Does more carbon mean that crops grow better?
This is a hard question to answer, because there are so many things that affect how crops perform. We have looked at this in two ways: how crop yields and soil properties vary in different parts of the same paddocks, and secondly, how crop yields and soil properties vary from paddock to paddock. Results comparing different poppy paddocks (Figures 1 and 2) show no clear relationship between soil carbon and yield. This is perhaps not surprising because other important factors like water, fertiliser, pest and disease management also varied from paddock to paddock.
Good management of these factors would tend to even out any differences due to organic matter. When yields were measured at a number of points within individual paddocks (Figures 3 and 4), a stronger but still variable effect of carbon could be seen. The within-paddock results were also confounded by the fact that the sample points were affected to varying extent by previous soil erosion. Nevertheless, in 4 out of 5 cases, higher carbon was associated with higher yield.Figure 1.
Association between poppy straw yield and soil organic carbon in sodosols in 2001/2 (Cotching et al. unpublished data). Each point comes from a separate paddock.Figure 2.
Association between poppy straw yield and soil organic carbon in ferrosols in 2001/2. Each point comes from a separate paddock.
(Cotching et al. unpublished data). Figure 3.
Association between carrot and onion yields and soil organic carbon in ferrosols in 2000/01 (Cotching et al. 2002). Each point with the same symbol represents a sampling point within the same paddock.Figure 4.
Association between poppy and pyrethrum active ingredient yields and soil organic carbon in ferrosols in 2000/01. Each point with the same symbol represents a sampling point within the same paddock.
(Cotching et al. 2002).
Long-term changes in soil carbon
If all of the major factors affecting soil carbon are kept the same, it follows that soil carbon will reach a characteristic equilibrium over the long term (Figure 5). At that equilibrium, additions of carbon through plant production will be balanced by losses due to decomposition of humus. Such an equilibrium will be upset if one of the influencing factors changes. For example, changing from a pasture system to cropping usually means less carbon is being produced and so less carbon is returned to the soil. The soil carbon level drops (Figure 6) until the equilibrium for the new rotation is reached. Unfortunately the data in Figure 2 are not sufficiently long term to show the new equilibrium. This can take 50-100 years, although most of the change usually happens in the first 10-20 years.
In practice, the almost constant readjustment by farmers of their crop rotations means that equilibria are hardly ever reached before the cropping system changes. On top of this, weather conditions can be drier or wetter than average for extended periods (e.g. 5-10 years), and this variation also prevents soils from reaching a steady state.Figure 5.
Generalised example of movement of soil carbon to a steady-state following an initial change from cropping to pasture.Figure 6.
Carbon change with increasing cropping in Tasmanian soils:
squares = ferrosols; circles = dermosols; triangles = tenosols
(Cotching and Sparrow unpublished data)Figure 7.
Sample output from a computer model of carbon change showing the impact of barley residue incorporation, and the inclusion of a green manure or a pasture, on long term carbon concentrations in a cropping system on a sand at Campbell Town.
(Rayner and Sparrow unpublished data).
- Organic matter is an important soil property. It is associated with a variety of other important soil physical, chemical and biological characteristics
- Tasmania's cool climate favours the retention of organic matter to a greater extent than is the case in warmer regions
- It is not easy to show a clear link between soil organic matter concentrations and crop yield and quality. Many other factors affect yield and quality to a greater extent
- Nevertheless, there is sufficient evidence linking organic matter and yield to suggest that farmers should strive to maintain or increase organic matter in their soils
- Maximising inputs of organic matter by incorporating crop residues and including green manures and pastures in the rotation, where practical, should be a goal for all farmers.