CHAPTER 5 RESULTS AND DISCUSSION
5.1 Season 1 Results and Discussion
5.1.8 Soil Water
5.1.8.5 Season 1 Soil Water Characteristics
the A horizon the Tukulu has the highest LL followed by the Westleigh, Bainsvlei and Avalon in decreasing order. The soil forms with a higher clay content are prone to have a low DUL and higher LL which may cause crop stress easily after high rainfall periods.
The DUL of the B horizon varies much more because the different soils have different subsoil horizons with different water retention capabilities. The LL of the B horizon does not vary much between the different soil forms, and no statistical significance is evident between the different soil forms. The post-hoc statistical analysis indicated that all the soil forms’ DUL differ from one another statistically. Due to the statistical difference between the different soil forms DUL the plant available water (PAW) varies between the different soil forms as well because the PAW is derived from the DUL and LL. Table 5-10 indicates the most significant differences in DUL and PAW between the different soil forms in the B horizon. The biggest statistical difference occurs between the Westleigh and Bainsvlei soil forms followed by the following comparisons in decreasing order:
Westleigh-Avalon, Tukulu-Bainsvlei, Westleigh-Tukulu, Tukulu-Avalon, and Bainsvlei-Avalon. All the comparisons have p-values<0,05 which indicates that they are all statistically different from one another.
Table 5-10: 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) Westleigh-Bainsvlei 0,0000032 (1) 0,1411698 0,0000032 (1) Westleigh-Avalon 0,0000267 (2) 0,9939721 0,0000267 (2) Tukulu-Bainsvlei 0,0001712 (3) 0,9732088 0,0001712 (3) Westleigh-Tukulu 0,0018994 (4) 0,0781262 0,0018994 (4) Tukulu-Avalon 0,0060613 (5) 0,11135 0,0060613 (5) Bainsvlei-Avalon 0,0377738 (6) 0,199188 0,0377738 (6)
‘* All the values with numbers indicate the rank of most significant comparison 5.1.8.5.2 Schietfontein Soil Water Retention Characteristics
Table 5-11 illustrates the soil water retention characteristics of the four dominant soil forms in Schietfontein. The dry bulk density varies between 1,50 and 1,64 for the A horizon, and between 1,44 and 1,52 in the B horizon. The statistical analysis indicated that there is statistical significance between all the soil forms in terms of both the A and B horizon dry bulk density with the p-value <0,05. The A horizon dry bulk density is higher than the B horizon dry bulk density and due to this, the infiltration of water is affected due to the higher density. The increase in the A horizon density is mainly due to the machinery used to work the soil and plant. The difference between the A horizons is not much due to the similar diagnostic topsoil horizon. The Avalon soil form has the highest B horizon dry bulk density. The Tukulu soil form has the second highest B
horizon dry bulk density, and this is due to structure of the neocutanic horizon which is prone to compaction and results in a higher dry bulk density.
The Avalon soil form B horizon is the counterpart of the Glencoe B horizon, with the only difference being the plinthic horizon being soft plinthite in the Avalon soil form and hard plinthite in the Glencoe soil form. This influences the density of the soil as it consists of more air and water which results in a lower density.
Table 5-11: Drained upper limit, lower limit, and dry bulk density of the soil forms.
Soil form Horizon Dry Bulk Density (g/cm³) DUL (mm) LL (mm) PAW (mm)
Tukulu A 1,50 82,60 36,83 45,77
Tukulu B 1,46 243,08 79,72 163,36
Avalon A 1,64 75,19 39,68 35,51
Avalon B 1,52 308,77 122,52 186,25
Glencoe A 1,53 66,20 33,06 33,15
Glencoe B 1,44 197,83 67,62 130,22
Oakleaf A 1,61 62,33 19,71 42,62
Oakleaf B 1,44 222,12 71,65 150,47
When considering the drained upper limit (DUL) in Table 5-11 it is evident that the Tukulu soil form has the highest DUL in the A horizon followed by the Avalon, Glencoe and Oakleaf soil forms in decreasing order. The DUL values of the A horizon does not differ much from one another since it is classified as the same diagnostic topsoil horizon. When considering the lower limit (LL) of the A horizon the Avalon has the highest LL followed by the Tukulu, Glencoe and Oakleaf soil forms in decreasing order. The soil forms that have a low DUL and higher LL which may cause crop stress easily after high rainfall periods.
The DUL of the B horizon varies much more because the different soils have different subsoil horizons with different water retention capabilities. In the Schietfontein field uniform soils in terms of the B horizon exist, but still yet variance exists. Considering Table 5-11 it is evident that the Avalon soil form has the highest DUL followed by the Tukulu, Oakleaf and Glencoe soil forms.
Considering Table 5-11 the Avalon soil form has the highest LL followed by the Tukulu, Oakleaf, and Glencoe. The difference between the LL of the other soil forms except the Avalon LL does not vary much from one another. The post-hoc statistical analysis indicated that all the soil forms’
DUL and LL differ from one another statistically. Due to the statistical difference between the different soil forms DUL and LL the PAW varies only between the Glencoe and Avalon soil forms.
Table 5-12 indicates the most significant differences in DUL, LL and PAW between the different
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