extent, the greater the problem in that greater quantities of compost are required, greater areas need an applcation, and greater time is required. It is tempting to suggest that smaller sized farms can, in this respect, be better managed.

Better managed for soil quality, that is.