Mill mud is a valuable resource and also a source of many questions from growers - about best practice and how to get the most out of mill by-products.
Our extension team in the Tully and Murray district recently pulled together all the available information to answer common questions and share findings.
What are the regulations?
Since we are growing sugar cane in a Great Barrier Reef catchment, application of nitrogen and phosphorous is regulated. Mill mud is also regulated because it has high concentrations of many essential nutrients.
The regulations state that for an application rate higher than 100 wet tonnes/hectare, fertiliser N must be reduced by 40kg/ha in the season applied, while fertiliser P must not be applied for the entire crop cycle.
The following table shows a comparison of these regulated amounts with Sugar Research Australia’s 6 Easy Steps recommendations (which are currently under review):
|Nitrogen - first year||40 kg per ha discount||80 kg per ha discount|
|Nitrogen - second year||40 kg per ha discount|
|Nitrogen - third year||20 kg per ha discount|
|Phosphorus||No fertiliser P for one crop cycle||No fertiliser P for two crop cycles|
|Potassium||40 kg per ha discount|
|Sulphur||10 kg per ha discount for 3 years|
|Calcium||Reduce next lime application by 2 tonnes per ha|
|Magnesium||No Mg required for one crop cycle|
What are the regulations for waste product use?
While growers use mill mud in a variety of different ways, some choose to collect their mill mud and compost it on farm over the wet season for application on their plant crop, which allows them to plant early.
However, storing mill mud on farm over the wet season means there is a greater risk of nutrient run-off and leaching.
The End of Waste Code (EOW Code) includes conditions for the storage of mill mud to minimise exposure to stormwater/runoff.
Any pond used for storage must be constructed and maintained so that ‘there is no release of resource over or through bed or banks of the pond to any waters; and a freeboard of not less than 0.5 metres is maintained at all times, except in an emergency’.
The EOW Code also includes a summary of mill mud composition critical maximum values as follows:
(Department of Environment and Science, 11, September, 2018)
The EOW Code also states that growers must use mill mud for a specific use (soil ameliorant, conditioner or as a feedstock in composting or soil conditioner manufacturing) and they must keep records of the following:
- Application location
- Application rate
- Application date
- Application method
How are other growers applying mill mud?
There are at least three application methods currently being used in the Tully district:
- Broadcast and banded application of fresh mud from the back of a tip truck driven through fallow blocks or recently harvested ratoon blocks. The application equipment can cater for row spacings from 1.52m – 1.9m.
- Composting the mill mud, with or without ash, for several months and applying this compost to the plant cane crop before filling-in.
- Sub-surface application which enables the grower to place the mill mud under the row before planting. Growers have seen benefits at rates as low as 30 wet tonnes/hectare.
Has mill mud composition changed over time?
Samples of mill mud produced at Tully mill are collected for laboratory analysis regularly.
The table below shows that although there has been some variation, the analysis has been broadly similar over the past decade.
|Kg of nutrient per 12 tonne truck load||N||P||K||S||Ca||Mg|
How accurate are application rates?
As part of this project we ran a small study to see how evenly and accurately the contractor applied mill mud using the banded application method.
Nineteen (20.6 cm x 14.7cm) plastic trays were placed at selected locations within the target area, and the quantity of mill mud which landed in each tray was weighed and converted to an equivalent weight per hectare.
We repeated this on successive days in October with the following results:
|Min t/ha (wet)||Average t/ha (wet)||Max t/ha (wet)|
Whilst the overall rate of 74.1 t/ha (wet) was close to the target of 75 t/ha (wet), this study shows the extreme variance in mill mud application between low and high areas.
- When using mill mud or other mill by-products always follow Six Easy Step recommendations
- Mill mud composition varies from day to day and year to year and is challenging to spread evenly over a paddock.
- Mill mud is a great resource and contains many positive characteristics, however it isn’t a silver bullet for increasing productivity.
- Mill mud should only be applied in situations where application of mill mud will benefit the crop.
Thanks to Simon Cristaudo, a QFF Extension Graduate Trainee, for pulling all this information together during his time working with WTSIP.