Soil Moisture Monitoring in Sand
Tensiometers fitted with 0 to 40 centibar gauges offer the cheapest and most direct output for soil moisture monitoring in sand. They provide direct measurement of soil water tension, are inexpensive, simple to install and read, and are not affected by salinity.
To check that applied irrigation rates are not supplying too little or too much water, sand tensiometers can be installed within crops and read daily before applying day-time irrigation in summer and any irrigation at all other times of the year.
Sand tensiometers (LT Irrometer model or similar with 0 – 40 centibar gauges) can be installed at 15, 30 and 45 cm depths to monitor the effectiveness of irrigation scheduling.
Irrigation rate and frequency should be guided by evaporation and crop factors but can be fine tuned based on tensiometer readings.
Tensiometer readings can be recorded just prior to any daytime watering or late in the day if no daytime watering is scheduled. The tensiometers are installed close to plant roots. Tensiometers need to be inspected regularly to ensure the fluid reservoir is not low and that no air is within the column. (Note that a 1 centibar correction is applied per 10 cm of tensiometer column to adjust for the column of water in the body of the tensiometer, however for practical purposes, guideline tensions are given as ‘uncorrected’ readings which ignore the correction.)
Generally, the 15 cm tensiometer is used to guide the frequency of irrigation and the deeper tensiometers are used to adjust the amount of water that is applied.
Tensiometer readings less than 8 cb for the 30cm depth and 10 cb for the 45 cm depth indicated that the soil is near field capacity at these depths. This information is summarised in Table 1.
Table 1. Tensiometer guidelines for irrigating carrots on coarse sandy soils
|Tensiometer Depth (cm)||Level at which irrigation should be applied (centibars)|
|15 cm||7 cb or higher|
|30 cm||11 cb or higher (< 8 cb indicates soil is near field capacity)|
|45 cm||15 cb or higher (< 10 cb indicates soil is near field capacity)|