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Fertilisers for organic farming

Fertiliser use is a major concern when farmers consider going organic.

The soil is a nutrient bank

Consider the soil, as a nutrient bank for plants - if you take nutrients out you must put them back. Some organic farmers see this as an over-simplistic view. They ague that it ignores the contribution the soil biomass makes to fertility. You’ll find it hard to find a farm, either organic or conventional that has maintained production over the long term without the use of solid conventional fertilisers.

The ongoing debate over water-soluble fertilisers

Organic farmers consider high-water-soluble fertilisers as almost a soil poison. Their argument is that these highly soluble fertilisers lower pH making soils more acid, which decreases the biomass leading to soil deterioration - eg compaction and erosion.

Organic farmers stipulate that plants are unable to absorb the nutrients fast enough from these quick release soluble fertilisers and the excess is leached out of the soil. As a consequence the use slow-release fertilisers to avoid this rapid absorption of nutrients by plants is encouraged. The principle is to feed the soil, then the plant can absorb minerals and nutrients as required.

Conventional science disagrees with these beliefs and can quote research that shows the opposite. There is no scientific evidence to show that water-soluble fertilisers harm soil microbes, decrease soil pH or lead to compaction or erosion. These defects can occur with any system. It’s the type of crop grown which is the key. But the argument continues.

Soil analyses

Most conventional soil analyses do not satisfy the information needs of organic farmers. Soil analyses are just that - a soil analysis. It does not provide you with a fertiliser recommendation. Please take your soil analysis to a consultant who understands the organic system and will supply you with a suitable practical recommendation. Some soil analyses are a little too complicated so use a laboratory that produces an analysis that you can understand!

Herbage analyses should be used for minerals and trace elements. Be careful not to get the herbage contaminated with soil or hand perspiration when cutting it. Wear rubber gloves. Get it to the lab immediately.

Use your eyes and ask WHY

Successful organic farmers firmly believe that if your animals are not performing, you should look at the plants, and if they’re not performing, then look at the soil. The state of your stock health is a reflection of your soils. Part of “sustainability” is keeping land in a better state for future generations than we took it over. Learn to see with your eyes and not just look!

Development and maintenance fertiliser

There are two stages in a fertility building exercise - Development and Maintenance. The Development stage is where fertility is building up rapidly to reach “optimum” levels. It is sometimes called “capital” fertiliser application. Once fertiliser levels are adequate then all that is needed is Maintenance applications.

Soil test results will give you an indication of what your Maintenance fertilisers needs are. Fertilisers are a major expense and are an investment in the future of your soil, so a pro-active approach is needed. A thorough understanding of your soil analysis is vital before accepting any recommendations.

“Locked up” nutrients

This is a very contentious issue. Evidence indicates that certain elements can be locked up in the soil, some of which eventually become available for plant use. Organic farmers in general appreciate that fertilisers used that are more favorable to the soil organisms will help unlock these nutrients, and as a consequence they become more available to the plant. Conventional soil science tends to disagree therefore the argument continues.

What is not well known is that some organic manures can release their N very quickly. For example poultry manure can decompose within weeks of application when the weather warms up.

Apparently “dead” soils

This is not an uncommon feature in modern farming - where, indicated by the soil analysis, levels of nutrients are optimal, but the pastures are not performing. It is regularly associated with high use of N fertiliser where more has to be applied over time to maintain a response. Consultants with an organic approach suggest the soil biomass has been damaged and a balanced fertiliser program needs to be instigated.

What fertilisers are approved for organic farming?

These may vary depending on the individual certification organisation standards.

  • Nitrogen (N) - Animal Effluent, Compost, Liquid Fertilsers., Blood and Bone, Natural N Fixation.
  • Phosphorus (P) - RPR & Bio-Phos
  • Potassium (K) - feldspar (12% K). Potassium sulphate (restricted)
  • Sulphur (S) - elemental sulphur
  • Gypsum - calcium sulphate
  • Dolomite - calcium and magnesium
  • Lime
  • Trace elements - magnesium sulphate
  • copper sulphate
  • cobalt sulphate
  • Other trace elements can be applied, the differing forms. Seek approval from your certification organisation.
Liquid fertilisers

These come from two main sources - seaweed and fish wastes. The general consensus is that liquid fertilisers should be used in conjunction with solid fertilizers or on a planned rotation. Remember that successful organic farming is not poverty farming. High fertility levels are essential for high production.

Some liquid fertilisers are sold as “foliar feeds” to feed plants through the leaves. If you cost the NPK units in a liquid fertiliser compared to a conventional one (not approved for organic farming), then the liquid fertiliser units are always much more expensive.

Liquid fertiliser supporters then say that despite higher cost/unit of each element, their product have extra plant growth stimulants and products that unlock nutrients in the soil. There is little scientific basis for these claims at the moment. The arguments continue.

Probably the main criteria for promotion of liquid fertilisers are their benefit to soil microorganisms - which then brings about a more healthy soil.

Major pollutants in soils

In some areas of New Zealand there are major pollutants present. These are elements like heavy metals, organophosphates (Ops), DDT and Dieldren. These are often the results of chemical dumps or previous farming practices. In some situations a heavy metal residue test is required prior to conversion of your property. If residues are above the limits, the land may be unsuitable for organic farming. (Check with your certifying organisation)

Fertiliser pollutants

To avoid contamination coming onto the certified area all non-dedicated vehicles used for spreading organically approved fertilizers are required to be adequately cleaned prior to loading.

Information provided by:
Mr Denis Cadwallader, Organic Farming Specialist. 22 May Avenue, Napier, New Zealand
Phone (06) 834-3405, Fax (06) 834-3406, Mobile 025-481-782, Email This email address is being protected from spambots. You need JavaScript enabled to view it.

Mr Cadwallader is guest tutor in Organic Farming at the Waikato Polytechnic.
Phone (07) 834-8806 for further information on courses.

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