One thing I have always found a bit fascinating is the vast array of terminology that we use in Agronomy. Maybe every industry has its own language; I wouldn’t know, this summer marks my 18th year working in the Agronomy field. Other than working on a farm and at the Sale Barn when I was in High School, it is the only industry I have ever known. And perhaps that leads to me having a jaded view about how we discuss the daily workings of agriculture. There is one thing I am certain of however, and that is that we use a lot of abbreviations, acronyms, and shorthand in Agronomy, as well as a lot of technical terms that can sound confusing, and even sometimes interchangeable.
When it comes to soil moisture and irrigation, I want you to be completely comfortable with the terms we use and what they mean. What we are trying to accomplish with the AquaSystems above all else is to demystify the process of Irrigation Management and to make the process as simple as possible. In order to monitor and save water, we need to understand how the following levels of soil moisture content correlate with the availability of water in the soil. Having a familiarity with these levels will help you understand the way soil holds water. This information will also help you set the thresholds for watering with a soil moisture sensor. They are Saturation, Field Capacity, Wilting Point, and Management Level.
Saturation: At the saturation level, nearly all of the spaces between soil particles are filled with water. After a soil has reached saturation, it does not become more saturated; it becomes flooded. As a rule, irrigators do not want to saturate the soil because it cuts off the plant’s supply of oxygen, which, in effect, drowns the plant. Do not confuse saturation with Field Capacity.
Field Capacity: When soil is at the field capacity level, it means that all excess moisture has drained freely from that soil. The amount of remaining moisture is the field capacity. It is the maximum water a soil can hold without gravity pushing it down and out. To manage water properly in a perfect world, we would turn off the irrigation or rain just before the soil in the root zone of the plants reaches field capacity so that we would not push nutrients through the profile with excessive water movement.
Wilting Point: The point at which wilting of the plant occurs because the volumetric water content is too low for the plant to remove water from the soil. About half of the water in the soil at field capacity is held too tightly to be accessible to plants. Soil at permanent wilting point is not dry. When the water content of a soil is below the permanent wilting point, water is still be present in the soil, but plants are unable to access it.
Crop Available Water: This is the available water plants can extract from the soil. This is simply Field Capacity minus Wilting point. Another term for this is crop available water.
So then, as we look at a probe report online, we will see these terms visualized, and begin to have more clarity. The area above the green is the equivalent of Saturation. We can at times show that we have water above that line, but we cannot keep it there as it falls off in fairly rapid and predictable manner. The top of the green area is our Field Capacity line. This is the point where our soil is full of available water. Then the area below our green box is our wilting point. We often set the bottom to be slightly above wilting point to give us a “fudge factor” so we aren’t living on the razors edge. In addition to that fudge factor we also want to take into account the amount of time it takes to make an irrigation so we are going to introduce one more term…
Management Line: This yellow line may or may not be present for every one of you. Some of our ACS Regional Managers use it and others don’t. What it represents is the point that we want you to take action. It is about making sure we have time on our side and don’t stress the plant.
None of this is voodoo or magic. All of these terms are based on soil properties and principals that have been around for years. I just want to make sure that when your FSA or someone on the ACS team says that we have 2.5 days left until we are at our management line, or if we say after an irrigation we want to see a 1” gap left in the profile for a rainfall, that you understand what that means. Our end goal is to make this feel second nature, not like we need to summon spirts to understand what our probe is telling us.