Compost can improve overall soil health and benefit crops. The organic matter may improve soil structure, infiltration and porosity.  The soil’s water and nutrient holding capacity may get a boost. Compost feeds the microbes in soil, which in turn increases soil fertility.

Compost and nutrients

Individual composts are different. The source materials and composting processes affect the final product. Good quality composts can supply some crop nitrogen (N), and most or all of the phosphorus (P), potassium (K) and trace elements. Test the nutrient status of composts before relying on them for crop nutrition.

The nutrient contents of composts and manures are usually reported as total amounts. Total values combine nutrients available to plants and those held in the soil organic matter (SOM). Some ‘rules of thumb’ to determine the nutrients available from compost in the first year are:

• 30% of the total N
• 70 – 80% of the total P
• 80 – 90% of the total K
• 90% of the trace elements.

About 25% of the remaining N becomes available in the second year after application. Another  10% becomes available in the third year. Apps can assist in working out the nutrients supplied in composts and manures – search for ‘compost calculator’.

Soil tests

It helps to conduct paddock soil tests before applying composts. You need to understand the current nutrient status of the soil to know if the compost will address all deficiencies, or if additional nutrients are required. Some indications that soil can benefit from adding compost are:

SOC levels < 2%.

Depending on soil type, soil organic carbon (SOC) levels below 2% could be a major productivity constraint. Most Australian cropping soils have SOC% <2%. Within conventional productive farming practices, it seems unlikely that SOC% can be substantially lifted. The best case scenario in most cropping paddocks is to maintain existing SOC levels. Where the SOC% is much higher than 2% it is questionable whether adding compost would help yield. It may apply excess nutrient that can be lost into the atmosphere or leached. This has been seen in some vegetable growing regions where excess compost is used.

Low CEC

Compost can increase the Cation Exchange Capacity (CEC) in soils with low clay content. High rates may be needed to make a useful difference.

Low nutrient levels

Compost can supplement and supply major nutrients over extended periods.

Trace element deficiencies

Compost can help address these, depending on its own trace element content.

Applying composts

Recommended application rates vary. Common rates are between five and ten tonnes per hectare. Ten tonnes per hectare will create a layer of compost approximately 1–2 mm thick. Compost on the soil surface is at risk of nutrient losses through washing away or gaseous losses of N. Incorporating compost into the soil or applying in subsoil bands reduces these risks.

Compost precautions

Observing grazing withholding periods is essential,  particularly with composts containing material from animals, including fish and birds. Each State and Territory has environmental legislation and regulations that control the application of compost to paddocks. Contact your State branch of the Environmental Protection Authority (EPA) for details of regulations.

More

GRDC

100: Does adding compost increase soil carbon?  (podcast)

Does combined use of organic and mineral phosphorus fertilisers support mycorrhizal colonisation?

The impact of various organic amendments on yield and soil properties

extensionAUS

Subsoil manuring provides long-term N supply

ACIAR

Opportunity to increase phosphorus efficiency through co-application of organic amendments with monoammonium phosphate (MAP)

EPA

Short report: The benefits of using compost for mitigating climate change

Acknowledgements

Virginia Brunton, MRA Consulting Group.

Photo credit: GRDC