DETERMINATION OF SOIL MOISTURE LEVELS Alberf W. Marsh 1 WHY MEASURE SOIL MOISTURE? irrigation before loss by deep percolation occurs. The purpose of irri.gation is to replenish the water Purpose4 stored in soil that has been extracted by living plants and Another purpose for soil-water measurementis to used by them for their growth. Soils store this irrigation detect problemsthat may exist relatedto water in the root water for future useby the plants.Soils, depending on their zone. The measurementscan frequently serveas a guide for texture and structure, can store and release limited correcting problems when they have been detected and quantities of water that varies among different soils. The determined. A soil may have poor drainage so that measurementof soil water is useful in severalways as a irrigation water causesan excessivelywet soil or possiblya guideto irrigation management. rising water table. Soil-water measurementcan alert the grower to the need for artificial drainageor guide him to Purpose1 limit irrigation until the soil returns to a normal wetness. The prime Ptlrposeof soil-water measurementis to Poor soil aeration will produce various tree symptoms determine the need for irrigation or the right time to difficult to explain. Poor ~ration is usually associatedwith irrigate. Irrigating at the correct time before the soil has or caused by excessivesoil wetness. a condition easily becometoo dry improvescrop performancecompared to shown by soil-water measurement.Steps to correct it can allowing the soil to become dry enough to cause plant be guidedby continuing soil-watermeasurement. stress. Soil-water measurement can improve irrigation efficiency and water conservationby informing the grower TYPES OF SOil-WATER MEASUREMENTS that his field does not need irrigating until some time later. Soil scientistsand many others recognizethat there are 2 distinct types of soil-watermeasurement that can be Purpose2 made (6). Simply stated, one measuresthe quantity of Another purpose of soil-water measurementis to water in soil, the other measuresthe degree of wetness measurethe amount of water at 2 separatedates to permit produced by the presenceof water in soil. Each will be calculation of water consumption by the crop between described,its usesstated and the methodsfor making the the dates of measurement.This tells the rate of water use measurementsoutlined. by the crop during the measuredperiod. The annualwater requirement of the crop is determined when such Quantity of Water measurementsare continued for a year or more. Quantity of water can be measuredas the weight fraction of water comparedto the weight of soil in which it Purpose3 is contained. It may be stated as the grams of water per The amount of water the soil is able to store from an gram of soil or pounds of water per pound of soil. Most irrigation can be calculated from the samemeasurements. often it is expressedas percent or gramsof water per 100 This tells how much water can be applied effectively at an grams of soil for which the symbol Pw is used meaning percentageof water on a weight basis.The soil is not dry when measuredbut the calculation is based on the dry 1Extension Irrigation and Soils Specialist, Cooperative Extension weight of soil in which the water is contained. Service, University of California, Riverside. The measurementis made by collecting a sampleof 20 21 the soil to be measuredfrom the appropriate depth and usually not reliable closer than 6 inchesto a discontinuity. location, placing it immediately in a water-tight container The radiation hazardto an oper~tormust be consideredbut so that it will not dry during collection of further samples is lessthan normal backgroundif directions for use of the or transport to a laboratory. The $ample is weighed as instrumentand its shieldare followed correctly. receivedin the laboratory and againafter drying in an oven The greatest advantage of the neutron meter is for 24 hours at 1050 C. The weight of water lost (W) and possibly the direct measurementof Pv . the form usually weight of dry soil (S) are obtained from which the Pw needed in irrigation and water use Studies.Values of Pw is calculated.P =W/S x 100from theseweighings. obtained from soil samplingmust be convertedto Pv by the IrrigationWwaterand rain water, the sourcesof soil formula Pv = PwDb. Db is the bulk density of the soil water, are usually measuredand consideredon a volume which is the ratio of the weight of a unit volume of undis- rather than a weight basis.It is, therefore, helpful to know turbed soil to the weight of the same volume of water. the volume fraction of water in soil suchas inches of water The bulk density must be determined by a separate in a selecteddepth of soil. This is actually a depth fraction measurement for each soil and each depth sampled. An and not a volume fraction but the 2 areequal when related undisturbed core of soil is carefully cut using a metal to an areaof soil surfacesuch as square meter, squarefoot, cylinder of known volume to make this measurement. The hectare,or acre. The symbol Pv is usedmeaning percentage soil is trimmed so the cylinder is exactly filled and its dry of water on a volume basis. weight determined in the laboratory. Quantity of water as the depth fraction, volume Soil-water content is used for Purposes 2 and 3 fraction, or Pv can be measureddirectly by usingan instru- where changes in water volume tell the rate of soil-water ment called a neutron probe (4,5). The probe is inserted depletion by growing plants and show the soil profile in previously prepared accessholes that are lined with a capacity for holding water without excessive Jossto deep metal casing.The probe contains radioactivematerials that percolation. radiate fast neutrons able to passunchanged through the metal casingand dry soil. The neutrons are changedto a Relati~ Wetnessof Soil (WaterPotential) lower energy form called slow neutrons when they Water is held in soil as films by attraction between encounterwater molecules.A portion of the slow neutrons water molecules and soil particles. The wetnessof soil are defl'ectedback to the probe wherethey are counted by dependson the thicknessof the water films, which depend a rate meter or cumulative counter. These counts when in turn on the amount of water and the number of soil properly calibrated tell the amount of water in the soil on particles on which it is spread.Thin water films are held a volume basis. The probe can be placed at successive more tenaciously by the soil than thick water films and, depths in the soil to measurethe water content at e~h therefore, require more energyfor removal.Soil physicists depth from which the total water content of the soil profile describethe energy condition of soil water as a potential, can be obtained. which is usuallycalled soil suction for easierunderstanding. The neutron device has severaladvantages over soil It equatesto the suction requiredto extract water from soil sampling for measuring water content. Soil sampling whether by a mechanicaldevice or a plant root. destroysthe particular spot from which the sampleis taken, Water moves in soil in responseto differences in by removing the soil and altering the immediate suction. Thin water films have a greatersuction than thick surroundingsso that a later measurementmust be made water films so water will move from soil with thick water elsewhere. The neutron device is nondestructive and films to soil with thin water films. Water films around the measurementsmay be taken repeatedly from exactly the root becomethinner when a plant root exerts suction to same soil and provides greater accuracywhen comparing extract moisture,thereby creatingmovement of water from water content changeswith time as in water depletion nearby soil where films are thicker. The rate at which this studies.Measurements in the field can be mademuch more movementcan occur is largely dependenton the thickness rapidly than taking samplesand needsno further laboratory of the films through which the water is transported.The effort. This advantage is especially great for moisture stressthat a plant may endureis as likely to result from the measurementsat depths greaterthan 2 or 3 feet (61 or 91 slow rate of water movementas the films becomethinner cm). as it is from the greatersuction required to extract water The soil measuredis a sphere around the probe. from thin films. The measurement will be incorrect if the sphere is Soil suction can be measureddirectly by instruments truncated by a sharp alteration or discontinuity of the soil called tensiometers,which havebeen adequatelydescribed such as the soil surfaceor a suddenchange of texture in the (7, 8, 9). Most tensiometers for commercial use read soil profile. Distanceof travel of neutronsvaries inversely directly in centibars(cb) of soil suction and havea practical with moisture content of the soil, but measurements are limitation of readingbetween 0 and 80 cb. Zero suction is a 22 state of soil saturation in soil-water terminology, and The tensiometer is pushed firmly into the prepared hole 15,000 cb is frequently equatedwith the soil suction at the and should reside with a tight fit so that the porous cup is permanent wilting point. It would seem that the in close contact with undisturbed soil that contains active tensiometer measuresonly a very smalr fraction of the roots. Some physical protection around the instrument is range-of soil water available to plants. Fortunately, the de$irable after installation to avoid damage from scaleis not equally divided and the rangefrom 0 to 80 cb equipment, field workers or possibly frost. includes from 50 to 90% of all of the water availableto The proper use of tensiometers involves taking plants in most soils. Water held at suctions greater than readings periodically, such as 2 or 3 times a week at the 80 cb movestoo slowly to supply the needswithout produ- same time of day, preferably before the day warms up, and cing stressfor sensitiveplants with small root systems.A keeping them filled with water as needed.