A survey of the impact of cropping on soil physical adn chemical properties n north-western New South Wales.
Many Australian soils used for cropping are fragile and highly degraded. Farming practices such as crop residue removal, poorly managed leys, and excessive tillage, are common and unsustainable. The high intensity rainfall, common during summer in northern New South Wales, often coincides with a disturbed fallow and further exacerbates the problem. The cultivation of soil which has previously supported native vegetation and/or pastures, generally leads to declining soil organic matter (SOM) and carbon (C) concentrations (Dalal and Mayer 1986), lower biological activity (Gupta et al. 1994), and deteriorating soil structure (Chan et al. 1992). Results presented in Whitbread et al. (1997) showed that there have been up to 75% and 79% declines in [C.sub.T] and [C.sub.L] respectively, relative to the native reference soils, after a long-term wheat-lucerne rotation. There were concomitant declines in hydraulic conductivity (K) and aggregate stability. These changes are mainly due to lower SOM concentrations brought about by the mixing of soil and organic matter during cultivation, resulting in increased microbial accessibility, reductions in the quantity and quality of added organic materials, soil erosion, and altered temperature, moisture, and aeration conditions.
A survey of the main arable soil types of the north-western NSW region was conducted to investigate the impact of cropping on soil chemical and physical fertility.
Materials and methods
The survey was aimed at assessing the physical and chemical fertility parameters of the main cropping soils of north-western NSW in a climatic and cropping zone similar to that of the McMaster experimental site described in Whitbread et al. (1997). The main soil types of the area are broadly identified as Black Earths and Grey Clays (Vertisols) and Red Earths (Alfisols). Typical clay mineralogy and particle size analysis of these soils can be found in Stace et al. (1972). Sites were located on a range of soil types on commercial farms with different management systems (Table 1). The cropped soils were selected on the basis that they had not been recently cultivated. The survey was undertaken from August to December 1995. At this time the cropped soils sampled had either been recently harvested or were due to be cultivated for the summer cropping season.
Table 1. Soil survey sites, their history, management, and current vegetation state
Site Location Years since Management and rotation clearing (%) Red Earth Reference Warialda 0 Cropped 15 Sorghum-cereal Reference Warialda 0 Cropped >40 Cereal-lucerne-sorghum Grey Clay Reference Warialda 0 Lucerne 15 Cereal-lucerne Sorghum >40 Sorghum-cereal, no fertiliser application Reference Croppa Ck 0 Cropped-short 2 Cereal-stubble mulch Cropped-long 43 Cereal-stubble mulch Reference Moree 0 Cropped >25 Direct drilled-stubble mulched Black Earth Reference Warialda 0 Cropped >40 Sorghum-cereal Reference Warialda 0 Cropped >25 Cereal-sorghum-lucerne Site Vegetation Cover (%) Red Earth Reference Uncleared native grass, lightly grazed 90 Cropped Grazed sorghum stubble 20 Reference Treeless, uncultivated lightly grazed 95 native grass Cropped Grazed sorghum regrowth 40 Reference Uncleared, lightly grazed native 100 grasses Lucerne 2nd year lucerne 80 Sorghum Grazed silk sorghum 70 Reference Uncleared native scrub, ungrazed 85 Cropped-short Wheat stubble 80 Cropped-long Barley stubble 80 Reference Cleared uncultivated grazed area 80 Cropped Grazed sorghum stubble 10 Reference Treeless, uncultivated lightly grazed 95 native grasses Cropped Grazed sorghum stubble 60 Reference Uncultivated lightly grazed native 95 grassed area Cropped Grazed sorghum stubble 70
The cropped sites were selected such that nearby, on the same soil type and topographic position, was a soil of the same type which had never been cultivated and which could be used as a reference area. The reference sites were generally located in uncleared and lightly grazed fencelines or roadsides. Generally, the reference sites were within 50 m of the cropped site and the actual areas sampled were chosen to be away from animal trails, eroded areas, and trees. The reference sites generally had 100% plant cover.
The experiments and soil samples were located within a 2 m by 2 m area within the paddock which was considered to be representative of the rest of the paddock. These sites were away from trees, fencelines, corners, and areas where stock trampling was likely to be heavy. Information regarding cropping history and management practices was collected from the farmers. Current vegetation cover was also estimated.
Soil physical parameters
Hydraulic conductivity (K) was determined on the basis of 4 replicate measurements made within the sampling area using disc permeameters at 4 tensions (40, 30, 20, and 10 mm) (Ankeny et al. 1991). Two soil cores (85 mm internal diameter, 41 mm depth) were taken from under each permeameter at the completion of the K measurements. The intact cores remained in the sampling rings until they were sufficiently dry to remove without damaging the soil structure. The 2 samples were then dried in a fan-forced oven at 40 [degrees] C and combined to give a composite sample. In order that all soil samples had a similar energy input during crushing, the soil was gently crushed on a board with small ridges on the side to maintain a 4-mm gap between the board and the roller. The samples were then sieved to [is less than] 4 mm and large particles of organic matter removed. Aggregate stability to wetting was measured using a wet-sieving technique with 5 sieve sizes (125, 250, 500, 1000, and 2000 /Am). Wet sieving was undertaken by placing a 30-g soil sample on top of the nest of 5 sieves, immersing the sieves in water, and sieving for 10 min with a vertical amplitude of 18 mm at 30 movements/min. The soil remaining on each sieve was then dried at 40 [degrees] C for 24 h and weighed. Water-stable aggregates (WSA) were defined as the proportion of the soil which remained on the sieves [is greater than or equal to] 250 [micro] m in size after wet sieving.
Nutrient and organic C concentrations
Subsamples of the soil collected for wet sieving were air dried and ground to [is less than] 500 [micro] m and the CT and total nitrogen (NT) were each measured in an automatic N and C analyser mass spectrometer system (ANCA-MS), consisting of a Dumas-type dynamic flash catalytic combustion sample preparation system (Carlo Erba NA1500), with the evolved gases separated and analysed by mass spectrometry (Europa Scientific Tracermass Stable Isotope Analyser). All Grey Clay and Black Earth soil samples were treated with orthophosphoric acid (2% w/v) prior to analysis of CT to remove inorganic C. The more labile soil organic C in the whole soil samples and each particle size fraction was measured by oxidation with 333 mm KMn[O.sub.4] (Blair et al. 1995). On the basis of changes in CT between a reference site and the cropped site, a C pool index (CPI) was calculated:
CPI = [C.sub.Tcropped]/[C.sub.Treference]
On the basis of changes in the proportion Of [C.sub.L] in the soil, a lability index (LI) was determined:
LI = [L.sub.cropped]/[L.sub.reference]
These 2 indices were used to calculate a C management index (CMI) (Blair et al. 1995):
CMI = CPI x LI x 100
The available phosphorus (P) (Colwell 1965), ammonium-N and nitrate-N (Adamsen et al. 1985), and the available sulfur (S) (Blair et al. 1991) were determined on a single composite mixture of the bulked soil samples collected from each of the 4 disc permeameter positions within each site.
At the time of sampling, all soils were in a similar phase of the cropping cycle and had not been cultivated for several months. The period since the soils were first cleared and cultivated, as well as the recent management, influenced the parameters. The results from each soil type are discussed separately.
The detrimental impact of cropping on the inherently infertile and weakly structured Red Earth soils is severe and probably occurs soon after the soils are first cultivated. At both sites, the loss Of [C.sub.L] had been greater than the loss of CT, and due to C losses and decreased lability, the CMI declined to 32 and 24 on the soils cultivated for 15 and 40 years, respectively (Table 2).
[TABULAR DATA 2 NOT REPRODUCIBLE IN ASCII]
The loss of [N.sub.T] was large, but the application of fertiliser had maintained available N, P, and S and actually increased available P in the area cultivated for 15 years (Table 3). The destruction of the macroaggregates is shown by a 53% and 56% decrease in the percentage of WSA of the soils cultivated for 15 and 40 years, respectively, relative to the reference sites (Table 4). There were declines in K of 86% on the Red Earth cultivated for 15 years and 66% on the Red Earth cultivated for [is greater than] 40 years. There was no significant difference in K for the Red Earth cultivated for [is greater than] 40 years, due to the large variation in the replicates at the Red Earth reference soil.
The Grey Clay, being inherently well structured and moderately fertile, is more resilient to cropping than the Red Earths of the area. Once the Grey Clay becomes degraded, however, erosion can become severe. The [C.sub.L] of all the Grey Clay soils had decreased by a greater proportion than the decrease in CT. The largest decline in [C.sub.L] was [is greater than] 60% on the soils cropped for 43 and 25 years. The Grey Clay, which had been cleared 2 years previously, had lost 43% and 26% Of its [C.sub.L] and CT, respectively, resulting in a CMI of 55. The sites sown to sorghum and lucerne had also lost C, resulting in CMIs of 44 and 52, respectively (Table 2).
Nitrogen, P, and S concentrations in all of the cropped soils had generally decreased relative to the reference sites. The Grey Clay sorghum site, which had never been fertilised since clearing approximately 40 years before, showed low P and ammonia-N concentrations relative to the fertilised lucerne site (Table 3).
Table 3. Colwell P, KCl-40 S, nitrate-N, ammonia-N, and total ([N.sub.T]) (all [micro]g/g) for reference and cropped (no. of years since clearing) soils (0.41 nun)
Site Location P S Red Earth Reference Warialda 7.3 2.6 Cropped (15) 19.2 4.6 Reference Warialda 12.3 4.7 Cropped (40) 10.7 1.5 Grey Clay Reference Warialda 20.6 6.1 Lucerne (15) 17.8 3.5 Sorghum (40) 8.6 3.1 Reference Croppa Ck 28.6 3.3 Cropped (2) 17.0 4.1 Cropped (43) 9.7 0.3 Reference Moree 16.3 5.8 Cropped (25) 22.8 1.4 Black Earth Reference Warialda 36.8 7.2 Cropped (40) 21.4 2.8 Reference Warialda 21.8 3.4 Cropped (25) 21.4 1.4 Site Nitrate-N Ammonia-N [N.sub.T] Red Earth Reference 0.0 2.9 1800 Cropped (15) 0.2 5.1 900 Reference 0.1 5.9 3200 Cropped (40) 0.3 3.7 1100 Grey Clay Reference 0.8 6.2 2800 Lucerne (15) 0.1 4.2 2300 Sorghum (40) 0.2 1.9 1900 Reference 0.2 3.2 1900 Cropped (2) 0.0 1.5 1400 Cropped (43) 0.9 1.4 900 Reference 2.5 6.2 2400 Cropped (25) 1.0 1.0 1000 Black Earth Reference 0.2 5.3 2000 Cropped (40) 0.6 8.6 1200 Reference 0.0 3.6 1700 Cropped (25) 0.0 2.7 1200
Although the soils at Warialda were under a pasture phase at the time of measurement, the K decreased from over 500 mm/h on the reference site to 230 and 90mm/h on the lucerne and sorghum areas, respectively (Table 4). The reduced K in the sorghum paddock, which was being grazed by cattle at the time of sampling, is probably the result of trampling causing soil compaction. The reference site had over 64% of its aggregates as WSA [is greater than] 250 pm and this was reduced to 36% and 39% on the lucerne and sorghum areas, respectively. The Grey Clay soil at Croppa-Creek, which had only been cultivated for 2 years, had K and aggregation values similar to the reference soil, but the adjacent soil which had been cultivated for 43 years had declined by [is greater than] 30% relative to the reference. The K and aggregation of another Grey Clay from near Moree, which had been cropped in a reduced tillage and then a direct drill system for [is greater than]25 years, had also decreased by [is greater than]50% compared with the reference.
Table 4. Percentage of water-stable aggregates (WSA) >250 pm and hydraulic conductivity (K) (10 mm tension) for reference and cropped (no. of years since clearing) soils.
Means followed by the same letter within sites are not significantly different according to DMRT at P < 0.05
Site Location WSA >250 [micro]m (%) Decline Red Earth Reference Warialda 43a Cropped (15) 20b 53 Reference Warialda 57a Cropped (40) 25b 56 Grey Clay Reference Warialda 64a, Lucerne (15) 36b 44 Sorghum (40) 39b 39 Reference Croppa Ck 33a Cropped (2) 35a -3 Cropped (43) 22b 33 Reference Moree, 54a Cropped (25) 24b 55 Black Earth Reference Warialda 49a Cropped (40) 14a 71 Reference Warialda 60a Cropped (25) 50a 16 Site Macropore K (mm/h) Decline(%) Red Earth Reference 399a Cropped (15) 55b 86 Reference 144a Cropped (40) 49a 66 Grey Clay Reference 507a Lucerne (15) 231b 54 Sorghum (40) 91b 82 Reference 216a Cropped (2) 219a -5 Cropped (43) 153a 29 Black Earth Reference 192a Cropped (25) 80b 58 Reference 377a Cropped (40) 213b 44 Reference 444a Cropped (25) 194b 56
Although the Black Earths are inherently well structured and self-mulching due to their high clay content and the nature of their clay, the loss of C due to cropping practices lowers chemical and physical fertility. [C.sub.T] and [C.sub.L] had decreased by 39% and 43% on the Black Earth cultivated for 40 years, and by 27% and 35% on the Black Earth cultivated for 25 years, respectively (Table 2). The proportion of [C.sub.L] lost due to cropping was larger than the loss of [C.sub.T] in all soils of the survey except in the Black Earth which had been cultivated for 25 years. This soil had a CMI of 76 due to some recovery in C and a higher lability, and probably represents better soil management and the benefit of several years of a well-managed lucerne ley.
The [N.sub.T], P, and S concentration of the Black Earth cultivated for 40 years decreased substantially relative to the reference, whereas the ammonia-N concentrations increased. The P concentration of the Black Earth cultivated for 25 years remained unchanged relative to the reference due to regular fertiliser application, but N concentrations had declined (Table 3).
The Black Earth, which had been farmed for 40 years, had the proportion of its water-stable macroaggregates reduced from 49% to 14% with a concomitant decrease in K (Table 4). On the Black Earth, which had been cultivated for 25 years, there was only a small decrease in aggregation, but a large decrease in the K. At the time of sampling, cattle were grazing the sorghum crop residues, which would have resulted in trampling and compaction of the soil surface.
Relationship between hydraulic conductivity, aggregation, and C
Surface infiltration is dependent on several factors including aggregation and stability, pore continuity, and the condition of the soil surface. There was a positive linear relationship between K and aggregation (Fig. 1). The large variation of points around this line is to be expected because K is influenced by factors other than aggregation.
[Figure 1 ILLUSTRATION OMITTED]
Since stable aggregation maintains K and pore size distribution, leading to better water retention and more favourable conditions for plant growth, the factors that control aggregation need to be identified. When all soils were included in the regression, a significant correlation ([r.sup.2] = 0.35, P [is less than] 0.05) was found between macroaggregates and [C.sub.L] [proportion of aggregates [is greater than] 250 [micro]m = (3.64 X CL) + 25.12]. A similar significant correlation ([r.sup.2] = 0.54, P [is less than] 0.01) was found between macroaggregates and [C.sub.T] [proportion of aggregates [is greater than]250 pm = (1.40x [C.sub.T] ) + 12.27]. Although a log relationship improved the [r.sup.2] value, linear correlations were used in order to be consistent with Figs 2 and 3.
[Figures 2-3 ILLUSTRATION OMITTED]
When the regressions were repeated separately for each soil type, large differences were found in the correlations between aggregation and [C.sub.T] or CL (Figs 2 and 3). This relationship was strongest for the Red Earth which has a low clay content ([is less than]20%) and where it is likely that SOM plays a dominant role in maintaining favourable soil physical properties. For the Grey Clay, a highly significant correlation was also present between aggregation and [C.sub.T] or [C.sub.L], but the relationship was not as strong as that determined for the Red Earth. There was no significant correlation found for the Black Earth. Since [C.sub.L] is a fraction of [C.sub.T] , similar correlations with aggregation were found, but the correlations were strongest when CL was used.
Decline in soil C and fertility
Since tillage operations and plant growth have a large impact on soil C and physical properties, the cropped sites were carefully chosen to be in a similar phase of the cropping cycle and not recently cultivated. Some sites had been harvested for cereal grain only weeks prior to the sampling (October-November 1995), whereas other sites had been harvested for summer crops 8-9 months previously. Murphy et al. (1993) found differences in hydraulic properties attributed to tillage or management to be most obvious when measurements were made on a settled seedbed in the period from flowering and into the post-harvest period. Most of the sites had been grazed, or were being grazed at the time of sampling. However, the paddocks were very large and it was possible to avoid localised areas of stock trampling.
When virgin soils are brought under cultivation and cropping, organic C content generally declines (Dalal and Mayer 1986). This is due to the reduction in organic residues added to the soil, the mixing of the SOM-rich topsoil with deeper layers, the disruption of macroaggregates, and the increase in microbial oxidation of C. The SOM content of soils has often been reduced to very low concentrations and may result in widespread land degradation (McLennan 1996) and low grain yields and quality (Verrell and O'Brien 1996). The rate of decline of SOM depends on the intensity of cultivation, management of crop residues, and soil type.
Overall, the loss of C from the Red Earth was greater than the loss of C from the soil containing more clay. Up to 75% Of [C.sub.L] and 69% of [C.sub.T] was lost from the cultivated Red Earth, with most of these losses being evident after 15 years of cropping. Soils such as the montmorillonite-dominant Black Earths and the montmorillonite-kaolinite-dominant Grey Clays provide protection to SOM as a result of clay and aggregate protection. The Grey Clay, which had been sown to lucerne or sorghum, showed high CMIs due to the smaller losses of carbon. The Croppa Creek Grey Clay, which had been cleared and cultivated for 2 years, showed declines in [C.sub.L] and [C.sub.T] of 43% and 23%, respectively, relative to the reference, and this emphasises the susceptibility Of [C.sub.L] loss following cultivation. On the Croppa Creek Grey Clay cultivated for 43 years, although cereal crop residues had been mulched back into the soil for the past 15 years, C concentrations were still much lower than those at the reference site. Results from the Warialda field experimental sites (Whitbread et al. 1998) indicate increased [C.sub.L] with 3 years of wheat residue retention. These experiments indicate the increase in C to be very slow and it is likely that these benefits may be easily lost in periods where stubble is burnt, or where soil is cultivated excessively or at an incorrect water content. The Croppa Creek Grey Clay was used for cotton production 2 years prior to the survey sampling and it is likely that SOM may have been lost during these periods of low organic inputs and frequent cultivation.
The Moree Grey Clay, which had been cropped for 25 years, had been in a no-till system for the previous 5 years and stubble mulched for the previous 15 years. Carbon concentrations were very low relative to the reference, resulting in a CMI of 33. The benefits of no-till systems on soil fertility and growth are well documented (Hamblin 1980), but the removal of crop residues by grazing stock may result in limited organic inputs into the system, which are essential for SOM improvement.
Both cropped soils of the Black Earth showed decreased C concentrations relative to the reference. By comparison with the other soils, the decrease in [C.sub.L] with cropping was smaller. This soil had recently been under a well-managed lucerne ley of several years duration and it is likely that there has been some recovery in C concentrations, especially [C.sub.L], due to organic matter inputs.
Many workers have found the rate of loss of labile fractions of SOM to exceed that of the total SOM (Dalal and Mayer 1986; Christensen 1992; Blair et al. 1995). Labile fractions of SOM include macro-organic matter such as plant residues and root material, fungal hyphae, microbial biomass, and their exudates such as polysaccharides. Since these fractions of SOM are easily oxidised, they provide an earlier indication of the consequences of land use than total SOM or [C.sub.T] . The soils of the survey showed greater losses in [C.sub.L] than in [C.sub.T] , except in the Black Earth cultivated for 25 years. In this soil it is likely that organic matter additions from several years of a well-managed lucerne ley have increased CL faster than [C.sub.T] . A well-managed lucerne ley would be one on which stock are rotationally grazed, weeds controlled, and organic matter inputs to the soil maximised. Blair et al. (1995) found an increase of 39.7%, 2.4%, and 8.5% in [C.sub.L], [C.sub.NL], and [C.sub.T] , respectively, over a 12-month period after mulch return in a sugarcane experiment. Through the measurement of [C.sub.L], cropping systems can be ranked on their ability to improve SOM concentrations. The improvement in C concentrations with the return of wheat residues was shown to be very slow in data presented from the Warialda residue management trial (Whitbread et al. 1998). Other management options such as green manuring, stubble mulching, and conservation tillage can be assessed on their ability to influence SOM concentrations.
The Black Earth and Grey Clay soils contain [is greater than]50% of their SOM in the clay size fractions. Aggregates are not only stabilised by SOM-clay complexes, but also by metallic cations on the exchange sites of clays and total soil solution concentration (Naidu et al. 1996). Although significant relationships between aggregation and organic C have been found, the dominant role of exchangeable cations is a factor in the stabilisation of aggregates in these soils.
In loams, the cohesive nature of clays and the shrink-swell capacity associated with colloidal particles create aggregates through drying and wetting cycles (Oades; 1993). The higher the clay content, the greater the shrink-swell capacity exhibited by these soils. In the self-mulching soils in the survey, such as the Black Earths, the creation of soil structure is dominated by the behaviour of clays. For the top few centimetres of the soil surface, biological factors are not important for either structural formation or stabilisation (Oades 1993).
For soils low in clay, such as the Red Earths, good correlations between aggregation and SOM have been observed. Tisdall and Oades (1982) showed an increase in aggregates [is greater than] 2 [micro]m with increases in SOM as a result of pasture-based crop rotations, and they calculated a highly significant linear regression equation:
Aggregate stability = 21.5 x (organic C) - 20.3 ([r.sup.2] = 0.93)
Hydraulic conductivity, aggregation, and soil type
Surface infiltration is dependent on a number of factors and includes aggregation and stability, pore continuity and stability, the existence of cracks, and the soil surface condition. The formation of cracks is most prevalent in high clay soils. The existence of WSA creates pores within and between aggregates. WSA reduce the incidence of surface crusting and hardsetting processes.
K measured at 10 mm tension was used to measure the flow through macropores [is greater than] 3 mm in diameter and therefore to infer the proportion of macropores that had been maintained or destroyed by land use practices (Murphy et al. 1993). Large pores, by their nature, are susceptible to collapse to form smaller and non-continuous pores. Murphy et al. (1993) also used sorptivity and K to quantify structural degradation due to farming practices.
A decline in K with cropping was shown here in every soil except for the Grey Clay which had been cleared only 2 years prior to sampling. A significant relationship was shown between K and aggregation (Fig. 1). The large amount of variation of data points around the fitted line was expected since K is influenced by a number of factors. In the high clay soils such as the Grey Clay and Black Earths, cracks may initially dominate the infiltration process until the soil swells and cracks close. Permanent macropores, which are dependent on WSA, are then the major water transmitters. Since K was measured at steady state, permanent cracks and macropores [is greater than] 3 mm were measured at 10 mm tension. The sites that were being grazed at the time of sampling (e.g. Grey Clay sorghum) displayed lower K, probably due to surface compaction from animal trampling. Whitbread and MacLeod (1994) showed K measured at 13 mm. tension to decrease from 75 mm/h on an ungrazed site to 20 mm/h on a site grazed with 20 sheep/ha.
This survey indicates that many of the farming practices common to the northern cropping zone have a detrimental impact on SOM concentrations, soil structure, and nutrient supply capacity. The consequences of this degradation can be seen in declines in yield and quality of agricultural produce.
The results of the survey have been reported relative to an uncropped reference area. The reference areas have been used to provide the original soil physical and chemical fertility levels. The SOM concentrations measured in the reference areas are not necessarily considered as the optimum level. SOM concentrations at which favourable soil physical properties are maintained, nutrient supply capacity is optimised, and crop yields are stable and sustainable need to be identified.
The soil survey highlights the degraded fertility of many cropped soils due to conventional cropping practices. Soils such as the Red Earths are most susceptible to degradation. Other cropped soils, such as Black Earths and Grey Clays which are generally considered inherently fertile and resilient, were also shown to be highly degraded. The cost of structural and chemical degradation, which can be measured in terms of reduced yields, increased production costs, and damage to the environment, is likely to be enormous.
The farmers Ken Brooks, David Moor, John McDonald, Philip Ledingham, Phil Kennedy, and Ian Johnston are thanked for the opportunity to survey and collect information about their land and farming practices. Financial support was provided by the Australian Centre for International Agricultural Research (ACIAR) and the Grains Research and Development Corporation (GRDC).
Adamsen, F. J., Bigelow, D. S., and Scott, G. R. (1985). Automated methods for ammonium, nitrate and nitrite in 2M KCl-phenylmercuric acetate extracts of soil. Communications in Soil Science and Plant Analysis 16, 883-98.
Ankeny, M. D., Ahmed, M., Kaspar, T. C, and Horton, R. (1991). Simple field method for determining unsaturated hydraulic conductivity. Soil Science Society of America Journal 55,467-70.
Blair, G. J., Chinoim, N., Lefroy, R. D. B., Anderson, G. C., and Crocker, G. J. (1991). A sulfur soil test for pastures and crops. Australian Journal of Soil Research 29, 619-26.
Blair, G. J., Lefroy, R. D. B., and Lisle, L. (1995). Soil carbon fractions based on their degree of oxidation, and the development of a carbon management index for agricultural systems. Australian Journal of Agricultural Research 46, 1459-66.
Chan, K. Y., Roberts, W. P., and Heenan, D. P. (1992). Organic carbon and associated soil properties of a red earth after 10 years of rotation under different stubble and tillage practices. Australian Journal of Soil Research 30, 71-83.
Christensen, B. T. (1992). Physical fractionation of soil and organic matter in primary particle size and density separates. Advances in Soil Science 20, 1-90.
Colwell, J. D. (1965). An automatic procedure for the determination of phosphorus in sodium hydrogen carbonate extracts of soil. Chemistry and Industry, 893-5.
Dalal, R. C., and Mayer, R. J. (1986). Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. 1. Overall changes in soil properties and trends in winter cereal yields. Australian Journal of Soil Research 24, 265-79.
Gupta, V. V. S. R., Roper, M. M., Kirkgaard, J. A., and Angus, J. F. (1994). Changes in microbial biomass and organic matter levels during the first year of modified tillage and stubble management practices on a red earth. Australian Journal of Soil Research 32, 1339-54.
Hamblin, A. P. (1980). Changes in aggregate stability and associated management properties after direct drilling and ploughing on some Australian soils. Australian Journal of Soil Research 18, 27-36.
McLennan, W. (1996). `Australian Agriculture and the Environment.' Australian Bureau of Statistics, Catalogue No. 4606-0. (Australian Government Publishing Service: Canberra.)
Murphy, B. W., Koen, T. B., Jones, B. A., and Huxedurp, L. M. (1993). Temporal variation of hydraulic properties for some soils with fragile structure. Australian Journal of Soil Research 31, 179-97.
Naidu, R., McClure, S., McKenzie, N. J., and Fitzpatrick, R. W. (1996). Soil solution composition and aggregate stability changes caused by long-term farming at four contrasting sites in South Australia. Australian Journal of Soil Research 34, 511-27.
Oades, 3. M. (1993). The role of biology in the formation, stabilization and degradation of soil structure. Geoderma 56, 377-400.
Stace, H. C. T., Hubble, G. D., Brewer, R., Northcote, K. H., Sleeman, J. R., Mulcahy, M. J., and Hallsworth, E. G. (1972). `A Handbook of Australian Soils.' (Rellim Technical Publications: Glenside, S. Aust.)
Tisdall, J. M., and Oades, J. M. (1982). Organic matter and water-stable aggregates in soils. Journal of Soil Science 33, 141-63.
Verrell, A. G., and O'Brien, L. (1996). Wheat protein trends in northern and central NSW, 1958 to 1993. Australian Journal of Agricultural Research 4T, 335-54.
Whitbread, A. M., Blair, G. J., and Lefroy, R. D. B. (1997). Management of legume leys, residues and fertilisers to enhance the sustainability of wheat cropping systems. In `Proceedings of the Farming Systems Research-Two Decades On'. 8th April 1997, Moree. pp. 47-53. (NSW Agriculture: Moree, NSW.)
Whitbread, A. M., Blair, G. J., and Lefroy, R. D. B. (1998). Managing residues and fertilisers to enhance the sustainability of wheat cropping systems. In `Proceedings of the 9th Australian Agronomy Conference'. Wagga Wagga, New South Wales. (The Australian Society of Agronomy, Inc.: Carlton, Vic.) (In press.)
Whitbread, A. M., and Macleod, D. A. (1994). The effect of grazing sheep on soil physical conditions and water relations in northern New South Wales, Australia. In `Transactions of the 15th International Congress of Soil Science'. Volume 2b. Acapulco, Mexico, 10-16 July 1994. pp. 259-60.
Manuscript received 24 March 1997, accepted 6 April 1998
A.M. Whitbread(A), Rod D. B. Lefroy(B), and Graeme J. Blair(A)
(A) Division of Agronomy and Soil Science, University of New England, Armidale, NSW 2351, Australia.
(B) Rod D. B. Lefroy, International Board for Soil Research and Management, PO Box 9-109, Bangkhen, Bangkok 10900, Thailand.
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|Author:||Whitbread, A.M.; Lefroy, Rod D.B.; Blair, Grame J.|
|Publication:||Australian Journal of Soil Research|
|Date:||Jul 1, 1998|
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