The impact of various organic amendments on yield and soil properties

Take home message

  • Regular, long term use of organic amendments can increase crop yields
  • Phosphorus and potassium nutrient levels can be increased over time using organic amendments

Background and aims

Despite a significant research effort aimed at clarifying the real impact of organic amendment (composts and manures) use, success has been limited. The complexity of the products, access to consistent and uniform products, different use and application methods, difficulty in measuring products and their impacts on crops as well as the length of time required to assess their impacts in terms of change in soil health and fertility, add high levels of variability to trials and reduce the consistency and significance of results. However, the uptake and use of these products by growers has continued to increase based mainly on field observations, practical experience and grower interaction.

Numerous research studies have shown that organic amendments can not only improve the nutrient status of soils but also their physical properties and organic carbon levels. However, these studies have tended to demonstrate soil benefits when application rates are much higher than are logistically and financially sensible to the majority of growers. Benefits associated with low application rates have been much more variable and unreliable. Despite the significant research effort, there are currently few general guidelines available to farmers in regards to their use.

In 2010 a large scale soil amendment farmer trial was established near Cecil Plains to replicate current farmer practice and to explore various compost and manure to determine the longer term effects of these practices.

Trial method

The trial was established on the property “Pittwater” in Cecil Plains. The randomized strip trial included six treatments with five replications. Each plot was 0.53 hectare in size. The block was surface irrigated on 2 metre beds. Treatments were (1) zero amendment (2) gin trash compost (3) raw poultry manure (4) poultry manure compost (5) feedlot manure and (6) feedlot manure compost.

5 tonne of product was applied prior to each crop rotation and worked in. All other cultural practices were uniformly applied across the trial including nitrogen fertilising and starter fertiliser applications. Five crops (2011-12 Maize, 2012-13 Cotton, 2013-14 Cotton, 2014-15 Cotton, 2015 Wheat) were grown over the trial period, yields recorded and soil samples taken in May of each year.



No significant difference for yield was identified for the maize crop in 2011-12 or cotton crops in 2012-13 and 2014-15, however a significant result was identified in the 2013-14 cotton crop. This crop was significantly impacted by flooding / waterlogging during the vegetative and early flowering period. It appears that the composted feedlot manure, feedlot manure and composted poultry manure treatments were able to respond more quickly following the waterlogging impacts. A significant 8.2% increase in yield for the composted feedlot manure treatment and a 7.5% yield increase for the raw feedlot manure were recorded from the nil treatment. A 4.3% yield increase for the composted poultry manure treatment was also recorded but this wasn’t significantly different from the nil treatment.

A significant yield difference was also identified in the wheat crop 2015. Composted feedlot manure (7.9% or average 377 kg/ha), feedlot manure (13.6% or average 645 kg/ha) and composted poultry manure (12.2% or average 580 kg/ha) and gin trash grower compost (5.9% or average 282 kg/ha) were significantly higher yielding than the nil treatment.

Table one summarises average yield results for all crops over the trial period and levels of significant differences. Although significant yield differences were not identified across all years and treatments, responses appear to be constantly higher than the nil treatment. Figures 1 and 2 show significant yield differences in cotton in the 2013/14 season, and in wheat in 2015.

Table 1. Average yields for all crops over the 5 year period of the trial. Averages with the same letter are not significantly different at the 5% level. NS = not significant.



Average Maize Yields (t/ha) 2011

Average Cotton Yields (bales/ha) 2012/13

Average Cotton Yields (bales/ha) 2013/14

Average Cotton Yields (bales/ha) 2014/15

Average Wheat Yields (t/ha) 2015






Nil Treatment



7.38 a


4.747 a


Gin Trash Compost rate @ 5t/ha



7.54 a


5.029 ab


Raw Poultry/barn Litter manure @ 5 t/ha



7.54 a


4.752 bc


Composted poultry/barn litter @ 5 t/ha



7.69 ab


5.327 c


Raw Feedlot Manure @ 5 t/ha



7.98 b


5.392 c


Composted Feedlot Manure @ 5 t/ha



7.93 b


5.124 c

Weir Duncan 1v2

Figure 1. Average cotton yield 2014.

Figure 2. Average wheat yields for treatments 2015.

Figure 2. Average wheat yields for treatments 2015.

Soil properties

Changes in several different soil properties have been identified over the trial period in both the 0-10cm and 10-30cm soil samples. Phosphorus and potassium (Colwell) levels for the 0-10cm soil depth showed significant increase over the trial period (figure 3 and 4) for all treatments. There were also significant differences between treatments.

Figure 3. Average Phosphorus levels 0-10cm soil depth over the trial period.

Figure 3. Average Phosphorus levels 0-10cm soil depth over the trial period.

Figure 4. Average Potassium levels 0-10cm soil depth over the trial period.

Figure 4. Average Potassium levels 0-10cm soil depth over the trial period.

Total organic carbon was trending up in the 0-10cm soil samples but was not significant (Figure 5) within the five year time frame. Soil tests were variable, but the composted feedlot manure and composted gin trash appear to have had the greater response. The increasing trend line for organic carbon appears to be more consistent in the 10-30cm soil samples however there wasn’t a significant difference between treatments but again the compost treatments appear to be having a greater impact.

Figure 5. Average organic carbon levels 0-10cm soil layer.

Figure 5. Average organic carbon levels 0-10cm soil layer.


The regular and continuous addition of organic amendments to a cropping system has shown to increase crop yields as well as improve soil nutrient levels. Yields for all treatments were generally greater than the nil treatment across all crops over the period of the trial. Significant yield responses to composted feedlot manure, composted poultry manure and raw feed lot manure were produced in cotton 2103/14. These responses followed a very significant flooding and waterlogging event.  This potentially indicates that the use of these amendments provided the soil with the resilience to be able to recover more quickly and better meet crop nutrient demand after this event. Further research would be required to confirm this outcome. There were also significant increases in 2015 wheat yields for most organic amendment treatments. The result supports the understanding that the benefits of using organic amendments (at commercial levels) are not immediate and regular, long term applications of organic amendments is required to increase yields and have a positive impact on soil nutrient status.

The use of composted feedlot manure and composted poultry manure has significantly increased phosphorus and potassium nutrient levels in the soil, particularly in the 0-10cm layer. However these nutrients increased for all treatments (although at a much lower level) and may be demonstrating nutrient stratification under a minimum tillage system. Further research would be required to confirm this outcome.

The impact of the treatments on organic carbon is inconclusive however there appears to be an increasing trend in both the 0-10 cm and 10-30 cm soil depths. Given that 25 tonne of each product has been applied over the 5 years this is probably understandable. Tracking changes in soil carbon is difficult as has also been found by another GRDC project (DAQ00182) investigating the impact different farming systems have on soil organic matter and carbon levels.  The aim of this trial is to determine the impact on soil carbon using feedlot manure when compared to granular fertiliser within a dryland farming system.  This trial began in 2013.  Although data is not presented in this paper it is interesting to note that manure applied at 5 t/ha every three to four years compared to granular fertiliser (aimed to supply similar level of nutrients as the manure) has had no impact on carbon to date and crop yields have appeared to be responding to the amount of nutrient rather than whether it was applied as manure or granular fertiliser. This trial will continue to be monitored.


The author would like to greatly thank the co-operator, Jan Lefrenz for all his support and effort in undertaking this trial. Thanks also must go to David Hall in the establishment of the trial and Martin Tower from Organic Nutrients.

The author would like to thank the CRDC for their support in undertaking this research. The author would also like to thank the GRDC for supporting research.

Contact details

Duncan Weir
Department of Agriculture and Fisheries (DAF)
203 Tor Street, Toowoomba, Qld, 4350
Ph: 0410 518 214