Modeling of Soybean Plant Sap Flow

Soybean (Glycine max. (L.) Merr.) sap flow during the growth stages in relation to soil moisture, nutrition, and weather conditions determine the plant development. Modeling this process helps to better understand the plant water-nutrition uptake and improve the decisions of efficient irrigation management and other inputs for effective soybean production. Field studies of soybean sap flow took place in 2017-2021 at Marianna, Arkansas using heat balance stem flow gauges to measure the sap flow during the reproductive growth stages R3-R7. Plant water uptake was measured using the lysimeter-container method. The uniform sap flow-based hydraulic system in the soil-root-stem-leaf pathway created negative water tensions with osmotic processes and water surface tensions in stomata cells as water evaporation layers increase are the mechanism of the plant water uptake. Any changes the factors like soil water tension, solar radiation, or air relative humidity immediately, within a few seconds, affect the system’s balance and cause simultaneously appropriate reactions in different parts of the system. The plant water use model was created from plant emergence, vegetative to final reproductive growth stages depending on soil-weather conditions, plant morphology, and biomass. The main factors of the model include solar radiation, air temperature, and air relative humidity. The effective sap flow uptake occurs around 0.8 KPa VPD. Further research is needed to optimize the model’s factors to increase the plant growth dynamics and yield productivity.

Relations among Sap Flow, Soil Moisture, Weather, and Soybean Plant Parameters in High Water Demand and Final Growth Stages

The dynamics of sap flow in relation to plant morphology and weather conditions during reproductive growth of soybean (Glycine max. L. Merr.) influence decisions pertaining to efficient irrigation management and other inputs for high yields. Field studies began in 2017 at Marianna, Arkansas to measure moisture dynamics of soybeans during seed fill (R5 to R7) using heat balance stem flow gauges. Sap flow was highly correlated to solar radiation with maximum rates observed during beginning seed fill (R5). A solar radiation efficiency (SRE) value, calculated as hourly sap flow rate per Watt-hour of solar radiation (g/Wh2), is proposed. The SRE relates to crop water demand and hydraulic resistance of the soil-root-stem-leaf-pod-seed pathway. SRE values ranged from 0 - 1.2 g/Wh2. Soil moisture, growth stage, time of day, and weather conditions influenced the SRE, with higher values observed in the morning, late afternoon, and during R5 growth. Peak sap flows of 39 g/h at R5, 25 g/h at R6, and 3 g/h at R7 occurred. The ratio of measured sap flow to estimated crop evapotranspiration was 0.9 to 1.3 during R5 to R6.9 (maximum dry matter), but dropped to 0.2 at R7. Further research is needed to better understand late season reproductive moisture dynamics in soybeans.

Response of Two Inexpensive Commercially Produced Soil Moisture Sensors to Changes in Water Content and Soil Texture

The use of low-cost (<200 USD) soil moisture sensors in crop production systems has the potential to give inference on plant water status and therein drive irrigation events. However, commercially available sensors in this price range vary in sensing methodologies and limited information on sensor to sensor relationship is available. The objective of this research was to test the response of the Watermark 200SS and Decagon 10HS sensors to changes in water content of three dissimilar soils representing common soils in Arkansas row-crop production in nine plastic, 19 L containers under variable environmental conditions. Both sensors were influenced by changes in soil temperature but the magnitudes of the temperature responses were small relative to the moisture responses. Furthermore, the 10HS sensor did not indicate a significant impact of soil texture on estimated volumetric water contents (VWCs). The small sphere of influence on the tested soil moisture parameters coupled with the substantial evaporative demands and temperatures under which this experiment was conducted resulted in suspected non-uniform drying and wetting of the tested containers. Subsequently, non-linear relationships were noted between 10HS estimated VWCs and actual container VWCs and the 200SS predicted lower water potentials than calculated by converting container VWC to soil water potential. The failure of the sensors to accurately predict container VWC highlights the importance of understanding the relatively small quantity of soil on which these sensors rely as well as the potential variability in soil moisture within a very limited volume. The authors caution users of sensors that soil variability may be one of the most important considerations in sensor deployment.