The Soil Environment

garden soil

The principal environmental variables affecting life in soils include moisture, tempera­ture, pH, aeration (i.e. presence or absence of sufficient oxygen), organic matter, and inorganic nutrients such as nitrogen and phosphorus. The balance of these factors controls the abundance and activities of the microbes and larger animals in soils which in turn have a marked influence on the critical processes of soil aggregation and degradation of plant and animal residues and the nutrient cycling that accompanies this latter process.

To understand life in the soil, the soil is best viewed as an ex­tremely heterogeneous collection of microhabitats. The soil is a matrix of solids including sand, silt, clay, and organic matter particles as well as aggregates of various sizes formed from them, and pore space, which may be filled with air or water. Thus, depending on the mix of environmental variables, prevailing conditions can vary quite markedly over distances on the order of a few millimeters (1 inch = 25.4 millimeters).

Soil Water

Soil water is usually derived from rainfall or some other form of overland flow. The amount of water that enters the soil is a function of soil structure which in turn is a function of texture and higher-order structure i.e. aggregation (e.g. well-aggregated, porous soils allow greater infiltration). Larger pores created by root channels and animal burrows (ant tunnels, crayfish tunnels and worm holes) as well as other types of macropores also greatly facilitate movement of water into and through the soil profile. This is why these soil animals are usually regarded as highly beneficial in soil and why some scientists often use the abundance of earthworms as an indicator of a "healthy soil", though it should be remembered that highly fertile, productive soils do not always contain large numbers of earthworms and other soil animals.

Water moves through soil by mass flow (i.e. fluid movement through larger channels) and by capillary action, the slower movement through the highly tortuous network of very small channels and pores. Most soil water is held in this system of capillary pores and channels and can be held quite firmly through physical interactions with the solid components of the soil matrix. Matric water is held in small (usually microscopic) pores and is often adsorbed onto particle surfaces that can be held rather tightly. Because these interactions with the soil matrix essentially bind the soil water, energy on the part of plants and microbes is required to extract water. These forces are usually given as a matric potential or pressure and they indicate how tightly the water is held. These values are usually expressed as a negative value because they are a measure of the amount of suction required to extract water from the soil. It is this adsorption of water to soil solids that explains why plants can wilt in a soil having substantial clay content, even when some matric water is still present. Beyond a certain point, the water is held too tightly for the plants to extract and the so-called wilting point is reached. An often-stated value for the matric potential at the permanent wilting point is approximately -15 bars (-1.5 mPa). Plants exposed to lower matric (more negative) potentials may wilt and not recover. It should be stressed that the matric potential is a function of soil texture and that fine-textured soils (silts and clays) can contain moisture at very low matric potentials because of the much greater numbers of small particles (greater surface area) and the finer pore network which adsorbs and retains water. Thus in clayey soils, water is not as biologically available as it is in coarser-textured soils. A corollary, however, is that coarse-textured soils do not retain as much water because the larger particles do not adsorb water as well and the larger pores drain more rapid­ly. Thus, nutrients and other chemicals may be retained in a clay soil whereas they may leach, i.e. move downward, with water more rapidly through sandy soils. These textural concerns are of extreme importance to our understanding of the movement of materials into ground water.

Water occupying the pore network in soil is often called the soil solution. It is never pure water but rather it is a solution containing dissolved salts and gases. This is the solution which is most available to plants and microbes and provides some of the nutrients needed for growth. Many nutrients actually reach the roots of plants through mass flow of the water to the vicinity of the roots. Too much water in the soil disrupts the ideal balance between air-filled and water-filled pores, i.e. soil aeration (discussed below). Salts dissolved in the soil solution give rise to an osmotic pressure or potential which can be measured. The osmotic potential can influence the movement of water into and out of cells and therefore it is important that the soil solution not be allowed to become too saline (salty) as it might in a soil experiencing increased salinity due to prolonged irrigation and poor drainage. Under low osmotic (high negative) potentials (salty or sugary solutions), water is actually drawn out of cells resulting in physiological impairment, that is, being dehydrated beyond some critical point. This is the basis of preserving foods by adding high concentrations of salt.

Osmotic and matric potentials are the two major factors controlling the availability of water to soil life and of these two, it is the matric potential which most affects soil microbes. As a soil dries, life in the soil must work harder to ex­tract water against ever increasing matric and osmotic potentials. For these reasons, adequate soil moisture is required for maximum rates of plant growth and to maintain the vital soil microbial processes essential for life on earth.

Soil Atmosphere

The soil atmosphere is the mixture of gases occupying that portion of the pore network that is not filled with water. It is derived from the overlying air but differs in a few prominent ways. Most notably, the soil atmosphere contains less oxygen than the overlying air and typically has 10- to 100-fold more carbon dioxide. You might surmise, and you would be correct, that these changes are driven by the aerobic respiration of the organisms inhabiting the soil. These changes are also a result of the physical characteristics of the soil environment. Generally, the rate of movement of oxygen back into the soil is not sufficient to keep up with all the respiratory demand because movement of gases is restricted by the highly elaborate network of fine pores and capillaries. Filling pores with water restricts the movement of gases even further, and it is for this reason that waterlogged soils often become anaerobic (i.e. depleted of molecular O). This condition should be avoided because it gives rise to a number of negative consequences, not the least of which is that plant roots are injured or killed, not just due to lack of O2, but to the accumulation of toxic products such as organic acids (e.g. acetic acid) or hydrogen sulfide (rotten egg gas) arising from anaerobic activities of soil bacteria.

Most soil organisms are aerobic, i.e. they require O2 to grow. However, some soil bacteria do not require O2 and in fact may be killed or inhibited by it. These bacteria are called anaerobes and they function when there are low concentrations of soil O2, for example, during periods of prolonged waterlogging. Still other bacteria, called facultative anaerobes, can grow in the presence or absence of O2.

This discussion of soil water and the soil atmosphere reveals why it is so desirable to maintain a soil in a highly aggregated state. Aggregation is indispensable for maintaining the balance between air- and water-filled pores that is essential for the maintenance of aerobic, beneficial microbes and soil animals.

Soil Temperature

Soil temperature is controlled by a number of factors not the least of which is the amount of sunlight incident upon it and the amount of water it contains. Soils are warmed by solar radiation. For this reason, dark-colored soils tend to absorb more heat and warm earlier in the spring than light-colored soils do. Obviously, vegetative cover plays an important role in controlling the amount of solar radiation that reaches the soil surface. We all know the value of a good shade tree on a hot summer afternoon. Because the soil can become quite warm in the surface few centimeters, particularly when free of vegetation, it is not surprising that numbers of microbes and soil animals are lower near the imme­diate surface. It is frequently just too hot and too dry in this narrow band of soil. Deeper in the solum the soil warms according to its moisture content and the amount of irradiance received on the surface.

Temperature affects soil life in several ways. First, all organisms exhibit preferences for certain temperature ranges; the so-called minimum, maximum and optimum temperatures. Soil microbes can grow across a very wide range of temperatures so that some portion of the population may be active nearly year round. At the lower extremes of temperature (near freezing) are microbes called cryophiles which actually require or prefer these cold temperatures. At the other extreme are thermophiles, organisms which thrive in high-temperature environments like the boiling geothermal hot springs of Yellowstone National Park at temperatures near 94 oC (water boils below 100oC because of the lower atmospheric pressure at the higher altitude). Thermophiles are also critical components of composting systems designed to accelerate the conversion of municipal refuse, paper etc. into compost for use in gardens and for disposal onto land. In the middle temperature range (15 to 40oC ) we find the mesophiles and, not surprisingly, ourselves (humans) and most other life forms. Most soil microbes are mesophiles. Most of life's important biological processes operate best in the mesophilic range and we find special adaptations in those organisms that live in colder or hotter environments.

Temperature affects soil microbes and microbial processes in another fundamental way. Namely, it governs the rate of chemical and biological reactions. Thus, biological reactions and processes tend to occur more rapidly as temperature rises to some point where some vital function (perhaps an important enzyme) of the cell is impaired or destroyed and the organism is killed. At colder temperatures, reaction rates slow down and may almost com­pletely stop. This is the well known basis for preserving food from spoilage by keeping it cold in the freezer or refrigerator. We mentioned above that temperature controls biological processes in soil. One can easily see the value of early warming of soils in the spring for germination of seed and an early start of a crop growth cycle. One can also look at maps of soil organic matter content and easily ascertain the combined effects of soil moisture and temperature on maintenance of organic matter in the soil. In warm, moist climates, soil organic matter is difficult to maintain because the conditions for growth of soil microbes are favorable for longer periods of the year. In colder climates, soil organic matter can accumulate because of prolonged periods when the soil is very cold or even frozen in the near surface layers where most microbes reside.

Lesson 7. Soil Biological Properties

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