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Learn soil’s water holding capacity
Dr. Victor Martin

As of this March 16th the Drought Monitor is showing marked improvement. Our area is out of abnormally dry conditions. Except for a sliver of Southwest Kansas, extreme drought is gone. The rains of last week covering much of the state were quite beneficial, especially for the western half of the state. Wheat is greening up nicely, a bit behind schedule but growing, and cool season grasses are also starting to green up and grow for pasture. The six to ten-day outlook (March 23-27) indicates normal to slightly below normal temperatures and slightly above average precipitation. The eight to 14 day outlook (March 25 to March 31) indicates below normal temperatures and precipitation. This week appears a bit unsettled with chances for rain.

With our climate and agriculture as the major economic force in Kansas, we tend to think about water a great deal. Water and temperature are key to production of crops and livestock. This is why each column starts off with a recap of drought conditions and the weather outlooks. Today, let’s focus on water and our water reservoir, the soil. Part one discusses what determines the soil’s water holding capacity, the terms used to describe it, how it affects plants.

• The soil’s water holding capacity is determined by several factors including soil texture (the relative proportions of sand silt and clay), organic matter content, and soil structure. Clay and organic matter “hold” water in soils. They possess net negative charges. Water is a polar molecule with a partially positive end (the two hydrogen atoms) and a partially negative end (the oxygen atom). As you learned in school, opposite charges attract so the positive side of the water molecule is attracted to the negatively charged soil materials. Then water is attracted to the water attached to the soil and so on. This creates a water film adsorbed to soil particles. Sand has no charge and therefore water isn’t attracted to it. Sandy soils hold less water than heavier soils with more clay and organic matter. The other item in holding soil water is structure, the amount and type of soil pores. There are large pores, macropores, and small pores, micropores. Macropores move water and are responsible for mass water movement in soil, drainage. Micropores are surrounded by clay and perhaps organic matter and are where soil water is held against gravity. Micropores are also responsible for water being able to move upward against gravity through capillarity. The smaller the pore radius, the greater the potential rise upwards.

Now, soil water terms:

• Saturation: All pores are filled with water. The soils is holding as much water as it can.

• Field capacity: The macropores have drained and the micropores are all full. This is as much water as the soil can hold against gravity.

• PWP (Permanent Wilting Point): Most of the micropores are empty and the water left in micropores is unavailable to plants, however, there are still microscopic water films around clay and organic matter.

• Saturation – Field Capacity = Gravitational Water or macroporosity

• Field Capacity – PWP = PAW (Plant Available Water)

• From Field Capacity down towards the PWP, there is a zone where water is readily available to plants termed Least Limiting Water. Below that to the PWP is stress zone.

Next week, how to improve soil water relationships for plants.

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.