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A Word About

Soil Layers

The international classification of soils is based on the soil horizon system (layer). As samples are taken from progressively deeper regions of the soil different layers come to light. Seven major soil horizons are differentiated; they have many sub-layers and combination layers. Not all horizons are in all soils, and the layers vary in thickness &emdash; this gives rise to differentiation of soils. (see also SOIL TAXONOMY)

The formation of soil layers is influenced by many factors. The major factors being:

Climate is one of the most influential factors affecting formation of layers. The effects of temperature changes, wind, and precipitation influence the weathering of mineral rocks. Depending on the rates of accumulation and decomposition the different layers are made. The climate also influences the microorganisms, the fauna, and plant growth.

Animals and plants accumulate and humify organic matter. The creation of different organic acids influences the weathering of the mineral rocks and differentiates layer composition.

Mineral substructure with climate directly influences the clay type formed. Water percolation rate is much dependent on this mineral substructure, in turn influencing layer formation.

International horizon classification uses seven horizons for soil differentiation. The horizons are defined as H-O-A-E-B-C-R, which are:

H is a horizon that accumulates organic matter under water saturated conditions (saturated for longer periods, not necessarily all the time).

O is the organic horizon, which has litter accumulation of plant and animal origin, like horizon H but with no saturation of water (only few days a year).

A is dark colored, high in fine organic matter and humus associated with clay.

E is high in sand and silt and lacks (it has been washed out) some clay (e.g. AlSi, or some oxide clay).

B horizon has a high concentration of humus or certain minerals (e.g. iron). Most often this horizon is defined further (humus B, or iron B). This is where materials washed from upper layers set up.

C horizon is basically the parental mineral material and is only affected by soil formation to a minor degree.

R is the underlying rock.

There may be combination layers (transitional) designated by the two layer abbreviation, like EB - this is a layer transition between E and B layers and it has more character of E than B.

Further there are sub-characteristics that are given a small letter after the horizon designation (Bt is a B layer with accumulation of clay, or Oi is an O layer with almost no organic matter decomposition). These designations are as follows:

a Highly decomposed organic matter

b Buried soil horizon

c Concretions or hard nodules

e Intermediately decomposed organic matter

f Frozen soil

g Gleying due to variations of oxidation

h Organic matter accumulation

i Little decomposed organic matter

k Carbonate accumulation (CaCO3, MgCO3)

m Cemented layer (hard pen by silica)

n Sodium accumulation

o Residual sesquioxide accumulation

p Plowing disturbance

q Silica accumulation

r Weathered or soft bedrock (usually due to ground water)

s Sesquioxide accumulation (Fe, Al)

t Clay accumulation

v Iron enriched subsoil material becoming hard and brittle

w Recognizable structure or color

x High density, brittle

y Gypsum accumulation

z General salt accumulation

In farmed soils the horizons become more or less homogeneous.

To understand the cycle of life in the soil and its importance to farming, we must first accept the scientific premise that all plants derive 95 percent of their dry matter from the air and water and only five percent from the soil. Yet without the five percent nutritional contribution of the soil, plants would not exist. To make soil rich in nutrition for healthy plants to grow, it is absolutely essential that soil be alive with its millions of species of miniature organisms, from algae to zymogenous fungi. It is these little workers that make plant food from the essential elements placed in the soil by nature and, more recently, man.

To learn how this very intricate soil life system works, we must visualize the top soil as four biological layers or zones as they occur in most farmed soils.

The uppermost layer or cover is thin, like a cloth or tarp, a carpet of skin over the top of the soil. It is lacking in life due to exposure to atmospheric influences, especially heat, sunlight, and wind. It should be as thin as possible, perhaps no more than an eighth of an inch thick. This thinness can be achieved and preserved by protecting the topsoil with a covering such as mulch, crop residues, or by applying a thin coating of barnyard manure. Fallowing land will deepen this infertile cover layer. For this reason, fallowing should be avoided.

The second layer below the top skin of the soil is the first decay zone, which may be from four to six inches deep. The dimensions of this and all other zones will vary considerably due to the influences of air, moisture, temperature, and the physical chemical and biological conditions that exist in the soil. In fact they are not truly separate, well-defined layers.

In this second layer, the first phase of decomposition of organic materials takes place&endash;the first stage of decomposition. If putrefaction occurs during the decomposition, various gases are formed and released that are harmful to some beneficial kinds of microorganisms and these gases hinder plant growth. This stage of disintegration will come to an end as soon as sufficient oxygen is present, which requires that the soil granules not be too small and not be disturbed too much from the position they naturally occupy.

Excessive deep plowing disturbs the first decomposition zone, slowing down or preventing oxygenation. This is the major reason deep plowing is harmful and should be avoided.

The third layer below the surface of the soil is the second decomposition zone. It may occur at depth of six to ten inches, but varies both in thickness, and depth below the soil surface. The micro-fauna, including mites and springtails among others, live in great abundance here. They further decompose the organic matter of the cell walls of the material consumed, leaving only the "cell pulp." In this layer are the beginnings of feeder roots, which are not more fully developed because decomposition in this layer has not reached the stage which makes nutrients available for plant use and thereby attractive to the roots.

The fourth layer, or third decay zone, is where innumerable armies of many kinds of microbes decompose the "cell pulp" into soil plasma, producing the desirable plant nutrition portion of stable humus. This becomes the true soil fertility. The production of soil fertility occurs primarily in this fourth layer and the third layer above it. For this reason, these two layers are the most important, but it is in the fourth layer where the most crucial stage in natureís cycle of life takes place. Here the perfect food, the ambrosia attractive to roots, is produced. As they mature, feeder roots thrust out eagerly in search of this nourishment.

Another way of thinking about the three zones of disintegration is to imagine that the living organisms in the soil are the farmerís tiny workers. It does not require much imagination, because that is what they really are. Now imagine that they are employed in shifts. They comprise the first, second and third shifts of natureís work day.

The warm and cold seasons of the year are natureís alarm clock which tells the microbes when to sleep, when to awake, when to eat, and when to multiply their numbers.

Of course, the picture we have been portraying is really oversimplified. Nature is more complex. All the processes of decomposition described here occur in a continuous fashion. The various kinds of microbes are intertwined and woven together within soilís microsites. Soil metabolism, which involves the process of catabolism (decomposition), and anabolism (rebuilding of decomposed matter into plant nutrition), forms an endless chain of life and death, of decomposition, of growth and rebuilding. Nature is working out all her processes that constitute the cycle of life in the soil. Her mass of organic material, both plant and animal, is like a childís lump of modeling clay. It can be molded into a whole array of forms, later to be broken down into a shapeless mass from which new forms can be molded anew.

The third and fourth layers of the soil are the carriers of the life force. Here we find soil with a desirable crystalline structure, the most fertile soil, the permanent or stable humus. In these layers, the soil contains the vibratins of life.

Mineral Subsoil

Below the four layers of topsoil is the "mineral" zone for the tap roots. This contains only partial life: deep roots and earthworms. They connect the underground mineral layer to the top layer of soil. Tilth and stable humus are not created in this layer.

Fertile topsoil, with its wonderful composition of life, should not be mixed with the minerals, and should be shallowly disced. Deep plowing the living fertile topsoil down into the earth and bringing the dead layer from the depths of the soil destroys the purpose of fertilization. Deep plowing may be beneficial only in special situations, and as long as it is not conducted yearly.

The microflora reduce and eliminate deficiencies in the fertile topsoil. The more active the microflora are, the deeper the zone of fertile topsoil.

If the underground soil is hardened by rains and deep plowing the soil must be loosened, but the surface soil should not be turned under. (This suggests the practice of ripping, which breaks up the compacted mineral layer but minimally disturbs the topsoil).

To enhance soil life activity in the second, third, and fourth layers it is suggested that an application of organic matter in addition to the leftovers from the previous crop be incorporated shallowly into the soil. This may include residue from other crops such as straw, wood chips, or manure, which should be composted prior to application. Composting is the decay of organic material (animal and plant residues) that has been decomposed from its original state into a form more directly utilizable by the crop plants. Compost should be made from at least 50% direct plant matter and not more than 50% animal manures.

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