Compare sandy, clayey, and loamy soil
based on texture, water, and air-holding
capacity
Answers
The water holding capacity of a soil is a very important agronomic characteristic. Soils that hold generous amounts of water are less subject to leaching losses of nutrients or soil applied pesticides. This is true because a soil with a limited water holding capacity (i.e. a sandy loam) reaches the saturation point much sooner than a soil with a higher water holding capacity (i.e. a clay loam). After a soil is saturated with water, all of the excess water and some of the nutrients and pesticides that are in the soil solution are leached downward in the soil profile.
Soil water holding capacity is controlled primarily by the soil texture and the soil organic matter content. Soil texture is a reflection of the particle size distribution of a soil. An example is a silt loam soil that has 30% sand, 60% silt and 10% clay sized particles. In general, the higher the percentage of silt and clay sized particles, the higher the water holding capacity. The small particles (clay and silt) have a much larger surface area than the larger sand particles. This large surface area allows the soil to hold a greater quantity of water. The amount of organic material in a soil also influences the water holding capacity. As the level of organic matter increases in a soil, the water holding capacity also increases, due to the affinity of organic matter for water.
In the past 100 years, many laboratory methods have been developed around the world to determine soil water holding capacity. These methods use a variety of special apparatus to determine how much water a soil will hold under various conditions. Most of theses methods start with a water saturated soil sample. The saturated sample is placed on a porous ceramic plate which is then placed in a closed chambers. A known amount whc2 of pressure is then put into the chamber, which forces water out of the soil sample and into the porous plate and out of the chamber (see 1/3 Bar picture). The water holding capacity of the soil is determined by the amount of water held in the soil sample vs. the dry weight of the sample. The amount of pressure applied in these different methods can be as low as 1/3 atmosphere of pressure (about 5 psi) up to 15 atmospheres of pressure (about 225 psi).
A few methods of determining the water holding capacity are conducted without external pressure being applied. The apparatus for “0 Bar” water holding capacity method is shown in the figure below. In this method, a soil sample is saturated with water from an adjacent container, with the water level being kept in the middle of the soil (see diagram below). Once equilibrium in this system is reached, the soil sample is weighed. The water holding capacity is calculated based on the weight of the water held in the sample vs. the dry weight of the sample. The “European” maximum water holding capacity method is another method that doesn’t use external pressure. In this method, the soil sample is saturated with water in a cylinder. The cylinder is placed on an absorbent membrane until the excess water is drawn away by gravity (see diagram at right). Once equilibrium is reached, the water holding capacity is calculated based on the weight of the water held in the sample vs. the sample dry weight. whc3
AGVISE Laboratories provides many methods of analysis for our customers to choose from. In an effort to clear up some of the confusion related to these different water holding capacity methods, we tested four samples by each of the eight methods we currently offer. As you can see from the tables (above left), there are large differences in the water holding whc4 capacity of a soil based on the method used. In most situations, our customers know exactly which method they require so there is no confusion. In other cases, we help our customers choose which methods will provide them with the most useful information. The most commonly requested methods for water holding capacity are the 1/3 Bar method, which is commonly referred to as field capacity and the 15 Bar method, which is known as the wilting point (see 15 Bar picture). In the future, these methods may change or become more standardized as research and testing become more global
Answer:
Soil moisture limits forage production potential the most in semiarid regions. Estimated water use efficiency for irrigated and dry-land crop production systems is 50 percent, and available soil water has a large impact on management decisions producers make throughout the year. Soil moisture available for plant growth makes up approximately 0.01 percent of the world's stored water.
By understanding a little about the soil's physical properties and its relationship to soil moisture, you can make better soil-management decisions. Soil texture and structure greatly influence water infiltration, permeability, and water-holding capacity.
Soil texture refers to the composition of the soil in terms of the proportion of small, medium, and large particles (clay, silt, and sand, respectively) in a specific soil mass. For example, a coarse soil is a sand or loamy sand, a medium soil is a loam, silt loam, or silt, and a fine soil is a sandy clay, silty clay, or clay.
Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates, which give soil its structure. Aggregates can be loose and friable, or they can form distinct, uniform patterns. For example, granular structure is loose and friable, blocky structure is six-sided and can have angled or rounded sides, and platelike structure is layered and may indicate compaction problems.
Soil porosity refers to the space between soil particles, which consists of various amounts of water and air. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil has bigger particles than a fine soil, but it has less porosity, or overall pore space. Water can be held tighter in small pores than in large ones, so fine soils can hold more water than coarse soils.
soil texture