So far we have focused on water in plants and water in the soil. Now let’s focus on water in the atmosphere, crop water needs, the soil, and tie it all together. It would seem, based on the last two columns that the amount of water a plant needs to produce a given amount of biomass, forage or grain, would be clear cut as would the soils ability to supply the plant’s moisture needs based on soil type, organic matter, structure, and so on. However, this is where the atmosphere and climate can throw in a monkey wrench.
If you check the literature, you will find guidelines as to how much water it takes to produce a bushel of corn or ton of hay. There are minor differences in terms of maturity and genetics but it is fairly well-established. As this is discussed, remember that water moves from the soil to the roots and out through the leaves in response to a vapor pressure gradient between the interior of the plant and the atmosphere. However, the following atmospheric/climate factors change this amount:
• Temperatures, both air and soil, affect crop water needs. Temperature determines the rate of growth and development. As you move towards the high and low extremes, development and water uptake are effected. At higher temperatures the water use of the plant increases, especially with the next two factors unless so high that the plant must shut down.
• Relative humidity of the atmosphere plays a major role in crop water needs. The higher the relative humidity, the less water needs to move through the plant to minimize stress and maintain plant growth. Even with higher temperatures, the plant is better able to function with a high relative humidity.
• As you can guess for Kansas, wind also plays a role. The higher the wind speed as a rule, the greater the rate the plant will move water through the roots, stems, and leaves into the atmosphere.
• The relative effects of these factors is a function of plant growth stage, soil moisture, and soil texture.
• Putting these factors together – high temperatures, lower humidities, and wind – results in the plant needing more water to produce a bushel of grain or ton of forage than where these factors are more favorable. So on average, it takes more water to produce a bushel of corn in Southwest Kansas than in the Northeast part of the state.
• Where do conditions tend to lead to more stress through temperature, humidity, and wind? The areas which tend to receive less precipitation and have higher water loss rates from the soil. If you look at the average precipitation across the state it is highest in the southeast and decreases as you move north and west, from around forty inches to twelve to fifteen inches. A map of irrigation allocations is the reverse of the precipitation map which makes sense from a crop growth standpoint, results in the problems faced today with groundwater depletion.
Next week’s column ties this all together.