Last week’s column discussed temperature and what it means for crop growth. This week will focus on moisture. While temperature determines the area of crop adaption, moisture determines the potential for growth and yield potential. However, it is not as simple as the amount of precipitation an area receives yearly and involves other factors besides rain or snow. And we are discussing the long-term average precipitation, not just one or two years.
· The average annual amount of precipitation naturally is important. It takes a certain amount of precipitation to optimize vegetative and reproductive growth. Plants need moisture extracted from the soil to transport nutrients, conduct photosynthesis, and maintain turgor pressure (keep from wilting). However, what may be adequate moisture in one area may not be for a given crop in another area because of factors discussed shortly. Also in the mix can be too much precipitation for a given crop that leads to yield reductions from waterlogged soils and often increased pest pressure.
· Rainfall distribution – When does the precipitation come? In our area most rainfall occurs from spring through mid-summer. So most years, rainfall comes when it benefits crops like winter wheat and canola or spring oats. Rainfall patterns in this area often lead to stress during the production of grain by summer crops and severely limit yields. That is why the development of efficient irrigation systems was so important in developing summer cropping to include corn, alfalfa and soybeans. On the positive side for summer row crops, our typically dry conditions in September and October help dry down harvested crops and maintain quality.
· Rainfall intensity – How “hard” or “fast” does the precipitation come? In spite of the severe weather experienced here, the majority of our precipitation events are actually low intensity (<0.25” per hour). This fact favors grass crops over broadleaf crops or fibrous shallow root systems over tap rooted crops and is one of the reasons native vegetation was primarily grass species.
· Humidity – How much water vapor is in the air helps determine the ability of a plant to grow and produce the desired crop. Low humidities combined with high temperatures make it difficult for plants to transpire adequate amounts of water from the roots up through the leaves. If the demand is too great, the plant “shuts down” causing wilting. Wilting is actually a defense mechanism to protect the plant until conditions become better. It’s when the plant doesn’t recover over night that problem becomes severe. The downside to this mechanism is that is reduces photosynthesis. Think of Ireland and Southeast Kansas. We think of Ireland as lush, green, and wet while SE Kansas not so much. They both receive similar amounts of precipitation. The difference is temperature regime and humidity. That leads to the last point.
· PET (Potential EvapoTranspiration) – This is how much moisture is lost from the ground surface through evaporation and by transpiration through plants if moisture wasn’t limiting. PET is determined primarily by humidity and air temperature. In Ireland PET is typically less than the amount of precipitation received while is SE Kansas PET is typically greater than the precipitation received. This is typical of most of Kanas in most years.
There is more but these are the basics of moisture and crop growth.