Application of Neutron Gauge in Agriculture

IntroductionThe subject of soil moisture has long been of interest in agriculture. For centuries the farmer has picked up and felt a handful of soil to determine the best time to plough his fields. The amount of moisture in the soil is also of great importance in hydrology, forestry, and soil- mechanics engineering. Consequently, much effort has been expended in developing methods and equipment for measuring soil moisture under field conditions.Determination of soil moisture is one of the most difficult measurements required in the field of hydrology. Measurement of soil moisture ranges from the method of feeling the soil to the use of complicated electronic equipment using radioactive substances. The development of equipment has been directed primarily toward instruments that continuously measure changes in moisture content at a single sampling point.

Accurate measurement of soil moisture is very critical to all field studies of soil-plant-water relationships. This is because soil moisture is a key variable in controlling the exchange of water and heat energy between the land surface and the atmosphere through evaporation and plant transpiration. As a result, soil moisture plays an important role in the development of weather patterns and the production of precipitation. Irrigation water management requires timely application of the right amount of water. Competition for water, high pumping costs, and concerns for the environment are making good water management more important. Managing irrigation water needs to combine a method of measuring soil moisture with some method of irrigation scheduling. Measuring soil moisture detects if there is a water shortage that can reduce yields or if there is excessive water application that can result in water logging or leaching of nitrates below the root zone. Measuring soil moisture also can build an awareness and knowledge of each irrigated field that is invaluable for planning and management. Monitoring soil moisture levels is required for effective irrigation water management. The measurement technique should be reliable, dependable, simple, cost-effective and non-destructive. The neutron gauge meets all these requirement.

There are several methods of soil moisture measurements;A. Gravimetric methodsB. Electrical-resistance methods. C. Heat-diffusion methodsD. Absorption methods E. Tensiometric methodsF. Penetration methods. G. Radioactive methods

GRAVIMETRIC METHODThe gravimetric method involves collecting a soil sample weighing the sample before and after drying it, and calculating its original moisture content. The gravimetric method is the oldest but still continues to be the most widely used method for obtaining data on soil moisture. Because it is the only direct way of measuring soil moisture, it is required for calibrating the equipment used in the other methodsELECTRICAL-RESISTANCEThe electrical-resistance "blocks" developed by those named above operate on the principle that resistance to the passage of an electrical current between two electrodes buried in the soil will depend upon the moisture content of the soil. Nylon or Fiberglas fabric or plaster of Paris surrounding the electrodes permits uniform contact with the soil moisture. When buried in the soil, the porous material of the block readily absorbs moisture or gives it up so that the moisture content of the block tends to stay in equilibrium with the moisture content of the soil. These moisture-content changes cause changes in electrical resistance which are measured by a meter at the surface. The resistance read on the meter is converted to moisture-content values by means of a calibration chart. The calibration chart is prepared by correlation, either in the field or in the laboratory, of gravimetric moisture-content values and resistance readings for the soil in which the blocks are buried. Laboratory calibration consists of drying and intermittently weighing soil cores in which blocks have been inserted. Field calibration consists of taking gravimetric samples as close as possible to blocks that have been buried in the field, and relating the moisture content of the sample to the measured resistance.HEAT-DIFFUSIONThe heat-diffusion method is based upon the principle that the heat conductivity of a soil varies with its moisture content. The temperature rise caused by an electrically activated heat source installed in the soil is measured by a sensitive temperature-measuring device and is correlated with moisture content. Wet soil will conduct heat rapidly away from the heat source in the cell and will thus have a smaller temperature rise than dry soil.ABSORPTIONLivingston and Koketsu developed porous points or blocks that would absorb moisture from the adjacent area when installed in the soil. The soil moisture was then estimated from the change in weight of the points or blocks. Wilson and Stoeckeler did additional work on the use of absorption blocks. Davis and Slater used an absorption block consisting of a porous chamber that contained a close-fitting plug that could be removed for weighing. The plug overcame the disadvantage of having to disturb the installations in the soil each time the blocks were to be weighed. Dimbleby later developed a pencil-type absorption block which is stuck into the soil; the moisture contents are estimated from the color changes of the "pencil." This method is more qualitative than quantitative and has considerable inherent error; it has never been used extensively.TENSIOMETRICA tensiometer consists of a porous point or cup (usually ceramic) connected through a tube to a pressure-measuring device. The system is filled with water and the water in the point or cup comes into equilibrium with the moisture in the surrounding soil, water flows out of the point as the soil dries and creates greater tension, or back into the point as the soil becomes wetter and has less tension. These changes in pressure, or tension, are indicated on a measuring device, usually a Bourdon-tube vacuum gauge or a mercury manometer. The tensiometer may also be attached to a pressure recorder or to an electronic pressure transducer to maintain a continuous record of tension changes.

PENETRATIONMoisture content may be estimated by relating it to the force required to push an instrument through the soil. Allyn and Work developed an instrument they called the "availameter" that measured the force required to drive a pair of needles into a soil core. Allyn reported a newly developed soil probe with which he found moisture-content estimation possible within 0.5 percent.RADIOACTIVE METHODSThis method is based on the principle of measuring the slowing of neutrons emitted into the soil from a fast-neutron source. The energy loss is much greater in neutron collisions with atoms of low atomic weight and is proportional to the number of such atoms present in the soil. The effect of such collisions is to change a fast neutron to a slow neutron. Hydrogen, which is the principal element of low atomic weight found in the soil, is largely contained in the molecules of the water in the soil. The number of slow neutrons detected by a counter tube after emission of fast neutrons from a radioactive source tube is electronically indicated on a sealer.Neutron Gauge The neutron gauge use neutron emitting radioactive material for moisture measurement which is in essence a measure of hydrogen-containing (hydrogenous) material. Water contains a large amount of hydrogen. The radiation of high-energy (fast) neutrons interacts with the similar sized nucleus of hydrogen atoms. The detector which is a gas filled chamber that is sensitive, not to high-energy neutrons, but to very low energy (thermalized) neutrons. The source and detector are fixed in position relative to each other and the measurement technique uses the backscatter geometry.The neutron source contains two materials: the radioactive, americium-241 (241 Am) and the non-radioactive, beryllium (Be). The 241 Am emits alpha particles and some photons. The alphas initiate the process that results in neutron emission. Since the 241Am and Be are intimately mixed together in the sealed source capsule, the alphas never escape. They do strike the Be and, in a process called "alpha-in, neutron-out", cause the Be to release a high-energy neutron [9Be (, n) 12C]. The individual neutrons may have any of several energies but they average about 5 MeV. These high-energy neutrons are emitted in all directions. Of the ones that enter the media (e.g. soil) to be measured, a small portion will interact with hydrogen nuclei and lose energy and change direction.The process of losing energy is called elastic scattering. Some of the neutrons will lose energy and become "thermalized". A thermal neutron has reached the mean temperature of the material, and has energy of about 0.025 eV. The detector is blind to high-energy neutrons, but is sensitive to thermalized neutrons. So, the thermalized neutrons that scatter back into the detector will be measured. The detector is a chamber filled with helium-3 (3 He) or boron trifluoride (BF) gas.