Wednesday, July 30, 2014

Living Soil

In any agricultural system the most important factors are sun, wind, water and soil. Out of these, soil is stressed the most and is perhaps the least well understood. In organic circles, it is usually considered sufficient to input large quantities of organic matter, and this is also how most home gardeners approach things. However, soil forms a complex ecosystem of its own which includes a variety of organisms from worms and bugs down to bacteria and fungi. Additionally, the roots of plants host complex ecosystems that are distinct from bulk soil, and which can dramatically affect the health of the plants themselves. Merely digging organic matter into the soil will not support these ecosystems, and may damage them instead. Managing soil to support these ecosystems requires first understanding how they work.

The Basic Soil Cycle

All soil starts out as rock. Either through glacial action or biological and rain action the rocks are gradually broken down into finer and more chemically available silt. Given sufficient conditions, plants then colonize this silt and extract its nutrients, combining them with carbon and nitrogen from the air in order to grow. Dead plant material that falls to the ground then breaks down into humus, which binds various nutrients as well as providing carbon and nitrogen for plants and soil organisms. Humus also softens the soil and holds water, which further benefits the growth of plants.

There are three main factors which determine how a soil will evolve: the parent material, temperature and rainfall. Parent materials which are richer in mineral nutrients tend to break down into more fertile clays than mineral-poor materials such as granite. Mineral-rich parent materials also support more plant life throughout their productive lifetimes. In nature, the richest soils are called 'loess', and are formed from deposits of silt which glaciers leave in their trails, and which are then blown by wind and deposited over large areas.

Temperature and rainfall determine the growing season. This is the period during the year in which plants are able to retain green foliage, and also the period during which decomposition is able to occur. In places which are either very cold or very dry, soil evolution occurs much more slowly and soils tend to have larger particle sizes and higher mineral contents. In places which are cold and wet or in which the soil is waterlogged much of the time, organic material may be deposited faster than it decomposes, leading to soils which are relatively poor in minerals but high in organic matter. In places which are warm and wet, such as the southeast United States or the rainforests of Brazil, decomposition tends to be faster than the rate at which organic matter accumulates, and the soils tend to be weathered all out of nutrients as well as low in organic material.

The Rhizosphere

Plants by themselves cannot generally extract nutrients from silt. In order to achieve this, plants secrete sugars from their roots which feed a host of bacteria and fungi which symbiotically solubilize and collect nutrients for the roots. Only certain species of fungi and bacteria have evolved to form this type of relationship with plant roots.

The most ubiquitous inhabitants of the rhizosphere are a class of fungi called 'mycorrhiza'. Mycorrhiza form a coating around root hairs that extend fine fungal threads into the soil which increase the effective surface area of plant roots and allow them to absorb nutrients and water more effectively. The fungal coating also buffers the roots against harsh environmental conditions and provides a measure of protection against root pests. The fungal networks tend to connect together, and through them plants can trade nutrients and water with other nearby plants.

Mycorrhiza are divided into three main groups, endomycorrhiza, ectomycorrhiza and ericoid mycorrhiza. Endomycorrhiza send fungal shoots into the living plant's cell walls to increase nutrient transfer. Most herbaceous plants and grasses associate with endomycorrhiza. Ectomycorrhiza only coat plant roots without fusing with the plant's cells. Most trees as well as roses and orchids associate with ectomycorrhiza. Ericoid mycorrhiza are similar to endomycorhiza, but are more specific to plants such as blueberry, cranberry, and azaleas.

There are also a number of bacteria which have evolved to be able to live symbiotically with plant roots, either directly in contact with the roots or in the vicinity of mycorrhiza. Rhizobia, bacteria which live inside special nodules in the roots of legumes and which fix nitrogen for the plants, are the most well known of these. While rhizobia can only fix nitrogen inside legume nodules, other bacteria such as Azospirillum spp., Azotobacter spp., Bacillus Polymyxa and Klebsiella Pneumonidae can fix nitrogen while living on the outside of any plant's roots. Other bacteria have been discovered that produce a variety of benefits for their host plants, including solubilizing phosphorous and other nutrients, attacking bacterial and fungal root diseases and forming a moisture-retaining slime layer around the roots.

The use of symbiotic bacteria and fungi is very effective and can convey many different benefits to crops. Field experiments have shown increases in yield, biomass and root growth, increases in vitamin, mineral and antioxidant contents (1.5-2x on average), increased resistance to pests, diseases and drought, and with nitrogen fixing bacteria crop nitrogen requirements can be cut in half. However, standard practices of chemical fertilizer (and other chemicals) and ploughing discourage their growth and proliferation. Soluble phosphorous fertilizers inhibit the growth of mycorrhiza, while chemical nitrogen fertilizers cause rapid changes in pH which are damaging for many beneficial organisms. Ploughing physically disrupts bacterial and fungal colony structures in the soil, and repeated ploughing and chemicals kills off beneficial organisms while favoring pathological ones (thus requiring more chemicals to 'fix').

The Detritosphere

In nature, the top layers of soil tend to accumulate all sorts of dead and decomposable matter. This includes things like the manure, sheddings and carcasses of insects and animals, dead leaves and other plant parts, and whole dead plants and roots. This dead material provides food and habitat for a number of organisms including worms, woodlice, earwigs, springtails and other "crawlies" as well as bacteria and fungi which collectively recycle nutrients within an ecosystem.

As organic matter is broken down, it is mixed into the soil by the constant burrowing activities of the various "crawlies". Natural soils have an "O horizon" or "mollic horizon" which is rich in humus that is gradually mixed down in this way. Depending on soil and climate conditions, mollic horizons can vary between nonexistant and three feet in depth. "Crawlies" also carry spores of symbiotic bacteria and fungi between the roots of different plants, and destruction of their ecosystem can halt this transfer.

Ploughing destroys the structure of the mollic horizon, kills many "crawlies" with each pass, buries their food where there isn't enough oxygen to support bugs, and mixes oxygen into the soil which promotes the consumption of humus and its conversion into carbon dioxide. Ironically the crawlies that are killed by ploughing would normally mix and aereate the soil naturally free of cost and labor. Chemical fertilizers, pesticides and herbicides can also kill off many or all of these beneficial organisms. Compost, which has already been broken down, also cannot provide food for these organisms and as such is not a sustainable method for building soil.


Certain minerals, particularly sulfur, can only be released from humus when it is fully broken down and converted into carbon dioxide. In most places, soil humus breaks down at an adequate or even excessive rate, and disrupting the soil is particularly damaging. However, in certain places such as the Congo and the British Isles, the soil is frequently waterlogged which prevents organic material from ever fully breaking down. In these systems, accelerating the breakdown of soil through aeration may be necessary to facilitate the release of nutrients to growing plants. 

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