Compost has many positive attributes including the addition of organic matter to the soil over time, helping to improve soil structure and water holding capacity — but how do the nutrient values of compost compare?
The major difference between conventional fertilisers and compost is the nutrient availability.
Nutrient availability refers to how quickly the nutrients can be taken up by the plant.
Conventional fertilisers, such as urea, contain nitrogen in a readily usable form, whereas compost has only a small proportion of nutrients readily available, the rest are released slowly or are unavailable to plants due to their high organic matter content.
Ask for a laboratory analysis for the compost you are purchasing.
How does nutrient availability of compost stack up?
Only a small proportion of the total nutrient content of compost is immediately available to plants. As the organic matter breaks down, other nutrients are slowly released. However, this process can take years.
In a typical compost, two to three years after application the nutrients available to the plant are about: nitrogen 0-15 per cent, phosphorus 20-40 per cent and potassium 80-100 per cent.
Therefore, if the analysis says it contains 100kg/tonnes DM of phosphorus then in the first three years only 40kg/ tonne will actually be available to be used by the plant, and only 0.2 per cent is available for immediate use by plants.
Compost is not a reliable source of nitrogen, particularly in the short term.
In fact, depending on the compost’s carbon to nitrogen (C:N) ratio, once compost is applied you may need to apply extra nitrogen because some of the soil nitrogen may bind with the compost’s organic matter (carbon). A C:N ratio greater than 25:1 will ensure nitrogen is not taken from the soil to decompose the carbon in the compost.
Compost can be an effective source of phosphorus, potassium and magnesium for plant growth over time.
However, heavy applications of compost are required to provide enough nutrients to replace conventional phosphorus and potassium fertilisers up front. This can be quite expensive and may create an imbalance of the different nutrients.
For example: To apply 40kg of phosphorus a hectare for the year from a conventional fertiliser, you would need to apply 455kg/ha of single superphosphate.
With a compost of 0.3 per cent phosphorus on a dry matter basis (as per table 1), you would need to apply 13.3tonnes of compost/ha.
Because compost normally contains 30 per cent moisture, that would need to be increased to 19 tonnes. If you were also only wanting to apply 17 kg/ha/year of potassium, by applying 19tonnes of compost you would be applying 48kg of potassium, more than twice as much as required.
Therefore, as with most recycled organic fertilisers, it can create an imbalance of nutrients.
Compost products can vary greatly, so when buying compost ask for an up-to-date analytical report for the product, which includes nutrient and heavy metals concentrations.
These will be on a dry matter basis, not a fresh weight/as received basis.
Typically compost contains around 30 per cent water, therefore you will be applying two-thirds of what the lab analysis listed.
If possible ask for the nutrients to be listed as kg of nutrients per cubic metre spread. This will tell you what you are applying: what is it made from? Was it made from prescribed waste (ie milk waste)? Does it meet the Australian Standards for compost (AS 4454)? Has the compost been pasteurised at the required temperatures for the required times?
Remember to test your soils, understand your nutrient needs and consider what is the right product for your situation.
This article has been adapted from the Understanding purchased compost products fact sheet produced by Dairy Australia. It is available at www.dairyingfortomorrow.com
For more information contact Sarah Killury, DEDJTR Maffra on 5147 0845 or firstname.lastname@example.org