Soil Acidity in the Wet Tropics

In this video: Learn about soil acidity, what it is, why its a problem for farms and what to do about it.

Many soil types have some soil acidity -  it's a natural part of the processes on earth. But in the Wet Tropics soil acidity can be especially challenging because the high rainfall really encourages the acidification of soil.

What is soil acidity?

Soil acidity means that its pH is below 7.

When we measure a soil's pH we're measuring how much active hydrogen is in the active part of our topsoil. The term used for this is exchangeable hydrogen, it's the really active hydrogen moving in the topsoil.

When there's a lot of exchangeable hydrogen, there tends to be more soil acidity.

pH is measured on a scale from 1 to 14 and 7 is neutral so anything below 7 is acid.

The problem with soil acidity

In some soil types, as it gets more acid, aluminium also gets really active in the soil, which is toxic to plants.

When soil is highly acid it also means there's not much active calcium or exchangeable calcium around in your topsoil. Calcium is a really important element for plants.

So soil acidity is a three prong issue in many soil types - it's a low pH, you may have aluminium as well and you often have low calcium in the tropics .

One of the big impacts of this is that it reduces your nutrient cycling and nutrient availability processes in the soil. It can also impact your soils biological community so you may not have an ideal community for plants and pasture roots to thrive in.

What's a good pH to aim for?

As a general rule around five and a half [5.5] and above is a good target to set.

However some crops are more sensitive to soil acidity so you need to set a higher target of six and above.

How do you monitor soil acidity?

There's three main ways you can do it.

  1. Use a field colour kit - this is a good way just to keep monitoring it actively through the year.
  2. Use a handheld meter and a soil probe that can just be pushed gently into the topsoil.
  3. Use a soil test - if you're not comfortable with using soil tests talk to your extension officer or your agronomist.

What to do about soil acidity

  1. Set yourself a target for soil PH - for your crop, your soil type and your climate because it will be different.
  2. Monitor the soil regularly - when it falls below your target then you're probably impacting on crop yield and soil health. The main approach to dealing with soil acidity is to lime - basically it's applying lime (calcium carbonate) to your paddock periodically. Lime works to address the exchangeable hydrogen in your soil and that also adds calcium - so it does a double whammy and it lifts just oil pH in the process.
  3. Make sure you have high organic matter in your soils - maintaining good levels of organic matter, and in particular humus, just helps to mitigate and buffer soil acidity and in some cases it can really slow down that acid process.
  4. Target your nitrogen fertilizer carefully - one of the things that drives an increase in soil acidity is high use of nitrogen fertiliser.

Soil acidity can really impact on yields. If it is an issue in some of your paddocks, adding  more fertiliser to them won't solve the problem.

You need to address the exchangeable hydrogen, the low calcium and exchangeable aluminium  and adding more fertilizer is not going to do that.

Talk to your agronomist or your local WTSIP Extension Officer to find out more.

soil structureWatch the next video in this series: Soil Structure in the Wet Tropics