School of Agriculture, Food and Ecosystem Sciences - Theses
Permanent URI for this collection
Search Results
1 - 2 of 2
-
ItemSubsoil physicochemical constraints and growth of cereals on alkaline soils in the Victorian Mallee( 2003)Alkaline soils used for dryland cropping across the semi-arid regions of south=eastern Australia typically have high levels of salinity (ECe), sodicity (ESP) and soluble boron (B) in the subsoil. The current research was undertaken to improve our knowledge of the physicochemical characteristics of these alkaline soils and assess what impact they have on the growth and water use of cereals. A survey of representative alkaline soils of the southern Mallee and Wimmera regions of Victoria, comprising 140 Calcarosol profiles and 10 Vertosol profiles, revealed high correlation between exchangeable Na+ and both ESP (r = 0.96) and B (r = 0.88). ESP and ECe (r = 0.71) and B and pH1:5 (r = 0.70) were also highly correlated. Readily measured properties (field texture, pH1:5, ECe and exchangeable Na+) were found to provide good estimates of ESP and boron in these soils. Overall, ESP was best defined as 1.47 + 2.68 x Na+ (r2 = 93.9) and boron by 0.34 +3.93 x Na+ (r2 = 76.7). A break point values for pH was defined such that soils with pH 1:5 < 8.1 have low levels of soil boron that are not potentially toxic to cereal growth. The growth and water use of the boron tolerant wheat cultivar, Frame, was monitored on these soils during the 1999-growing season. A descriptive model that explained 54% of the variation in grain yield (range: 1.3 - 6.1 Mg/ha) was established using ridge regression analysis, which is not confounded by the correlation that exists between the physicochemical factors (collinearity). The statistical analysis identified rainfall around anthesis, available soil water in the 0.10-0.40 m layer at sowing, nitrate in the 0-0.10 m layer and salinity and sodicity in the 0.60-1.00 m layer as significant explanatory variables for grain yield and water use. Subsoil salinity (ECe) and sodicity (ESP) were effective surrogates for extractability of water in the deep subsoil. It is estimated that crops can make use of water at depth if subsoils have ECe <8 dS/m and ESP <19%. Levels of soluble soil boron (reaching concentrations of up to 52 mg/kg), were not significantly correlated with root growth, water uptake or yield of wheat. It is proposed that the boron tolerance of cv. Frame rendered high boron as non-limiting and that the high Na+ in these soils had an overriding effect in driving variation in crop yield. The impact of boron tolerance, watering regime and soil disturbance on the growth and water use of near-isogenic lines of wheat and barley was assessed using large intact soil cores (0.3 m diam. x 1.0 m height), containing an alkaline soil (Calcarosol) sampled from the southern Mallee. Within the subsoil (0.80-0.90 m) ECes, ESP and B was 8.1 dS/m, 29% and 31 mg/kg respectively. In the shallower (0.61-0.71 m) layer respective levels were 7.2 dS/m, 22% and 29 mg/kg. Crop root growth between these layers decreased significantly with depth and no net water extraction occurred beyond 0.80 m, irrespective of crop boron tolerance. As the concentration of soil boron was equivalent across these layers, it was discounted from being a constraint to water extraction by crop. Rather, the increase in EC0 and more so ESP suggests these factors were constraining water use at depth. Grain yield per unit of applied water for wheat and barley grown under low water supply was 1.5 times that of their high water counterparts, suggesting inefficiencies in water capture by crops under the high water regime. Deep ripping had no effect on grain yield. Importantly these results showed boron tolerance provided little benefit to cereals on soil where constraints in addition to high boron exist. A glasshouse trial was conducted to assess how interactive effects of boron and salinity and cultivar affected the early growth of wheat using a range of soil salinity and soluble boron levels observed in the field survey. The three cultivars, Frame, BT Schomburgk and Schomburgk, varied significantly in tolerance to boron, with critical soil concentrations were estimated at 53, 32 and 27 mg/kg respectively in the absence of salinity. These 3 varieties did not differ in tolerance to salt in the absence of high boron, where cultivars equally tolerated ECe = 9 dS/m. Boron and Na+ in shoot tissue could not be used to define critical concentration for toxicity. This trial demonstrates the value Of using genetic variation for adaptation of wheat to high levels of boron, but appears not the case for salt tolerance here. This thesis confirmed that, due to the strong intercorrelation that exists between the physicochemical factors, salinity, sodicity and soluble boron, that these factors are likely to operate simultaneously to reduce cereal growth. Adequate boron tolerance currently appears to exist in commercial wheat cultivars i.e. Frame, enabling these varieties to withstand the high boron levels encountered in the alkaline cropping soils of north-western Victoria. High. levels of salinity and sodicity, however, are more likely to be constraining growth through osmotic, toxic and physical impediment. This thesis indicates the need for improve tolerance of cereals to salinity and the need for pyramiding tolerances for crops targeted to alkaline soils where constraints exist together.
-
ItemEffects of soil and water management on the productivity of irrigated pastures in the Goulburn Valley( 1986)The water status and productivity of perennial pastures were monitored during irrigation cycles. Measurements of leaf water potential reflected the water status of the pastures and when this deteriorated with the developing shortage of water after irrigation, various responses of the pastures were recorded. These included the rate of leaf elongation, canopy conductance, and the rates of net photosynthesis and evapotranspiration. The Parameters of gas-exchange were Measured using open-system, field chambers. The studies were designed to characterise and Compare the responses of these Pastures as water deficit developed, and to quantify their,effects on productivity. Experimental plots included swards of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and paspalum (Paspalum dilatatum) both as monocultures and as mixtures of the three species. These swards were growing on two soil profiles, both of which were Lemnos loam. The first profile was in its normal state and had been under pasture for fifteen years. The second had been structurally and chemically modified in 1979 to minimise the limitations to plant growth. It had been re--sown to pastures in 1980. The productivity of monocultures and mixed swards of each species on each profile was investigated as water shortage developed. In all species the responses to water shortage were the same, involving a reduction in the rate of development of leaf area,, followed by a reduction in photosynthesis per unit leaf area as the lack of water became more severe. The sensitivity of each species was distinct, with white clover being the most sensitive, ryegrass intermediate and paspalum the least. On modified soil, all species were less affected than on the normal profile but the order of sensitivity was the same. Overall, these experiments showed that after a typical irrigation cycle, which is about 8 days, water shortage reduces the productivity of white clover by 50%, ryegrass by 20% and paspalum by 5-10%. These limitations to productivity were overcome to some extent by modifying the profile, so that after an equivalent period the productivity of white clover was only reduced by about 208 and paspalum was not affected at all. The ability of paspalum to maintain high levels of productivity during an irrigation cycle had the effect of promoting this species in a mixed sward, its dominance becoming greater as the shortage of water became more severe. In order to achieve balanced pastures of maximum productivity, farmers in the Goulburn Valley need to reduce the limits to growth imposed by the physical and hydraulic characteristics of the soils. This may be achieved by irrigating more frequently or by undertaking some form of profile modification.