Soil Organic Matter in the Wet Tropics

In this video: Learn about what organic matter is, how to assess it and how to improve it on your farm.

Soil organic matter is one of the most important parts of your soil. Without soil organic matter, plants really struggle to grow well and the soil doesn't have good structure, doesn't hold water well and doesn't cycle its nutrients.

Where organic matter comes from

Organic matter starts with plants. Plants do a very important process - photosynthesis. Photosynthesis is where plants grab carbon out of the air, combine it with water and nutrients that they've taken up from the soil, and create biomass.

When plants grow they create different types of organic matter or organic biomass. They create:

  • Hard woody material, which is high in carbon and lignin
  •  Soft leafy matter, which was breaks down much more easily.

It's not just happening above ground - the same thing is also happening underground. All of the roots of plants are contributing to your soils organic matter below the ground as well.

When all this organic matter hits the soil two things happen:

  1. The stuff that's easily broken down  and is low in carbon decomposes quite quickly and most of the carbon in it gets breathed off by all the microbes in the soil that eat it up.
  2. The really woody stuff breaks down more slowly and some of it ends up being  humus. Humus is the long term soil organic matter that is a nice dark color.

You need both types in your soil.

Benefits of organic matter

Farming systems that improve their organic matter often see real benefits.

  1. It helps to create soil structure,
  2. It drives the nutrient cycling in your soil. Without organic matter nutrient cycling and the management of fertilizers is really problematic. And in fact the leaking and loss of nutrients from your farm is increased as your organic matter levels lower.
  3. It allows your soil biology to thrive.
  4. Good levels of humus in your soil helps build resilience and plant health. This means that in dry periods and in flooding periods the plants and the crops are able to bounce back better.

How much organic matter do you need?

As a general rule of thumb at least two and a half percent [2.5%] soil organic matter is a minimum target to set.

How to improve organic matter on your farm

  1. Set yourself a target.
  2. Start to monitor soil organic matter by using a soil test.
  3. Put practices in place to help build and maintain soil organic matter add organic matter to your soil. Add things like mill mud, mill ash, compost, mulch and reduce your tillage. When you till the soil you pretty much just put all the decomposition into turbo and they break down all that organic matter even faster.
  4. Reduce your nitrogen fertiliser. When you add a lot of nitrogen to your topsoil it really stimulates all the microbes in your soil and they all start to decompose at a faster rate. Start trying to reduce your nitrogen based fertilisers a bit and target your nitrogen management better.
  5. Keep living cover on your paddocks for as much as possible through the year. It's the root volume of the cover crops that's probably more important than the material and the trash on the surface.

If you want to find out more contact your local WTSIP Extension Officer.

managing nitrogen on farmsWatch the next video in this series: Managing Nitrogen on Your Farm