Great Bend Tribune
Published March 12, 2023
The drought monitor report as of Tuesday, March 7 indicates a continuing extreme and exceptions drought in Western Kansas. Not much hope for relief either. The six to ten-day outlook (to March 14 to 18) indicates normal temperatures and 33 to 50% chance of leaning above normal precipitation. The eight to fourteen-day outlook (March 16 to 22) indicates a 40 to 50% chance of below normal temperatures and a 33 to 40% chance of leaning to above normal precipitation. Hopefully the rain will happen.
One of the main functions we want our soils to do is hold water and nutrients. Everyone from producers to home gardeners want a soil that provides water and nutrients. Today, how does soil hold water and nutrients? Briefly and this is a primer.
- First, porosity and pore size distribution matter in terms of water movement and water holding. Macropores, large pores, allow for easy water movement and drainage. Micropores are responsible for holding soil water and having it available for plant roots. These small pores are also responsible for “capillary rise” which is the ability of water to move upwards against the force of gravity and the smaller the pore, the greater the rise. The how is next.
- As odd as this might sound, water and nutrients are held in the soil electrostatically. Soil texture is defined by the three soil separates: sand, silt and clay. Clay and decomposed organic matter, humus, are classified as soil colloids. They have a large surface area per unit volume. An average teaspoon of clay spread one layer thick could cover a football field. One thing clay and organic matter have is numerous positive and negative charges. Under normal conditions, the negative charges outnumber the positive ones and result in an overall net negative charge. Remembering your electricity, opposite charges attract. So, positive charges are attracted to the colloids – calcium, magnesium, potassium, ammonium, iron, etc. These cation nutrients are held in the soil, don’t leach easily, and available for plants. Some soils are able to hold negative ions, anions, that are plant nutrients such as nitrate, sulfate, and phosphate. Our typical soils here have little anion exchange capacity. Next – water.
- Water has an overall neutral charge; however, the electrons spend more time closer to the oxygen nucleus than the hydrogen nuclei. This results in the oxygen end being partially negative and the hydrogens partially negative. Water is a polar molecule with a positive and negative end. So, water molecules are attracted to each other and why water forms a crystalline structure as a solid and less dense as a solid than as a liquid – it is cohesive. The hydrogens being partially positive are attracted to the negative soil colloids and held there. Then the hydrogen ends of water molecules are attracted to the oxygen of the water adsorbed to the soil colloid. And they in turn attract other water molecules and a water film develops. This is how plant available water is held in soils.
- So, soils with organic matter and clay will hold more water than soils low in organic matter with little or no clay. And it’s not just the amount but also the type of clay matters. Certain clays have a low net negative charge while others possess a very large one. Finally as a soil become more acid, the ability to hold cations decreases.