This thesis reports work on the effects of added, decomposable organicudmatter as a possible lime substitute on exchangeable and soil solution aluminium inudrelation to phosphorus availability and growth of perennial ryegrass (lolium perenne.udL) in acid krasnozems on Tertiary basalt, North-Western Tasmania. Soil wasudcollected to a depth of 15 cm at three sites in a sequence of increasing elevation andudrainfall, representative of the Burnie, Lapoinya and Yolla series.udThe soil materials were first screened for their suitability for later work byuddetermining certain basic soil properties and by measuring effects on these propertiesudof glasshouse incubation for four weeks of soils with organic matter added as barleyudstraw or lucerne chaff at rates equivalent to 0, 3.5 and 7.0 t/ha. There were significantudeffects, relative to initial levels of soil parameters, in terms of reduced exchangeableudaluminium, increased available phosphorus and increased soil solution (pF2) ionicudstrength, with effects due to the addition of organic matter. Differences in cationudexchange capacity sum of basic exchangeable cations, and soil reaction, were notudsignificant.udBecause it was highest in exchangeable aluminium, lowest in availableudphosphorus, and was strongly acid, Burnie soil material was used to study effects ofuddifferent rates of added barley straw or lucerne chaff on soil aluminium, phosphorus,udand soil solution ionic strength after different periods of incubation (0, 4, 8, 16udweeks), with and without added urea to bring the C/N ratio of the organic matter to audvalue of 12. The pH of the soils with added organic matter was higher than theudcontrols, both with and without added urea, but the effect of urea on pH decreasedudwith time. The effect of urea on the increase of pH was obvious in the first 4 weeksudand then slightly varied from week 4 to week 8. Thereafter, the pH was on theuddecline. Exchangeable aluminium was lower in soil incubated with organic matter but uddifferences due to rate of added organic matter were not significant, lucerne chaff haduda greater early effect but the difference versus barley straw was reduced after fourudweeks. There was a sharp rise in available phosphorus measured after four weeks in udall soils with added organic matter but this was followed by a rapid fall so that levelsudat the end of the experiment (16 weeks) were well below those at its commencement.udAvailable phosphorus of control soils decreased regularly throughout. There was littleuddifference in effects due to kind of organic matter. Ionic strength of soil solutionudextracts (pF2) from the soils with added organic matter rose sharply and levels wereudmaintained in soils with added organic matter.udBurnie soil was again used to follow effects of added organic matter onudactive aluminium in relation to native available phosphorus and growth of "Nui"udcultivar of perennial ryegrass. Urea was not added in this experiment. Solubleudaluminium was added as Al2(SO4)3. 14H20 after four weeks of incubation of soiludplus organic matter (Sequence (a)) and together with organic matter (Sequence (b)),udboth sequences being incubated for a further four weeks before sowing of ryegrassudseed. Differences between the two sequences in effects on pH, soil solutionudphosphorus (pF2), phosphorus uptake and dry matter production by the test plantudwere not significant. However, there were marked effects within each sequence ofudadded organic matter in increasing values of each of these parameters in relation toudcontrols. Exchangeable and soil solution aluminium, especially monomericudaluminium, were markedly reduced by added organic matter at all rates of aluminiumudapplication and these effects were greater in Sequence (a) than in Sequence (b) soils.udAluminium remaining in solution was almost wholly in organic complexed forms.udConversely, available phosphorus was higher. Lucerne chaff was slightly butudconsistently more effective than barley straw but these differences were notudsignificant.udFinally, Lapoinya soil from the "museum" area of Elliott Research Stationudwas used in a study of effects of both added organic matter and phosphorusud(KH2PO4) on soil aluminium in relation to growth of ryegrass because of its low pH,udvery low level of available phosphorus and high level of active aluminium. Reductionudof exchangeable and monomeric aluminium occurred with rate of phosphorusudapplication, the effect being greater in the presence of added organic matter. Again, udnearly all of the soil solution aluminium was in organically complexed forms in theudpresence of added organic matter. Differences between the effects of lucerne chaff andudbarley straw were small but consistent, although not statistically significant, withudlucerne chaff having the greater effects. Soil reaction again increased with addedudorganic matter and an inverse relationship between levels of aluminium (bothudexchangeable, monomeric form) and available phosphorus depended on pH. Plantudgrowth and phosphorus uptake and content responded by increasing sharply with rateudof phosphorus application and were further enhanced greatly by added organic matter.udPlant symptoms of aluminium toxicity/phosphorus deficiency were not evident in theudpresence of added organic matter, even at the highest level of aluminium applicationud(100 ppm, 0.D.).udBoth forms of organic matter used whether fresh or partly decomposed, gaveudbeneficial effects within the period of experimentation in suppressing aluminiumudtoxicity and enhancing growth of perennial ryegrass in the acid soils used.udApplication of organic matter at practical rates may allow reduction of the quantity andudfrequency of lime additions.
展开▼
