There is considerable global interest in using recycled organic materials because of perceived benefits to soil health and environment. However, information on the effects of organic waste products and their optimal application rates on the quality of heavy clay soils such as Vertisols is sparse. An incubation experiment was therefore conducted using five organic amendments at various rates to identify their optimal application rates, which could improve the quality of the Vertisol. Cotton gin trash, cattle manure, biosolids (dry weight basis 7.5-120 t/ha), chicken manure (dry weight basis 2.25-36 t/ha) and a liquefied vermicast (60-960 L/ha, volumetric basis) changed the soil chemical, physical and microbiological properties compared with a control where no amendments were applied, viz. higher light fraction of organic matter, nutrient content (N and P) and soil microbial activity. Higher application of chicken manure resulted in an increase in dry-sieved mean weight diameter. Increasing rates of cattle manure increased exchangeable Na concentration and ESP. Although vermicast itself did not contribute a significant amount of N into the soil, when applied at higher rates (60-960 L/ha), its application resulted in increased concentration of NO₃-N in soil by amounts ranging from 43 to 429%. Optimal application rates for cattle manure and cotton gin trash were 30 t/ha, whereas for biosolids and chicken manure, the optimum rate was 60-18 t/ha, respectively.

Concerns about declining soil organic carbon (SOC) and increased greenhouse gas emissions due to practices such as intensive tillage and bare fallows have encouraged the adoption of practices such as no-tillage, crop rotations and residue retention. However, whilst no-till farming is suited for broadacre crops, it has not been widely adapted for most vegetable production systems. Vegetable production systems, especially organic ones, routinely use tillage to prepare beds and manage weeds. These tillage operations break soil structure and aggregates, which is known to accelerate losses of SOC stocks. Despite requiring multiple tillage operations, the vegetable systems are also characterised by little or no crop residue input, potentially further reducing SOC stocks. The effect of sweet corn ('Zea mays L. var. rugosa') residue management (RM; i.e. incorporation or removal) in a corn-cabbage ('Brassica oleracea' L.) rotation on SOC parameters in two soil management systems (SMS; i.e. organic and conventional) was examined because crop residue incorporation and application of organic fertilisers could be ways to counteract loss of SOC due to tillage in vegetable systems. The principal aim of this thesis was to examine the effect of RM in the two SMS on soil total carbon (TOC) concentrations and stock, soil carbon fractions and microbial biomass carbon (MBC) through a field experiment of a corn/cabbage rotation over two years. A laboratory experiment was performed to separate the confounding factors of SMS in the field experiment, i.e. herbicide and mineral fertilisers in the conventional SMS, and cultivation and organic fertilisers in the organic SMS. To supplement the field experiment, another laboratory experiment focused on how two potentially opposing determinants of TOC, residue incorporation and simulated tillage (sieving), influence the emission of CO₂-C. Although, the research objectives of this thesis are focused on SOC, agronomic and fertility parameters, the essential components of a crop production system, were also considered.

Soil organic carbon (SOC) is the part of carbon (C) in the soil that is derived from living organisms and plays an important role in the C cycle (Paustian et al. 1997). Soil is a major reservoir of soil C, at 3.3 times the size of the atmospheric pool of 760 pentagrams (Pg) and 4.5 times the size of the biotic pool of 560 Pg (Lal 2004). Soils act as a reservoir of SOC and the level of storage within an ecosystem is mainly dependent on the soil type, climate, land use history, and current management practices. The quantity of SOC stored in a particular soil is dependent on the quantity and quality of organic matter returned to the soil matrix, the soil's ability to retain SOC (a function of texture and cation exchange capacity), and abiotic influences of both temperature and precipitation (Grace et al. 2005). SOC is essential for maintaining fertility, water retention, and plant production in terrestrial ecosystems with different land uses (Grace et al. 2006). Soil organic matter (SOM) maintains soil structure and productivity in agroecosystems (Lal 2010). Maintaining high levels of SOM is beneficial for all agriculture and crucial in improving soil quality. SOM has been widely used as an effective indicator of the functional response of soils to land use intensification (Dalal et al. 2003).

In Australia, the surface and subsurface soils of the majority of cotton ('Gossypium hirsutum' L.)-growing regions are sodic. Application of organic amendments can be an option to stabilize the structure of sodic Vertisols due to their potential positive effect on soil physical properties. An incubation experiment was conducted for 4 wk in a temperature-controlled (30°C) growth chamber to study the effect of organic amendments on the properties of two Vertisols with different sodicity levels. The exchangeable Na percentages (ESPs) in these Vertisol soils collected from the Australian Cotton Research Institute (ACRI) near Narrabri, New South Wales, and a commercial cotton farm near Dalby, Queensland, were modified such that three different sodicity levels resulted, i.e., nonsodic (ESP<6), moderately sodic (ESP 6-15), and strongly sodic (ESP>15). The organic amendments used were cotton gin trash (60 Mg ha⁻¹), cattle manure (60 Mg ha⁻¹), and composted chicken manure (18 Mg ha⁻¹), as well as an unamended control. The organic amendments improved the physical properties of both Vertisols by decreasing clay dispersion. In the Dalby soil, cotton gin trash produced the largest decrease (29%) in the dispersion index over the control at the moderate sodicity level, whereas in the strongly sodic soil, the lowest dispersion index resulted from the application of chicken manure. Nutrient availability (N, P, and K) was also increased significantly at higher sodicity levels for both the ACRI and Dalby soils by using organic amendments. These results indicate that using organic amendments can be beneficial for the amelioration of sodic vertisols and also to sustain soil quality.

Annual horticultural systems rely on frequent and intensive tillage to prepare beds, manage weeds and control insects. But this practice reduces soil organic carbon (SOC) through accelerated CO₂ emission. Crop residue incorporation could counteract this loss. We investigated whether vegetable systems could be made more resilient by including a high-residue grain crop such as sweet corn ('Zea mays' L. var. 'rugosa'), in the rotation through the use of conventional (no residue, no soil sieving) and organic (residue incorporated and soil sieved) soil management scenarios. We evaluated short-term emission of CO₂-C and soil C content in incubated Chromosol and Vertosol soils (Australian Classification) with and without sieving (simulated tillage) or the incorporation of ground sweet corn residue. Residue treatment emitted 2.3 times more CO₂-C compared to the no-residue treatment, and furthermore, sieved soil emitted 1.5 times more CO₂-C than the unsieved across the two soil types. The residue incorporation had a greater effect on CO₂-C flux than simulated tillage, suggesting that C availability and form can be more important than physical disturbance in cropping soils. The organic scenario (with residue and sieved) emitted more CO₂-C, but had 13% more SOC compared with the conventional scenario (without residue and unsieved), indicating that organic systems may retain more SOC than a conventional system. The SOC lost by soil disturbance was more than offset by the incorporation of residue in the laboratory conditions. Therefore, the possible SOC loss by tillage for weed control under organic management may be offset by organic residue input.

One of the alternatives to conventional (Conv) farming system is organic (Org) farming to prevent or to mitigate negative environmental impacts of intensive agriculture. Organic farming systems are claimed to be more resilient to weather extremes and can outperform conventional systems in weather extremes such as floods and droughts due higher levels of soil organic carbon (SOC).Whether organic or conventional, crop residue management (RM) plays an important role in maintaining SOC in horticulture, especially where annual crop rotations rely on frequent tillage. Stubble retention, incorporation and burning are the main three stubbles management practices in Australia. The effects of tillage and RM are often complex and difficult to separate. Theoretically, the effects of the two practices on SOC dynamics differ and can be opposing: no-tillage reduces the rate of organic carbon breakdown and potentially can reduce soil carbon losses, while stubble retention/burning directly affects the rate of organic input. The argument that organic farming is better than conventional farming for SOC storage have been challenged by critics due to its high reliance on cultivation (tillage) for weed control although its fertility management requires the addition of high levels of organic materials.

Crop residue management (RM) plays an important role in maintaining soil organic carbon (SOC) in horticulture, especially where annual crop rotations rely on frequent tillage. A trial investigating the short-term effects of sweet corn ('Zea mays' L. var. 'rugosa') residue incorporation on crop yields in a corn-cabbage ('Brassica oleracea' L.) rotation using organic (Org) and conventional (Conv) soil management systems (SMS) was established on 14 December 2009 in two contrasting soil types (Vertosol and Chromosol). The effect of mulched corn residue incorporation on weed biomass production was also studied. Corn was grown under the two SMS and residue was retained (+RES) or removed (-RES) after harvest on 23 April 2010. Cabbage was then grown from 4 May to 14 October 2010, under the same SMS in a three-way factorial design (SMS x RM x soil type). In both systems, equal quantities of macro-nutrients were supplied. Crop yields and weed biomass and apparent electrical conductivity (ECa) of soil were measured. There was no significant difference in total corn biomass for SMS or soil type. However, cabbage yield was significantly greater at the Chromosol site. The SMS x RM x soil type interaction was significant for weed biomass in cabbage, with Org having less weed biomass at the Vertosol site, especially in -RES. The +RES treatment had reduced weed biomass by 20 and 64% in conventional and organic SMS, respectively, in comparison to -RES in Chromosol. Soil ECa was significantly different for soil type only. The reduction of weed biomass in +RES treatment could be attributed to the mulching effect of the incorporated corn residue, the differences in weed seed bank and drainage between two sites. In conclusion, crop yields and soil ECa were not influenced by SMS or RM in short-term, but incorporation of residue in soil reduced weed biomass.

Conventional soil management systems (SMS) use synthetic inputs to maximize crop productivity, which leads to environmental degradation. Organic SMS is an alternative that is claimed to prevent or mitigate such negative environmental impacts. Vegetable production systems rely on frequent tillage to prepare beds and manage weeds, and are also characterized by little crop residue input. The use of crop residues and organic fertilizers may counteract the negative impacts of intensive vegetable production. To test this hypothesis, we evaluated the effect of sweet corn ('Zea mays' L. var. 'rugosa') residue incorporation in a corn-cabbage ('Brassica oleracea' L.) rotation on crop yields, nutrient uptake, weed biomass and soil nutrients for organic and conventional SMS in two contrasting soil types (a Chromosol and a Vertosol). Yields of corn and cabbage under the organic SMS were not lower than the conventional SMS, possibly due to the equivalent N, P and K nutrients applied. Macro-nutrient uptake between the organic and conventional SMS did not differ for cabbage heads. Corn residue incorporation reduced the average in-crop weed biomass in cabbage crops by 22% in 2010 and by 47% in 2011. Corn residue-induced inhibitions on weed biomass may be exploited as a supplementary tool to mechanical weed control for the organic SMS, potentially reducing the negative impacts of cultivation on soil organic carbon. Residue incorporation and the organic SMS increased the average total soil N by 7 and 4% compared with the treatments without residue and the conventional SMS, respectively, indicating the longer-term fertility gains of these treatments. Exchangeable K, but not Colwell P, in the soil was significantly increased by residue incorporation. The clayey Vertosol conserved higher levels of nutrients than the sandy Chromosol. Yields under organic SMS can match that of conventional SMS. Residue incorporation in soil improved soil nutrients and reduced weed biomass.

Application of organic waste products as amendments has been proposed as a management option whereby soil quality of Vertisols could be improved. An incubation experiment was, therefore, conducted for 4 weeks under controlled temperature conditions (30°C) to identify those potential organic amendments that might improve the quality of a Vertisol. Twelve organic amendments were investigated: cotton gin trash from three sources, cattle manure from two sources, green waste compost, chicken manure from three sources including a commercial product, biosolids and two commercial liquefied vermicomposts. Except for the biosolids, no other organic amendments had any effect on soil microbial biomass and respiration. Compared with NO₃₋N levels in the control, there was a 50% decrease in soil amended with 10 t ha⁻¹ green waste compost (65 μg g⁻¹). The three different types of chicken manures increased the NO₃₋N concentration from 75% (228 μg g⁻¹) to 226% (424 μg g⁻¹) over the control. Approximate recovery of P added by the amendment as resin-extractable soil P was 53% for cattle manure and 39% for chicken manure. Application of cattle manure resulted in a 22% increase in soil-exchangeable K over levels found in control. Organic amendments application also resulted in a significant increase in exchangeable Na concentration. Some of the organic wastes, viz. cotton gin trash (10 t ha⁻¹), cattle manure (10 t ha⁻¹), biosolids (10 t ha⁻¹) and composted chicken manure (3 t ha⁻¹) have value as a source of nutrients to soil and hence showed potential to improve Vertisol properties.

There is increasing interest in the use of organic amendments in the Australian cotton ('Gossypium hirsutum L.') industry because of perceived benefits to soil health and the environment. A 2-year field experiment was conducted at the Australian Cotton Research Institute (ACRI), near Narrabri, NSW, using three locally available organic amendments applied at typical farmers’ rates to irrigated cotton. The amendments used were cattle manure (10 t/ha), composted cotton gin trash (7.5 t/ha), and a commercial liquefied vermicompost (50 L/ha), and their effects on soil quality characteristics were compared with those of control soil where no amendment was added. The soil (0–0.10 m) was sampled on six occasions and analysed for selected chemical and microbiological properties. The physiological characteristics and nutrient uptake of mature cotton plants were also examined. The organic amendments did not have a significant effect on microbiological properties as measured by microbial biomass and respiration. Of the chemical properties measured, manure-amended plots showed higher nitrate-nitrogen, available phosphorus, and exchangeable potassium (K) concentrations over 2 years. Exchangeable K was 28% higher where cattle manure was applied than in control plots during the active growth stage of cotton in the first year of experiment. Higher nutrient uptake by mature cotton plants and lower nutrient concentration in soil were observed in the second year. Cotton physiological properties and lint yield were not significantly affected by the application of organic amendments. Seasonal parameters had a strong effect. The results suggest that there are few short-term benefits to be gained in terms of soil quality from application of organic amendments to Vertosols at the rates used in these trials.