CHAPTER 5 RESULTS AND DISCUSSION
5.1 Season 1 Results and Discussion
5.1.8 Soil Water
5.1.8.6 Pretorius Rust Soil Water Regime
followed by the following comparisons in decreasing order: Oakleaf-Avalon, and Tukulu-Avalon.
Concluding this significant difference, the main difference is between the Avalon soil form and the other soil forms. Only the values with ranking numbers yielded statistically significant values.
Table 5-12: Tukey post-hoc comparison test of the DUL, LL and PAW of the B horizon of the soil forms.
Comparison P-value (DUL) P-Value (LL) P-value (PAW) Glencoe-Avalon 0.0024578 (1) 0.0001649 (1) 0.0135857 (1) Oakleaf-Avalon 0.0068521 (2) 0.0000637 (2) 0.1802760 Tukulu-Avalon 0.0147699 (3) 0.0003630 (3) 0.1503898 Oakleaf-Glencoe 0.8294128 0.6717053 0.3131373 Tukulu-Glencoe 0.5097131 0.8219014 0.3679890 Tukulu-Oakleaf 0.9314531 0.2586072 0.9991052
‘* All the values with numbers indicate the rank of most significant comparison
before remaining constant. The soil profile was above DUL during most of the growth stages, especially during the flowering and boll formation stage when the cotton is most susceptible to stress.
Figure 5-14: Soil water regime on the Avalon soil form in Pretorius Rust.
5.1.8.6.2 Bainsvlei
Figure 5-15 illustrates the soil water fluctuation throughout the cotton production season of the Bainsvlei soil form created from the averages of the repetitive site measurements. During October 2019 the total amount of soil water was slightly above the lower limit. During the beginning of November 2019, the first rain was observed which assured that planting can commence. The cotton was planted on the 4th of November 2019 during which the soil form was well above lower limit. Frequent rainfall events followed the planting which ensured that soil water was available for uptake during the emergence of the cotton. Between the emergence and flowering growth stages the soil water was readily available for plant uptake and soil water conditions above DUL is observed towards the middle of January 2020. The amount of soil water kept increasing above the drained upper limit which raises the possibility of plant stress. During the period from November 2019 to February 2020 the evapotranspiration demand increased as the growth stages progress towards the flowering stage, but due to the frequent high rainfall events this increase had no real effect as the cotton water needs are rather low. During February 2020 a peak in soil
June 2020 as the soil water needs of the cotton increased as the growth progressed to the boll formation and boll burst growth stages.
Figure 5-15: Soil water regime on the Bainsvlei soil form in Pretorius Rust.
The period above DUL lasted from the time just after flowering (January 2020) to just before boll burst commenced in April 2020 before reaching a point of water becoming freely available for plant uptake without the risk of planting stress. In the period from boll burst commencement to harvest the soil water content kept decreasing, but after harvest increased for a short period before remaining constant. The soil profile was above DUL only for a short period in between flowering and boll formation but only slightly which might not have a significant effect in terms of planting stress.
5.1.8.6.3 Westleigh
Figure 5-16 illustrates the soil water fluctuation throughout the cotton production season of the Westleigh soil form created from the averages of the repetitive site measurements. During October 2019 the total amount of soil water was slightly above the lower limit. During the beginning of November 2019, the first rain was observed which assured that planting can commence. The cotton was planted on the 4th of November 2019 during which the soil form was still slightly above lower limit. Frequent rainfall events followed the planting which ensured that soil water was available for uptake during the emergence of the cotton. Between the emergence
and flowering growth stages, the soil water was readily available for plant uptake and soil water conditions above DUL is observed during the beginning of December 2019. Soil water contents above DUL is observed very early in the growth process of the cotton. The amount of soil water kept increasing above the drained upper limit which raises the possibility of plant stress. During the period from November 2019 to February 2020 the evapotranspiration demand increased as the growth stages progress towards the flowering stage, but due to the frequent high rainfall events and waterlogged conditions this increase had no real effect as the cotton water needs are rather low. During February 2020 a peak in soil water content was observed and the total amount of soil water is approximately three times the DUL of the soil form. The soil water content decreased drastically from February 2020 to June 2020 as the soil water needs of the cotton increased as the growth progressed to the boll formation and boll burst growth stages. The soil water content above DUL lasted from the time just after emergence in early December 2019 until after harvest where it remained constant. The soil water content was above DUL during all the growth stages except emergence. The soil profile, being highly above DUL throughout the season, should have led to plant stress due to waterlogged conditions that indicates signs of plant stress.
Figure 5-16: Soil water regime on the Westleigh soil form in Pretorius Rust.
5.1.8.6.4 Tukulu
Figure 5-17 illustrates the soil water fluctuation throughout the cotton production season of the Tukulu soil form created from the averages of the repetitive site measurements. During October 2019 the total amount of soil water was slightly above the lower limit. During the beginning of November 2019, the first rain was observed which assured that planting can commence. The cotton was planted on the 4th of November 2019 during which the soil form was well above lower limit. Frequent rainfall events followed the planting which ensured that soil water was available for uptake during the emergence of the cotton. Between the emergence and flowering growth stages the soil water was readily available for plant uptake and no waterlogged conditions were observed. The amount of soil water kept increasing above the drained upper limit, but only for a short period during February 2020. During the period from November 2019 to February 2020 the evapotranspiration demand increased as the growth stages progress towards the flowering stage, but due to the frequent high rainfall events this increase had no real effect as the cotton water needs are rather low. During February 2020 a peak in soil water content is observed. The soil water content decreased drastically from February 2020 to March 2020 as the soil water needs of the cotton increased as the growth progressed to the boll formation growth stage. A gradual decrease in soil water is visible from March 2020 to harvest. In the period from boll burst commencement to harvest the soil water content kept decreasing, but after harvest increased for a short period before remaining constant. The soil water content was above DUL only for a short period in between flowering and boll formation but only slightly which might not have a significant effect in terms of plant stress.
Figure 5-17: Soil water regime on the Tukulu soil form in Pretorius Rust.
5.1.8.7 Schietfontein Soil Water Regime