Soil acidity, aluminium and added organic matter in krasnozen topsoils in relation to phosphorus availability and plant growth
Creators: name: Bunmongkolrat, S
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis reports work on the effects of added, decomposable organic matter as a possible lime substitute on exchangeable and soil solution aluminium in relation to phosphorus availability and growth of perennial ryegrass (lolium perenne. L) in acid krasnozems on Tertiary basalt, North-Western Tasmania. Soil was collected to a depth of 15 cm at three sites in a sequence of increasing elevation and rainfall, representative of the Burnie, Lapoinya and Yolla series. The soil materials were first screened for their suitability for later work by determining certain basic soil properties and by measuring effects on these properties of glasshouse incubation for four weeks of soils with organic matter added as barley straw or lucerne chaff at rates equivalent to 0, 3.5 and 7.0 t/ha. There were significant effects, relative to initial levels of soil parameters, in terms of reduced exchangeable aluminium, increased available phosphorus and increased soil solution (pF2) ionic strength, with effects due to the addition of organic matter. Differences in cation exchange capacity sum of basic exchangeable cations, and soil reaction, were not significant. Because it was highest in exchangeable aluminium, lowest in available phosphorus, and was strongly acid, Burnie soil material was used to study effects of different rates of added barley straw or lucerne chaff on soil aluminium, phosphorus, and soil solution ionic strength after different periods of incubation (0, 4, 8, 16 weeks), with and without added urea to bring the C/N ratio of the organic matter to a value of 12. The pH of the soils with added organic matter was higher than the controls, both with and without added urea, but the effect of urea on pH decreased with time. The effect of urea on the increase of pH was obvious in the first 4 weeks and then slightly varied from week 4 to week 8. Thereafter, the pH was on the decline. Exchangeable aluminium was lower in soil incubated with organic matter but differences due to rate of added organic matter were not significant, lucerne chaff had a greater early effect but the difference versus barley straw was reduced after four weeks. There was a sharp rise in available phosphorus measured after four weeks in all soils with added organic matter but this was followed by a rapid fall so that levels at the end of the experiment (16 weeks) were well below those at its commencement. Available phosphorus of control soils decreased regularly throughout. There was little difference in effects due to kind of organic matter. Ionic strength of soil solution extracts (pF2) from the soils with added organic matter rose sharply and levels were maintained in soils with added organic matter. Burnie soil was again used to follow effects of added organic matter on active aluminium in relation to native available phosphorus and growth of \Nui\" cultivar of perennial ryegrass. Urea was not added in this experiment. Soluble aluminium was added as Al2(SO4)3. 14H20 after four weeks of incubation of soil plus organic matter (Sequence (a)) and together with organic matter (Sequence (b)) both sequences being incubated for a further four weeks before sowing of ryegrass seed. Differences between the two sequences in effects on pH soil solution phosphorus (pF2) phosphorus uptake and dry matter production by the test plant were not significant. However there were marked effects within each sequence of added organic matter in increasing values of each of these parameters in relation to controls. Exchangeable and soil solution aluminium especially monomeric aluminium were markedly reduced by added organic matter at all rates of aluminium application and these effects were greater in Sequence (a) than in Sequence (b) soils. Aluminium remaining in solution was almost wholly in organic complexed forms. Conversely available phosphorus was higher. Lucerne chaff was slightly but consistently more effective than barley straw but these differences were not significant. Finally Lapoinya soil from the \"museum\" area of Elliott Research Station was used in a study of effects of both added organic matter and phosphorus (KH2PO4) on soil aluminium in relation to growth of ryegrass because of its low pH very low level of available phosphorus and high level of active aluminium. Reduction of exchangeable and monomeric aluminium occurred with rate of phosphorus application the effect being greater in the presence of added organic matter. Again nearly all of the soil solution aluminium was in organically complexed forms in the presence of added organic matter. Differences between the effects of lucerne chaff and barley straw were small but consistent although not statistically significant with lucerne chaff having the greater effects. Soil reaction again increased with added organic matter and an inverse relationship between levels of aluminium (both exchangeable monomeric form) and available phosphorus depended on pH. Plant growth and phosphorus uptake and content responded by increasing sharply with rate of phosphorus application and were further enhanced greatly by added organic matter. Plant symptoms of aluminium toxicity/phosphorus deficiency were not evident in the presence of added organic matter even at the highest level of aluminium application (100 ppm 0.D.). Both forms of organic matter used whether fresh or partly decomposed gave beneficial effects within the period of experimentation in suppressing aluminium toxicity and enhancing growth of perennial ryegrass in the acid soils used. Application of organic matter at practical rates may allow reduction of the quantity and frequency of lime additions."