MEASURING SOIL MOISTURE

Press, Volume XCI, Issue 27670, 28 May 1955, Page 5

MEASURING SOIL MOISTURE

SUCCESSFUL NEW SYSTEM EVOLVED Accurate measurement of the microclimate in which pasture species grow has for a long time escaped the scientists concerned with pasture plants, but a new system evolved at the Grasslands Division of the Department of Scientific and Industrial Research and the Dominion Physical Laboratory appears to solve this tricky problem effectively. The new instruments give an accurate record of temperature and moisture just and just below the surface of the ground.

t The micro-climate of particular importance to pasture plants extends from about 16 inches below the surface to about a foot above it. Most meteorological records do not give enough information on this band, in which conditions may be very different from those of the general climate that the instruments record. The range of temperature during one day in the band about an inch deep just at the surface of the soil, for instance, has been found to vary from freezing point to 120 degrees Fahrenheit, whereas the range recorded by normal instruments removed from this band would be perhaps only a few degrees. One of the m,ost important considerations in studies of plant growth is the amount of moisture available to the plant. Soil moisture has usually been assayed by taking samples, weighing them, then drying them in an oven, and determining the quantity of moisture present by the difference in the two weights. This system gives total moisture, which is a very different thing to the amount of water in the soil available for the use of plants. At the Grasslands stations at Lincoln, Palmerston North, Gore, Ruakura, and Kaikohe, the instruments have been installed and will give an accurate mesaurement of the moisture available to plants on a variety of soils ranging from a sandy silt to an undrained clay, and eventually stations will be set up throughout the country. Electrical System

The system is electrical. A small cylinder of plaster of Paris has imbedded in it the terminals of an electrical circuit. The plaster of Paris takes up moisture as a plant would take it, and the interference with the electrical circuit through the presence pf moisture is shown on the dial of an instrument. There have been many attempts to use the absorbent qualities of plaster of Paris for this work before, but new techniques have been adopted to overcome the difficulties encountered earlier.

At the five stations, measurements of available soil moisture are taken once a day from 1,2, 4,8, and 16 inches below the surface, -a range which covers the depth occupied by most of the root system of the common pasture plants. A new form of electrical thermometer evolved in Holland is used for measuring temperatures over the same range of depths below the surface of the soil, and also at 1,3, and 12 inches above the ground. These thermometers deliver their, readings to the same instrument as the moisture determinations. Similar instruments are also used to measure humidity at 1 and 12 inches above ground level. The immediate applications of the new system of determining soil moisture and temperature, and temperature and humidify just above the ground, will be scientific. Partly because no satisfactory method of measuring conditions in the narrow band about the earth’s surface in which pasture plants live has heretofore been evolved, remarkably little is known of these conditions. There is an obvious practical application for the irrigation farmer in the measurement of the available moisture status of his land. The system would not only tell him when to irrigate, but could also tell him with considerable accuracy how much water the landneeded.