Australian soils tend to be old and nutrient-poor meaning that farmers often need to use fertilisers on their crops to supply them with the nutrients essential to growth – such as nitrogen, phosphorus and potassium.

But what is the best source of phosphorus for plants? Is it better to use organic amendments such as animal manure or poultry litter, or conventional manufactured fertilisers?

Jessica Mackay, a PhD candidate in the School of Agriculture, Food and Wine here at Waite, is researching the impact of organic amendments on phosphorus (P) nutrition in cereal plants. She has found that using a combination of conventional mineral fertiliser and a manure-based organic amendment is likely to produce the best results.

Uptake of nutrients is enhanced in many plants by mycorrhizal fungi which colonise the roots, creating a vast connection between the plant roots and the soil around them. Mycorrhizal fungi effectively increase the surface area of the roots, collecting nutrients from the soil beyond the reach of plant roots alone, and transfer these nutrients back to the plant.

“Wheat plants grown in soil to which only mineral fertiliser (phosphoric acid) was added took up more than double the amount of P taken up by plants in pots where only an organic amendment had been added. This indicates that P from organic amendments is generally not available to plants in the first weeks of growth but becomes available later in the growing season as soil organisms begin to convert it from organic to soluble inorganic forms.”

Jess’s research indicates that using an organic amendment with a top-up of sufficient mineral fertiliser to meet a crop’s P needs not only supplies phosphorus essential for plant growth, but could benefit the overall soil condition and health through boosting the development and action of mycorrhizal fungi.

However, growers looking to use an organic amendment such as poultry litter as a source of P should also apply a low rate of mineral fertiliser to provide ‘starter’ P to ensure the crop has adequate P available for early root establishment.

This project is funded by the Grain Research and Development Corporation.

Jess not only looked at what type of fertiliser provides plants with the best P nutrition, but also how different amendments affect the development of mycorrhizal fungi and their ability to help the plant acquire nutrients.

“I found that using an amendment to supply part of a crop’s P needs improves mycorrhizal colonisation,” she said.

“Wheat fertilised with an organic amendment such as a manure or compost plus a conventional mineral phosphorus (P) fertiliser has greater levels of colonisation by mycorrhizae than wheat fertilised only with mineral P.”

But Jess also found that not all organic amendments are equal when it comes to the type of P it provides and how much of it is actually available to the plant in a form it can use.

“Different forms of P become available at different rates and the form of P in an amendment might be more significant than its total amount of P,” Jess said. “The form of P-containing compounds in an organic amendment or manure is an important indicator of the suitability of an amendment as a source of P for wheat.”

“Wheat plants provided with a combination of mineral P fertiliser and poultry litter were able to source adequate P but an organic amendment alone did not meet crop P demands, partly because very little P from an organic amendment is available in the first few weeks after it is applied,” she said.


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