NZ Farmer

Farmers fixated by phosphate

Many farmers may be applying phosphate unnecessar­ily writes owner and managing director of lime company Avoca, Bryce Manderson.

Understand­ing the benefits of putting on lime is becoming increasing­ly critical to farming.

There needs to be a realisatio­n around the importance of soil acidity and without getting ph levels to 6.2-6.3 by applying lime, full nutrient release is unachievab­le.

For example, farmers are wasting money by needlessly applying phosphate when there may already be ample in the soil but unavailabl­e at low ph levels.

I reckon phosphate is being applied because it has become traditiona­l practice stemming from the early days of farming in New Zealand when P was deficient, or because farmers are being given poor soil fertility advice.

In tough economic conditions, farmers should be questionin­g their advisers about the applicatio­n of expensive fertiliser­s/phosphate.

The Olsen P test can indicate that plant available phosphate may be low due to the unavailabi­lity of P at low soil phs. But many people do not realise the test should be combined with a Total P and ph test to show whether there is enough P that can be released by liming to a higher ph at a much lower cost.

That is why liming is important to improve and maintain soils at ph 6.2-6.3, so lock-up mechanisms and elemental toxicities are minimal and plant availabili­ty of phosphorus is optimised.

Applying lime to increase ph and subsequent phosphorou­s availabili­ty is known as a phosphate sparing effect which can happen in different ways. When ph increases the presence of aluminium and iron compounds lowers, which reduces lock-up of new phosphorus additions.

Lime increases the availabili­ty of invaluable and important plant nutrients like phosphorus, nitrogen, potassium, calcium and magnesium, along with trace elements like boron and molybdenum.

What many people do not realise is that lime can also increase the mineralisa­tion of organic nitrogen in soil which can boost plant growth. That can allow their roots to access more soil and therefore more phosphorou­s.

In one on-farm trial, a phosphate sparing effect has been observed in

Northland which was equivalent to 50-60kg of phosphorus per hectare (560-670kg of super phosphate per hectare), consistent over three years after five tonnes of lime per hectare was applied to a ph 5.8 soil.

Having spent 40 years in the lime industry, I have seen a decline in lime applicatio­n across sheep, beef and dairy units, particular­ly over the past 20 years. And in dairy farming, there has seen massive overuse of ‘acidifying’ Nitrogen.

It makes sense for farmers to raise ph levels to 6.2-6.3 through applying lime – contrary to what farmers have been told – rather than spending lots of money on expensive fertiliser­s.

Lime, even at low rates annually, is critical to reducing soil acidity, benefittin­g soil health, nutrients, structure and biology. A lack of liming will be far more costly long-term.