Can't meet carbon storage goals without proper fertiliser

In recent times some media have repeated claims that the way to build soil organic carbon (SOC) is not to use fertilisers like superphosphate, graze via strict rotational grazing and use multi species pastures.

"Important" people are often quoted as supporters of such claims. In the above example I knew one of these "key people" and he informed me he was wrongly associated with such claims.

Research, published in scientific publications, has noted that the better the pasture, provided sensibly managed, including retention of at least minimal groundcover, the more likely they build SOC. SOC is around 50 percent of organic matter. Superphosphate, to correct soil sulphur and phosphorus deficiency has been an important aspect of better pastures and therefore greater SOC.

Example of research assessing SOC build up is the long term "MASTER" trial near Wagga Wagga. Despite seasonal fluctuations in SOC stocks and severe droughts, overall there was an increase in SOC of 0.5 t C/ha/year in the most productive annual and perennial pastures that also involved superphosphate use.

Another valuable study showing the importance of high soil fertility in a tropical grass pasture is the Boggabri research undertaken by Dr Rob Banks.

SOC increased from 58t/ha on native unimproved pasture to 84 t/ha on tropical grass pasture with winter legume, plus soil phosphorus and sulphur deficiency corrected via superphosphate.

There are other than superphosphate ways to correct soil sulphur and phosphorus deficiency. For example feedlot and poultry manure, provided used at high enough rates to supply similar amounts of deficient elements to what products like superphosphate supply compared to their more normal rates around 100 to 150 kg/ha.

Research has often shown that products that don't supply sufficient levels of elements like sulphur or phosphorus to correct soil deficiencies neither impact on production or SOC. For example comprehensive research conducted by Yass LLS (previously NSW DPI) agronomist Fiona Leech, noted superphosphate outperformed all tested alternative products. Products only helped pastures in relation to their ability to supply sufficient levels of available forms of deficient elements phosphorus and sulphur.

Claims that multispecies crops and pastures better build SOC seems logical if it results in greater plant biomass, but so far this claim is not backed by much Australian research.

In some favourable environments multi species pastures can be reasonably long term persistent. Many native pastures, including improved ones via fertiliser and legumes, have proven productive with a multitude of persistent species. However in many environments it is pointless sowing a multitude of pasture species when one or two are more productive and persistent. A multitude of species can also prove more costly to sow.

Several studies have not shown SOC levels are higher for high intensity rotational grazing verses other suitable grazing systems, for example flexible rotational grazing. It seems sensible to assume that best grazing for given species and circumstances is best for SOC accumulation. Many grazing management studies have found various strategies as ideal for a given species. Not overgrazing is vital to all grazing systems.

For many pasture species, especially perennials, a reasonable degree of rotational grazing helps keep them strong as well as productive and good quality. For annuals, keeping them between around 500 -1000 kg/ha drymatter and up to around 4.0 t/ha is good management with rotational or set stocking for extended periods less critical.

A recent paper "An everyman's guide for a landholder to participate in soil carbon farming in Australia" authored by leading Australian soil scientists Robert White, Brian Davidson and Richard Eckard, with endorsement from many other scientists, is I feel a must for farmers interested in pursuing science based SOC schemes.

Next week: Reduce risk of pasture dieback.