CHAPTER 5 RESULTS AND DISCUSSION
5.1 Season 1 Results and Discussion
5.1.8 Soil Water
5.1.8.7 Schietfontein Soil Water Regime
5.1.8.7 Schietfontein Soil Water Regime
During February 2020 a peak in soil water content is observed and the total amount of soil water is approximately two times the DUL of the soil form. 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 burst growth stages. The soil water conditions above DUL period lasted from the period just after emergence around the end of January 2020 until after harvest where it remained constantly oversaturated. This extensive above DUL soil water conditions creates water stress in the plant and results in lower yield.
5.1.8.7.2 Avalon
Figure 5-19 illustrates the soil water fluctuation throughout the cotton production season of the Avalon soil form created from the averages of the repetitive site measurements. In November 2019 the total amount of soil water was slightly below 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 13th of November 2019, but the soil water measurements had not yet commenced in Schietfontein. 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 waterlogged conditions is observed towards the end of January 2020 just before flowering commenced. 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 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. The soil water conditions above DUL lasted from the period just before flowering (January 2020) to just after boll formation (March 2020) before reaching a point of water becoming freely available for plant uptake without the risk of plant stress. In the period from boll burst commencement to harvest the soil water content kept decreasing until after harvest. The soil profile was above DUL only during the flowering and boll formation growth stages, especially during the flowering and initial boll formation stage when the cotton is most susceptible to plant stress.
Figure 5-19: Soil water regime on the Avalon soil form in Schietfontein.
5.1.8.7.3 Glencoe
Figure 5-20 illustrates the soil water fluctuation throughout the cotton production season of the Glencoe soil form created from the averages of the repetitive site measurements. During November 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 13th of November 2019, but the soil water measurements had not yet commenced in Schietfontein. 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 end of December 2020. Soil water conditions above DUL are reached 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 soil water conditions above DUL, this increase had no real effect as the cotton water needs are rather low.
Figure 5-20: Soil water regime on the Glencoe soil form in Schietfontein.
During February 2020 a peak in soil water content is observed and the total amount of soil water is approximately two times the DUL of the soil form. 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 burst growth stages. The soil water conditions above DUL lasted from the period just after emergence around the end of January 2020 until after harvest where it remained constantly above DUL. This extensive soil water conditions above DUL creates water stress in the plants and results in lower yield according to Ninari et al. (2004).
5.1.8.7.4 Oakleaf
Figure 5-21 illustrates the soil water fluctuation throughout the cotton production season of the Oakleaf soil form created from the averages of the repetitive site measurements. During the beginning of November 2019, the first rain was observed which assured that planting can commence. The cotton was planted on the 13th of November 2019, but the soil water measurements had not yet commenced in Schietfontein. During the beginning of November 2019, the first rain was observed which assured that planting can commence. 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 middle of December 2019. The soil water conditions above DUL are reached 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 soil water conditions above DUL, 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 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 March 2020 as the soil water needs of the cotton increased as the growth progressed to the boll burst growth stage. The soil water conditions above DUL lasted from the period just after emergence in early December 2019 until after harvest where it remained constantly far above the DUL. The soil water content was above DUL during all the growth stages except emergence. The soil profile being high above DUL throughout the season indicated signs of plant stress.
Figure 5-21: Soil water regime on the Oakleaf soil form in Schietfontein.