Increasing population pressure is increasing the demand on agricultural systems in many parts of the world and this has often led to the degradation of the soil resource. Soil carbon (C) is a major determinant of sustainability of agricultural systems and changes can occur in both total and active, or labile, C pools. A procedure is presented to determine the degree of lability of soil C. By treating a ground sample of soil with 333 mM potassium permanganate (KMnO4) to oxidize a proportion of the carbon and by determining the total carbon by combustion, two fractions of C can be measured. These fractions represent carbon of different lability, with fraction I representing the Labile C (CL), which is oxidized by 333 mM KMnO4, and fraction I1 representing the non-labile C (CNL), which is not oxidized by 333 mM KMnO4. On the basis of changes in total carbon (CT), a Carbon Pool Index (CPI) is calculated and, on the basis of changes in the proportion of labile C in the soil between a reference site and those subjected to agricultural practice or research treatments, a Lability Index (LI) is determined. These two indices are used to calculate a Carbon Management Index (CMI), with CMI = C Pool Index (CPI) xLability Index (LI) x 100. Analyses of paired samples (cropped and uncropped) from three sites in northern and central New South Wales, Australia, have shown a decline in CPI, a greater decline in LI and hence a decline in the CMI with cropping. Introduction of a legume into a wheat cropping system restored the CMI from 22 to 37 at the Warialda site. Analyses of paired samples from a sugarcane area in north Queensland have shown a decline in CMI in systems dominated by trash burning, but an increase in CMI in systems dominated by green cane trash management. Similar data from Brazil showed no increase in CT with mulching but a 48% increase in CMI due to an increase in the lability of C in the soil. The fractionation procedure and CMI outlined can be used to determine the state and rate of change in soil C of agricultural and natural systems.

Soil testing for S has generally been unsuccessful when using extractants that remove only sulfate from the soil.An assessment of a range of extractants to predict S status was undertaken on soil samples taken from 18 field trials in northern N.S.W. The extractants were water, 0 . 0 1 M monocalcium phosphate (MCP) and 0 . 5 M NaHC03, 0.25 M KC1 heated for 3 h at 100, 80, 40, or 25°C. The highest correlation between soil S test level and % maximum yield was found in the 40°C KC1 extractant (r2 = 0.73). This compares with an r2 value of 0.47 for the widely used MCP extractant.A study using a soil from a pot experiment where rice was grown showed that the KC1 extract removed more S from the H1 reducible (ester sulfate) fraction than did MCP. This S fraction is believed to be important in supplying S to plants.A comparison of the specific radioactivity of soil extractants and rice plants confirmed that the KC1 40°C extract removes S from similar soil pools as do plants. The procedure is recommended for wider evaluation.

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No yield differences in tops or roots were measured between ammonium and nitrate nitrogen sources in flow ‘United 108’ grown for 14 or 28 days in constant flow culture solution. There was a greater uptake of N from the nitrate source but approximately 25% of the total uptake remained as nitrate within the plant. Higher plant levels of phosphorus and sulphur were measured in the ammonium treatment and higher Ca++ and Mg++ in the nitrate treatment indicating a predominately cation‐anion balance effect.

The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength K M n Q has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in 813C ratio have been used to measure C turnover in systems which include C3 and C4 species.

Abstract. Seventeen field experiments were conducted on alkaline soils in eastern Australia between 1997 and 2000 to evaluate irrigated cotton response to phosphorus (P) fertilisation. Only 3 experiments demonstrated significant (P < 0.05) increases in crop P uptake or lint yield with P application. Comparison of several soil P tests revealed that Colwell (bicarbonate) P provided the best correlation with P uptake at early flowering and lint yield. Soil P may limit cotton growth where Colwell-P concentrations are <6 mg/kg. Soil P concentrations at most of the sites were well above this critical limit, so P fertiliser application was not required. Average P uptake at physiological cut-out and P removal in seed cotton was 21 and 15 kg P/ha, respectively. Apparent P fertiliser recovery was variable (0–67%) and may have contributed to the lack of response that was observed in 14 out of the 17 experiments. It is recommended that at least 40 kg P/ha be applied to soils with Colwell-P concentrations <6 mg/kg to increase soil P reserves. Application rates of at least 20 kg P/ha are recommended where Colwell-P falls between 6 and 12 mg/kg to maintain soil P fertility.

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