Soil Stability Index Standard Operating Protocol 2018-03
Alan Franzluebbers USDA-Agricultural Research Service 3218 Williams Hall NCSU Campus Box 7620 Raleigh NC 27695 Tel: 919-515-1973 Email: [email protected]
Purpose Stability of soil is important for controlling wind and water erosion, promoting adequate water infiltration, minimizing sediment and nutrient loss to surface waters, and providing a stable habitat for soil organisms. Methodology for aggregate distribution and stability has been the least standardized of most soil quality properties, despite its strong importance for soil physical quality. Variation in methodology has been partly due to the strong importance of aggregation in both dry environments (wind erosion control) and wet environments (water erosion control) and lack of universally adopted sieving apparatus. The method proposed here has advantages of (i) standardization of the pre-treatment of soil, (ii) determining both drystable and wet-stable aggregate distribution to be relevant to dry and wet environments, and (iii) normalizing the differences in soil texture, mineralogy, antecedent water content, and other physical factors by calculating the ratio of mean-weight diameter (MWD) of wet-stable and dry-stable fractions [9 – see attached PDF]. Note: The method is not a true aggregation method, as single-unit particles are not deleted from calculations. However, the method relates well to water infiltration, because infiltration depends on size distribution irrespective of whether particle or aggregate.
Materials a. Sieve with 4.75-mm openings – to homogenize soil and take subsample for analysis Dry-aggregate distribution b. 20-cm diam. sieves (1, 0.25, and 0.053 mm openings) – to collect aggregate fractions c. Shallow 20-cm diameter pan – to collet <0.053 mm fraction d. Motorized sieving apparatus (CSC Scientific Sieve Shaker) – to uniformly shake soil e. Plastic weighing boats – to collect aggregate fractions for weighing f. Top-loading balance – to weigh aggregate fractions Wet-aggregate distribution g. 13-cm diam. sieves (0.25- and 1.0-mm openings) – to collect aggregate fractions h. 20-cm diam. sieve (0.053-mm openings) – to collect aggregate fraction in water suspension after initial oscillation step i. Motorized wet-sieving apparatus – to uniformly oscillate soil j. Drying oven (55 °C) – to dry aggregate fractions k. Plastic weighing dishes – to collect aggregate fractions for weighing l. Top-loading balance – to weigh aggregate fractions
Method 1. Sieve oven-dried soil (55 °C, 2-3 days) to pass a 4.75-mm sieve (all material should pass the screen, except stones and large plant residues) 2. Weigh a 100 + 0.02 g portion of each soil sample into a labeled vessel Dry-aggregate distribution 3. Place the pan (bottom), 0.053-mm (bottom middle), 0.25 (top middle), and 1.0 (top) mm sieve into the holding device of the sieving apparatus. Load the top sieve with 100 g of soil 4. Shake for 1 minute on Level 6
5. At the end of 1 minute, pour soil from 1 mm screen into a plastic weighing boat and record weight to nearest 0.01 6. Repeat recording of weight for each of the 3 remaining fractions Fraction (i) = weight of 1-4.75 mm aggregates Fraction (ii) = weight of 0.25-1 mm aggregates Fraction (iii) = weight of 0.053-0.25 mm aggregates Fraction (iv) = weight of <0.053 mm aggregates 7. Return the 100 g of soil back to vessel for subsequent determination of water-stable aggregation 8. Calculate mean-weight diameter of dry aggregate distribution [MWDdry (mm)] as follows: = [fraction (i) * 2.875] + [fraction (ii) * 0.625] + [fraction (iii) * 0.1515] + [fraction (iv) * 0.0265] Wet-aggregate distribution 9. Place the 0.25- and 1.0-mm sieves into the holding device of the sieving apparatus 10. Fill the containers enclosing the sieves with water so that at the bottom of the stroke, the top rim of the 1.0-mm sieve is elevated above the water level by ~1 cm 11. Position the sieves at the top of the stroke prior to addition of soil 12. Pour the soil uniformly onto the 1.0-mm sieve and oscillate apparatus for 10 minutes 13. At the end of 10 minutes, carefully remove both sieves from water and place in oven until visibly dry (~1 hour) 14. At the end of 1 hour, transfer soil from sieve into a shallow pan and scrape all soil material from the sieve 15. Transfer soil into a labeled plastic weighing dish and dry further for 24 hours 16. Re-suspend the soil passing through the 0.25-mm sieve by swirling solution; pass all of the suspension over a 0.053-mm sieve and rinse lightly 17. Transfer the soil retained on the 0.053-mm sieve to a labeled plastic weighing dish with a squirt bottle; place in oven at 55 °C for 24 hours 18. Determine oven-dried weight of soil retained on the 1.0-mm sieve, of soil retained on the 0.25-mm sieve, and soil retained on the 0.053-mm sieve 19. Calculate the fraction of total soil on an oven-dry basis as (i) 1.0-4.75 mm, (ii) 0.25-1.0 mm, (iii) 0.053-0.25 mm, and (iv) <0.053 mm (i.e., total soil minus other three fractions) 20. Calculate mean-weight diameter [MWDwet (mm)]as follows: = [fraction (i) * 2.875] + [fraction (ii) * 0.625] + [fraction (iii) * 0.1515] + [fraction (iv) * 0.0265] Note: Stability of macro-aggregate fraction (>0.25 mm) can also be calculated if a simplified procedure is desired.
Costs Equipment (convection oven - $2.5K, twenty-four 13-cm diam. sieves - $2.4K, manufactured sieving apparatus - $1K, toploading balance - $1K, three 20-cm diam. sieves - $400, four stainless steel buckets - $200) – total = $7,500 Operating (paper bags, envelopes for 100-sample batches) – total = $20 Space requirements = 4 m2 floor space for soil processing and analyses Time to get 100 samples analyzed = 10 days Labor to get 100 samples analyzed = 36 hours
Sensitivity to management Stability of soil is responsive to differences in tillage type [1, 2, 4, 5, 7, 8], tillage frequency , cover cropping , land use , and pasture management [3, 8, 9]. The normalization procedure allows soils with different texture, antecedent moisture content, and mineralogy to be compared and evaluated simultaneously.
Dynamic nature This soil stability index varied logically with time of pasture management – both from a few years of evaluation and from a multi-decadal chronosequence . It also appeared to be seasonally influenced by root and water variations during a 60week growth study .
Relationship to soil function Soil stability index reflects the functional capability of soil to provide physical stability and resistance against wind and water erosion. It also creates surface structure to promote rapid water infiltration to avoid sediment loss and water and nutrient runoff. Complex macroaggregates store greater quantity of soil organic C [3, 4, 5, 6], harbor soil microbial biomass, and protect C from rapid decomposition by soil microorganisms .
Production lab capability The method requires standardization with unique, specialized sieve shaker and oscillating apparatus. The latter is typically not in large-scale production, but a prototype for standardization could be manufactured routinely if there were demand. No other specialized equipment is needed and operating costs are low assuming labor is available, making the method very affordable.
References  Causarano HJ, Franzluebbers AJ, Shaw JN, Reeves DW, Raper RL, Wood CW. 2008. Soil organic carbon fractions and aggregation in the Southern Piedmont and Coastal Plain. Soil Science Society of America Journal 72, 221-230.  Franzluebbers AJ. 2002. Water infiltration and soil structure related to organic matter and its stratification with depth. Soil and Tillage Research 66:197-202.  Franzluebbers AJ. 2006. Short-term responses of soil C and N fractions to tall fescue endophyte infection. Plant and Soil 282, 153164.  Franzluebbers AJ, Arshad MA. 1996. Water-stable aggregation and organic matter in four soils under conventional and zero tillage. Canadian Journal of Soil Science 76, 387-393.  Franzluebbers AJ, Arshad MA. 1997. Soil microbial biomass and mineralizable carbon of water-stable aggregates. Soil Science Society of America Journal 61, 1090-1097.  Franzluebbers AJ, Brock BG. 2007. Surface soil responses to silage cropping intensity on a Typic Kanhapludult in the piedmont of North Carolina. Soil and Tillage Research 93, 126-137.  Franzluebbers AJ, Langdale GW, Schomberg HH. 1999. Soil carbon, nitrogen, and aggregation in response to type and frequency of tillage. Soil Science Society of America Journal 63, 349-355.  Franzluebbers AJ, Stuedemann JA. 2008. Soil physical responses to cattle grazing cover crops under conventional and no tillage in the Southern Piedmont USA. Soil and Tillage Research 100, 141-153.  Franzluebbers AJ, Wright SF, Stuedemann JA. 2000. Soil aggregation and glomalin under pastures in the Southern Piedmont USA. Soil Science Society of America Journal 64, 1018-1026.