 |
|
|| Home || || Contact || Free Research Publications || Eshop || Privacy Statement ||
Summary of full report
by Chris Penfold
RIRDC Project No. UA-49A
July, 2000
A dilemma in soil nutrition has arisen amongst established broadacre organic farmers. Soil available phosphorous has declined to levels where yields are being compromised, and the sustainability of the farming system is being questioned. This is despite the best efforts of farmers to address the issue using inputs allowed under the present certification standards.
The observed decline in available phosphorus is supported by scientific studies, which have found phosphorus nutrition to be limiting production by as much 60%. In organic / biodynamic systems which are deemed to be sustainable and producing product of the highest quality from nutritionally balanced soils, there are some significant questions that must be answered quickly to ensure the long term productivity and profitability of these farming systems.
On organic farms, it is preferable to close the nutrient cycle, whereby surplus nutrients in grains and forage that are fed to livestock, are returned to the soil as organic manure. Alternatively, if the product is used off-farm, waste product from the next link in the consumption chain (eg manure from a dairy to which the organic farmer has sold their hay or grain) is returned and spread on the organic property. This system is used and works well on many small European farms, or where they have vertically integrated within their own farming unit. The Australian broadacre farmer does not have this luxury, nor do they generally have access to large amounts of readily composted manures to replace those nutrients which have been exported. The alternative is the use of fertilisers permitted under the organic standards, which are either expensive per unit of nutrient, of poor performance or both.
With regards to soil nutrition, the management of phosphorus is the primary concern of organic farmers for the following reasons:
Despite the apparent
difficulties, it is believed that some improvements can be made in the
nutritional management on many organic farms, by utilising known practices
within a farming systems package. It is therefore the aim of this
project to provide some background to the intricacies of phosphorus in
the soil/plant ecosystem, and means of addressing the issue given the constraints
of availability and cost of certified products. It is also intended to
provide researchers with a basis for the further investigation of strategies
for overcoming this vexing problem.
Outcomes
The provision of adequate
phosphorus to plants using conventional practices is comparatively easy
in Australia, so long as the cost is readily redeemed in the product sold.
Water soluble phosphatic fertilisers have been used with considerable success
in Australia for over 100 years, and remains the standard by which others
are judged in all the farming regions. In some situations they can
adversely affect plant growth, by for example inducing trace element deficiencies,
or disrupting legume nodulation, but this is rare. More often however,
inadequate phosphorus is likely to be limiting plant productivity.
The efficiency of phosphorus use is low, with only 10-20 % of that applied being used by the growing plant. This fact, coupled with historical applications of phosphorus beyond plant requirements, has led to a bank of soil phosphorus in many of our agricultural soils. In some situations, this may mean that plants are no longer responsive to further fertiliser application. This is the preferred position for a person to begin farming organically, as it should be possible to maintain this level of soil phosphorus using certified phosphate rock products. On the other hand, using these same products, it would be a challenging and expensive proposition to raise plant available phosphorus to adequate levels in highly responsive soils. The plant growth response to the different phosphate rock fertilisers also varies considerably between plant species, soils and climatic regions.
There are available however, numerous management options available to the farmer which will assist in phosphorus nutrition. These start with the location of the property, as having soils which are below pH 6 and in medium to high rainfall areas is an advantage. Legume dominant pastures (annual or perennial) will cycle P, and if these are green manured prior to cropping, then phosphate rock should be broadcast before ploughing down. All seed sown should be of the highest quality, as this will greatly assist the plants early growth pattern. Where possible, seeding should occur as early as permissible, using crop species known to perform well in a particular environment, which will probably mean a genotypic adaptation to the P status of soils in that area. Organic matter is all important in general soil nutrition, and with phosphorus it is no exception. Enhancing the soil organic matter levels through the growth of healthy crops and pastures with a high legume component will ultimately enhance P nutrition.
When sourcing fertilisers, a ready source of compostable manure nearby is desirable, but this is rarely the case. Other possibilities must therefore be explored. The potential of using citrate soluble superphosphate should be investigated with the certifying bodies and the fertiliser companies. Otherwise, the most reactive phosphate fertiliser available should be used. Despite a lot of work investigating phosphate solubilising micro-organisms, their is uncertainty about their future role in Australian farming systems. However, the potential of humic acids should be explored further, particularly for the areas of alkaline soil where rock phosphates are of little apparent value.
Through using some or all
of these practices, it is expected that phosphorus nutrition can be improved
on broadacre farms in Australia. This review has also identified
many research areas which require further investigation or validation for
Australian conditions. Overcoming the problems of phosphorus availability
in broadacre organic farming is of vital importance, as it is a major constraint
to the further expansion of this burgeoning industry. As such, it
is hoped that appropriate levels of funding will soon be directed to this
field of investigation.
 
|