Measuring aggregate stability

The method used is that proposed by Le Bissonnais and Le Souder (1995) and granted AFNOR standardisation in 2005 (NF X31-515).

This method includes three tests that characterise dry aggregate stability versus the degrading action of water:

• Rapid soaking test: the aggregates are suddenly immersed in a volume of water. 
• Mechanical disaggregation by agitation test: the aggregates are first saturated with ethanol then immersed in water and manually agitated in an erlenmeyer.
• Slow soaking test: the aggregates are slowly soaked by capillary action.

After each of these tests, the proportions of the residual aggregate size classes are evaluated by screening and the results are expressed in terms of the average weighted diameter (AWD) calculated according to :

	: the average diameter measured between two screeners
wi	: proportion of the total mass of residual aggregates in each screener (2, 1, 0.5, 0.2, 0.1 and 0.05 mm)

The more the aggregates are resistant to the destructive action of the water and remain large in size, the higher the AWD is and the more stable the soil's structure is considered to be.

For each test, AWDs are calculated: AWD_RS, AWD_MD, AWD_SS for, respectively, the rapid soaking, mechanic disaggregation and slow soaking tests. The AWD values of each of these tests were averaged to give the AWD_avg. 

Change in aggregate stability

Figure 1 shows the change in the average AWD calculated based on the AWDs from the three aggregate stability tests, giving an overall picture of the effect of treatments on the structural stability of the soil.

Figure 1. Change in aggregate stability based on the average of the three tests (AWD_avg).

In 1999, the values measured in the three different treatments are similar: an average of 0.43 mm, which is characteristic of an unstable loamy soil. The differences between the AWD_avg in 1999 can be attributed to a combination of spatial variability and the accuracy of the measurement.
This measurement from 1999 is considered as the initial state, even though the first input took place in September 1998. This is because the organic products were rapidly buried by surface tillage and the first ploughing. It is therefore considered that these first few centimetres of soil (from which samples are taken) are not very influenced by the organic products input.
Although there is significant annual variability in the structural stability, over time there is a positive effect from OWPs compared with the control treatment (without the input of OWPs). In 2000, only the MSW treatment had a positive effect compared with the control. From 2003, all of the OWP treatments had a positive effect on soil stability. In the last measurements, the BIO treatment produced the most pronounced effect compared with the control. The overall positive effect observed in 2003 could be due to a consolidation of aggregates following the long period of dryness. Low rainfall in the spring of 2005 and 2007 could have promoted the continuation of this effect.

The effectiveness of OWPs is ordered as follows:
- During the first 9 years:  FYM=GWS < BIO < MSW,
- In 2007 :  FYM < MSW < GWS < BIO.

The effectiveness of the ROPs is linked with their action on the different soil parameters such as: total organic carbon, the microbial biomass, polysaccharides and lipids. These parameters are monitored in the QualiAgro test and enable the origin of the effects of the ROPs to be determined (Parameters involved in structural stability).