Abstract
This paper is the second of three reports dealing with the nature cracking paddy field after drainage of residual water.
The first paper presented the effect of developing cracks on drying. The test was done with a drying box in a near steady state conditioned room. In order to examine these properties in field conditions, several tests were made with a lysimeter (2×2×2 m, two kinds of clayey soils were used respectively), which represented field conditions after the lapse of several years.
Soil moisture was measured with a tensiometer which was set at depths of 5, 10, 20, 30, 40, and 50 cm in the lysimeter to know drying characteristics of clayey soil. A glass block electrode was used to measure a loss of soil moisture from the cracked surface by an electric resistance method.
And at the same time, the width of cracks which developed in the soil was measured at the points where the cracks intersected lines set at intervals of 25 cm both longitudinally and transversely in a 1×1 m square selected in the lysimeter.
To evaluate variations of soil hardening with drying, a penetrometer analysis was conducted on alternate days in the drying period.
The tests were done without cropping, and the results of the tests were as follows;
1) Drying characteristics of clayey soil in the lysimeter are described with dividing into two layers, an upper layer (0-20 cm) and a lower layer. Drying of the upper layer is divided into two or three stages: the stage in which the soil dries quickly to pF 2 in soil moisture and the following stage in which it dries slowly. And on the lower layer, the soil does not dry till the soil on the upper layer dries to pF 2 in soil moisture. In such clayey soil, most of moisture is lost by evaporation from soil surface.
2) A crack, when its width becomes 1.0 cm or so, accelerates drying of soil markedly, especially in the aforementioned second stage.
Cracks developed in the soil in the lysimeter cause the evaporation surface to increase more than twice.
3) There seems to be some relation between the total water loss from soil and the maximum width of cracks.
The drying of clayay soil is accelerated by the existance of cracks at the begining of drying period.
4) According to the results of the penetrometer analyses, there seems to be some functional relation between the pF value of soil moisture and the corn index with the initial bulk density used as a parameter.
5) The corn index of the lower layer increases gradually with the repetition of cyclic drying.
All these data show that the drying of clayey paddy field ought to be recognized an associated behavior of soil-water system, such as drying-shrinking-cracking-accelerating of drying= increasing of soil hardness.
