CHAPTER 5 RESULTS AND DISCUSSION
5.1 Season 1 Results and Discussion
5.1.8 Soil Water
5.1.8.3 Pretorius Rust 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. The results correlate with the
hydropedological classification of the soils by van Tol (2020) and their associated responses to the flow of soil water.
5.1.8.3.1 Westleigh
Figure 5-6 illustrates the soil water depth profile of the Westleigh soil form. It is evident that there is an increase in soil water from the pre-season, planting to boll formation, followed by a decrease from boll formation to harvest and post-season. During all these periods a loss of soil water is visible between 1100–1200 mm due to deep drainage. From Figure 5-6 it is evident that there is a slight increase in soil water with depth during the pre-season.
Figure 5-5: Total soil water and precipitation of the Schietfontein site.
When considering the time of harvest the amount of soil water increases with depth throughout the soil profile. 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. During this section of the Westleigh soil profile, the soft plinthic horizon becomes more prominent and the effect of the gleyic limiting horizon becomes more inevitable. At the depth between 1100-1200 mm the amount of soil water decreases once more indicating the interflow of soil water on the limiting layer and the extraction of water by the taproot system of the cotton throughout the boll formation growth stage.
5.1.8.3.2 Tukulu
Figure 5-7 illustrates the soil water depth graph of the Tukulu soil form. It is evident that there is an increase in soil water from the pre-season to planting and to boll formation. This is followed by a sharp decrease in soil water up to harvest. During the boll formation there is a significant water loss trend at 1000-1200 mm because of the water consumption of the cotton roots and internal drainage of the soft plinthic horizon. During post season there is a decrease from 300–700 mm in comparison with the harvest period. This is due to the downward flow of the soil water to deeper levels and evaporation of surface water. The post season and harvest soil water content from 800–1100 remained constant. The post-harvest curve corresponds to the harvest curve indicating small soil water loss from harvest to post-harvest. This is mainly due to the neocutanic soil horizon having a structure that allows for water storage. Between 1100–1200 mm a decrease is visible, which is due to the interflow of soil water post-season on the gleyic soil horizon.
5.1.8.3.3 Avalon
Figure 5-8 illustrates the soil water depth graph for the Avalon soil form. When considering the pre-season and planting curves it is evident that the amount of soil water throughout the soil profile is constant up to 1100 mm. From 1100–1200 mm a decrease in soil water is visible which is due to the interflow and internal drainage properties of the Avalon soil form, specifically the soft plinthic soil limiting horizon. 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 an increase of soil water with depth from 300-700 mm, from which there is an abrupt increase in soil water from 700-1200 mm indicating accumulation of water on the limiting layer. No soil water loss occurs post season in the Avalon soil form. When considering the post-harvest curve this curve indicates that more soil water was left post-harvest than there was pre-season.
5.1.8.3.4 Bainsvlei
Figure 5-9 illustrates the soil water depth profile for the Bainsvlei soil form. By considering the pre-season and planting curves it is evident that there is a slight increase in soil water with depth.
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. The harvest curve indicates similar indications of soil water flow as the Avalon soil form but seem to be more constant. No abrupt changes occur in the soil water content between harvest and post season which indicates that the Bainsvlei soil form had less interflow of soil water, thus yielding
the red colour in the subsoil horizons. Similar findings were described by van Huyssteen et al.
(2010).
Figure 5-6: Soil water depth profile of the Westleigh soil form in Pretorius Rust.
Figure 5-7: Soil water depth profile of the Tukulu soil form in Pretorius Rust.
Figure 5-8: Soil water depth profile of the Avalon soil form in Pretorius Rust.
Figure 5-9: Soil water depth profile of the Bainsvlei soil form in Pretorius Rust.
By considering the harvest curve it is evident that from 800-1100 mm the amount of soil water increases but decreases slightly from 1100–1200 indicating interflow. This indicates that a higher level of saturation on the limiting layer is evident. The post-harvest curve has a similar trend to the harvest trend, but with an increased soil water content. Losses in this section of the soil profile tend to be associated with gravel layers which in turn allow flow of water horizontally. The amount of soil water left post-harvest is more than pre-season and planting.