Under undisturbed veld, soils on the sugar estate studied were calcareous and vertic and had high pH values (8 to 9.5) and high exchangeable Ca and Mg contents. There was evidence of some accumulation of soluble salts in the surface 0.15 m. Under furrow-irrigated sugar production, over-irrigation has resulted in a marked rise in the watertable. The watertable is closer to the soil surface at the lower ends of the gently sloping fields, where it can be within 0.2-0 .3 m of the surface. The high watertable,particularly at the lower ends of the fields presumably resulted in death of roots due to anaerobic conditions in the subsoil. As a result, the effective crop rooting depth probably decreased progressivelyfrom the high to the low ends of the fields.
Due to capillary rise of salts, soluble salts have accumulated near the soil surface particularly at the lower ends of the fields. This has resultedin sodic and saline-sodic conditions developingin the surface soil horizons. Soil pH was often between 9 and 10 particularly in profiles from where sugarcane grew poorly or had died. That sugarcane yields were not significantly related to ECe but were negatively correlated with ESP suggests sodicity was a more limiting factor for sugarcane growth than salinity. Sodicity did not appear to be limiting plant growth through induction of nutrient imbalances or deficiencies since foliar analysis of sugarcane tissues showed no substantial differences in macro- or micro-nutrient content between good and poorly-growing crops. The sodic, and sometimes saline, conditions may have limited plant growth through ion toxicities (i.e., Na" and HC03-) which could inhibit root growth and function and impede physiological processes within plants. In some cases, salinity could also have induced water stress.
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. In addition, soils tended to disperse,and dispersion was most apparent where high ESP and SAR
e
values occurred in association with elevated pH values and relatively low ECevalues. These measurements confirmed observations at the site oflow infiltration rates and restricted drainage particularly on the lower ends of fields where sugarcane had died.
Itis concluded that the combination of a high watertable, restricted rooting depth, poor physical conditions and sodic and saline-sodic conditions in the surface soil resulted in a declining sugarcane yield from the high to the low ends ofthe fields, with crop death occurring at the lower ends.
Most research dealing with the effects of sodicity and salinity on soil fertility have dealt with their negative influence on soil chemical and physical properties. Research reported here showed clearly that irrigation-induced salinity and/or sodicity caused a marked decrease in the size and activity of the soil microbial biomass and in the activity of key enzymes involved in C,N, Sand P mineralization. The wide diversity of microorganisms present in soils meant that a significant microbial community remained in soils with a very high ESP and ECcvalues even though the sugarcane had died. The microbial metabolic quotient, however, increased suggesting that a smaller, more stressed, less metabolically efficient microbial community had been induced. That measures ofthe size and activity ofthe microbial biomasswere equally or more closely negatively correlated with ECethan ESP suggested that high soluble salt concentrations were more important in inhibiting microbial activity than sugarcane growth.
The implication of these findings is that increasing salinity and/or sodicity have an extremely adverse effect on the size and activity ofthe soil microbial community which is so essential for the maintenance of soil quality. Decreases in the availability ofN, Sand P through a reduced rate of organic matter decomposition and mineralization are likely. It is interesting to note that, soil organic matter content tended to decrease, rather than increase,with increasing ECe and ESP. This was attributed to a combination of lower organic matter inputs to the soil because of decreasing crop growth, and the possible solubilization and leaching of organic matter induced by the high soil pH and often high Na content.
In order to ameliorate the problems now evident at the site, the watertable will first need to be lowered substantially. This will require hydrological and engineering expertise and will be expensive to achieve. In association with this, irrigation scheduling and practice will need to be improved so that water-use-efficiency is maximised and future downward movement into the groundwater is minimised. The sodic and saline-sodic conditions will also need to be ameliorated.
The most obvious method is applications ofgypsumwith subsequent leaching events. Lowering of the watertable is also important so that the soils can be allowed to 'dryoff and crack so that macroporosity can be restored on a regular basis. This will help improve and/or maintain adequate soil physical properties in the crop rooting volume.
In the lowest-lying parts of the fields it may prove unpractical and/or uneconomic to effectively ameliorate the poor soil conditions. In these areas, the use of salt-tolerant sugarcane cultivars or more salt-tolerant crops such as cotton or bermuda grass could be considered. The watertable will, however,still need to be lowered in order to allow for an adequate crop rooting depth.