Plants generally obtain nutrient elements and water from soil. Thus, soil features that affect the availability and uptake of nutrients and water are of great importance to plants. Below is a brief overview of the soil features that most greatly affect plant growth.
Mineral particles in soils can be classified on the basis of their size (diameter). Clays are very small (<0.002 mm) particles, silts are larger (0.002-0.05 mm), and sands yet larger (0.05-2.0 mm). The percentage of each of these major particle classes soil determines the texture of a soil. Textures range from those predominantly containing one of the three major particle sizes and thus named for them (silt, sand, clay textures) or various intergradations (sandy clay, etc.). One major textural class (loam), which is ideal for plant growth, is not named for a predominant particle class because loam soils have similar amounts of all three particle classes. Texture is important for plant growth because it influences water availability and soil fertility, that is, the ability of a soil to supply nutrients to plant roots. Open spaces among soil particles represent the pore space of a soil. This pore space may be filled by water, air, or plant roots. Thus, the amount of pore space greatly influences the amount of water that a soil can contain. The tightness with which water is held in pore spaces determines whether or not the water and dissolved mineral ions will drain through the soil profile or remain in place and be available for uptake by plant roots. Coarse textured soils (e.g., sandy soils) have large pore spaces that do not hold water tightly enough to prevent gravity from pulling that water into deeper soil layers. Very fine textured soils (e.g., clay soils) have very small pores. Small pores hold water strongly and thus retain much water despite the pull of gravity. However, they also hold water against the pulling power of plant roots and so provide only small amounts of plant-available water. Soil texture is also an important determinant ofthe cation exchange capacity (CEC) of a soil, that is, the ability of a soil to adsorb and exchange mineral ions essential for plant growth. Soils with higher percentages of clay or silt particles generally have a greater CEC.
The three-dimensional arrangement of soil particles gives rise to soil structure. In many soils, groups of particles are held together to create lumps of soil materials termed peds. The space between peds can be important in allowing penetration of water and roots to deeper soil layers.
Soil depth can greatly influence the types of plants that can grow in them. Deeper soils generally can provide more water and nutrients to plants than more shallow soils. Furthermore, most plants rely on soil for mechanical support and this is especially true for tall woody plants (e.g., shrubs, trees). A classic example of the influence of soil depth on plant communities is seen on granite rock outcrops in the southeastern US. As the granite weathers, it can form pools of soil that vary in depth from a few millimeters at the margin to tens of centimeters in the middle. The shallow marginal soils support certain annual plants, whereas deeper soils support herbaceous perennials and still deeper soils are colonized by woody plants. Plant zonation in these soil pools can be striking (Figure 1). Some soils can develop special soil horizons (horizontal soil layers characterized by distinct chemical and physical features) that limit the soil depth available to support plants. These special soil horizons include clay-pans, zones of soil which contain large amounts of clay, and hardpans, layers of soil particles that have been cemented together by the deposition of mineral materials. Hardpans include calcic horizons (commonly called caliche), in which calcium carbonate cements the soil particles. The net effect of these dense horizons is to impede or prevent root growth and thus limit the effective depth of the soil. They also may affect soil oxygenation by restricting drainage at times in which large amounts of water are present.
Organic matter in soils ranges from recognizable plant parts (roots, leaves, stems) to humus, which is partly decomposed plant material that is amorphous and spongy in nature. Organic matter contributes to a soil's ability to retain nutrients and water (i.e., soil's CEC). It aids in holding nutrients because negatively charged compounds in humus attract and hold positively charged plant nutrient ions. It helps provide water because humus can absorb 80-90% of its weight in water and therefore contributes to a soil's ability to hold water under drought conditions.
Figure 1 (a) A small soil pool (about 2 m wide) on a granite outcrop in east-central Alabama. Shallow soil at the margins is dominated by lichens. The deepest soil in the center of the pool has been colonized by Senecio tomentosus, a yellow-flowered herbaceous perennial species. (b) A larger soil pool on the same granite outcrop shown in (a). Deep soil on the left (behind the children: Jenny and Kristina Boyd) is occupied by woody plants (shrubs and trees). The soil pool becomes more shallow to the right, where striking zonation of smaller plants can be observed. The most shallow soil on the extreme right is occupied by the small red-colored annual Sedum smallii. Slightly deeper soil to the left of the Sedum zone is dominated by moss (Polytrichum commune) and white-flowered annual Arenaria species. Still deeper soil between that zone and the woody plants is dominated by perennial grasses along with some Senecio tomentosus. Credit: R. S. Boyd.
Soil pH is an extremely important ecological parameter. Its most important effect on plant growth is its influence on ion availability in the soil solution. Ions in soils are important for two major reasons. One is that many soil ions contain elements required for plant growth. These elements, called essential nutrients, are primarily obtained from the soil. The second reason is that plants obtain most of their water from soil and the amount of dissolved ions in soil water can influence a plant's ability to take up water (see the following section). The influence of pH on ion availability stems primarily from the influence of pH on the solubility of the various compounds present in the soil. In general, soil compounds containing some elements are more soluble at some pH values than others. For example, iron is relatively insoluble at pH values of 8 or greater and plants with a high iron requirement may perform poorly in soils with high pH values. Similarly, many heavy metals become increasingly available for plant uptake at pH values of 4-5. Thus, plants growing in low pH soils are more susceptible to heavy metal toxicities.
Although we mentioned ion availability under pH (above), we should also mention that some ions are abundant in some soils primarily because they have been deposited in those soils in great amounts. Certain salts (often Na, Mg, or Ca salts) may be abundant in some soils in quantities that greatly affect plant growth. These salts include those from seawater (as in salt marshes) or those that build up in desert soils from evaporative concentration of relatively freshwater (e.g., the Great Salt Lake of Utah). Extensive irrigation of land in regions where there is high evapo-transpiration can also lead to accumulation of salts on the soil surface (i.e., secondary salinization). Salty soils (e.g., saline, sodic, saline-sodic soils) can impact plant growth by affecting water uptake, nutrient uptake or by causing toxicity due to specific ion effects. Water uptake can be slowed because the high ion concentration in the soil impedes water movement into plant roots. Nutrient uptake can be affected because ions can competitively inhibit the uptake of essential ions of similar size (e.g., Na+ vs. K+, Mg2+ vs. Ca2+). Excess ions can also have specific toxic effects on plants by directly inhibiting essential physiological processes.
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