Relationship Between Temporal Patterns of Soil Moisture and Microclimatic Conditions in Plasticulture Grown Poblano Pepper Under Tensiometer-scheduled Irrigation

Managing irrigation using soil moisture sensors for automation can allow for application of water to soils, at the appropriate amounts, rates, frequency and timing to meet crop water use while avoiding drought stress and maintaining optimum yields in high value crops. Frequent monitoring of soil water status within the root zone in response to atmospheric conditions is essential to evaluate the sensitivity of any sensor based irrigation scheduling method. Eight-week old transplants of poblano pepper (Capsicum annuum) ‘Don Amelio’ were planted 9 June 2010 at the University of Kentucky Horticulture Research Farm in Lexington, KY.  Transplants were set into 10-12 cm high raised beds covered with 0.025 mm-thick black plastic mulch (1.22 m wide) with a single line of drip irrigation tubing placed approximately 2.5 cm below the soil surface in the center of each bed. Automated irrigation was managed using switching tensiometers and which were placed equidistant (12-15 cm) from pepper plants and the edges of the raised beds, and at a depth of 20 cm from the upper surface of the bed.  On/off set points for the five automated irrigation treatments were as follows, -30/-25, -40/-35, -50/-45, -60/-55 kPa.  Irrigation treatments were implemented after plants were well established. The frequency and duration of the automated and manual irrigation events were recorded with data loggers (Hobo U9 State Data Logger). The soil was a well-drained Maury silt loam series. Soil moisture was logged at depths of 15 and 25 cm and environmental measurements of the study area were recorded at an hourly interval. Soil water potential was well correlated with environmental factors such as air temperature, relative humidity, dew point temperature, and solar radiation. Trend of soil water decline was in line with rise in VPD and/or ET especially during sunny and non-rainy periods of the study. Probes set at depths of 15 cm responded earlier to microclimatic conditions than probes set at 25 cm in all irrigation treatments suggesting the faster soil water depletion by evapotranspiration and timely replenishment by irrigation in this layer. The rate of change in soil water content decreased with increase in On/Off threshold soil water potentials as lower setpoints (-30/-25 kPa) hold more water and stay wetter than higher setpoint values (-60/-55 kPa) and respond  quicker to VPD and/or ET variations. The study confirms the prompt response of soil moisture sensors to varying atmospheric conditions while scheduling automated irrigation.