Within each field, sugarcane growth was identified as dead and dying (D), poor (P), satisfactory (S), or good (G). Within each field, sugarcane growth was identified as dead and dying (D), poor (P), satisfactory (S), or good (G).
DECLARATION
ACKNOWLEDGEMENTS
ABSTRACT
INTRODUCTION
Soils derived from gneiss parent materials have been classified as sialite soil or chrome luvisols in the FAO legend (Clowes and Breakwell, 1998; Nyamudezaetal., 1999). These developed as a result of poor irrigation management on these soils, resulting in the development of saline, sodic or "saline-sodic" conditions.
REVIEW OF LITERATURE
Introduction
- Soil acidification
- Bulk density and compaction
Deficiencies in soil N are common in many cultivated soils, including cultivated heavy clay soils (Stephens and Donald, 1958; van Wambeke, 1991) and there is considerable evidence that crops grown on heavy clay soils respond to applications of N -fertilizer, especially when other deficiencies (especially P), have been corrected (Hubble, 1984; Probert et al., 1987). Due to their mineralogy, the structure of heavy clay soils varies greatly with moisture content.
Soil Salinity and Sodicity
- Development of Salinity and/or Sodicity
- Consequences for Soil Chemical Properties and Processes
- Consequences for Soil Physical Properties
- Consequences for Soil Biological Properties
- Consequences for Crop Growth
- Alternative Uses for Saline and/or Sodic Soils
This can be detrimental to crops sensitive to waterlogging and poor aeration, such as cotton (Probert et al., 1987). The flocculating effect of CaCO 3 can sometimes override the dispersive effect of high pH (Gupta et al., 1984b).
Conclusions
IRRIGATION-INDUCED SALINITY AND SODICITY AND ITS EFFECT ON SUGARCANE YIELD
Introduction
In addition, sodicity in arid regions is often associated with alkalinity (i.e., high levels of dissolved carbonate and bicarbonate in soil solution, resulting in a high soil pH; Suarez et al., 1984). The creation of alkalinity depends on clay mineral type, exchangeable Na levels and the concentration of the soil solution. The amount of land planted to sugar cane has consistently increased over the last century (Bramley et al., 1996).
In fact, van Antwerpen and Meyer (1996) considered increased soil salinity and/or sodicity as the most important soil chemical processes leading to soil degradation under irrigated sugarcane. Salinity generally causes water stress through osmotic effects while sodicity results in an increased pH, nutrient imbalance and clay distribution resulting in poor water, air and root penetration, low readily available water holding capacity and difficulty in timely and effective tillage (Gupta and Abrol, 1990; Nelson and Ham, 2000). In the vertical soils of the study area in lowland Zimbabwe, the decline in sugarcane yield is a major problem.
The most important factors leading to this yield decline are thought to be soil salinity and/or sodicity caused by over-irrigation (Hussein, 1998). Visual observations revealed that yields decrease from high to low ends of the pre-irrigated fields and that crop dieback at the bottom end is associated with accumulation of salts at the soil surface and/or dispersion and loss of soil structure. The aim of this study was to investigate the cause of yield reductions and crop dieback in these countries with specific reference to irrigation induced soil salinity and sodicity.
Materials and Methods
Foliar samples of sugarcane in these plots were taken when the cane was about 22 weeks old in the following period. The first fully expanded leaves from the top of the cane plants (usually the third leaf down the stem) were collected, the midribs removed, the leaves from each plot bundled and the tops and bottoms cut off (Clowes and Breakwell, 1998). Extracts of the saturated paste were prepared, the electrical conductivity (ECe) of the extracts was measured and the content of Ca, Mg and Na was analyzed by atomic absorption spectrophotometry.
As all the soils had apH(water) above 8.0, exchange acidity was not measured. Liquefaction and degree of dispersion are recorded at each time interval and aggregates are classified according to Emerson's procedure, with the modified inclusion of subclasses. ii) soil samples «2 mm) of soil leaching but not spread in (i), moistened and equilibrated to a suction water content of -10 kPa overnight. These are also immersed in distilled water and the amount of dispersion is noted after 2 and 20 hours.
Subclasses of classes 2 and 3 are determined by the proportion of the aggregate that has spread, i.e. best-fit regression functions are presented in the chapter together with the best-fit lines and the statistical significance of the relationships. The original intention was to use two years of data, but the uncertain political situation in Zimbabwe at the time prevented the collection of data for the second year.
Results and Discussion
- Soil Chemical Properties
- Soil Physical Properties
- Relationship between Soil Chemical Properties and Sugarcane Yield
- Foliar Analysis
The accumulation of exchangeable Na in the soil profiles under dead, weak, and to a lesser extent, satisfactory sugarcane growth is evident from the data presented in Table 3.2. Salinity was concentrated in the surface layers (0-0.3 m) of soils especially at sites 1 and 2 under dead and weak sugar cane and under satisfactory sugar cane at site 1 (0-0.15 m). This relationship was reflected in the results of this study where regression analysis revealed a highly significant (n= 20, r exponential) relationship between pH and ESP in the 0-0.9 m of the profile (Figure 3.1b).
This is the result of the significantly lower pH values found in the 0-0.15m layer under dead and dying sugarcane at site 1 compared to these at sites 2,3 and 4 (Table 3.1). The poor physical conditions of the heavy clay soil in the study area are related in another way to poor irrigation management. However, in the alkaline, saline-sodium soils used in this study, sugarcane growth was more closely related to sodicity than salinity.
In both of the above studies, sodicity and salinity generally increased with increasing depth in the soil profile. However, in this study, sodic soil conditions were frequently encountered throughout the profile, and there was a tendency for Na to concentrate in the surface 0.3 m. First, nutrient toxicities and/or imbalances may occur primarily due to the high Na content in the soil solution.
EFFECTS OF IRRIGATION INDUCED SOIL SALINITY AND SODICITY ON SOIL MICROBIAL ACTIVITY
- Introduction
- Materials and Methods
- Results and Discussion
- Conclusions
Microbial biomass C was determined by the method of Vance et al. 1987) based on the difference between C extracted with 0.5 M K2SO4 from chloroform-fumigated and non-fumigated soil samples with a K, factor of 0.38. With increasing salinity, proportionally greater levels of C are required to meet the energy demands of the microbial biomass (Lavahunet al., 1996). The negative relationship between microbial biomass C and metabolic quotient, as observed here, is common (Garcia et al., 1994; Sparling, 1997).
Many workers have found that the size and activity of microbial biomass is positively related to soil organic C content (Dicket al., 1988; Haynes, 1999). This is because organic matter is the energy and C source for most of the heterotrophic microbial community. Although irrigation-induced salinity reduced the size and activity of the soil microbial community, it is apparent (Figures and 4.4) that significant microbial activity persisted under saline soil conditions.
This relationship between soil enzyme activity and the content of organic matter in the soil has been found by many researchers (Schnureret al.,1985; Dicket al.,1988; Martenset al.,1992;Haynes, 1999). In contrast, Nelson et al. (1997) found that sodicity had a slightly negative effect on the degradation of added residues. In contrast, measurements of microbial community size and activity were equally, or more closely, negatively correlated with ECethan ESP (Table 4.1).
GENERAL CONCLUSIONS
It is concluded that the combination of a high water availability, limited 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 of the fields, with crop death occurring at the lower ends. Research reported here clearly demonstrated that irrigation-induced salinity and/or sodicity caused a marked decrease in the size and activity of soil microbial biomass and in the activity of key enzymes involved in C,N, Sand P mineralization. The implication of these findings is that increasing salinity and/or sodicity has an extremely negative effect on the size and activity of the soil microbial community, which is so crucial for the maintenance of soil quality.
Reduced availability of N, Sand P through a reduced rate of decomposition and mineralization of organic matter is likely. In order to improve the problems now evident at the site, the water level will first have to be lowered significantly. In this regard, irrigation planning and practice will need to be improved so that water use efficiency is maximized and future downward movement in groundwater is minimized.
This will help to improve and/or maintain suitable physical properties of the soil in the rooting volume of the plants. In the lower parts of the fields it may be impractical and/or uneconomical to effectively improve poor soil conditions. However, the water table will still need to be lowered to allow adequate crop rooting depth.
Soil nitrogen kinetics in some Darling and Western Downs fissured clays, Queensland. Effects of extending periods of intensive vegetable production on biological, chemical and physical indicators of soil quality. Association of soil microbial biomass and activity with fertilization practice and yield of three ultisols.
The effects of soil compaction due to field transport on cane yield and soil physical characteristics. Soil microbiological parameters as indicators of soil quality under improved fallow management systems in southwestern Nigeria. An overview of saline/wet soil management and improvement at Mhlume (Swaziland) Sugar Company.
Depth-weighted average pH (wat.rj' exchangeable cations (cmol, kg:'}, soluble cations (crnol, kg"), electrical conductivity (EC.), exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR.) and exchangeable cation exchange capacity (ECEC ) (cmol, kg") of the O-O.3m depth layer of the soil of four sugarcane fields. Sugarcane growth within each field was identified as dead and dying (D), poor (P), satisfactory (S) or good (G ) Soil samples were also taken from adjacent undisturbed field V. Carbonate and bicarbonate levels of 1:5 soil:water slurries (cmol, kg·l) of the O-O.15m depth layer of soil from four sugarcane fields.
Ammonium acetate extractable cations (NH40Acin cmol, kg-i), soluble cations (mmol, L-1) and saturation percentage (%) of the O-O.15m depth layer of soil from four sugar cane fields. Ammonium acetate extractive cations (NH40Acin cmol, kg-I), soluble cations (rnmol, L-I) and saturation percentage (%) of the 0.3-0.6m depth layer of soil from four sugarcane fields.
