The effects of irrigation-induced salinity and/or sodicity on sugarcane yield, and two growth parameters, namely stalk height and number of nodes per stalk , were investigated on a sugarcane estate in the Zimbabwean lowveld. The effects of soil salinity and/or sodicity on the size, activity
and metabolic efficiency of the soil microbial community was also studied. Furrow-irrigated fields which had a gradient in soil salinity and/or sodicity which increased from the upper to lower ends of the fields were selected for this study. This gradient was recognized by decreasing sugarcane
growth down from the upper to the lower ends and the appearance of salt on the soil surface at the lower ends of fields. Sugarcane growth was classified as either dead, poor, satisfactory or good; and soil samples (0-0 .15 m, 0.15-0 .3 m, 0.3-0 .6 m and 0.6-0.9 m) were taken from each of these areas. Soils from under adjacent areas of undisturbed veld were also sampled. Sugarcane
growth and yields in micro-plots of the various areas of the fields were measured. Foliar samples of sugarcane were taken at 22 weeks of age and analysed for nutrient content. Soil salinity and sodicity were quantified by measuring pH(water), electrical conductivity (ECe) and cation content of saturation paste extracts and the exchangeable cation content. From this information, the
sodium adsorption ratio (SARe)and exchangeable sodium percentage (ESP) were also calculated.
The calcareous, vertic soils in the study area under undisturbed veld were found to have high pH
values (8 to 9.5), very high exchangeable Ca and Mg concentrations and there was evidence of
accumulation of soluble salts in the surface 0.15 m. Under sugarcane production, irrigation induced
salinity and sodicity had developed. Under poor and dead sugarcane, high values for ECe,
SARe, and ESP were generally encountered in the surface 0-0 .3 m of the profile. In addition, the
pH values under sugarcane were often between 9 and 10 particularly in profiles where sugarcane grew poorly or had died. As expected, pH was positively related to ESP and SARe, but negatively related to ECe.
Measurements of aggregate stability by wet sieving, the Emerson dispersion test and the Loveday
dispersion score all showed that soils from the study sited tended to disperse and that dispersion
was most apparent where high ESP and SARe values occurred in association with elevated pH
values and relatively low ECe values. These measurements confirmed observations at the sites of
low infiltration rates and restricted drainage particularly on the lower ends of fields where sugarcane had died. In addition to the above measurements it was also observed that there was a rise in the watertable
under furrow irrigation and that the watertable was nearest to the surface at the lower ends of the
fields. In some cases the watertable was observed to be only 0.2 to 0.3 m from the surface. Thus,
death of roots due to anaerobic conditions could be occurring to a greater extent at the lower ends
of the fields. Another consequence of the high watertable was that these vertic soils were
observed to remain in a permanently swollen state. This limits air and water movement in the soil
profile as such soils need to be allowed to dry out and crack regularly so that macroporosity can be restored.
Sugarcane yield, stalk height and number of nodes per stalk were not significantly related to ECe.
Sugarcane yields were, however, significantly correlated with ESP and pH while stalk height and
number of nodes were negatively correlated with ESP, SARe and pH. These results suggested
that sodicity was a more limiting factor for sugarcane growth than salinity. Foliar analysis of leaf
tissue did not reveal substantial differences in macro- or micro-nutrient content between good and
poorly-growing sugarcane.
It was concluded that the gradient of decreasing sugarcane growth down the furrow-irrigated fields, with crop death at the lower ends, was the result of a combination of factors. That is, the
watertable had risen due to over-irrigation and it was nearer the surface at the lower ends of the fields. Due to capillary rise of salts, this resulted in sodic and sometimes saline-sodic conditions
in the surface soil. These conditions could limit plant growth through ion toxicities, plant water
stress and inhibition of root growth and function and physiological processes. These would be
induced by the high pH and high salt, Na and HC03- concentrations in soil solution. Poor
physical conditions associated with sodicity and the continually swollen state of the soils
presumably limited infiltration and aeration in the surface soil, and probably restricted root
growth. In addition, it is likely that the high watertable limited effective crop rooting depth to
about 0.2 m at the lower ends of the fields. The net result was that sugarcane died at the lower
ends. A negative effect of soil salinity and/or sodicity was also observed on the soil microbial
population. Significant negative correlations were obtained with ECe SARe and ESP with
microbial biomass C and microbial activity (as measured by FDA hydrolytic activity or arginine
ammonification rate). The activity of enzymes involved in C (P-glucosidase), P (phosphatase) and
S (arylsulfatase) mineralization and potential nitrogen mineralization (as determined by aerobic
incubation) were also negatively correlated with these factors, with the exception of arylsulfatase
activity and ESP. All the above mentioned microbial population measures were also positively
correlated with soil organic C content, besides potential nitrogen mineralization. The metabolic
quotient, which provides an indication of stress and efficiency of the microbial community,
increased considerably with increasing salinity and sodicity and decreased with soil organic C.
Thus, increasing salinity and/or sodicity resulted in a smaller, more stressed, less efficient
microbial community, while the turnover rate and cycling of C, N, P and S also decreased. It was
concluded that salt affected soil not only causes a decline in sugarcane yield through raising the concentration of soluble salts in soil solution, but also has a detrimental effect on microbial activity and on mineralization of soil organic C, N, Sand P. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/4920 |
| Date | January 2001 |
| Creators | Rietz, Diana Nicolle. |
| Contributors | Haynes, Richard J. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
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