Impact of clay on crust crushing energy
Significant differences in crust crushing energy were found between 0 and 2% clay amendment treatments for the four soil series according to ANOVA (Table 2). However, no statistical significant differences in crust crushing energy were found between 2 and 4% clay amendment treatments for two silt loams and between 4 and 8% and 8 and 16% clay amendment treatments for the four soil series except Athena silt loam. This suggests that the effectiveness in increasing crust crushing energy diminished with progressive increase in clay amendment.
The relationship between clay amendment and crust crushing energy for the four soil types examined in this study is illustrated in Figure 5a. The relationship between clay amendment and crust crushing energy appeared to be nearly a binomial function for all soil types. This binomial trend was consistent with previous studies for a variety of soil types (e.g., Diouf et al., 1990; Hagen, 1995; Pi et al., 2019b). Clay particles play an important function in cementing sand grains together which then can cause considerable change in crust strength. Crust crushing energy increased with increasing clay content. However, Skidmore and Layton (1992) reported crust crushing energy steadily increased with the increasing clay content until clay content reached 33%. In this study, the clay content for all the soil types was much less than 33%, thus we did not find a decrease in crust crushing energy with increasing clay amendment.
Regression analysis revealed similar trends with ANOVA in that the rate of change in crust crushing energy diminished with progressive increase in clay amendment. Crust crushing energy increased by 226, 357, 306 and 7473% for Athena, Walla Walla, Warden and Farrell when clay amendment increased from 0 to 2%. Nonetheless, crust crushing energy increased by only 69, 41, 218 and 144 % for the four soil types when clay amendment increased from 2 to 4%.
The rate of change in crust crushing energy with progressive increase in clay amendment varied among soil types based on regression coefficients. The relationships between clay amendment and rate of change in crust crushing energy for the four soil types examined in this study is illustrated in Figure 5b. The relationship appeared to be nearly a linear function for all soil types. The higher regression coefficients suggest the rate of change in crust crushing energy with progressive increase in clay amendment was higher for Farrell sandy loam than Warden sandy loam, followed by Athena silt loam, and Walla Walla silt loam. Clay amendment was more effective in increasing crust crushing energy for Farrell sandy loam than other soil types. Actually, clay amendment was more effective in increasing crust crushing energy for two sandy loams than two silt loams. For example, crust crushing energy increased by 30 and 17 fold for Athena silt loam and Walla Walla silt loam and by 32 and 229 fold for Warden sandy loam and Farrell sandy loam in the presence of 16% clay amendment. This was expected because of lower clay content for sandy loam than silt loam.