Aggregate stability

Aggregate stability was measured on air-dried soil samples using three different methods:

(i) The wet sieving method with multiple sieves proposed by De Leenheer and De Boodt (1959) and adjusted by Hofman (1973)

(ii) The three treatments of the method by Le Bissonnais (1996)

(iii) The wet sieving method using a single sieve based on Kemper and Rosenau (1986).

Chapter 3

34 All analyses were replicated three times for each sample. For the method of De Leenheer and De Boodt (1959), abbreviated here as dLdB, 100 g of aggregates divided in three fractions were used. The aggregate fractions of 40 g with diameter between 4.75 – 8.00 mm, 32 g of 2.80 - 4.75 mm and 28 g of 2.00-2.80 mm, were prewetted to field capacity by drops falling from a height of 50 cm. Raindrops are formed at 5 mm tip of a capillary tube with inner diameter of 0.4 mm.

After pre-wetting, the different aggregate size fractions were incubated for 24 h at 20 °C and 98-100% relative humidity. Finally, each aggregate size fraction was placed on its corresponding sieve. Three extra sieves with mesh sizes of 1, 0.5 and 0.3 mm were added and all the sieves were gently shaken under water at a constant speed (automatically controlled) for 5 min. The aggregates remaining on each sieve were washed off the sieve and weighed after drying.

The results were expressed in terms of the mean weight diameter (MWD) and the stability index (SI):

t n i

1

i i i

m d m (mm)

MWD 



^ (3-1) where mi = mass of the stable aggregate fraction i; di = mean diameter of fraction i; mt = total weight of the sample.

The instability index (IS) was calculated as the difference between the initial MWD and the final MWD. The inverse of the IS, the SI, was taken as another measure of the stability of the aggregates:

SI IS1 (3-2)

Classification of the aggregate stability based on SI (De Leenheer and De Boodt, 1959), for medium-textured Belgian soils includes the following rating: >1 = excellent; 0.8 - 1 = very good; 0.66 - 0.8 = good; 0.5 - 0.66 = unsatisfactory; and < 0.5 = bad.

The procedure of Le Bissonnais (1996), shortened here as LB, involves three treatments, which represent different wetting procedures: fast wetting (LB1), slow wetting (LB2) and mechanical breakdown by shaking after pre-wetting (LB3). Briefly, air dried aggregates of 3-5 mm in diameter, were calibrated by putting them in the oven at 40 °C for 24 h.

Chapter 3

35 LB1 involves the immersion of 5 g of calibrated aggregates in 50 ml of deionized water for 10 min. Then the water was cautiously extracted and the soil material was transferred to a 50 μm sieve for wet-sieving in ethanol (gently moved five times) in order to measure the fragment size distribution. For LB2 the 5 g of calibrated aggregates were put on a filter paper on a tension table at a matric potential of -0.3 kPa for 30 min and then transferred to the 50 μm sieve immersed in ethanol.

In the case of LB3, 5 g of calibrated aggregates were immersed in 50 ml of ethanol for 10 min. After this, the ethanol was extracted and the soil material was transferred to a flask with 200 ml of deionized water and agitated end over end 20 times. The mixture of water and soil was left for 30 min for sedimentation, then the water was extracted and the soil material was transferred to the 50 μm sieve immersed in ethanol as the other treatments of LB.

After wet-sieving in ethanol, the > 50 μm soil material was collected, oven-dried and gently dry-sieved by hand on a set of six sieves: 2.0, 1.0, 0.5, 0.2, 0.1 and 0.05 mm.

The mass percentage of each size fraction was then calculated; the fraction < 50 μm was the difference between initial mass and the sum of the six other fractions.

The aggregate stability resulted from the three treatments was expressed by calculating MWD and SI from equations (3-1) and (3-2), respectively. Le Bissonnais (1996) suggested the following classes of stability according to MWD values measured with the three treatments: > 2 = very stable; 1.3 - 2 = stable; 0.8 - 1.3 = medium; 0.4 - 0.8 = unstable and < 0.4 = very unstable.

Finally, the Yoder method modified by Kemper and Rosenau (1986), denoted here as KR, calls for air-drying and rewetting the soil samples prior to wet sieving in deionized water to determine the recovery of aggregated particles on a single sieve (0.25 mm). Fast wetting (FW) and slow wetting (SW) were applied to determine the aggregate stability using the wet sieving apparatus by Eijkelkamp Agrisearch Equipment (the Netherlands).

The SW of aggregates was performed on a tension table at a matric potential of – 0.33 kPa for 30 minutes. For both pre-treatments, 1 - 2 mm air-dried aggregates were wet sieved in deionized water for 3 minutes at a constant, automatically controlled speed. After mechanical shaking, the soil sample that remains on the sieve (0.25 mm) was shaken again in a solution of sodium metaphosphate until the aggregates were fully dispersed.

This was in order to conduct the correction of sand fraction. Results were expressed as MWD.

In this study, for dLdB and LB methods a very stable soil was considered as having

> 70% of WSA remained on the sieve of 0.5 mm and those above it. An unstable soil has <

50% WSA remained on the sieve of 0.5 mm and those above it. For KR method, a stable soil was considered having > 70% of the aggregates remaining on the sieve of 0.25 mm after wet sieving, and an unstable soil has < 50%.

Chapter 3

36 3.2.4. Structural stability index

Particle size distribution and SOC content were used to calculate the structural stability index (StI) suggested by Pieri (1992), which expresses the risk for soil structural degradation associated with SOC depletion:

Silt 100 Clay

SOC 1.724

StI 



  (3-3)

Where StI is the structural stability index expressed in %, SOC is the soil organic carbon content (%) and Clay + Silt is the soil’s combined clay and silt content (%). StI < 5%

indicates a structurally degraded soil; 5% < StI < 7% indicates high a risk of soil structural degradation; 7% < StI < 9% indicates a low risk of soil structural degradation; and StI > 9%

indicates sufficient SOC to maintain the structural stability.

In document Integrated assessment of soil structural quality (Page 55-58)