Microcontroller-based setup for soil wetness measurement

Citation

(2008), "Microcontroller-based setup for soil wetness measurement", Sensor Review, Vol. 28 No. 1. https://doi.org/10.1108/sr.2008.08728aaf.002

Microcontroller-based setup for soil wetness measurement

Microcontroller-based setup for soil wetness measurement

The wetness of the soil and its retention over a period of time is one of the most crucial considerations while choosing the type of crop. The soil wetness can also be used for estimating the water and energy fluxes and provides vital information regarding moisture to the atmosphere for precipitation, and controlling the pattern of groundwater recharge. Moisture content of the soil is also considered as one of the major factors in determining plant growth, especially in irrigated systems (Allison et al., 1983). The literature survey reveals many varied methods for determining soil water content, but most of them fall either in volume basis measurement or a tension basis measurement (Gardener, 1986; ICT International Application Note, 2006). As the moisture status of the soil is a critical factor influencing plant production and also for correct irrigation scheduling, it has fascinated many researchers.

A quick comparison of the different soil wetness measurement reveals pros and cons of the respective measurement technique. The capacitance and time domain reflectometry are the most popular techniques used in practice to estimate soil water content. The capacitance sensor-based measurement poses several advantages like higher sensitivity, ease of measurement by lowering the sensor into the access tube, well proven signal conditioning techniques by embedding the sensor in an oscillator circuit, etc. However, the main challenges are utilization of the full area of the sensor to make measurement accurate, further processing of the oscillator output to make it compatible with the microcontrollers, difficulty in calibration and last but not the least the overall cost of the setup.

The present paper describes a low cost, easily installable, soil wetness measurement system based on the 89S52 microcontroller. The system facilitates easy calibration with in situ water content sensing at a particular depth, or off-line measurement by feeding the soil in the sensor cap. It is successfully tested on soils of different composition and found not to alter soil densities. The response is fairly fast with good long-term stability.

Capacitive transducer

The sensing mechanism is based on a capacitor probe. A parallel plate capacitor has been designed in which the test sample is placed. The wetness conditions of the sample influences the dielectric constant or permittivity and in turn changes the capacitance of the capacitor transducer.

The transducer is concentric cylinder type of structure with a plastic case at the center used as a sample holder and two parallel aluminum plates are machined in parallel to form the capacitor.

The working of the transducer is governed by the following equation of the capacitance of parallel plate:

where, A – area of plates; d – distance between two plates; 1 – permittivity of medium.

The measurement setup comprises of the following blocks:

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  capacitive transducer;

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  signal conditioning;

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  analog to digital converter;

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  microcontroller; and

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  display unit (LCD).

The capacitive transducer is cylindrical in shape with a plastic cap at the center to hold the soil sample. The soil wetness varies the capacitance that in turn modulates the frequency of the oscillator in which the transducer has been embedded (Figure 2). The output is displayed on the LCD in the form of soil wetness in percentage from 0 to 100 for the soil sample under investigation.

Figure 2 Modulation of oscillator frequency with soil wetness condition

Soil wetness is one of the most important ecological factors for the farmers. In spite of research work for several years in this area, its biologically meaningful assessment still poses a challenge for the agriculturalist. The poor farmers from the developing countries cannot afford high-cost systems presently available in the market.

The authors have come out with a low- cost microcontroller-based soil wetness measurement system. The system is rugged, useful for all types of soils and exhibits good sensitivity as well as long- term stability. The calibration process is straightforward and with minor modifications in the code the instrument can be calibrated on the field itself. Accuracy is not an important issue in the soil wetness measurement as most of the time the system has to measure dynamic wetness status rather than the absolute values. The system is also useful to measure the moisture in case of grains, bulk solids, and wood. With the field application of the system it was found that it is not only useful for monitoring the absolute amount of water in the soil but also for several other purposes such as examining the amount of water used by plants, estimating the amount of irrigation water needed to refill the root zone, estimating the volume of water draining below the root zone and monitoring the soil-water storage in order to complete water-balance studies.