Om om om om: not a chant but feeds the soil

Om om om om: not a chant but feeds the soil

Carbon, nitrogen, sulphur and phosphorus — a smorgasbord of delectable nutrients for crops.

Plants need them to survive and thrive and, to help deliver them, growers need organic matter (OM), which also improves soil structure, binds pollutants, and influences soil-buffering capacity.

Many soils, particularly light soils, suffer from relatively low OM according to Daniel Murphy, a researcher with the Centre for Land Rehabilitation at the University of Western Australia who is investigating the role of biologically active fractions of organic matter in soil.

Supported by growers and the Federal Government through the GRDC, Dr Murphy is monitoring the impact of WA agricultural practices on OM and will identify and report best practices for improving organic matter fertility, allowing growers to access the benefits of greater OM for their cropping systems.

A primer on OM

OM comprises decomposed and decomposing vegetation, plus microorganisms such as bacteria and fungi.

These micro-organisms are the living component of the OM and, as they break down the dead OM, they release nutrients into the soil in a form which plants can absorb. They also use nutrients, such as ammonium, out of the soil.

It is the net difference between the nutrients provided and used by the microorganisms that determines plant availability.

To raise OM levels, new organic content (plant debris or animals) needs to be added to the system, supplanting stocks that have been completely decomposed.

Green or brown manuring are two methods growers can use to build OM. In both cases, a crop is grown to build up organic matter. For green manuring, the crop is ploughed in. For brown manuring, the crop is killed with chemicals and left on the surface (an option for no-till farming).

No-till systems are also beneficial as they protect against soil erosion — the upper layers of soil being home to the majority of OM.

Cultivation tends to accelerate the rate of decomposition, prematurely draining the soil of OM.

Perennials and warm season crops (where they can be grown) prolong the length of time that plant debris and root turnover are available to micro-organisms. This helps maintain activity over the summer.

The GRDC research will continue over the next two years, but already Dr Murphy is getting some interesting results which should help WA growers improve future soil performance.

Microbial biomass is the living component of the soil OM. It comprises 1-5 per cent of the total OM. Although it is a small pool it is a more sensitive measure of land management-induced changes in OM than measurements of total carbon (C).

&nbspMicrobial Biomass-C% Increase kg C / ha (0-5 cm)
Continuous wheat163
Crop rotation (direct drill)1768
Crop rotation (no-till)19017
Crop/pasture (grazed) rotation23544
Annual pasture25959
Perennial pasture26160

This data from another GRDC-supported trial managed by AGWEST.

Early results indicate:

  1. Considerable increases in microbial biomass (and total soil OM) can be achieved with the inclusion of pasture phases in rotation (44 per cent increase over continuous wheat).
  2. Microbial biomass was greater under no-till (in the surface soil layer) compared to direct drill.
  3. At this stage there is no difference between microbial biomass under annual compared to perennial pasture. (However, 31 per cent more nitrogen was supplied from the soil under perennial vs annual pasture.)

Program 4.2.1 Contact: Dr Daniel Murphy 08 9380 7083 email

Region North, South, West