Comparison of tests

Results from analyses of the correlation of structural stability tests and the parameters involved in structural stability are shown in Table 1.

Table 1. Coefficients of correlation (r) between structural stability tests, the microbial biomass, polysaccharides extracted from warm water and the hydrophobicity of aggregates.

Structural stability, microbial biomas and polysaccharide tests: measured en 2003, 2004 and 2007 (n=50),
Hydrophobicity: measured in 2003 and 2004 (n=35),
Significant differences at 5% (*) and 1% (**).

The three aggregate stability tests are significantly correlated and reveal links between the structural stability mechanisms in the presence of different agents. The relationship of these tests with the microbial biomass is negative on the whole, although the literature shows positive relationships between these parameters. Polysaccharides are significantly positively correlated with the microbial biomass. This is because these substances are the result of microbial activity. They are also positively correlated with the rapid soaking test, but negatively so with the slow soaking test. Hydrophobicity is significantly correlated with the rapid soaking test, showing the protective effect of these substances against water.

OWP action on structural stability

All OWP act on structural stability. Their cumulative effects are visible from one year to the next. Laboratory work conducted alongside the QualiAgro test highlighted the different modes of action of OWP based on the degradeability of the OM that they contain (Figure 1).

Figure 1. Diagrammatic presentation of the variable effects of ROPs on the stabilisation of soil aggregates according to the biodegradeability of their organic matter (Annabi et al., 2007).

The incorporation of an ROP containing easily biodegradeable organic matter (RMW) would have a significant and rapid effect on the stabilisation of aggregates, an effect that would gradually diminish over time if inputs are not renewed. The action of this type of ROP is primarily a result of its easily biodegradeable organic matter, which stimulates the microbial biomass in the soil and would also increase the level of humified carbon in the soil, after stabilisation of the OM provided. The regular provision of this type of ROP increases the structural stability over time by accumulating the effects of each input.
The incorporation of an ROP containing slightly degradeable organic matter (GWS or MAN) is slower and more regular. Microbial activity is not significantly stimulated and as a result, a larger proportion of carbon will be stored in the soil. In the case of these ROPs, regular inputs also gradually increase the structural stability of the soil.
Over the long term, the effect of slightly biodegradeable products and that of highly biodegradeable products are combined. Data from the QualiAgro test appear to show that after this period, ROPs containing slightly biodegradeable OM (GWS and MAN) would have a more pronounced effect than easily biodegradeable ROPs (RMW). Until 2005, RMW plots had the highest structural stability. From 2007 the trend was reversed, with the GWS and BIO plots displaying better structural stability.