dash soil has minimum water holding capacity
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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).
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Why is soil water holding capacity important?
Maintaining soil water holding capacity can mean increased profits to farms.
Soil water holding capacity is a term that all farms should know to optimize crop production. Simply defined soil water holding capacity is the amount of water that a given soil can hold for crop use. Field capacity is the point where the soil water holding capacity has reached its maximum for the entire field. The goal for agricultural producers is to maintain the field at or near capacity. When there is a deficit in the amount water in the soil, the soil profile needs to be replenished by precipitation or irrigation. The key is for farmers to understand the nuances of soil water holding capacity and how to manage it so that the farm does not need to irrigate or suffer from a drought.
Soil texture and organic matter are the key components that determine soil water holding capacity. In terms of soil texture, those made up of smaller particle sizes, such as in the case of silt and clay, have larger surface area. The larger the surface area the easier it is for the soil to hold onto water so it has a higher water holding capacity. Sand in contrast has large particle sizes which results in smaller surface area. The water holding capacity for sand is low.
Soil organic matter (SOM) is another factor that can help increase water holding capacity. Soil organic matter has a natural magnetism to water. If the farm increases the percentage of soil organic matter, the soil water holding capacity will increase. SOM is decayed material that originated from a living organism. SOM can be increased by adding plant or animal material.
The question that farmers should ask themselves is, “What can I do to increase the soil water holding capacity on my farm?” Unfortunately changing the soil texture of the field is not a viable option. Soil texture can be changed naturally by erosion, but that usually changes soil texture in a negative way. The best option for a farm is to increase their soil organic matter. Here are some basic ways that a farm can increase their SOM:
Use cover crops
Change to conservation tillage practices, for example no-till or minimal tillage
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