Magic of Manure: The Value of Organic Amendments
For generations, farmers have recognised the value of organic amendments, but questions remain about how best to use them in modern farming systems.
How much nutrient value do manures really provide? Can phosphorus from manure replace mineral fertilisers? And can organic amendments do more than simply feed a crop?
In a recent episode of Wheatbelt NRM’s Get the Dirt series, Professor Terry Rose from Southern Cross University discussed his research project, New Organic Based Amendments for Enhanced Nutrient Use Efficiency and Soil, exploring the role of animal manures in improving nutrient efficiency and soil health.
The project, supported by the Cooperative Research Centre for High Performance Soils, investigated how phosphorus in animal manures compares with traditional phosphorus fertilisers such as MAP and superphosphate.
The project began with the question how much phosphorus value is contained within animal manures, and how available is that phosphorus to crops?
Professor Rose explained that while some growers apply manure occasionally as a one-off soil amendment, others have regular access through nearby feedlots of livestock industries.
“The main aim of what we were looking at was how available is phosphorus in animal manures?” Terry explained.
A key challenge was understanding the variability between different manure sources.
“There’s almost as much variation within batches of manure from the same source or from the same animal as there can be between pig or beef or poultry manures,” Terry said.
For growers using manure, this highlights the importance of testing each batch before application.
While testing manure may seem complicated, Terry explained that it is a relatively simple process.
“All you do is grab a sample from ten or twenty spots in the pile, put it in a bag, send it to a lab,” he said.
Understanding the nutrient content allows farmers to make informed decisions about fertiliser applications and avoid unknowingly over- or under-applying nutrients.
The research team focused on total nutrient levels rather than only plant-available nutrients, giving growers a clearer picture of what they were adding to their soil.
One of the most significant findings from the project was that phosphorus contained in manure can provide similar value to phosphorus from mineral fertilisers.
“Across all of our trails... they’re pretty comparable,” Terry said. “If you apply 20 units of phosphorus as manure, it’s pretty much the same as 20 units of phosphorus as superphosphate or as MAP.”
The research examined crops including wheat and canola and found that phosphorus availability from manure was generally equivalent to mineral fertiliser.
However, there may be a short-term delay in phosphorus availability because manure also provides carbon, which stimulates microbial activity in the soil.
“When you add it to soil, the soil microbes have a field day,” Terry explained.
Microbes use phosphorus as they break down carbon-rich organic material, temporarily storing some nutrients before they are eventually released back into the soil system.
Despite this potential early delay, the long-term availability of phosphorus was comparable.
“For a field crop that grows for six to eight months, you can treat phosphorus in manure exactly the same as you would treat phosphorus in a mineral fertiliser,” Terry said.
While phosphorus may be equivalent between manure and mineral fertiliser, organic amendments provide additional benefits.
Manures contain nitrogen, potassium, micronutrients and carbon, all of which contribute to soil function.
“You’re getting all these added benefits of other nutrients and carbon into your soil,” Terry said.
The addition of carbon was particularly interesting in soils with high phosphorus fixation, where phosphorus can become strongly bound and less available to plants.
The research found that manure applications may improve longer-term phosphorus availability by changing soil conditions.
In the second year of one trial, phosphorus from manure had a higher residual value than phosphorus from mineral fertiliser, suggesting the carbon and other components of manure may have influenced soil properties.
While organic amendments offer many benefits, Terry highlighted the importance of applying them carefully.
Large applications of nutrient-rich materials can sometimes produce excessive vegetative growth, particularly in dryland cropping systems.
“You can blow your crop up in terms of producing too much biomass too early, and in dryland environments that can be dangerous,” Terry said.
Too much nitrogen can encourage rapid plant growth, which may use available water before grain filling and increase the risk of haying off.
For this reason, understanding the nutrient concentration of manure before application is essential.
Beyond nutrient supply, organic amendments may also contribute to longer-term soil resilience.
Terry discussed the potential value of placing organic amendments deeper in the soil profile, particularly where growers are undertaking soil amelioration.
While the research did not directly measure deep placement, he explained that incorporating carbon deeper into the soil may provide greater opportunities for long-term carbon storage and encourage deeper root growth.
“You’re giving your plants and the roots a reason to want to go deeper into the soil,” he said.
Deeper roots can help crops access more water and nutrients, improving resilience in challenging environments.
The research highlights that animal manures can be a valuable nutrient source when managed carefully.
Rather than simply viewing manure as waste, growers can see it as a resource that provides phosphorus, additional nutrients, carbon and potential soil health benefits.
The key message for farmers is simple: test what you apply, understand the nutrient contribution, and consider the broader benefits organic amendments can bring to the farming system.
As Terry reflected during the discussion, who would have thought so much could be talked about when it comes to “poo”!
Published eNews #417, August 2026