The benefits of organic matter in soil are well known. Organic matter improves factors including water holding capacity, nutrient holding capacity and structure. But organic matter can be made up of more longer lasting humus through partially broken down material to fresh material from plants and animals that has recently entered the soil. This fresh reactive fraction is more likely to be a major supplier of nitrogen to a pasture as it is broken down.
How much of each is likely to be present in a pasture soil? A recent study, Culman et al, 2012, has found that permanganate oxidizable carbon in soils correlates well with widely used measurements of microbial biomass and particulate organic matter. Permanganate oxidizable carbon is also a good indicator of variation in management and environmental factors.
It is relatively cheap and easy to measure the reactive fraction of soil organic matter by permanganate digestion. A simplified method is outlined in detail in the Archive for March, 2012.
In a preliminary study soil was sampled at three sites on a dairy farm in West Gippsland.
Site 1: Pasture soil mid way down a slope, known to be poorly drained. Mixed pasture species including some perennial ryegrass and poorly developed white clover. pH measured at approx 5.5. The soil has a heavy texture but becomes powdery when dry.
Site 2: Elevated pasture with mixed species. Chosen for its contrast to Site 1. More typical West Gippsland red-brown soil. Distinct crumb structure with pH around 6. This is the site in the picture.
Site 3: Another red-brown soil in an elevated position considered to have good pasture. pH approx 6. Good crumb structure.
Partially dried samples were sieved to remove roots. Two tests were carried out: digestion with 30% hydrogen peroxide for a ‘total’ organic matter measurement and, digestion with potassium permanganate for a reactive organic fraction.
|Organic matter||Reactive||Total||% reactive||Approx|
|Site||total w/w %||org C ppm||org C ppm||org C||level *|
* representative values can be seen by following the SOM Method link in the Archive for March, 2012 ‘A simple test for reactive soil organic matter’.
Across the farm, levels of total and reactive organic soil matter varied from low to high. The lowest at Site 1 and the highest at Site 3. The percentage of total organic matter weight for dry weight in the soils ranges from 5.3 to 9.5.
The percentage of reactive soil organic matter was significantly higher at Site 3 (3.9% of total). However a meta-analysis of a range of figures for total and reactive soil C from the Archive for March, 2012 shows that typically the reactive component ranges from 3.8 % to 10.6 %. Therefore overall, soils on the dairy farm in this study have low or lower than expected levels of reactive soil organic matter.
This study has provided some comparative figures for soil organic matter fractions on a dairy farm. Reliability will be improved with more tests per paddock and wider testing over the farm will be useful as part of pasture and feed management on the farm. Many of the factors that determine organic matter levels in the soil can be identified like grazing, pasture, crop and fertilizer history. This information along with tests for key nutrients can help to better understand how the current situation has developed.
Culman et al, 2012, Permanganate oxidizable carbon reflects a processed fraction that is sensitive to management, 2012, Soil Science Society of America Journal.