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Archive for June, 2012

Fertilisers are a bit of a black box for most people.  Walk down any hardware or garden centre aisle and be confronted with an endless array of products all designed for different plants and situations.  But is it really that complicated?

Plants all need the same elements – nitrogen (N) and potassium (K) in the greatest amounts and often in about the same quantities give or take.  Phosphorus is only required in about 10-20% of the amount of N and K. Next come magnesium and calcium and then a whole array of others – sulphur and trace elements.  While plants might take up nutrients in differing amounts it often doesn’t matter what ratio they are in the soil, they will take up what they need.  Or in the case of nitrogen often more – that is called luxury consumption.  Unlike humans, plants don’t get fat though they just get overly leafy, sappy and prone to pests and diseases.

What about the type of fertiliser?  Are organics better than chemical fertilisers?  What about slow release or controlled release fertilisers?  And liquid versus granulated?

Liquid fertilisers – those that you buy as a powder or liquid and dilute with water are the ultimate in instantly available and quick acting.  Unfortunately in sandy soils the next time you irrigate, or if it rains, they will all be gone.  They are good for seedlings that have a small root ball because you can place it just where its needed and you can apply as little as you need.  So for a typical 6-8 pack type seedling you might only give each plant 50mL max but you might do that every 2, 3 or 4 days in the first 2-3 weeks.  No point in fertilising the whole bed, most will be totally wasted.  Just the plant.

Granulated fertilisers like NPK Blue are good when the plants get slightly bigger.  Sprinkle around the canopy area and do every 1-2 weeks for veges.

Sheep or other animal manures are also good but can be relatively high in phosphorus and contrary to popular belief, a lot of the N, P and K in them is water soluble and therefore instantly available and liable to be leached.  Animal manures may have to be aged to avoid burning from ammonia and they may carry weed seeds.

Slow release fertilisers are great for plants that don’t need to be pushed and are long lived.  So most garden plants, fruit trees if you wish, pot plants etc.  They are available in many formulations including low phosphorus for natives.  So anything from 3-4 month to 8-9 and there are even tablets that last 12 months.  The way these all work can vary.  Some are plastic coated and rely on the slow breakdown of that coating to work.  For others the fertilisers are embedded in a slowly soluble matrix.  Temperature ultimately controls the rate of release and for most the time frame on the label is worked out at about 21ºC.  In our hot summers it will be much quicker.  The disadvantage of these types of fertiliser is the rate of release may be too slow for some quick growing crops but otherwise they are excellent.

It is possible to get single element slow release fertilisers.  The most common available to the home gardener is nitrogen.

More on plant nutrition next time.

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