Fertiliser myths exposed according to research

Crops and pastures are constantly being developed for higher yield combined with traits such as better quality and greater disease resistance. It stands to reason that the higher the yield potential a variety has, the greater soil needs to be fertile to reach its potential. Plants respond to higher soil fertility, whether it be via manures, for example, poultry or cattle derived, from soils still high in natural fertility or from fertilisers such as urea and MAP.

It is important, no matter what the farming or pasture system, to correctly identify soil deficiencies and apply appropriate products to correct deficiencies. For example, phosphorus deficiency for a given paddock and crop or pasture may require 15 kilograms per hectare of added available phosphorus. It doesn't matter to the plant where that supply comes from. From a product like poultry manure, it may be 2t/ha, from MAP around 70kg/ha. Research has repeatedly shown that products with little nutrients but commonly marketed under titles that infer "stimulants" or the like have failed to perform.

Fertilisers like superphosphate, a mainstream pasture product for correcting phosphorus and sulphur deficiency, are also commonly wrongly criticised for only being immediately available to pastures but having no lasting ability. Again research does not support this criticism.

For example, in one of my trials, which commenced in my days as a NSW DPI agronomist, SF45, a sulphur fortified superphosphate, continued to boost legumes in a native grass pasture 15 years after the last fertiliser application. Soil sulphur and phosphorus levels remained high enough for near maximum production.

Read more: Pulse crops a standout feature of southern NSW rotation trials

Soil chemistry and biological function are complex, with millions of fungi and other microorganisms found in almost any small soil sample. Claims that adding various biological products can replace products with sufficient elements like phosphorus have yet to be largely supported by scientifically published research.

An example of independent research that assessed a wide range of products, including various ones termed biological or similar, was published in CSIRO journal Crop and Pasture Science, Volume 70 (12). Authors included former NSW DPI Agronomist, now Yass LLS Senior Agronomist Fiona Leech, Dr Alan Richardson CSIRO, and Dr Michael Kertesz, University of Sydney. The research found pasture production was mainly related to a given product's level of available phosphorus and sulphur, with single superphosphate the standout product.

Claims that so-called synthetic fertilisers are not good for soil quality have also been largely refuted by good research. A recent example is the PhD studies conducted at Boggabri by Dr Robert Banks. A degraded sandy loam soil with poor-quality native pasture converted to high-quality tropical grass plus winter legumes resulted in soil carbon build-up of over 30t/ha. Superphosphate used to correct soil deficiencies was a vital part of the program. Other long-term pasture improvement studies, such as the long-term Wagga Wagga NSW DPI one, showed improved pastures increased soil carbon by around 0.5t/ha per year.

Another common criticism of superphosphate is that it acidifies soils. Again this is a complex issue but is also largely refuted by good science. What commonly acidifies soils, especially those with low clay content (low cation exchange capacity) and those in their natural state with a tendency to acidity, is what occurs with pasture and crop improvement. That is additional soil organic matter created by correcting soil deficiencies (commonly so-called artificial fertilisers but can include others), greater product removal (grain, hay, meat), and sometimes leaching of nitrogen, often created by legumes in the rotation.

As thousands of farmers are doing, supported by detailed research, lime application as part of the system is part of the long-term sustainable cropping and pasture system on these soils.

It's difficult to speculate why conventional fertilisers attract such a relentless campaign of opposition to their use. I speculate sometimes it is vested interests, sometimes it's part of opposition to everything modern. The best guide to fertiliser use always depends on verifiable credible research.

Next week: Lentils; capable of lifting cropping income.