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Soil Components
All soils are composed of four components: organics, inorganics, air and water. The percentage of each component in the soil may vary from location to location but these four components are always present in true soils. Organic matter includes plant material, roots, bacteria, fungi, molds, algae, protozoa, insects and worms. In a cubic inch of soil there are more individual living things than there are people in the world. Air and water fill the spaces in the soil particles. Among the gaseous components is carbon dioxide at one hundred times the concentration found in the open atmosphere, which originates from the decomposition of plants and animals in the soil. Green plant tissues contribute the oxygen
component via photosynthesis. The inorganic component is weathered rock particles from the underlying bedrock Along with this weathering process comes many nutrients, such as potassium and calcium. Soil Profile
Soils of the Earth are found in layers, which have developed uniquely due to time, air, topography, moisture, organic materials and parent rock minerals. Each soil layer, or horizon, is distinguishable by color, particle size and structural characteristic. Each horizon in a particular area may range from a few inches to several feet thick. Soil profiles typically have three layers, identified by the letters A, B and C.
The A horizon is the uppermost layer. This is where most of the organic matter can be found, along with roots and animals. This layer can be further subdivided into layers A0, A1, and A2. The top layer, A0, consists of decaying leaves and plant matter. Layers A1 and A2 have more decomposed organic matter.
The B horizon, has less living organisms and less humus than the A horizon. Clay minerals accumulate in this zone resulting in a soil composition that is harder when dry and stickier when wet.
The C horizon consists of partly weathered rock beneath the B horizon and above the bedrock layer. As weathering proceeds, the C layer becomes the B layer. Physical Properties of Soils
One method of describing soil is by particle size. Clay particles are defined as particles having a diameter of 0.002 millimeters or less. Silt particles are between 0.002 and 0.05 millimeters. Sand particles are between 0.05 and 2 millimeters. Particles greater than 2 millimeters in diameter are considered gravel. The percentage of clay, silt and sand found in a particular soil sample determine the soil texture.
Fig. 1 - soil composition
Different physical properties can be attributed to various soil textures. Sandy soils are highly permeable, meaning that water rapidly moves through them, allowing the soil to dry out quickly. Water holding capacity, called porosity, is directly proportional to the sum of the particle surface area. Clay soils have the greatest surface area, have a much greater water holding capacity, and thus can be described as being very porous. Infiltration of water in clay soils can be very slow, they easily become water logged, and plants drown from the lack of oxygen in the soil pores. Loams posses a balance of sand, silt and clay. Soil Structure
Soil structure describes how soil particles join together in larger units. These larger units, or aggregates, are called peds. Peds have been classified into four types - blocky, platy, granular and prismatic.
Fig. 2 - soil structure
Roots cause peds to form granular units, creating a well aerated, porous, permeable soil. This is further improved by microorganisms which produce strong binding substances via decomposition of organic residues in the soil. These binding substances decompose at various rates, and to maintain a soil structure the microorganisms require a continuous supply of organic food. Soil Colloids
Soils become acidic (pH less than seven, i.e. a presence of a higher concentration of hydrogen cations (positively charged) than hydroxyl anions (negatively charged) through the process of leaching. Over time, hydrogen cations flow downward, gradually replacing calcium and magnesium cations. The more water moving through the soil, the faster the removal process takes place. Thus, the more humid the climate, the more the soil is acidic.
Clay soils prevent the rapid movement of water and possess more basic minerals, held by colloids. Colloids are microscopic particles found in clay and humus, and are the center of chemical activity in soil. They have a negative charge on a surface area of less than 0.1 micron in diameter. This negative charge attracts positively charged particles such as magnesium and calcium, resisting the effects of leaching. Due to their small size and large surface area per volume, they have the capability of holding comparatively large quantities of nutrients for plant use. The plant draws on this micronutrient source by exchanging for them with hydrogen cations (a positive charge for a positive charge).
The colloid, also referred to as the clay-humus complex, holds different cations with different affinities. Calcium is the most strongly bound, and sodium is the most loosely held. Potassium and magnesium are both moderately held. Clay soils with a wealth of humus have the highest capacity to hold exchangeable cations. This is measured as cation exchange capacity. Sandy soils, which are colloidally poor, are easily leached of their micronutrients, especially in humid environments since there is nothing to prevent their removal.
Fig 3 - soil molecules
Thus pH is a good measure of the soil's clay-humus complex. If the pH is low, this implies that hydrogen cations have already replaced the important micronutrients. If the pH is high, the soil is high in its available exchangeable nutrients. Different species of plants function better at different pH levels, but overall, the measurement of the pH is merely a guideline of micronutrient availability. In container plant propagation, pH levels are insignificant when compared to the importance of availability of macro and micronutrients.
Next issue, the science column will continue with an overview of different bonsai artist's soil preferences from around the world, with an attempt to understand why there are so many lenses through which to examine this complex topic.
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