By Nick Gladden, Hannah Grubbs, Sarah Beth Thompson, and Wesley Porter
Even though after I had this post together, we finally received rain across most of the state, it still was not enough to have a significant impact on the current conditions. By the time this will be posted, we are expected to receive more rain throughout Friday (9/26/2025), it still isn’t a significant amount considering the conditions we have been in. Also, looking at the long-term forecast, we are not predicted for any rainfall in the near future. So I say all of this to tell you DO NOT GET COMFORTABLE with your soil moisture levels and back off now. Let’s finish these crops off to the best of our abilities.
High temperatures, periods with little to no rainfall, low humidity, and moderate winds have all contributed to complicating end of season irrigation in both cotton and peanuts. In the production guide for both crops, the crop water requirement curve provides an estimate of water needed in each respective week of growth. For example, peanuts in the 18th week of growth would require 0.7 inches of irrigation per week according to the production guide (Figure 1), but we have seen that in conditions that have been hotter and had lower humidities than normal the amounts reflected below do not reflect the actual crop water usage nor recover to optimal soil moisture conditions.
During these water usage periods with prolonged drought, once there is depletion of moisture deeper in the soil profile it is very difficult, if not impossible, to recover with irrigation alone. This raises the question that even if you have the irrigation capability to keep up with more frequent irrigation, is it economical? Are you recovering more in yields than you are losing from the costs associated with operating the irrigation system? Unfortunately, at this time we do not have the final answer to that. The situation that we are currently facing is very different from what we normally face during the fall. Yes, we have dry periods, but in most cases, they only last a few weeks at the most. At the time of drafting this, the last rainfall we received in Tifton was around September 3, or over 21 days ago. The 30-day percentage of normal precipitation for Georgia is shown below in figure 2. This map shows abnormally low rainfall occurred over the previous 30-days for most of the state.
As of September 23rd, 68 percent of the state is experiencing abnormally dry conditions, 24 percent experiencing moderate drought conditions, and 3 percent experiencing severe drought conditions as shown in Figure 3. These drought conditions are affecting the main crop production areas of the state with moderate and severe drought conditions in the southwest corner of the state.
In research at the Stripling Irrigation Research Park near Camilla, Georgia we have both cotton and peanut studies installed with soil water tension sensors. In peanut, we have a static 45 kPa threshold all season (optimal soil moisture). Between September 8-15 (123-130 DAP) we triggered irrigation three times to maintain optimal soil moisture in that treatment. At this point in the season, sensor depths are weighted as 40%, 40%, and 20% for the 4-, 12-, and 20-inch depth respectively. When the 12- and 20-inch depths are not being recovered by rainfall events, weighted averages show less response to irrigation events overall because water is not moving deeper into the soil profile. By scheduling daily using these weighted averages, it shows a need for increased frequency of irrigation events. Basically, the plant depleted the deep moisture, and we have struggled to recover it with irrigation. The recovery was impossible without multiple back-to-back events which some farm irrigation systems are not capable of.
The cotton trial was planted on June 10th, later in the season. There are several soil moisture sensors installed into the field with varying irrigation thresholds. Sensor readings are at the 8 in, 16 in, and 24 in depths. In the 45 kPa static threshold, we have two sensors installed and the graphs of these below show a really good story of soil moisture depletion throughout the season, especially during the start of peak water use.
In figure 4 below starting around August 7th, the green horizontal line for the sensor at the 24-inch depth starts a steady decline. This date is also when the field was estimated to be at 50% first bloom. Irrigation was triggered on August 22nd. A response was observed in both of the shallower depths represented by the blue (8 in) and black (16 in) lines. Each irrigation event is based on a weighted average of each depth. When irrigation is applied at this site 0.75 inches is applied. For this part of the season, it is weighted at 40% for both the 8 and16 in depths and 20% for the 24 in depth.
Irrigation was triggered again on September 3rd, 8th, 9th, 16th, and 22nd, as shown in figure 5 below. There is a significant response in the shallower depths indicated by the blue and black lines; however, each irrigation event did not reach the deeper 24-inch depth.
This demonstrates a major benefit of soil moisture sensors which is being able to monitor conditions throughout the soil profile. If a producer was irrigating using the water requirement curve, in these conditions the crop would be underwatered. At the surface level, it may appear to have ample moisture, but sensors have shown that in these conditions, water is not reaching deeper into the soil profile.
This is a very tough situation to be in because the crop is depleting the deep moisture. We have a recommended irrigation amount from the Checkbook, which was developed based on a 15 year average, but we are no longer in an average year. Irrigation is only affecting the shallower sensors, so we are using deep moisture without replenishing it. As stated earlier, we are in a deficit irrigation scenario. In this case the question is are we reducing yield due to the lack of adequate moisture deep in the soil profile or is shallow moisture enough to maintain an adequate yield level? This situation shows that once we fall behind on replenishing our deep moisture, we cannot catch up with irrigation alone. It is critical to monitor your crop and stay on top of your irrigation requirements in these conditions. In these cases, even using sensors, we were unable to stay in an acceptable soil moisture condition for the crops.
If you have specific questions about your current situation reach out to your UGA County Extension Agent.
