Cotton is now a significant crop in the irrigated Riverina region of southern NSW, Australia, increasing in area from approximately 39,000 ha in 2013 to 90,050 ha in 2018 and contributing from between 10-30% of Australia’s total cotton production (Cotton Info, 2018). Many cotton crops and processing facilities in the region (Whitton, Hay, Carathool) are located in close proximity (< 50 km) to chicken production (meat and eggs), which has also increased significantly in recent years. Around Griffith, NSW, it is estimated that poultry production is currently generating approximately 10,000 m3/week of manure and chicken litter (raw manure + bedding material which is usually rice hulls or sawdust). There is quite significant variability in the nutrient content and dry matter mass of manures and wastes that are cleaned out of chicken sheds (Griffiths, 2007). However, considering average values of plant total nutrients as 2.9%N, 1.2% P, 2.8 % K and 60% dry matter, our estimates suggest that on an annual regional basis, there maybe 5400 tonnes of TN, 2246 tonnes of total P and 5200 tonnes of total K. These are sizeable amounts of nutrient resources which have the potential to be relatively easily reused for productive purposes in an economically attractive way for farmers. Coincidentally, this re-use may contribute to offsetting the amount of artificially produced fertilizers that are applied regionally, which offers positive environmental outcomes.

A significant number of growers (> 50%) are applying the available chicken wastes to their land, mainly with a view to maintain or enhance yield through the soil fertility benefits that organic amendments can offer (nutrient efficiency and biological and physical property maintenance and improvement). Typically application rates are between 2-5 t/ha, spread and surface incorporated early in the winter when field conditions are optimal and nutrient mineralisation and losses are minimised due to low microbial activity under relatively cold winter conditions. When temperatures start to increase in spring, mineralisation of nutrients and other physical and biological beneficial effects of manure amendments are considered to potentially become available from planting through the growing season and long term.

The organic materials are not usually applied to fields by growers as fertilizer replacements, rather, in addition to conventional fertilizers as there is a lack of adequate manure management recommendations for use in cotton production under different soil types and cropping systems. The usual rates of 2-5 t/ha of animal manure applied are convenience based rather than research based and can’t be relied upon in high yield cotton systems without supplemental N and other nutrient fertilization. Application rates of chemical N and other fertilizers are determined from field tests, petiole tests and experience. However, there are few detailed studies on how amendments impact on soil processes and thus effect crop production (Flavel and Murphy, 2006). The productivity responses of cotton crops in major irrigated soil types of the Australian southern region to organic amendment types, rates and frequency and placement of application, alone or in combination with fertilizers is unknown. The nutritional value, extra profit and soil sustainability measures that the organic products provide in the short and long term compared with fertilizers are ambiguous and difficult to predict. One particular reason for this is that manures vary greatly in their composition and degree of stabilization. Chicken litter available N, P, K composition and moisture content can vary by 2-10 times from one batch to another (Griffiths, 2007, Azeez and Averbeke, 2010). To successfully manage nutrient cycling from chicken litter it is necessary to know their decomposition rate and the influence that may have on the biogeochemical processes in the soil to which they are being applied.

Overall, there is a large body of research that shows that application of organic amendments improves soil fertility (see reviews of Murphy, 2014, Macdonald and Baldock, 2010 and Krull, 2004). Soil organic matter affects water holding capacity, nutrient retention, cation exchange capacity, aggregate stability and buffering capacity to acidification. It has a clear effect on nutrient supply, nutrient cycling, soil strength and compaction, water infiltration and gaseous exchange. A number of studies have reported the positive effect of soil organic carbon on cereal, potato, corn and rice in tropical and sub-tropics, Russia, China, Argentina (Johnston et al, 2009, Lal, 2010, Chen et al 2018). However, there are now several reputable publications detailing rigorous meta-analysis of well-known long term crop trials that conclude, on average there are insignificant increases in yield which can be attributed to organic inputs (Edmeades, 2003; Oelofse et al, 2014; Hijbeek et al 2017). The major finding that came out of the original work of Rothamstead was that organic fertilizers gave the same yield as farmyard manure, not that synthetic fertilizers were necessarily better than organics or vice versa (Johnston and Poulton, 2018). Johnston et al (2015) has countered some of the misconceptions surrounding the effects of SOM by finding that as crop cultivars with increased yield potential have been introduced, yields in many Rothamstead long term experiments are now larger on soils with more SOM. In contrast, Oelofse et al (2014), argues that the evidence from a lot of published organic amendment trials is quite variable and obtained from one location and therefore generalisations are difficult. In many studies, the analyses are dependent on, or fail to account for other factors that can affect yield such as the mineralizable C fraction, soil fertility status and managerial inputs such as nutrients and water (Lal, 2010).

Favourable agronomic, profit and soil conditioning effects for poultry litter application in cotton production have been documented within 3-5 years, in Alabama and Mississippi on silt loam soils (Reddy et al 2007; Tewolde et al, 2007; Tewolde et al 2016) and degraded upland soils in Louisiana (Lofton et al 2014). In other studies, the benefit of poultry litter to cotton production was stated as far exceeding the nutrient concentrations through soil conditioning (Tewolde et al 2010). Mitchell and Tu (2005) found broiler litter increased cotton yield in both no till and conventional till over a 13 year period in silty clay loam non-irrigated soils and residual effects in the year after application were found to be beneficial to yield.

Fertilizer replacement value of manure is affected by a number of factors including the form of nutrients, organo-metallic complexes, in the amendment, soil type and pH, crop type, application method, timing and manuring history (Jensen, 2013). It is well known that organic amendments including chicken litter cannot be depended on in commercial situations as a soul source of crop nutrients, even when applied at high rates due to nutrient imbalances and asynchronic release of nutrients that do not match plant requirements. Supplementary synthetic fertilizers are necessary in commercial food and fibre crops but little systematic research has been undertaken to evaluate the fertilizer replacement value of organic amendments and any improvements in nutrient use efficiency they may offer.

The current project has aimed at more thoroughly establishing how to optimise the management and application of manure that is available in the southern cotton growing region for tangible productivity and nutrient use efficiency benefits. Field experiments have focussed on incorporating the use of chicken manure and litter into farmer agronomic practise and specific site issues. One focus has been on the fertilizer replacement value of the organic wastes when used in combination with chemical fertilizers for N, P and micronutrients Zn, Mn and Cu. A second focus has been on the effects that supplementary chicken litter or manure application may have to rectify productivity and nutrient deficiencies in newly developed land formed fields where ‘cuts’ can have a significant effect on productivity for several years.

Replicated cotton field trials on two soil types, red and grey chromosols with a range of chicken manure and litter amendment rates, have been established to evaluate productivity, nitrogen efficiency and macro and micro-nutrient uptake. The trials have been used to assess crop responses, examine the contributions and availability of important nutrients other than nitrogen (P, K, S, Zn, Cu) water stable aggregates and any negative or inconsequential impacts according to manure rate treatments. The work has provided regionally specific information which can be used to refine existing general manure management guidelines developed for broad acre cereals. These may assist cotton growers in the Murrumbidgee Valley to better estimate how manure may be applied to reduce synthetic fertilizer inputs without compromising yield and quality outcomes.