Great Bend Tribune
Part I
Published March 21, 2021
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 to 27) indicates normal to slightly below normal temperatures and slightly above average precipitation. The eight to fourteen-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 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 the 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 small the pore radius, the greater the potential rise upwards.
Now, soil water terms:
- Saturation: All pores are filled with water. The soil 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 the stress zone.
Next week, how to improve soil water relationships for plants.
Part II
Published March 28, 2021
As of March 23, the Drought Monitor is showing even more improvement. Our area and much of Western Kansas is now is out of even abnormally dry conditions. Part of Northwest and Southwest Kansas improved to abnormally dry and only the extreme western counties and parts of the next are in drought conditions. Rains after Tuesday aren’t included. The rains midweek were beneficial as wheat continues to progress, alfalfa is growing, and there should be adequate moisture for planting corn. The six to ten-day outlook (March 30 to April 3) indicates normal temperatures and below-average precipitation. The eight to 14-day outlook (April 1 to 7) indicates below normal precipitation and above normal temperatures. A decent forecast for wheat and preparing for spring planting.
Last week we discussed Soil Water: Part I, what determines the soil's water holding capacity (soil texture and organic matter, amount of porosity, and types of pores) and soil moisture terms (saturation, field capacity, permanent wilting point, gravitational water, plant available water). Today two more terms and what we can do to increase water going into the soil.
- Infiltration – The source of soil water is precipitation. Infiltration is the process of water moving into the soil from the soil surface. You want the infiltration rate to be as high as it can be so precipitation isn’t lost to evaporation or runoff. The infiltration rate is highest at the beginning of a rainfall event and decreases to a steady state once all soil pores are full. That is determined by the saturated hydraulic conductivity. The saturated hydraulic conductivity is determined by several factors including total pore space, amount of macropores, and the connectivity of the pores. How do you maximize infiltration? Develop a pore system that includes large, connected pores by minimizing or eliminating tillage, eliminate soil compaction, having the soil covered by surface residues and a growing crop. Residues absorb the impact of rainfall which protects the soil surface, prevents soil crusting, and slows down the rainfall.
- Percolation – Once water is in the soil, its movement is termed percolation. Percolation occurs in all directions. In saturated soils, percolation is initially downward with gravity through macropores. Once these have drained, movement is confined to micropores. This water, held to clay and organic matter by adhesion can move in any direction, including upward against gravity, termed capillarity. Capillarity occurs only in micropores and the distance it can move upward against gravity is a function of pore size. The smaller the pore radius and the straighter it is, the higher it can move.
- One last thing: water really doesn’t necessarily move from more to less water but from where the water has a higher energy potential to where it has less. So the water in wetter soil beneath drier soil has a greater energy potential and will move upward.
Next week: How do we maximize soil water holding capacity?
Part III
Published April 4, 2021
As of March 30, the Drought Monitor is showing improved conditions remaining. The predicted unsettled pattern of this week may continue to help. Alfalfa and wheat continue to progress and it’s definitely time to monitor alfalfa for insects. The six to ten-day outlook (April 6 to 10) indicates temperatures well above normal and slightly below-average precipitation. The eight to 14-day outlook (April 8 to 14) indicates slightly below normal precipitation and normal temperatures. A decent forecast as we head into spring planting season. Today, after describing soil water holding capacity and getting precipitation into the soil, how do we improve soil moisture?
Naturally, unless you have irrigation, you don’t control what precipitation a field receives but you can maximize the chances of it staying where it falls and get it into the soil profile. And once you keep it there, how do you keep it in the profile for crop use? Some suggestions include:
- Leaving crop residue on the soil surface and/or cover crops help slow rainfall down and allow it to infiltrate into the soil. And to keep snow in place, have standing stubble over winter.
- Minimizing tillage or transitioning to no-till or strip-till if possible achieves several objectives. Residue on the soil surface serves as a vapor barrier to minimize evaporative loss in addition to keeping the soil cooler which also decreases evaporation. This also helps promote the buildup of organic matter in topsoil which increases soil water holding capacity and helps prevent soil crusting which aids infiltration. It also helps build a stable porous soil structure which leads to the next item.
- By employing conservation tillage you develop a more porous soil structure with stable aggregates containing more large pores that are continuous down into the soil. This allows for a greater infiltration rate once the soil is saturated. It also moves water deeper into the soil profile.
- You can’t realistically change the soil texture (sand, silt, and clay) positively to increase water holding capacity but you can increase, slowly over time, but increasing organic matter accumulation in topsoil through reduced tillage, cover crops, green manures, and not so green good old-fashioned manure.
- To allow water to infiltrate on sloped ground plant perpendicular to the slope and with the contour. Especially if you have worked ground, building terraces reduces the length of slope with water flow which keeps the water, and soil in place.
- During fallow periods, keep the ground weed-free, preferably through chemical control or heavy residue cover.
- If you must till, any aggressive or deep tillage should be as far ahead of planting as possible with succeeding tillage passes as shallow as possible.
Naturally, there are other techniques but this provides a starting place.