CHAPTER 5 RESULTS AND DISCUSSION
5.3 Season 2 Results and Discussion
5.3.8 Soil water
5.3.8.3 Hoekblok Soil Water Depth Profiles
All the repetitive sites of the four dominant soil forms volumetric water content were averaged of the growth stages and plotted in the soil water depth profiles.
5.3.8.3.1 Bainsvlei
Figure 5-29 illustrates the soil water depth profile of the Bainsvlei soil form. It is evident that there is an increase in soil water from planting to boll formation and a decrease from boll formation to harvest, followed by an increase from harvest to post-season, concluding more soil water is present in the profile post-season that pre-season. A significant increase in soil water in the topsoil is present during boll formation, but it is due to a rainfall event that occurred during this period.
When considering the boll formation stage, it is evident that there is a clear increase in soil water with depth, but the soil water content stays constant from 1500-2000 mm as anoxic conditions occur at this depth and the cotton roots does not thrive in this part of the soil profile. A clear increase in soil water with depth is evident during all these periods, except with planting where a constant soil water content is visible from 1400 – 2000 mm. The most significant soil water loss from boll formation to harvest occur from 500-1500 mm due to the soil water needs of the cotton during the vegetative growth stages as well as deep internal drainage of the soil horizons. There is a slight decrease in the topsoil water content from harvest to post-season due to evaporation.
It is evident that from planting the soil water was extracted from the topsoil to approximately 1600 mm where an increase in soil water is observed for the boll formation, harvest, and post-season period. The clear increase in soil water from 1600–2000 mm in both the harvest and post-season periods suggest that soil water accumulated on the soft plinthic horizon which forms a water table.
This feature is due to the interflow capabilities of the subsoil horizon. This soil profile had a big amount of soil water throughout the season which illustrates the effect of the yellow and red colours in the yellow-brown apedal horizon.
5.3.8.3.2 Bloemdal
Figure 5-30 illustrates the soil water depth graph of the Bloemdal soil form. It is evident that there is a decrease in soil water from planting to harvest. During post season there is an increase in soil water throughout the soil profile in comparison with the harvest period. This is due to the downward flow of the soil water deeper in the soil profile. There is a slight decrease in the topsoil from harvest to post-season due to evaporation. The harvest soil water line indicates a decrease in soil water with depth from 300–700 mm and then increases slightly from 700–800 mm to a soil water content the same as in the topsoil which illustrates that the cotton extracted most water from 300–700 mm in the soil profile and drainage from the subsoil horizons occured. From 700–
800 mm the gleyic horizon becomes prominent and soil water extraction by the taproot harder.
The planting and post-season curve which deems to be similar indicate that approximately the same amount of soil water is present during planting than what were present in the post-season.
5.3.8.3.3 Sepane
Figure 5-31 illustrates the soil water depth graph for the Sepane soil form. The planting soil water line illustrates a clear increase in soil water with depth. When considering planting and post- season curves it is evident that the same trend is followed, except less soil water is left post season that by planting time. In the boll formation curve, it is evident that a significant water loss occurs from boll formation to harvest because of the increased soil water needs of the cotton during this growth stage. In the harvest curve it is evident that there is a decrease of soil water
with depth from 300-700 mm, from which there is a slight increase in soil water from 700-800 mm indicating that soil water was extracted by the cotton from 300–700 mm. In the section from 700–
800 mm slight water accumulation occurs. When considering the post-season curve this curve indicates that less soil water was left post-harvest than there was during planting in the Sepane soil form due to interflow, drainage and cotton soil water consumption.
5.3.8.3.4 Westleigh
Figure 5-32 illustrates the soil water depth profile of the Westleigh soil form. It is evident that there is a decrease in soil water from the planting to harvest and post-season. When considering all four the lines it is evident that there is an increase in soil water with depth. This implies that the amount of soil water did indeed increase from the time of planting up until harvest even with the extraction of soil water by the cotton. In the boll formation curve, it is evident that a significant water loss occurs from boll formation to harvest because of the increased soil water needs of the cotton during this growth stage. There is a slight decrease from 300–400 mm in the soil water content due to evaporation. The effect of the soft plinthic and gleyic subsoil horizons is evident, but not clear indication exists of water accumulation on the gleyic limiting horizon. Less soil water is present post-season than by planting time.
Figure 5-29: Soil water depth profile of the Bainsvlei soil form in the Hoekblok.
Figure 5-30: Soil water depth profile of the Bloemdal soil form in the Hoekblok.
Figure 5-31: Soil water depth profile of the Sepane soil form in the Hoekblok.
Figure 5-32: Soil water depth profile of the Westleigh soil form in the Hoekblok.
5.3.8.4 Teerpadblok Soil Water Depth Profiles