What the research says on conventional fertilisers

Popular comment, often seen in various media outlets, including city focused ones, would suggest fertilisers like superphosphate, urea, MAP and the like are not good for soil nor pasture or crop. Good science refutes these claims. Not correcting soil deficiencies issues correctly is a recipe for lack of profitability, and commonly also inability to improve soil quality.

An example of the importance of correcting soil deficiencies is a current research project I am involved with, as part of a nation MLA funded CSIRO research project. 8.5 t/ha winter legume drymatter was grown where sulphur and phosphorus deficiency were corrected, compared to less than 1.5 t/ha where deficiencies were not addressed. That data, from a replicated randomised experiment on a light to medium textured acidic soil near Boggabri, in a native perennial grass pasture, shows how critical it is to properly address soil deficiencies.

Not only has appropriate fertiliser, in this case single superphosphate, resulted in vastly improved productivity, but also vastly improved legume growth leads to likely 160 kg/ha extra fixed soil nitrogen. This is important for grass growth and quality, and also leads to improved soil health, (biological activity and soil carbon).

This production gain and improved soil quality, via independent research (no fertiliser company interests), are all related to science via many studies. Research supports that appropriate fertiliser use to address soil deficiencies is commonly the difference between a productive profitable business with good land management versus one with poor returns and high vulnerability to overgrazing because of lack of feed.

Commonly, advocates of non-manufactured fertilisers, promote replacement by various "biological" products or even options lacking useful quantities of required missing soil elements, but not at rates sufficient to correct deficiencies.

Through the years, a lot of credible independent research has assessed a wide range of fertiliser products, including various ones termed biological or similar.

A most noteworthy, relatively recent paper published in CSIRO science journal Crop and Pasture Science, Volume 70 (12), is especially relevant. Authors were noted ex NSW DPI agronomist and now Yass LLS senior agronomist, Fiona Leech.

Joint authors of the study were Dr Alan Richardson CSIRO, Dr Michael Kertesz University of Sydney, Beverley Orchard, formerly NSW DPI, Dr Samiran Banerjee North Dakota State University, and Phil Graham, formerly NSW DPI.

Research was in conjunction with the Binalong Landcare group on properties "Kia-Ora", (Bruce and Noelene Hazell), "Te Kooti" (Gary and Hansie Armour) and "Glenroy".

The research found pasture productivity was almost entirely related to a given product's ability to supply missing elements like phosphorus and sulphur at rates recommended by product advocates.

Across the six years of the study comparing superphosphate to a multitude of products, many marketed as biological, on the three pasture sites, superphosphate was the standout product. In essence the research showed you should know your deficiency, and choose products that economically correct them.

Note research supports products like animal manure and some composts can adequately correct soil deficiencies. The main proviso is to have accurate product nutrient analysis and to apply them at sufficient rate to correct deficiencies like phosphorus and sulphur. This commonly means application rates of over 1t/ha, and often 2t/ha to compare with products like superphosphate at around 120kg/ha.

Also read:

More from Bob Freebairn: Paspalum defies big wet

Tiny world of beneficials could yield better yields

A further aspect is if a product is any good, research should be available to support it. Some products advocated as fertiliser alternatives have been promoted for decades without any independent validation.