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 Soil Health: Organic Matter and Humus
Excerpt from The Soul of Soil, by Joe Smillie & Grace Gershuny
Soil health and humus are indivisible: health is the vitality of the soil's living population, and humus is the manifestation of its activities. As the cornerstone of the soil ecosystem, humus influences and is influenced by every other aspect of the soil. Building soil humus improves its physical and chemical properties as well as its biological health.
All humus is organic matter, but not all organic matter is humus. Raw organic matter consists of the waste products or remains of organisms that have not yet decomposed. Humus is one form of organic matter that has undergone some degree of composition. There is no hard and fast dividing line, but a continuum, with fresh, undecomposed organic materials-manure, sawdust, corn stubble, kitchen wastes, or insect bodies-at one end, and stable humus, which may resist decomposition for hundreds of years, at the other.
Humus is dark brown, porous, spongy and somewhat gummy, and has a pleasant earthy fragrance. Chemically, it is a mixture of complex compounds, some of which are plant residues that don't readily decompose, such as waxes and lignins. The rest are gums and starches synthesized by soil organisms, primarily bacteria and fungi, as they consume organic debris. Humus is highly variable in its composition, depending on the nature of the original material and the conditions of its decomposition.
"Humus" is actually more a generic term than a precise one. Its qualities will reflect different origins and composition. Just as wine can vary widely in quality, so can humus. And, just as different wines are suitable for different culinary purposes, the varieties of humus serve varying soil functions.
Several classification schemes for humus have been suggested. Theories differ as to how it is formed, why it behaves as it does, and how it should be measured. Humus that can still decompose readily is known as effective or active humus. It consists of a high proportion of simple organic acids (fulvic acids), which will dissolve in either acids or bases. This type of humus is an excellent source of plant nutrients, released as soil organisms break it down further, but of little consequence for soil structure and long-term tilth. This kind of humus is mainly derived from the sugar, starch, and protein fraction of organic matter.
Humic acids, which dissolve in bases but not in acids, characterize more stable or passive humus; humins, which are highly insoluble and may be so tightly bound to clay particles that microbes can't penetrate them, are the main constituents of the most stable humus. Because stable humus resists decomposition it does little to add nutrients to the soul system, but it is essential to improving the soil's physical qualities. Carbon-14 dating has revealed that very stable humus complexes may survive unchanged for thousands of years. Stable humus originates from woodier plant residues, which contain lots of cellulose and lignin.
The status of soil organic matter and humus is a dynamic one, continually changing through the activities of all the creatures that live there. Ideally, there should be a rough equilibrium among the different kinds of humus at any one time, with the more active fractions predominant when plant nutrient needs are highest, then giving way to more stable forms after harvest or when plants are dormant. Fungi and actinomycetes, which are more abundant that bacterial decomposers under cool, damp conditions, are also more important in the creation of stable humus.
The changes are fastest under optimum conditions for soil biological activity, and fresh supplies of raw organic matter must continually be added to keep the cycles moving. Anything that harms or disrupts one member of the soil community can lead to a form of "indigestion" in the soil. For example, if large amounts of nitrate fertilizer flood the soil system, the bacteria responsible for converting protein fragments into nitrates will be suppressed, in turn "backing up" the whole organic decomposition process. They will recover after a while, but if this process is repeated year after year, the capacity of that soil to digest fresh organic matter will be seriously damaged.
The process by which organic matter and humus breaks down in the soil is called mineralization. While humus is the product of organic matter mineralization, it too can be mineralized under the right conditions. Organic matter management, discussed in chapter 3, requires that you understand what conditions speed up or slow down mineralization.
Mineralization occurs quickly when conditions are perfect for bacteria to reproduce: high aeration, adequate moisture, good pH, and balanced mineral nutrients. Cultivation speeds it up by introduction air; if soil is dry, irrigation will also stimulate mineralization. Increasing soil temperature with dark mulch or row covers, or actually heating the soil in a green house bed, also encourages the faster release of nutrients to plants.
As is true with fertilizing, it's important to understand the concept of "enough" when you choose to stimulate mineralization. Too quick a release of nutrients from organic matter can cause problems, which parallel those of overfertilizing: excess plant nitrate uptake or possible leaching of nutrients into groundwater. It's also important to avoid "burning up" vital, stable humus reserves by making sure to add enough organic matter to replenish what is mineralized.
Humus tends to accumulate fastest under conditions unfavorable to mineralization: cool temperatures, low pH, and poor aeration. While to some extent this is desirable, the extreme example of going too far is the case of a peat bog, composed of almost pure humus. The key here is balance: an active, healthy biological population will continually be mineralizing humus at the same time that it is being formed. As you become attuned to the sighs of biological activity and health in your soil, as well as the rhythms of growth and rest in your crops, you will develop a better sense of "enough" when it comes to humus formation and decay.
Benefits of Humus
• Humus can hold the equivalent of 80 to 90 percent of its weight in water, so soil rich in humus is more drought-resistant.
• Humus is light and fluffy, allowing air to circulate easily, and making soil easy to work.
• The sticky gum secreted by microbes while forming humus hold soil particles together in a desirable crumb structure.
• Humus is extremely effective at holding mineral nutrients from being washed away in rain or irrigation water, and in a form readily available to plants. Ample reserves of humus also provide additional plant nutrients in times of need.
• Humus is able, because of its biochemical structure, to moderate excessive acid or alkaline conditions in the soil-a quality known as buffering.
• Many toxic heavy metals can be immobilized by soil humus, and prevented from becoming available to plants or other soil organisms.
• Although the color of humus can vary, it is usually a dark brown or black color, which helps warm up cold soils quickly in the spring. |
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