Soil fertility rules

Whatever happens in the future with fertiliser soil scientists Drs Robert McBride and Doug Edmeades say the truth about soil fertility and pasture nutrition will not change because it is a mature science.

Agknowledge’s independent soil consultants and scientists have listed the following seven key facts farmers need to know to get the best bangs for their fertiliser bucks.

1 Soils store nutrients, not make them. If nutrients are removed they need to be replaced.

2 All farmers need to do is to feed the bugs.

Get the soil fertility right, it will feed the bugs. Data from experiments going back to 1950 show as the rate of superphosphate is increased so is the microbial biomass and earthworms.

If 10 tonnes of drymatter (DM)/ha of pasture is grown and 80% utilised, then 2t/ha DM goes back to the soil as organic matter. Most of it will be carbohydrates (sugar) and that’s what feeds the bugs.

Snake-oil sellers are trying to sell farmers alternative fertilisers and products containing probiotics (humates). They are supposedly designed to enhance soil biology, but are a waste of time.

The soil is already teeming with microbes, a teaspoon contains millions of them.

3 Plants need only 16 essential nutrients to grow. They are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, magnesium, molybdenum, calcium, copper, zinc, boron, magnesium, iron and chlorine.

They are the ‘active ingredients’ in fertiliser which make plants grow. Farmers scrutinise active ingredients in weedicides and pesticides, and should do the same with fertilisers.

Nutrients such as selenium and cobalt are not essential for plant growth but added to fertilisers for animal health.

4 Von Leigbig’s law of the minimum. A 19th century German scientist, Professor Justus von Liebig, the father of organic chemistry, popularised the principle: A plant will only grow as fast as allowed by the most limiting nutrient.

There is no point adding more phosphate when a lack of potassium or molybdenum is the problem. It would be a waste of money.

So the old recipe used by a farmer may not work anymore, especially in South Island soils where potassium levels are lower than they used to be.

5 Clover is king. It fixes nitrogen for free and is a better stock food. More milk or meat will come from clover than grass.

6 Clover is low cost. It costs 4-5c/kg DM to produce ryegrass and clover pasture. Feed crops are 15-20c +/kg DM and supplements are about 30c/kg DM.

7 However, clover is also the canary in the mine. If there is not 30-40% in a pasture then there is a soil fertility problem.