Irrigated row crop acres in South Carolina have steadily increased over the last 10 years. Furthermore, groundwater regulation in SC has also increased with regard to well permitting. In order to maximize soybean yield while reducing water usage an appropriate irrigation schedule coupled with a scheduling tool needs to be incorporated into irrigation strategies. By determining appropriate soil moisture sensor thresholds specific to SC coastal plain soil textures and soybean, growers who adopt these technologies and recommendations can maximize profit by maximizing yield and improving irrigation water use efficiency.

• To determine the optimum soil moisture sensor threshold value using soil tension (Watermark) soil moisture sensors.

• To evaluate season-long irrigation schedules.

• To determine the return on investment and payback period for soil moisture sensor technologies and irrigation schedules utilized in soybean production in South Carolina.

The expected output/deliverables of this research will be shared with soybean growers in South Carolina and throughout the southeast. Information exchange will be through the use of social media (Twitter and Facebook), extension publications, Clemson Edisto REC Seminar Series, field days, and grower meetings.

Updated April 19, 2021:
Limited progress has occurred on this project because the soybeans have not yet been planted. However to date, site selection has been made and the field has been sprayed with burndown herbicide, supplemental fertilizer has been applied as well as lime. Seed has been ordered and delivered to EREC. Soil moisture sensors were purchased and built in the field season of 2020, so they are ready to be installed after planting occurs. We anticipate planting soybean the middle of May 2021.

Updated July 15, 2021:
SCSB Quarterly Report

General Information
Principal Investigator(s) Name(s): Michael Plumblee and Matt Inman
Organization: Clemson University – Edisto REC
Date: July 15, 2021
Quarter: 2nd

Proposal Information
Title: Determining the Optimum Soil Moisture Sensor Threshold in Soybean
Amount Expended to Date: $3,825.57 (Most of the expenses thus far have been to purchase lime, fertilizer, herbicides, and begin paying a partial stipend of a graduate student)

Progress Assessment
Wendy Buchanan, the M.S. graduate student working on this project has been actively checking soil moisture sensors since planting.

On-Station Location
A field (B10A) at Edisto REC in Blackville, SC has been selected for this trial. This is the same location as the 2020 trial so that soil texture is similar. This field is irrigated with overhead irrigation (lateral move like center pivot). As of July 15, 2021, the field has been sprayed with burndown herbicide application and the field has been strip tilled. Soybean was planted on June 16th with Asgrow 69XF0 at 130,000 seed per acre on 30-inch row spacing. Soil moisture sensors were installed immediately following soybean emergence. Post emergence herbicides have been applied and timely rainfall has prevented any irrigation events from going out on as of 7/15/21. As we progress through the growing season, we anticipate water use to increase and cause sensor thresholds to trigger.

Key Performance Indicators
Key performance indicators for this study will be determined at harvest (to determine if irrigation treatment and threshold value had any direct yield benefit). The water use efficiency data and yield data from this trial will help determine if there is an optimum soil moisture threshold that can be utilized for irrigated soybean. These data will be compared with the data collected and generated in the 2020 growing season. This research will help develop irrigation scheduling recommendations where soil moistures sensors are incorporated. Furthermore, this research will allow for a threshold to be selected that maximized yield and water use efficiency. As of now key circumstances impacting this research are continuous rainfall or lack thereof, and continued maintenance of the plots.

Next Steps
Over the next quarter we plan to continue with the management of the soybean crop and implementing irrigation treatments (irrigation, scouting for pests, etc.).

Updated October 16, 2021:
SCSB Quarterly Report

General Information
Principal Investigator(s) Name(s): Michael Plumblee
Organization: Clemson University – Edisto REC
Date: October 15, 2021
Quarter: 3rd

Proposal Information
Title: Determining the Optimum Soil Moisture Sensor Threshold in Soybean
Amount Expended to Date: $5,823.42 (Most of the expenses thus far have been to purchase lime, fertilizer, herbicides, and begin paying a partial stipend of a graduate student)

Progress Assessment
Wendy Buchanan, the M.S. graduate student working on this project has been actively checking soil moisture sensors since planting.

On-Station Location
As the growing season has progressed several irrigation thresholds have triggered, even though timely rainfall has occurred in Blackville throughout the summer. The most aggressive threshold (-15 kPa) had approximately 5.25 inches of irrigation applied; the average threshold (-30 kPa) had approximately 3.75 inches of irrigation applied; the most passive threshold (-60 kPa) had approximately 2.5 inches of irrigation applied. The majority of irrigation applications this year occurred in August and September when soybean reached reproductive growth stages and water use was maximized. Soybean was sprayed with Besiege (insecticide) and Approach Prima (fungicide) at the R3 growth stage. Overall, soybean plots look good, and we anticipate harvesting at the end of October or beginning of November.

Key Performance Indicators
Key performance indicators for this study will be determined at harvest (to determine if irrigation treatment and threshold value had any direct yield benefit). The water use efficiency data and yield data from this trial will help determine if there is an optimum soil moisture threshold that can be utilized for irrigated soybean. These data will be compared with the data collected and generated in the 2020 growing season. This research will help develop irrigation scheduling recommendations where soil moistures sensors are incorporated. Furthermore, this research will allow for a threshold to be selected that maximized yield and water use efficiency. As of now key circumstances impacting this research are continuous rainfall or lack thereof, continued maintenance of the plots, and getting the plots harvested.

Next Steps
Over the next quarter we plan to harvest the soybean plots and begin the data analysis component of this project.

Updated December 16, 2021:
SCSB Quarterly Report

General Information
Principal Investigator(s) Name(s): Michael Plumblee
Organization: Clemson University – Edisto REC
Date: December 15, 2021
Quarter: 4th

Proposal Information
Title: Determining the Optimum Soil Moisture Sensor Threshold in Soybean
Amount Expended to Date: $6,756.50 (Most of the expenses thus far have been to purchase lime, fertilizer, herbicides, and begin paying a partial stipend of a graduate student)

Progress Assessment
Wendy Buchanan, the M.S. graduate student working on this project has been actively checking soil moisture sensors since planting.

On-Station Location
Soybean was harvested on November 12, 2021. All irrigation was terminated at the R6.5 growth stage in soybean and no further irrigation was applied since the last progress report. From a preliminary assessment of the data, the rainfall that we received throughout the growing season appeared to be sufficient in producing high-yielding soybean, thus a significant yield effect may not be observed in the 2021 data, however meaningful data on water use and water use efficiency will still be conducted and shared. All data has been entered and is in the process of being analyzed and written into a publication. Wendy Buchanan, a student working on this project anticipates graduating in the spring of 2022 and is actively writing her thesis which contains this project.

Key Performance Indicators
Key performance indicators for this study will be determined at harvest (to determine if irrigation treatment and threshold value had any direct yield benefit). The water use efficiency data and yield data from this trial will help determine if there is an optimum soil moisture threshold that can be utilized for irrigated soybean. These data will be compared with the data collected and generated in the 2020 growing season. This research will help develop irrigation scheduling recommendations where soil moistures sensors are incorporated. Furthermore, this research will allow for a threshold to be selected that maximized yield and water use efficiency. As of now key circumstances impacting this research are continuous rainfall or lack thereof, continued maintenance of the plots, and getting the plots harvested.

Next Steps
Over the next quarter, we plan to complete all data analysis and writing components of this project. We also plan on presenting the results of this project over the winter at producer meetings throughout the region.

Updated January 17, 2022:
SC Soybean Board Final Report

General Information
Principal Investigator(s) Name(s): Michael Plumblee
Organization: Clemson University
Date: 1/17/2022
Quarter: Final

Proposal Information
Title: Determining the Optimum Soil Moisture Sensor Threshold in Soybean
Amount Expended to Date: $6,996

Project Summary
Irrigated row crop acres in South Carolina have steadily increased over the last 10 years. Furthermore, groundwater regulation in SC has also increased with regard to well permitting. To maximize soybean yield while reducing water usage an appropriate irrigation schedule coupled with a scheduling tool needs to be incorporated into irrigation strategies. By determining appropriate soil moisture sensor thresholds specific to SC coastal plain soil textures and soybean, growers who adopt these technologies and recommendations can maximize profit by maximizing yield and improving irrigation water use efficiency. Irrigation was applied with an overhead lateral move irrigation system. Irrigation was then scheduled with Watermark soil moisture sensors installed at depths to monitor soil moisture within the top 24 inches of the rooting zone. All soil moisture sensors were installed within the planted row after planting. Treatments consisted of three thresholds 1). 25% Maximum Allowable Depletion (MAD), 2). 50% MAD, 3). 75% MAD, and a non-irrigated treatment for comparison purposes.

The 2021 growing season would be considered an above-average rainfall for Blackville, SC. Even with good rainfall throughout most of the growing season, all three soil moisture sensor thresholds were met and irrigation applications went out. Plots that were held to a -15 kPa threshold received 5 inches of irrigation, -30 kPa received 3 inches of irrigation, and the -60 kPa received 1.5 inches of irrigation. All of these irrigation applications occurred during the months of August and September, until that point soil moisture levels never met the threshold in any of the plots. During these months is when soybean reaches its maximum water use, so it is not surprising that sensor thresholds were triggered at that point in time.

After harvesting soybean on November 12th all data was analyzed. From the 2021 trial, similar results were obtained to the 2020 trial where no significant differences in plant height or total nodes were measured at the mid-season (7/23) or harvest timings (11/3) regardless of how the treatments were irrigated (Figures 1 and 2). Furthermore, no significant yield differences were observed among treatments, and soybean yield ranged from 57 bushels/ac to 65 bushels/ac including the non-irrigated treatment (Figure 3). Taking into account the amount of water applied for each irrigation treatment, the -60 kPa threshold provided the greatest irrigation water use efficiency (IWUE) compared to both the -30 and -15 kPa thresholds, and the -30 kPa threshold out preformed the -15 kPa threshold. This is not surprising since more water was applied at the -15 and -30 kPa threshold compared to the -60 kPa threshold and no significant yield differences were obtained. Last, when comparing the irrigation water use efficiency of each treatment to the net return above irrigation costs associated with each of the three threshold treatments, this data suggests that the -60 kPa threshold would use the least amount of irrigation water while providing the most profit for the producer at $12 per bushel soybean price. Given that in both 2020 and 2021, rainfall was above average for the area, these results may differ in a below-average or dry year.

Key Performance Indicators
Key performance indicators for this study were determined at harvest (to determine if irrigation treatment and threshold value had any direct yield benefit). The water use efficiency data and yield data from this trial helped determine if there was an optimum soil moisture threshold that can be utilized for irrigated soybean. These data were compared with the data collected and generated in the 2020 growing season. This research helps develop irrigation scheduling recommendations where soil moistures sensors are incorporated. Furthermore, this research allows for a threshold to be selected that maximized yield and water use efficiency. Factors that impacted this research in 2021 were similar to the 2020 growing season, where timely and above-average rainfall resulted in crop water use demand being met through rainfall rather than supplemental irrigation. An additional site-year during a dry year would be beneficial to this data set.

Next Steps
The next steps in this project would be to implement similar treatments again in a year where below-average rainfall occurs to compare the results of the 2020 and 2021 growing seasons to that particular year. If such a year were to occur it would help validate the sensor threshold recommendation for soybean in a year where significant quantities of irrigation water are being applied to soybean. Additionally, these results will be shared with consultants, soybean farmers, and other irrigators throughout the region to reiterate the fact that in an above-average rainfall year soybean does not need to be irrigated excessively and a threshold of -60 kPa would result in the most profitable threshold that could be used. This research has been presented at the Edisto REC field days, CCA training, and will be written in an Extension report/bulletin and shared on the SC Crops Blog. With an additional year of data, the PI would like to publish the findings in an appropriate academic journal.

View uploaded report

Irrigated row crop acres in South Carolina have steadily increased over the last 10 years. Furthermore, groundwater regulation in SC has also increased with regard to well permitting. To maximize soybean yield while reducing water usage an appropriate irrigation schedule coupled with a scheduling tool needs to be incorporated into irrigation strategies. By determining appropriate soil moisture sensor thresholds specific to SC coastal plain soil textures and soybean, growers who adopt these technologies and recommendations can maximize profit by maximizing yield and improving irrigation water use efficiency. Irrigation was applied with an overhead lateral move irrigation system. Irrigation was then scheduled with Watermark soil moisture sensors installed at depths to monitor soil moisture within the top 24 inches of the rooting zone. All soil moisture sensors were installed within the planted row after planting. Treatments consisted of three thresholds 1). 25% Maximum Allowable Depletion (MAD), 2). 50% MAD, 3). 75% MAD, and a non-irrigated treatment for comparison purposes.

The 2021 growing season would be considered an above-average rainfall for Blackville, SC. Even with good rainfall throughout most of the growing season, all three soil moisture sensor thresholds were met and irrigation applications went out. Plots that were held to a -15 kPa threshold received 5 inches of irrigation, -30 kPa received 3 inches of irrigation, and the -60 kPa received 1.5 inches of irrigation. All of these irrigation applications occurred during the months of August and September, until that point soil moisture levels never met the threshold in any of the plots. During these months is when soybean reaches its maximum water use, so it is not surprising that sensor thresholds were triggered at that point in time.

After harvesting soybean on November 12th all data was analyzed. From the 2021 trial, similar results were obtained to the 2020 trial where no significant differences in plant height or total nodes were measured at the mid-season (7/23) or harvest timings (11/3) regardless of how the treatments were irrigated (Figures 1 and 2). Furthermore, no significant yield differences were observed among treatments, and soybean yield ranged from 57 bushels/ac to 65 bushels/ac including the non-irrigated treatment (Figure 3). Taking into account the amount of water applied for each irrigation treatment, the -60 kPa threshold provided the greatest irrigation water use efficiency (IWUE) compared to both the -30 and -15 kPa thresholds, and the -30 kPa threshold out preformed the -15 kPa threshold. This is not surprising since more water was applied at the -15 and -30 kPa threshold compared to the -60 kPa threshold and no significant yield differences were obtained. Last, when comparing the irrigation water use efficiency of each treatment to the net return above irrigation costs associated with each of the three threshold treatments, this data suggests that the -60 kPa threshold would use the least amount of irrigation water while providing the most profit for the producer at $12 per bushel soybean price. Given that in both 2020 and 2021, rainfall was above average for the area, these results may differ in a below-average or dry year.

From this research, we expect to establish soil moisture sensor threshold values for soil tension type soil moisture sensors for soybean grown in the coastal plain of SC.