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 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.
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 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. We will do our best to keep our customers up to date on any modifications of these methods. If you have any questions on the different methods used to determine water holding capacity or have questions on other methods of analysis, please give one of our technical staff a call.