Investigating the effects of soil moisture sensors positioning and accuracy on soil moisture based drip irrigation scheduling systems

Most soils in the Central Florida Ridge (CFR) area are Entisols that contain >95% sand, <3% clay, and <2% organic matter. Field capacity (θ fc ) is commonly ∼0.08 m 3 m -3 . Therefore, accurate estimation of soil water content (θ v ) is important in these soils. The objective of this study was to evaluate the performance of ECH 2 O probes when estimating θ v for scheduling irrigation in CFR soils. Probes were tested for (i) probe-to-probe output variability, (ii) soil volume sampled, (iii) sensitivity to salinity, temperature, and air pockets close to the sensor surface, (iv) pockets of very dry soil close to the sensor surface, and (v) performance after installation in the field. According to the calibration, a 1% change in water content corresponds to a probe output of 17 mV. Laboratory testing suggested that output variability from probe to probe can be a problem in these soils. The sampling volume of the probe was within 1.5 cm from either side of the sensor surface. Salinity induced during fertigation increased the output by about 200 mV, and for each 1°C drop in temperature, the sensor output dropped by 2.3 mV. When the bulk density was changed from 1.56 to 0.94 Mg m -3 , the output decreased by 3.5 MV for each 1% drop in air-filled porosity. When very dry soil lenses with <0.01 m 3 m -3 θ v were associated with the probe surface, the probe failed to sense the wet soil even 1 cm away from the sensor surface. Sensor failure was common due to water leaking into the circuit when sealing material deteriorated or casing material was damaged by insects. These issues need to be addressed before the probes can be considered reliable to estimate θ v or used in automated irrigation.

Performance of a capacitance-type soil water probe in a well-drained sandy soil

An automated soil moisture based drip irrigation system has been developed and tested
on a tomato crop. The system employs a quantified irrigation controller (QIC) developed by the
University of Florida to interface soil moisture monitoring devices with an irrigation timer.
Prescheduled low volume high frequency events are either allowed or blocked if in-field soil moisture
status is below or above a set threshold set point respectively. 
The field experiment was conducted during the winter season of 2004/2005 in Miami-Dade County.
Soil moisture based irrigation treatments used capacitance soil moisture probes and tensiometers,
and time-based schedules used historical evapotraspiration and local farmer practices. Fertilizer was applied by fertigation and automatically injected using venturi injectors. Drainage lysimeters
were used to collect leachate from the root zone to quantify differences in nutrient leaching. Large
savings in applied water occurred for the soil moisture based treatments, without a reduction in total
marketable yield compared to the time-based treatments (up to 80%). The soil moisture based
treatments resulted in 30% reductions in both volume of leachate and phosphorous load leached for
the season, when compared to grower practices. No significant differences were found for the load
of nitrogen leached due to high variability within treatments. The soil moisture based system
provides a viable means to increase water use efficiency, and reduce nutrient leaching from the root
zone, which reduces operation costs and the potential for groundwater contamination. Further
testing is suggested to determine the reduction in nitrogen leaching.

The Development and Testing of an Automated Drip Fertigation System for Sustainable Agriculture

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The Development and Testing of an Automated Drip Fertigation System for Sustainable Agriculture