managing soil organic matter - Grains Research & Development ...

35 MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE Figure 4.3 Phosphorus cycle in agricultural systems.While the sustained release of nitrogen associatedwith organic sources might be expected to moreeffectively raise grain protein than inorganicnitrogen fertiliser applied at the start of the season,environmental and management factors make itdifficult to predict when the organic nitrogen willbecome available and matching organic nitrogensupply to crop demand is not straightforward.The polyphenol and lignin content of organic matteralso influence the amount of nitrogen released fromorganic residues, with increasing amounts of thesesubstances limiting microbial access and nitrogenrelease (Ha et al. 2008).PHOSPHORUSPhosphorus is required for cell growth duringearly plant development. Demand for phosphorusincreases once phosphorus reserves in the seedhave been exhausted following plant establishment.Australian soils are inherently low in phosphorusand significant amounts of inorganic phosphorusfertiliser have been historically added to agriculturalsoils to support profitable crop and pastureproduction. Very soon after application, inorganicphosphorus reacts with clays and iron or aluminiumoxides rendering it ‘fixed’ and unavailable to plants(see Figure 4.3).Precipitation also results in a range of insolublesecondary phosphorous compounds forming inthe soil and as a result as little as five per cent ofthe phosphorus applied is available to crops inthe year of application due to these processes.These are distinguishable from direct losses suchas product removal, run-off and leaching becausethe phosphorous though relatively unavailableremains in the soil (see Figure 4.3). Plant-availablephosphorus in the soil solution is dominated bynegatively charged orthophosphate ions (H 2PO 4-and HPO 42-) though small quantities of solubleorganic phosphorus compounds might also bepresent. As a consequence, plant growth responsesto phosphorus fertiliser are common despite soil