This paper reviews most aspects of soil analysis, with particular emphasis on soil chemical testing in Australia. Water quality, sample contamination, and the effects of soil drying, soil storage, and particle size are recognised as important components in the laboratory preparation of soil samples for analysis. The subsequent effects of choice of soil to solution ratio, leaching v. equilibration, soil shaking equipment, and the choice of extracting and digesting solutions are reviewed with examples.The review includes an overview of key chemical soil tests including pH, electrical conductivity, chloride, phosphate, sulfur, exchangeable cations, and cation exchange capacity. There is an examination of field v. laboratory tests and comment on analytical quality assurance. The recent release of the Australian Laboratory Handbook of Soil and Water Chemical Methods and the emerging activities of the Australian Soil and Plant Analysis Council should ensure the direction of soil testing in Australia remains positive.
Seventeen short-term field experiments were conducted over a five year period in south-east Queensland in which rates of up to 60 kg P ha-1 as monocalcium phosphate were topdressed onto established, previously grazed, grass-white clover (Trifolium repens) pastures, Increases (P < 0.05) in yields of white clover were obtained at seven sites, but concurrent increases in grass production occurred at only four sites. Higher total pasture production resulted at six of these sites. One quantitative (total) and two empirical (0.005 M H2SO4 and 0.5 M Na HCO3) estimates of phosphorus status in 0-10 cm soil samples, collected prior to topdressing treatments, were separately correlated with relative yield responses of white clover, grass and total pasture components. Although soil phosphorus levels by all methods were statistically intercorrelated (P < 0.01), acid-extractable and total phosphorus tests were generally unsuitable for predictive purposes, having low coefficients of determination for regressions and Cate-Nelson separations of responsive from non-responsive sites. Bicarbonate-extractable phosphorus proved the most suitable soil test. It accounted for 60 and 44% of the variance in relative yields of white clover and total pasture, respectively, but was poorly correlated with relative yields of grass. The suggested critical level of soil phosphorus (bicarbonate extraction) for white clover is 28 ppm P. For total pasture, responses are likely below 22, unlikely above 28 and uncertain between 22 and 28 ppm P, respectively. Percentage variance in relative yields already explained by both empirical tests was not significantly increased by inclusion of terms for pH and exchangeable calcium into the X variable.
Dry matter responses by component species of 18 established, commercial Macroptilium atropurpureum cv. Siratrolgrass pastures to gypsum topdressing treatments (0 or 25 kg S/ha) were assessed from field experiments conducted over a four-year period in south-eastern Oueensland under rain-grown conditions. The objective was to establish diagnostic criteria for the assessment of sulfur status by relating pasture yields to agronomic attributes and soil and plant chemical tests. Beneficial responses to gypsum were small (maximum of 32% in Siratro) and restricted to fewer than 25% of sites, whereas at disadvantaged sites (28% of total), grass yields were more severely depressed than Siratro yields. It was not possible to predict these effects from past sulfur fertilizer history, Siratro percentage in the pasture, or pasture age. Significant correlations between Siratro relative yields (100 x yield without sulfur/yield with sulfur applied, attenuated at 100 for model fitting) and both soil sulfate and plant sulfur concentrations confirmed the predictive value of these laboratory data. For Siratro, best prediction of responsiveness was provided by sulfur concentrations in either whole tops (R2 = 0.65) or diagnostic samples (tips of runners back to the fifth to sixth fully expanded leaf; R2 = 0.65). Critical value for diagnostic samples was 0. 16% S while for whole tops of Siratro the value varied with mathematical model from 0.13 to 0. 15% S. Phosphate-extractable sulfate was the most effective soil test but irrespective of sampling depth, it accounted for less than 50% of the variation in Siratro relative yields. Whether sampled at 0- 10 cm or 0-90 cm, a critical range of 3-5 ppm phosphate-extractable sulfate was indicated for the Siratro component. Grass and total pasture relative yields were not correlated with the chemical tests employed.
Phosphorus topdressing experiments (rates to 60 kg P ha-1) on 18 commercial Desmodium intortum cv. Greenleaf/grass pastures were conducted over a 4-year period in south-east Queensland. The aim was to determine whether yield responses, which occurred only in the Greenleaf component at six sites, could be predicted using soil or plant chemical tests. Acid-(0.005 M H2SO4) and bicarbonate-(0.5 M NaHCO3) extractable tests of phosphorus status in 0-10 cm soil samples each explained about 60% of the variance in Greenleaf relative yields. The residual variance was not significantly reduced by the inclusion of terms for total soil nitrogen, total soil phosphorus, exchangeable calcium and pH into the independent variable. These empirical soil phosphorus tests had higher predictive value than plant tests based on phosphorus concentrations in tops and diagnostic samples of Greenleaf. With both acid- and bicarbonate-extractable phosphorus, yield responses are likely in the Greenleaf component when phosphorus levels in most soils are below 22 ppm. Above 29 ppm, no response would be expected
Details are given on the effect of topdressed phosphorus, at rates up to 60 kg P ha-1, on macronutrient concentrations and phosphorus uptakes of components of white clover based pastures from phosphorus responsive and non-responsive sites. These data were obtained from 17 short-term field experiments conducted over a five year period in south-east Queensland. Plant indices for predicting yield responses to phosphorus by these pastures were derived from nutrient concentrations in tops and in 'diagnostic' samples of white clover collected during spring to early summer. Mathematical approaches used had little effect on critical values obtained. For white clover tops, which comprised fresh leaves, petioles and flowers, a critical phosphorus concentration of from 0.28 to 0.30% was established. Alternatively, an N : P ratio of 15 effectively separated responsive from non-responsive sites. The critical phosphorus concentration based on 'diagnostic' samples, which differed from tops in that flowers were excluded, was 0.30%. There was no practical difference between diagnostic indices based on white clover or total pasture production.
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