AN ELECTRICAL RESISTANCE METHOD FOR THE CONTINUOUS MEASUREMENT OF SOIL MOISTURE UNDER FIELD CONDITIONS

Abstract

THE ELECTRICAL RESISTANCE OF SOILS WAS INVESTIGATED TO : (1) FIND SOME MEANS OF MINIMIZING THE EFFECTS OF SALT CONTENT, COMPACTION, TEXTURE AND TEMPERATURE ON SOIL RESISTANCE MEASUREMENTS, (2) STUDY THE RELATIONSHIP BETWEEN RESISTANCES AND SOIL MOISTURE, AND (3) DEVISE AN ACCURATE, RAPID METHOD OF MAKING A CONTINUOUS MEASUREMENT OF SOIL MOISTURE UNDER FIELD CONDITIONS. THE APPARATUS DEVELOPED CONSISTS OF ABSORPTION BLOCKS AND A SPECIAL ADAPTATION OF THE WHEATSTONE BRIDGE WITH WHICH TO MEASURE RESISTANCES. THE ABSORPTION BLOCKS ARE MADE OF PLASTER OF PARIS. THE POROUS ABSORPTION BLOCK PLACED IN THE SOIL WILL REACH EQUILIBRIUM BETWEEN THE MOISTURE IN THE SOIL AND THE MOISTURE IN THE BLOCK. THE MOISTURE CONTENT OF THE BLOCK FOLLOWS THE CHANGES TAKING PLACE IN THE SOIL. THE MOISTURE CONTENT OF THE ABSORPTION BLOCK DETERMINES ITS ELECTRICAL RESISTANCE. SINCE THE DENSITY, TEXTURE, COMPACTION, AND SALT CONTENT OF THIS BLOCK REMAIN FAIRLY CONSTANT, A CHANGE IN RESISTANCE INDICATES A CHANGE IN THE MOISTURE CONTENT OF THE BLOCK AND THUS, A CHANGE IN SOIL MOISTURE. THIS METHOD, DEPENDING FUNDAMENTALLY UPON MOISTURE EQUILIBRIUM BETWEEN THE BLOCK AND THE SOIL OFFERS A NEW TOOL IN THE STUDY AND CONTROL OF PLANT-SOIL RELATIONSHIPS. IT OFFERS A CONTINUOUS MEASUREMENT THROUGH WHICH FLUCTUATIONS IN SOIL WATER MAY READILY BE FOLLOWED. THIS IN SITU METHOD PRESENTS A MORE REPRESENTATIVE PICTURE OF ACTUAL SOIL CONDITIONS THAN MAY BE ASCERTAINED BY SAMPLING. PROCEDURES FOR USE OF THE METHOD ARE REVIEWED.