Study on the Expression of Soil Moisture during Research for Mechanism of the Farm Machinery. (1)

Relation between the Soil Structure and pF Value

Abstract

The study is taken up to look over the connection between soil structure and the energy with which moisture is held by a soil. To indicate the energy, applying the pF-value. The pF-value is the logarithm of the hieght, in centimeters, of a water column necessary to produce a force epual to the energy to draw water apart from a soil.
The soil, in natural state, has a very complex structure, so the making study of a connection between soil structure and pF-value, is in need of simplification the factor of structure.
The test soil is picked out from the volcanic light soil, and is divided into these three classes by sieving in water, 2-1mm, 0.5-0.25mm, and below 0.25mm in diameter.
The apparatus to measure the pF-value is to use sucking system; in which manometer, constant temperature box, pressure adjusting flask, burette, and soil vessel with filter are connected. Each three kinds of soil are filling up to the vessel first time densely and second time loosly. Thus, the soil structure changes to six varieties. The results of measurment are as Fig. 6.
This diagram says that the pattern of curves are decided by a diameter of the soil grains, not by the factor of “loosely” or “densely”.
The moisture is of a narrow range at low stage of the pF-value, and it becomes wide at high stage of pF-value.
The curves of “loosely” are placed rightwards to the curves of “densely”. It means the energy with which moisture is held by a soil, changes according to the soil structure.