2) Discussion about relation between draft force and moisture content.
Refer to ASAE standard 497.4, draft force was estimated through the empirical equation.
Parameters such as c1-c3, Fi, for plow were obtained by regression using test results. Which done during the primary and secondary tillage at typically planting season, which optimal moisture content of soil. However, the case study shown in this article was specific case, since it was done in dry season (Jan-April), which is the common tine to do the primary tillage for sugar cane. During this time, the moisture content was low, 3.62%wb or 5.61%vol (the soil moisture content from Table 3, experiment of plot 3-Trash elimination disks plow), while the field capacity (FC) of the soil was 9.46%wb or 14.85% vol. Thus, primary tillage for the sugarcane in Thailand was mostly done when the soil is dryer than the operation in offer crops.
Refer to Kesawadkorn P. (2000), bearing capacity of this type of soil (silty sand) increased from 5.9 to 10.2 T/sq.m when the moisture content decreased from 15% to 9%wb, since it was collapsible soil which strength of soil greatly varied by moisture content. When we apply the above relation to extrapolate bearing capacity of this type of soil at 3.62%wb, bearing capacity could rise to 2.4 time of the bearing capacity at 15%wb moisture content. Consequently, the measured predicted draft as a results, A soil texture parameter (Fi) was calculated to be 2.4 time of medium-course texture’s Fi (Fi=0.58) mentioned in the form section.