FABRICATION AND TESTING OF GYPSUM BLOCKS FOR SOIL MOISTURE DETERMINATION

Ajayi A. S.*, Eleta P. O., Duweni E. C. and Ehiomogue P. Department of Agricultural and Bio-Environmental Engineering Technology, Auchi Polytechnic Auchi Department of Agricultural Technology, Auchi Polytechnic Auchi Department of Civil Engineering, University of Ibadan, Oyo State Nigeria Department of Agricultural Engineering, Michael Okpara University of Agriculture, Umudike, Abia State Nigeria *Corresponding author: ajayistan@gmail.com, +2348039714411 ABSTRACT Over the last decade, many new developments of soil moisture sensors have been evolved, especially those based on the frequency domain reflectometry (FDR) capacitance technique, due to the rapid developments in the microelectronics industry. This project work is aimed at field study of gypsum blocks for soil moisture determination. Three types of GBs were fabricated and used in this study. The dimension for GB as given by Michael (1978) was


INTRODUCTION
In irrigation water management, water use represents a substantial opportunity for agriculture water savings. Automation of irrigation systems, based on Soil Moisture Sensors Systems (SMSS) has the potential to provide maximum water use efficiency by maintaining soil moisture at optimum levels. Fast and accurate monitoring of soil moisture plus the ability to do depth measurement is vital in this age of water scarcity. There is tremendous pressure and challenge to produce more out of less water, at the same time protecting and reducing risks to the environment. Some of the desirable attributes of the technique include being accurate, rapid, reliable, simple and cost-effective (Charlesworth, 2000). Over the last decade, many new developments of soil moisture sensors have been evolved, especially those based on the frequency domain reflectometry (FDR) capacitance technique, due to the rapid developments in the micro-electronics industry. This resulted in many relatively cheap and small sensors being manufactured, giving much more options to the traditional neutron scattering technique, which was the most commonly used method since its development in the 1950s (Gardner and Kirkham, 1952;Van Bavel et al., 1956;Gardner, 1986).While the new sensors claim to be accurate with minimal skill to operate, costeffective, and many have logging capability, their performance under different soil and cropping systems is only slowly being tested; few papers described comparisons of these methods with the traditional neutron-probe technique. However, it is well known that this category of sensors in general have a small sphere of influence and are very sensitive to small air gap around the tubes during installation, cracks and macrospores created by root activities, as well as positional changes in orientation within the tubes. Because of this, good sensor-tube-soil contact for reliable estimation of soil moisture is extremely critical (Evett and Steiner, 1995;Charlesworth, 2000). Soil moisture is a major measurable parameter to be considered in making irrigation management decisions. To this regard, methods on how to provide adequate irrigation water have to be careful selected. Soil moisture measurement is one of the best and simplest ways to get feedback to help make improved water management decisions (Shock et al., 1998). Soil moisture monitoring optimizes irrigation by helping the irrigation manager keep soil water content within a target range. This practice reduces the potential for excess soil water and leaching of agrichemicals, but it requires selection of a suitable method for soil moisture estimation (Muñoz-Carpena et al., 2002;2003).
The need to know how much water present in the soil arises frequently in many agro-ecological and agrohydrological investigations. As a result of the importance of knowing soil moisture status, so many methods and devices have evolved over the years to either directly measure or estimate soil moisture content. These methods and devices have been broadly classified by Pritchard (2005) into two: those that measure and express the soil moisture content quantitative (i.e. they indicate how much water present in the soil), and those that measure and express the soil moisture content qualitatively (i.e. they indicate how tight the water is held with the soil pores). Some of the quantitative methods and devices include the gravimetric method, neutron scatter method using the Neutron probe or Hydro-probe, dielectric constant method using devices like hand-push probe or Theta probe, Time Domain Reflectrometer (TDR) or Frequency Domain Reflectrometer (FDI). The qualitative methods and devices include the use of suction plate apparatus method, pressure plate and pressure membrane apparatus methods, use of tensiometers, porous blocks, and electrical resistance blocks commonly referred to a Gypsum Block (GB).
The GB has been around since the 1940s making it one of the oldest methods of soil moisture measurement (MEA, 1997). Ross (2007) described GB as electrical resistance device which uses gypsum (CaSO4) as a porous material in which electrodes are embedded. The electrical resistance between the electrode changes with changes in moisture content. Thus, the measured electrical resistance can be calibrated to moisture content or tension in the soil (Ross, 2007). According to Majumdar (2004)), the electrical resistance of dry gypsum is nearly infinite, but when the gypsum is permeated with water, the electrical conductivity approximate that of an average textured soil at the same water content (Bouyoucos, 1965). The principle of operation then relies on hydraulic content between water in the porous block and soil water. Starting with a saturated soil and a saturated GB, the two systems are in equilibrium. As the soil dries, it matric potential becomes more negative, setting up a hydraulic gradient that results in water being removed from the gypsum block. With less water in the block, the electrical resistance increases. The opposite happens when the soil water content increase, the soil matric potential become less negative, water flows into the gypsum block and the electrical resistance decreases (Wood et al., 1998). This paper aims to fabricate and evaluate gypsum blocks for soil moisture measurement.
Two shapes of gypsum blocks are common: cylindrical and rectangular shapes with concentric or parallel electrodes (Godwin, 2000). The sizes of the rectangular shapes are about a match box (Campbell-Clause, 2005). Michael (1978) gave the dimension of the rectangular gypsum block as 5.5 cm long, 3.75 cm wide, and 2.0 cm thick, and acknowledged that the sensitivity of the block is affected by its size. However, the magnitude of the effect with respect to size of blocks was not reported. Michael (1978) also made mention of the use of a pair stainless wire mesh as electrodes in the gypsum block. The stainless wire mesh electrodes are to be placed 2.0 cm apart within the block and connected to electrical cables of desired length which the electrical resistance is measured. Another electrode material that has been suggested by Measurement Engineering Australia MEA (1997) is stainless steel rod like nails or motorbike spokes.

MATERIALS AND METHODS Fabrication of gypsum block
Three types of GBs were fabricated and used in this study. Table 1 shows the description of the blocks as seen in plate 1. The dimension for GB as given by Michael (1978) was used as reference. The other two sizes were two-third ( ) and one and half (1 ) of the Michael's size, respectively as shown in Rectangular molds were first prepared to the different sizes of blocks constructed using a soft plywood as shown in plate 2. Pair of electrodes (either wire mesh or stainless steel rod) was cut to size so that they can be completely buried within the blocks and covers 75 % of the total length of the block. The pair of electrode for each block was placed 2 cm apart and connected to electrical cable cut to the desired length. Two parts of CaSO4 powder was properly mixed with one part of water forming a slurry or paste as shown in plate 3, and was carefully poured into the molds making sure that the positions of the electrodes did not shift. The blocks were then allowed to dry under the sun for 48 hours, after which the molds were removed. The blocks were left in water for 24 hours, after which were allowed to dry in the open air at room temperature. While they were drying at room temperature, the changes in resistance were monitored twice a day for three days. This was done to test if the blocks were working, particularly to ascertain that the electrical cables were not disconnected from the electrode while casting the blocks.
seen from the Table that the electrical resistances were also changing with changes in soil moisture content. This implied that all the block types were responsive and sensitivity to changes in moisture content. It may be observed from Table 3 the Type III block (1 times the dimensions of the reference block size) recorded the least values of electrical resistance, while the Type II block ( times the dimensions of the reference block size with wire mesh electrodes) recorded the highest values of electrical resistances. The implication of these results is that block size do affect the response of the gypsum block. Blocks of smaller sizes gives higher values of electrical resistance while blocks of larger sizes tend to give lower electrical resistance values for the same soil moisture content. One reason while the smaller block size may show higher resistance values may be because it holds less water as the moisture content of the soil decreases compare to the blocks of larger size.   Vol.9, No.3, 2019 74

Calibration of the blocks
The table shows that Type I gypsum block measured moisture content more accurately than others, R 2 of 0.93 was obtained which is closer to unity (1) for perfect fit between observed and resistance based moisture content.

CONCLUSIONS
Three sizes of rectangular gypsum blocks were fabricated and tested to measure soil moisture content for Agricultural Engineering Demonstration Farm. The electrical resistance of one of the three blocks types with dimensions 5.5 cm long, 3.75 cm wide, and 2.0 cm thick and stainless wire mesh as electrode material (being the specifications given by Michael, 1978), was used as reference for the other two blocks. The electrical resistance of the gypsum blocks was found to be affected by block size.