Clay-humus complex

Topic in Gardening courses. By John Eagles.
This page is under construction.

The clay-humus complex as described in soil study refers to the formation of aggregates of organic fragments such as humic substances with inorganic particles such as clay minerals. Both clay minerals and humic substances are soil colloids.

From: Humic substances are capable of individual soil particles connect to a crumb structure and thus the soil structure to stabilize. These aggregate structures are made ​​by combining Tonkolloiden calcium bridges formed with humic substances into humic topsoil. The resulting organo-mineral compounds are known as clay-humus complex, or lime-clay-humus complex. The combination of organic and mineral substances from live as crumbs obstruction occurs only at high activity of soil organisms . Crumb structure is optimal in the humus form "Mull" to find. While the clay minerals and Huminsäurereste (organic dipoles) of humus are negatively charged, is the Ca 2 + ion or other multivalent cations, such as Mg 2 + - a neutralizing bridge.

The resulting complex stabilizes the soil against erosion and siltation, creates the pore volume of the soil conditions favorable for the air-and water balance and thus results in a higher soil fertility . The binding of the positively charged ions, these nutrient salts are important for the plant materials are not washed by rain into the deeper soil layers. They are later released by the plant through an ion exchange of the THK's and utilized.

Conveniently, the elasticity of the crumb structure formed. The tilth of the soil life is the formation of clay-humus complexes promote sustainable than, for example, when a frost action is possible. The physical structures of the related Segregatgefüge are only temporarily favorable for crop production, because they can crack quickly lose their function and further decay.

Excessive tillage, however, leads to the disintegration of every crumb structure - in this culture, horticulture is error as "Totfräsen" refers to a soil.

From: Clay and humus form colloids that act like a sponge for water and nutrients. Clay particles consist of flat plates -- their surfaces are "sticky" to water molecules. Chemically, clay consists of aluminum silicate compounds with varying amounts of potassium. This potassium can be available to plants but only after chemical decomposition of the clay -- another vital role played by soil bacteria. The chemical composition of the clay influences its "sponginess" or ability to store and release nutrients. Kaolin-type clays are relatively poor at storing nutrients, while bentonite, a montmorillite type clay, is very absorbent. Our area has both types of clay -- the kaolin types formed from very old basalt flows that have been well leached by our high rainfall. Volcanic ash that fell into the drier climate of Eastern Washington weathered into sticky bentonite clay that was brought here by glacial floods. All the lower elevation areas in the Willamette Valley were covered by these floods, resulting in the heavy, soggy soils we have during winter. The clay has the potential to be a nutrient powerhouse but only if it can be opened up with organic matter. The clay sponge holds on to a wealth of different chemical ions that can be used by plants. The holding ability is referred to as the Cation Exchange Capacity (CEC) and can be tested by soil laboratories. An ideally balanced soil CEC is calcium 60-75%, magnesium 6-12% (these two totaling 80%), 3-5% for potassium, trace elements and hydrogen. Calcium and magnesium are nutrients that act in opposition to each other, yet the plant needs both. The ideal ratio is about 7 parts calcium to one part magnesium. Our soils tend to be low in both nutrients, so adding dolomitic limestone with both calcium and magnesium is usually a good idea. Even better than amending the soil is adding minerals to the compost pile. That way soil life organisms have a chance to start breaking down the minerals. The mineral-holding ability of the soil (CEC) is what keeps water from dissolving away the nutrients. Mineral nutrients tend to increase with low rainfall, but nitrogen (a proxy for Organic Matter) is low under desert conditions. The best soil lies in the Midwest, with high nutrients and also high organic matter. The Willamette Valley, with high rainfall, does well for organic matter but is low for all the mineral nutrients. That's why we need to consider adding soil minerals. The amount of hydrogen in clay sponge is a statement of the soil acidity. Minerals are most available if the soil is slightly acid -- at extremes, the minerals can be locked up and unavailable to plants.

From: Humification of leaf litter and formation of clay-humus complexes

Humus which is readily capable of further decomposition is referred to as effective or active humus. It is principally derived from sugars, starches and proteins and consists of simple organic (fulvic) acids. It is an excellent source of plant nutrients, but of little value regarding long term soil structure and tilth. Stable (or passive) humus consisting of humic acids, or humins, on the other hand, are so highly insoluble (or tightly bound to clay particles that they cannot be penetrated by microbes) that they are greatly resistant to further decomposition. Thus they add few readily available nutrients to the soil, but play an essential part in providing it's physical structure. Some very stable humus complexes have survived for thousands of years. Stable humus tends to originate from woodier plant materials, eg, cellulose and lignins.

Humus should not be thought of as 'dead'- rather it is the 'raw matter' of life- the transition stage between one life form and another. It is a part of a constant process of change and organic cycling, thus must be constantly replenished- for when we are removing prunings and crops for the kitchen we are depriving nature's cycle of potential humus. This is why we need to substitute compost and other sources of organic matter to maintain the fertility of our productive land.

--- From: Now comes the final stage in humus formation. Numerous species of earthworms eat their way through the soil, taking in a mixture of earth, microbes, and the excrement of soil animals. All of these substances are mixed together, ground-up, and chemically recombined in the worm's highly active and acidic gut. Organic substances chemically unite with soil to form clay/humus complexes that are quite resistant to further decomposition and have an extraordinarily high ability to hold and release the very nutrients and water that feed plants. Earthworm casts (excrement) are mechanically very stable and help create a durable soil structure that remains open and friable, something gardeners and farmers call good tilth or good crumb. Earthworms are so vitally important to soil fertility and additionally useful as agents of compost making that an entire section of this book will consider them in great detail.

From: Humus This process of decomposition continues until all the organic matter is reduced to carbon dioxide, water, minerals and humus. The humus arises from a small proportion of the fibrous (‘brown’) organic matter which is highly resistant to decomposition; the lignin and other resistant chemicals form a collection of humic acids which forms a black colloidal (jelly-like) material. The humus coats soil particles and gives topsoil its characteristic dark colour.

This colloidal material has a high cation exchange capacity and therefore can make a major contribution to the retention of exchangeable cations, especially on soils low in clay (see sands). It also adheres strongly to mineral particles, which makes it a valuable agent in soil aggregation. In sandy soils it provides a means of sticking particles together, whereas in clays it forms a clay-humus complex that makes the heavier soils more likely to crumble. Its presence in the soil crumbs makes them more stable, i.e. more able to resist collapse when wetted, and it increases the range of soil consistency. Bacteria eventually decompose humus so the amount in the soil is very dependent on the continued addition of appropriate bulky organic matter.

See also

External links