Busting common fertiliser myths

04 December 2015

Since the superphosphate price spike during 2008-09, producers have considered an ever-increasing range of alternatives for improving soil health, pastures and livestock productivity. However, with the myriad products and information available, it is easy to become confused as to what will be best for your farm.

A recent review of MLA projects, led by Dr Lewis Kahn of AIMS Agriculture, has sought to provide an explanation of the popular myths surrounding fertiliser choices and their application. Here is what he reported in his findings A guide to fertilisers and soil treatments for beef and sheep meat production systems.

MYTH: Conventional fertilisers damage the soil.

TRUTH: No they don’t. Fertilisers that increase pasture growth usually improve root growth and soil biology.

MYTH: Applying single super acidifies the soil.

TRUTH: Superphosphate does not have a direct acidifying effect, but may lead to acidification by encouraging legumes which fix nitrogen that can be leached from the root zone. So, rising soil acidity is a consequence of higher phosphorus levels, more legumes and the potential leaching of nitrogen and calcium in pasture situations where perennial species are minimal or absent. It is not caused by the single super application itself.

MYTH: Soil biology can provide all the nutrients required for good pasture growth.

TRUTH: Soil microbes are critical for nutrient cycling and play a key role in increasing the plant availability of a number of major nutrients through mineralisation of organic matter.  However,  it is rare for added microbes to actually increase pasture growth. The microbes in most microbial products are only a tiny fraction of that already present in the soil and it is likely they disappear quickly with few remaining a week after application.

MYTH: Conventional fertilisers don’t work anymore.

TRUTH: No additional pasture response can be a sign the nutrients in the soil are already at or above critical threshold levels and applying more will not give you a further pasture response. Conduct a soil test to confirm this. Alternatively it may also be that other nutrients, not included in the fertiliser, or toxicities are limiting pasture growth such as potassium, sulphur or soil acidity. Again, conduct a soil test. Finally, pasture composition can have an effect. Large pasture growth responses to phosphorus rely on the presence of legumes.

MYTH: Nutrients in conventional fertilisers lock up in the soil.

TRUTH: Locking up of nutrients is a natural chemical pathway and it particularly applies to phosphorus products (conventional and alternative). Some nutrients, like phosphorus, are naturally bound (also called adsorbed) to clays and other elements like iron and aluminium. This process is an aid in soil fertility management. The locking up of phosphorus applies to all fertiliser products. Products containing slow release forms of phosphorus are actually applying phosphorus in an already locked up state and require it to be solubilised before plants can access it. By comparison, products such as single superphosphate contain phosphorus that is more water soluble and when applied to the soil it rapidly enters the plant available pool. Once phosphorus is in the plant available pool, regardless of the product source, it is at risk of being adsorbed onto clays, iron and aluminium compounds etc.

MYTH: Spraying nutrients on leaves can increase pasture growth.

TRUTH: Foliar sprays are more commonly used for the application of micronutrients in crops, not pastures. It is difficult to deliver sufficient quantities of major nutrients to pastures with foliar sprays. Leaves generally act as a barrier to keep things out, rather than to take them in. Seek evidence of the product’s nutrient content and performance and advice on any grazing issues following use.

MYTH: There are no risks with using composts and animal wastes.

TRUTH: Composts and animal wastes may contain antibiotics, heavy metals, salt, microbial pathogens and weed seeds. Seek advice on their content from the supplier before purchase and application.

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