It is firm when moist and hard when dry. Water-holding capacity is controlled primarily by soil texture and organic matter. Soils with smaller particles silt and clay have a larger surface area than those with larger sand particles, and a large surface area allows a soil to hold more water. In other words, a soil with a high percentage of silt and clay particles, which describes fine soil, has a higher water-holding capacity.
The table illustrates water-holding-capacity differences as influenced by texture. Organic matter percentage also influences water-holding capacity. As the percentage increases, the water-holding capacity increases because of the affinity organic matter has for water. Water availability is illustrated in the figure by water levels in three different soil types. Excess or gravitational water drains quickly from the soil after a heavy rain because of gravitational forces saturation point to field capacity.
Plants may use small amounts of this water before it moves out of the root zone. Available water is retained in the soil after the excess has drained field capacity to wilting point.
This water is the most important for crop or forage production. Plants can use approximately 50 percent of it without exhibiting stress, but if less than 50 percent is available, drought stress can result. Clay soil has small, fine particles, which is why it retains the most amount of water. Sand, with its larger particles and low nutritional content, retains the least amount of water, although it is easily replenished with water.
Silt and loam, with medium-size particles, retain a moderate amount of water. The amount of organic matter in soil also affects how much water the soil is able to retain.
This is because organic matter has a natural attraction to water. So the more organic matter a soil contains, the greater the affinity it has with water. On the other end of the spectrum, clay soil has the highest levels of organic matter.
Furthermore, soil texture is also characterized by particle size distribution. Soil is a combination of various particles: sand, silt, and clay. Determining the soil texture type means knowing the predominant particle in the soil.
For example, soil that has 50 percent clay, 30 percent silt, and 20 percent sand will be categorized as clay soil. Sand particles measure between 0. Silt particles have a diameter between 0. Thus, silt and clay are the smallest particles. A large surface area in the soil makes it more conducive for absorbing water. Iron redox concentrations in the form of rusty orange pore linings. Figure 6. Gunmetal blue manganese redox concentrations in the form of ped surface coatings and pore linings.
Figure 7. Example of pale bluish gray redox depletions. Timing of Inputs. The amount and timing of precipitation ultimately governs soil moisture content, availability and flow. The temporal nature of moisture dynamics dictates ecosystem response and land-use decisions.
Soil moisture regimes are used in Soil Taxonomy to describe annual variability in moisture as dictated by climatic factors and indirectly by soil and landscape factors.
There are five general soil moisture regimes: aquic, udic, xeric, ustic, and aridic. These moisture regimes have detailed definitions Soil Survey Staff General working definitions are described below. Figure 8. Integrative Case Study. The type of soil moisture regime can be used to infer the direction of water movement in soil, which affects soil development and morphology.
The following case study illustrates this point. Figure 9 shows two soils, Mollisols Xerolls and Ustolls with mm of mean annual precipitation that differ in the timing of precipitation i. Mollisols are typically described as grassland soils that have large stocks of below ground soil organic matter. The Ustoll is a Mollisol with an ustic soil moisture regime that formed in a bunchgrass prairie east of the Rocky Mountains Figure 9a.
The Xeroll is a Mollisol with a xeric soil moisture regime that formed in a bunchgrass prairie on the Columbia Plateau in eastern Washington Figure 9b. These soils share similarities in soil forming factors including parent material, vegetation, age, and topography. The notable difference between these two profiles is the nature of the B horizons subsurface zones of accumulation of clay, salts or iron. This semi-soluble salt tends to accumulate in subsoils where deep percolation is minimal Jenny The Xeroll has a Bw horizon , a subsoil where salts have been removed by deep percolation and the development of soil structure and slight accumulation of clay are detectable.
Figure 9. Examples of Mollisols grassland soils formed from loess. Figure Climatic information and soil water balance for an Ustoll a and Xeroll b. Water storage dynamics and flow facilitate the four basic soil forming processes: translocations, transformations, additions and losses of soil constituents in a soil profile.
These processes determine the chemical, morphological and physical properties of soil such as the variation of texture with depth.
Hydrological processes active in soil contribute to weathering processes, and indicators of these processes are preserved by the soil profile in the form of observable and measurable soil characteristics, similar to those discussed in the case study O'Geen et al. Thus soil resource inventories e. B horizon : Subsurface soil horizons that are zones of accumulation of soil constituents such as clay, iron, or salts.
Bk horizon : B horizons that display an accumulation of pedogenic calcium carbonate Bw horizon : B horizons that display the initial stages of pedogenesis such as the development of soil structure, oxidation Capillary forces : Water held by small pores due to cohesive attraction between water molecules and adhesive forces between water molecules and solids.
Texture : The relative proportion of sand, silt and clay. Ustoll : Mollisols with an ustic soil moisture regime. Xeroll : Mollisols with a xeric soil moisture regime.
References and Recommended Reading Brady, N. Hillel, D. Introduction to Soil Physics. Jenny, H. Factors of Soil Formation. Soil Science Society of America. Glossary of Soil Science Terms Madison WI, Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.
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