BRAM GOVAERTS
How to evaluate cropping management practices
IN I ER 1AI lONAL
MAIZE AND WHEAT
lMPROVEMENT CEi 'TER
The cook book
version 1
Introduction
Caste/lanos~Navafrete, A, and Govaerts, B.
Castellanos-Navarrete, A, and Govaerts, B.2008. Soil bulk density. In: Govaerts, B. (Ed.), How to evaluate cropping management
practices; A cook book, OMMYT, Mexico, D.F., Mexico.
Protocol: Soil Bulk Density
Soil compaction is a form of physical soil degradation where an increase in soil bulk density, and
a decline in percentage and stability of aggregates as well as porosity and pore continuity, is
verified (Kooistra and Tovey, 1994). Possible problems associated with soil compaction are:
decreased aeration (increased proportion of soil pores filled with water) (Stepniewski el aL, 1994),
decreased water infiltration, increased surface runoff and erosion, as well as poor crop
establishment (seed germination and early root growth) and root development {Lal and Shukla,
2004; Logsdon and Karlen, 2004). C.Ompaction can occur when heavy farm machinery circulates
over the field, especially under wet conditions (Hom el al., 2006). Tillage has also been found to
cause soil compaction through plough pan formation. Because of its importance, soil bulk density
(Blake and H:ange, 1986) is very frequently included in analysis of soil quality (e.g. Govaerts el al.,
2005). There have even been attempts to establish soil bulk density (SBD) threshold values, to
indicate when compaction is occurring (e.g. 1.55 Mg m·3 in silt and silt loam soils). This is based
on the rationale that if bulk density is higher than a critical level, considering variations caused by
soil texture, compaction would be present (USDA-NRCS, 1996). However, increased SBD values
are not necessarily related to compaction since this parameter is dependent on a wide array of
factors such as type of parent material, the crop being grown, soil organic matter content and
type of present and past management (Logsdon and Karlen, 2004). Management, for example,
has an overarching effect on soil physical properties, including soil packing density. Under
conservation agriculture (CA), soil is not tilled and a protective residue cover is left over the soil's
surface. Recent studies (Logsdon and Karlen, 2004; Osunbitan el al., 2005; Mati and Kotorov;l,
2007) found that soil bulk density values were higher under CA when compared to conventional
systems. Soils treated under CA are denser but characterized by stable macroporosity (formed by
soil macrofauna and decayed plant roots) with significant effects on soil hydraulic conductivity.
Greater total porosity in tilled soils is not related to greater water infiltration given their lack of
connectivity, lower proportion of macropores and temporaiy character (Osunbitan et al., 2005).
1bis illustrates how soil bulk density is not necessarily related to compaction and threshold values
designed for conventional tillage (CI) are not necessarily applicable to CA In conclusion,
measures on aggregation, water infiltration and crop performance have to complement SBD
values. But also temporal variations in soil bulk density values due to changing conditions,
particularly at the topsoil where agricultural management and environmental conditions have
their greatest impact (Figure 1), makes repeated measures throughout the season highly
recommended. For example, Logsdon et al. (1999) and Logsdon and Carnbardella (2000) showed
significant temporal changes in near-surface incremental bulk density for tillage systems in a sub
humid climate.
Materials
• Metal rings ( 100 cm3
volume and 5 cm diameter) • Plastic bags • Trowel • Flat-bladed knife • Scale • Plates
Procedure
A Field sampling
Carefully drive the ring straight
into the soil (to avoid compaction)
(Figure 2). Once the soil is located
-+ Soil depth (cm)
5
10
15 V\ A g,
20 ~ 0 ..,
B ;::r 25 0
:::s V>
30
: Figure 1. Soil bulk density sampling at the topsoil. I ------------------------------ ---------------------------------------------------------- ----------------------------------~
within the ring, dig around, and with the trowel underneath it, carefully lift out to prevent any
loss of soil (Figure 3). Remove excess soil from the sample with a flat-bladed knife (if losses
include soil within the ring, repeat the procedure at a new sampling point). Place the sample in a
bag {touching the sample as little as possible) and label.
B. Measurement
Weight the wet soil sample in its bag (i.e. the weight of the bags must be known) and record data.
Oven-dry for 48 hat 105 lJ C in an open metal can. Once dried, weigh the sample (i.e. weight of
the metal can must be known). The soil samples are weighed when wet to calculate water mass
present in soil (g).
Figure 2. Soil bulk density sample extraction. Figure 3. Example of incomplete soil bulk density sample.
C. Calculus
Soil bulk density (SBD) is calculated as follows:
SBD = Sc1ry Vol
with, SBD =soil bulk density (g/cm3)
Sctry = oven-dried soil sample (g)
Vol= volume metal ring (cm3)
Gravimetric moisture (GM) is calculated as follows:
(swet -Sdry) GM=---
with, GM= gravimetric moisture(%)
S wet = wet soil sample (g)
Sctry = oven-dried soil sample (g)
Volumetric moisture (VM) is calculated as follows:
(swet -sdry) VM=---
Vol
with, VM = volumetric moisture (g cm-3)
S wet = wet soil sample (g)
Sctry = oven-dried soil sample (g)
Vol= volume metal ring (cm3)
References
Blake, G.R, and Hartge, K.H, 1986. p. 363-375.Bulk density. In. Klute, A, Campbell, G.S.,
Jacson, RD., Monland, MM, and Nielsen, D.R (Eds.) Methods of Soil Analysis. Part I ,
ASA and SSSA, Madison, WI, USA, 1986. pp. 363-375.
Govaens, B., Sayre, K.D., and Deckers, J., 2006. A minimum data set for soil quality assessment
of wheat and maize cropping in the highlands of Mexico. Soil Till. Res. 87: 163-174.
Hom, R, Hartge, K.H, Bachmann, J., and Kirkham, MB., 2006. Mechanical stress of soils
assessed from bulk-density and penetration resistance data sets. Soil Sci. Soc. Am. J. 71:
1455-1459.
Kooistra, MJ., and Tovey, N.K., 1994. Effects of compaction on soil microstructure. In: Soane,
B.D., and Van Ouwerkerk, C (Eds.), Soil G>mpaction in Oop Production. Elsevier, New
York, pp. 91-111.
Lal, R, and Shukla, MJ., 2004. Principles of Soil Physics. Marcel Dekker, New York, 2004, viii+
716 pp. ISBN 0-8247-5324-0.
Logsdon, S.D., Kaspar, T.C, and Cambardella, CA, 1999. Depth-incremental soil properties
under no-till or chisel management. Soil Sci. Soc. Am. J. 63: 197-200.
Logsdon, S.D., and Cambardella, CA, 2000. Temporal changes in small depth incremental soil
bulk density. Soil Sci. Soc. Am. J. 64: 710-714.
Logsdon, S.D., and Karlen, D.L., 2004. Bulk density as a soil quality indicator during conversion
to no-tillage. Soil Till. Res. 78: 143-149.
Mati, R, and Kotorovci, D., 2007. The effects of tillage system on soil bulk densityand other
physical and hydrological characteristics of gleyic fluvisols . J. Hydrol. Hydromech. 55:
246-252 ..
Osunbitan, J.A, Oyedele, D.J., and Adekalu, K.O., 2005. Tillage effects on bulk density, hydraulic
conductivity and strength of a loam sand soil in sounhwestem Nigeria. Soil Till Res. 82:
57-64.
Stepniewski, W., Glinski, J., and Ball, B.C., 1994. Effects of compaction on soil aeration
properties. In: Soane, B.D., and Van Owerkerk, C. (Eds.), Soil C.Ompaction in Oop
Production. Elsevier, New York, pp. 167-169.
USDA-NRCS, 1986. Soil Quality Resource C.Oncems: C.Ompaction. USDA-NRCS Soil Quality
Ints., Ames, IA, USA http://www.statlab.iastate.edu/survey/SQI/sqihome.shtml
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Introduction
Verhulst, N.1 Castellarws~Navarrete, A./ and Govaerts, B.
Verhulst, N., Castellanos-Navarrete, A. and Govaerts, B.2008. Soil aggregate distribution. In: Govaerts, B. (Ed.), How to evaluate cropping management
practices; A cook book, OMMIT, Mexico, D.F., Mexico.
Protocol: Soil aggregate distribution
From a physical point of view, the soil matrix is generally conceptualized as constituted by soil
aggregates (filled spaces) or secondary soil units, and pores (empty spaces) (Soil Science Society
of America, 1997; Lal and Shukla, 2004). Dry sieving provides an indirect static measure of field
aggregate size distribution that can be expressed as the mean weight diameter (MWDds) (van
Bavel, 1949), or as a percentage of aggregates. Aggregates can be divided into large
macroaggregates (> 2mm), small macroaggregates (250 µm - 2 mm), microaggregates (53 - 250
µm) and free silt+clay( <53 µm) (Six et al., 2004).
Factors that differ between studies and lead to variation in results are sieve load, duration of
sieving treatment (particularly on fragments sizes larger than 16.00 mm and smaller than 4.75
mm), as well as number and size of sieves (Dfaz-Zorita et al., 2007). To allow comparison, these
factors must be kept constant between samples. However, the disintegrating forces occurring
during sample taking, preparation, and analysis do not duplicate the field phenomena.
Gmsequently, the relationship between aggregate-size distribution obtained in the laboratory and
the reality in the field is somewhat empirical (Kemper and Rosenau, 1986). In Central Mexico's
El Bacin Research Station, OMMYT, Govaerts et al. (2007) found how management, and in
particular tillage management, can lead to significant differences in MWD of dry sieved samples
between permanent bed planting systems with full residue retention (MWDds = 2.5 mm) and
conventionallytilled beds with residue incorporated (MWDds = 2.5 mm).
Various authors indicate that dry sieving provides a way of measuring soil erosion. Chepil (1953)
stated that resistance to wind erosion is positively related to the percentage of dry soil structural
units greater than 0.84 mm. However, percentage of these aggregates would vary according to
disturbance intensity (i.e. strength and duration) during dry sieving. The application of stress to
soil fractures the soil matrix in zones where the bonds between particles are weaker than the
force of the applied stress. O:msequently, the size of fragments will depend upon the applied
stress (Dla.z-Zorita et aL, 2002). Given the variability of methodologies applied and the
complexities of processes related to wind erosion, the relation between dry sieving and soil
erosion is unclear.
Materials
• Shovel • Large rectangular sampling boxes • 8 mm sieve for sample preparation • Sieves with openings of 4.75, 2.00, 1.00, 0.50, 0.25 and 0.053 mm, lid and container
• Brush • Plates of known weight • Scale • Stopwatch • Machine to shake samples
Procedure
A Field sampling
Field sampling for soil structure studies must be done carefully to avoid structure disruptions as
that could distort results. When different management practices are compared, all samples should
be collected the same day (i.e. similar water content of soils with variability only caused by
management). Samples are taken with a shovel to avoid compression and disturbance of the
sample (i.e. as would be caused by an auger) and ensuring minimum wall surface area to volume
ratio to decrease compaction risk Avoid operations such as hammering the shovel which could
result in sample disruption. Samples are then located on rigid large sampling boxes (avoiding two
or more layers of samples that would cause compression of soil). Then, prior to analysis, samples
are stored in locations with constant conditions (i.e. temperature and humidit)7.
B. Sample preparation
After field sampling, samples are air-dried at room temperature for a few hours and big clods ( >
5 cm) are gently broken along natural planes of weakness into natural aggregates These samples
are air-dried during two weeks and passed through an 8 mm sieve. Coarse plant residues, roots
and any stones >8 mm are removed. A sub-sample of 200 g is then taken for further analysis.
C. Measurement
Sieves are placed in a stack (i.e. 4.75, 2.00, 1.00, 0.25 and 0.053 mm) within the machine (Figure 1)
with the top sieve covered by a lid The stack of sieves is shaken at a speed of 210 cycles/ min-
1,for 5 minutes (Figure 2). Afterwards, sieves are emptied in their corresponding plates (and
cleaned with a brush to ensure all soil is placed on plates. Plates are then weighed.
I I I I I
, ' ' ,
--------
Soil sample 200 g (sieved 8 mm)
•' 250 cycles min-1
~
~
~
~
MWD
mples. I Figure l Machine used to shake dry soil : Figure 2. A 200g soil sample ~eft) is introduced at the top of the stack
: (right) of sieves and shaken for two minutes. Soil left in each sieved is •· .... ·-··-······-····-···· ·- ----- - : ~~ig~~4-~~-~-~-~ig~! -~~!~!~)~~~~~~c:L - ······- .
D. Calculus
n
MWDds = L < d >;W; i =I
with, MWD ds = mean weight diameter (mm) of dry sieved soil
d =mean diameter of each siz.e fraction siz.e i (mm) (e.g. soil found in 2.00 mm sieve has
4.75 mm as maximum diameter and 2.00 mm as minimum diameter. Thus, mean weight
diameter for such sieve is 3.375 mm). For the selected group of sieves, mean siz.e
fractions are: 6.375 mm, 3.375 mm, 1.50 mm, 0.75 mm, 0.375 mm, 0.152 mm, and 0.0265
mm.
w = proportion of total sample weight occurring in the siz.e fraction i (g)
n = number of siz.e fractions
References
Auberbot, J.N., Diirr, C, Kieu, K., and Richard, G., 1999. Characterization of sugar beet seedbed
structure. Soil Sci. Soc. Am. J. 63: 1377-1384.
Chepil, W.S., 1953. Field structure of cultivated soils with special reference to erodibility by wind.
Soil Sci. Soc. Am. Proc. 17: 185-190.
Dfaz-Zorita, M, Grove, J.H, and Perfect, E., 2007. Sieving duration and sieve loading impacts
on dry soil fragment size distribution. Soil Till. Res. 94: 15-20.
Govaerts, B., Sayre, K.D., Lichter, K., Dendooven, L., and Deckers, J., 2007. Influence of
permanent raised bed planting and residue management on physical and chemical soil
quality in rain fed maize/wheat systems. Plant Soil 291: 39-54.
Kemper, W.D. and Chepil, W.S., 1965. Size distribution of aggregates. In: Methods of Soil
Analysis. Part I. Agronomy Series No. 9. Black, CA (Ed). ASA, Madison, WI, USA
499-509.
Lal, R, and Shukla, MJ., 2004. Principles of Soil Physics. Marcel Dekker, New York, 2004, viii+
716 pp. ISBN 0-8247-5324-0.
Limon-Ortega, A, Sayre, K.D., Drijber, RA, Francis, CA, 2002. Soil attributes in a furrow
irrigated bed planting system in northwest Mexico. Soil & Tillage Research. 63, 123-132.
Sandri, R, Anken, T., Hilfiker, T., Sartori, L., and Bollhalder, H, 1998. Comparison of methods
for determining cloddiness in seedbed preparation. Soil Till. Res. 45: 75-90.
Six, J., Bossut, H, Degryze, S., and Denef, K., 2004. A history of research on the link between
(micro)aggregates, soil biota, and soil organic matter dynamics. Soil Till. Res. 79: 7-31.
Soil Science Society of America, 1997. Glossary of Soil Science Terms 1996. Soil Science Society
of America, Madison, WI, 138 pp.
van Bavel, CHM, 1949. Mean weight diameter of soil aggregates as a statistical index of
aggregation. Soil Sci. Soc. Am. J. 17: 416-418.
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Introduction
Caste/Janos-Navarrete~ A.; and Govaerts, B.
Castellanos-Navarrete, A. and Govaerts, B.2008. Soil stability. In: Govaerts, B. (Ed.), How to evaluate cropping management practices; A cook book, OMMYT, Mexico, D.F., Mexico.
Protocol: Soil aggregate stability
Soils are subjected to spatial and temporal alterations in aggregates and pores in relation to
natural (pedogenesis) and anthropogenic (management) factors (Lal and Shukla, 2004). In fact,
soil aggregate distribution is a static measure that provides information on dynamic changes, and
has to be repeated over time. A practical alternative is to undertake measures on soil stability.
Structural stability is often measured as the stability of soil fragments exposed to stresses (Dfaz
Zorita et al, 2002). Particularly imponant is the soil's ability to retain its arrangement of solids
and void space during rainfall events. Soil structure can collapse: rn by direct impact of raindrops
that break surface soil aggregates and results in soil surface crusts; and (ii) by spontaneous slaking
of breakdown of soil aggregates during rapid wetting both at the soil surface (contributing to soil
crust formation) and within the soil (resulting in soil compaction) (Arshad and Mermut, 1988;
FAO, 2003; Lal and Shukla, 2004). In both cases, the break-down of aggregates into small
panicles leads to clogging of soil pore formation of swface seals, reducing the hydraulic
conductivity of the soil (Lal and Shukla, 2004). This problem is further exacerbated in arid and
semi-arid regions due to the rapid drying of soil. The consequence of degradation at the soil
surface and at the sub-soil is the reduced rainfall water infiltration into the soil and thus increased
incidence of runoff and soil erosion leading to low water use efficiency. In other words,
erodibility of soil increases as aggregate stability decreases (Kemper and Rosenau, 1986).
Aggregate collapse also influences solute transpon processes in the soil, as well as resistance to
penetration by roots and shoots in seedbeds (Rathore et al, 1983; Schneider and Gupta, 1985;
Nasr and Selles, 1995; Diaz-Zorita et aL, 2002).
Wet sieving has been proposed as a methodology measure to study aggregate stability against
water erosion (Yoder, 1936; Kemper, 1966; Kemper and Rosenau, 1986). This method includes
cyclically submerging and sieving soil in water, which emulates the natural stresses involved in the
entry of water into soil aggregates. Since disruption strongly varies according to the moisture
content of samples (Bruce and Beare, 1993; Marquez et al.., 2004), two sample pre-treatments
have been proposed: slaked and capillary-wetted pre-treatments. Direct immersion of dry soil in
water at atmospheric air pressure causes a great disruption of aggregates into smaller aggregates
and primary particles. Weak aggregates are disrupted as a consequence of the sudden release of
internal air displaces with water (panabokker and Quirk, 1957; Cambardella and Elliott, 1993;
Gale et al., 2000). If the purpose of the aggregate analysis is related to infiltration rates of flooded
soils or the formation of soil crusts, immersion of air-dry soil is probably the best procedure.
Capillary-wetted pre-treatment offers a better analysis of soil stability under rain-fed conditions,
also when soil under the surface is studied. A much smaller and gradual degree of disruption
occurs when soil is slowly wetted (e.g. through an aerosoQ because the bonding is still sufficiently
strong to hold most of the primary particles together in aggregates (Kemper and Rosenau, 1986).
For pre-treatment, distilled water must be used since salt can cause changes in the ionic status
and stability of soils, except in the case of sodic soils (Shainberg et al.., 1981). Results can be
expressed as the mean weight diameter (MWD) (van Bavel, 1949) or as an aggregate percentage.
To avoid miscalculations, it is also imponant to avoid the inclusion of primary textural panicles
(i.e. sand, gravel, etc.) or products from processes different than fragmentation (e.g. abrasion)
(Diaz-Zorita et al., 2002). Gravel and coarse sand in samples can be calculated through the sand
correction procedure, in which samples are subjected to chemical dispersion through immersion
in Na-hexametaphosphate (HMP) (Diaz-Zorita et al., 2002). Chemical dispersion is based
primarily on the concept of particle repulsion, which results from the elevation of the particle
zeta potential. This process is usually accomplished by saturating the exchange complex with
sodium (Gee and Bauder, 1986).
Materials
• Shovel • Large rectangular sampling boxes • 8 mm sieve for sample preparation • Sieves with openings of 2.00, 1.00, 0.50, 0.25 and 0.053 mm, lid and container • Distilled water (5 1 per sample) • Atomizer • Plates of known weight • Scale • Stopwatch • Brush
Procedure
A Field sampling
Field sampling for soil structure studies must be done carefully to avoid structure disruptions that
can diston results. When different management practices are compared, all samples should be
collected the same day (i.e. similar water content of soils with variability only caused by
management). Samples are taken with a shovel to avoid compression and disturbance of the
sample (i.e. as would be caused by an auger) and ensuring minimum wall surface area to volume
ratio to decrease compaction risk Avoid operations such as hammering the shovel which could
result in sample disruption. Samples are then located on rigid, large sampling boxes (avoiding two
or more layers of samples that would cause compression of soil). Then, prior to analysis, samples
are stored in locations with constant conditions (i.e. temperature and humidity?.
B. Sample preparation
After field sampling, samples are air-dried at room temperature for few hours and big clods ( > 5
cm) are gently broken along natural planes of weakness into natural aggregates These samples are
air-dried during two weeks and passed through an 8 mm sieve. C.oarse plant residues, roots and
any stones >8 mm are removed. A sub-sample of 20 g is then taken for further analysis.
C. Measurement
For each sieve, there would be a plate (properly marked) where soil would be located. The soil
sub-sample is remoistened with an atomizer (Figure 1) over the top of the stack of the five sieves
(i.e. 2.00, 1.00, 0.50, 0.25 and 0.053 mm) (capillary-wetted pre-treatment) (Figure 2). Sieves are
then immersed (when not remoistened this corresponds to slaking pre-treatment) and moved up
and down through a venical distance of 3.5 cm and a rate of 35 strokes/ min-1 (Figure 3). Each
size fraction is oven-dried at 75 DC (Figure 4), weighed and expressed as a percentage of total
soil after correction for the presence of sand and coarse fragments (which consists in shaking the
samples with 5% sodium hexametaphosphate (1:3 soil-liquid ration) for 18 hand determining the
weight retained on respective size screens).
D. Calculus
n
MWD11~ = L < d >;W; i=I
with, MWDc1s =mean weight diameter (mm) of dry sieved soil
d =mean diameter of each size fraction size i (mm) (e.g. soil found in 1.00 mm sieve has
2.00 mm as maximum diameter and 1.00 mm as minimum diameter. Thus, mean weight
diameter for such sieve is 1.50 mm). For the selected group of sieves, mean size fractions
are: 5.00 mm, 1.50 mm, 0.75 mm, 0.375 mm, 0.152 mm, and 0.0265 mm.
w = proportion of total sample weight (g) - sand and coarse fragments weight (g)
occurring in the size f raciion i
n = number of size fractions
: Figure 2. Stack of sieves.
3. Immersion of stack of sieves in distilled ' Figure 4. Soil samples after wet-sieving to be dry in : oven.
References
Arshad, MA, and Mermut, AR, 1988. Micromorphological and Ph~ico-chemical
Characteristics of Soil Gust Types in Nonhwestem Alhena, Canada. Soil Sci. Soc. Am. J.
52: 724-729.
Cambardella, CA, and Elliott, E.T., 1993. Carbon and nitrogen distribution in aggregates from
cultivated and native grassland soils. Soil Sci. Soc. Am. J. 57: 1071-1076.
Dfaz-Zorita, M, Perfect, E., and Grove, J.H, 2002. Disruptive methods for assessing soil
structure. Soil Till. Res. 64: 3-22.
FAO, 2003. Optimizing soil moisture for plant production; The significance of soil porosity. By
Francis Shaxson and Richard Barber. FAO Soils Bulletin No. 79. FAO, Rome.
Gale, W.J., Cambardella, CA, and Bailey, T.B., 2000. Root-derived carbon and the formation
and stabilization of aggregates. Soil Sci. Soc. Am. J. 64: 201-207.
Gee, G.W., and Bauder,J.W., 1986. Panicle-size Anal~is. p. 383 - 411. In: Klute, A, Campbell,
G.S., Jacson, RD., Mortland, MM., and Nielsen, D.R (Eds.) Methods of Soil Anal~is.
Pan I, ASA and SSSA, Madison, WI, USA, 1986. pp. 363-375.
Kemper, W.D., 1966. Aggregate stability of soils from western United States and Canada. USDA
ARS. Technol. Bull. 1355. US Gov. Print. Office, Washington, DC
Kemper, W.D. and Rosenau, RC, 1986. Aggregate stability and size distribution. p. 425 - 442.
In: Klute, A, Campbell, G.S., Jacson, RD., Mortland, MM, and Nielsen, D.R (Eds.)
Methods of Soil Anal~is. Pan I, ASA and SSSA, Madison, WI, USA, 1986. pp. 363-375.
Lal, R, and Shukla, MJ., 2004. Principles of Soil Ph~ics. Marcel Dekker, New York, 2004, viii+
716 pp. ISBN 0-8247-5324-0.
Limon-Onega, A, Govaens, B., Deckers, J., and Sayre, K.D., 2006. Soil aggregate and microbial
biomass in a permanent bed wheat-maize planting s~tem after 12 years. Field Crop. Res.
97: 302-309.
Nasr, HM, and Selles, F., 1995. Seedling emergence as influenced by aggregate size, bulk density,
and penetration resistance of the seedbed. Soil Till. Res. 34: 61-76.
Panabokke, CR, and Quirk, J.P., 1957. Effect of water content on stability of soil aggregates in
water. Soil Sci. 83: 185-195.
Rathore, T.R, Ghildyal, B.P., and Sachan, RS., 1983. Effect of surface crusting on emergence of
soybean ( Gl~ rrux L. Merr.) seedlings. I. Influence of aggregate size in the seedbed. Soil
Till. Res. 3: 111-121.
Schneider, E.C, and Gupta, S.C, 1985. C.om emergence as influenced by soil temperature, matric
potential, and aggregate size distribution. Soil Sci. Soc. Am. J. 49: 415-422.
Shainberg, I., Rhoades, J.D., and Prather, R.J., 1981. Effect of low electrolyte concentration on
clay dispersion and hydraulic conductivity of a sodic soil. Soil Sci. Soc. Am J. 45: 273-
277.
van Bavel, CHM, 1949. Mean weight diameter of soil aggregates as a statistical index of
aggregation. Soil Sci. Soc. Am J. 17: 416-418.
Yoder, R.E., 1936. A direct method of aggregate analysis of soils and a study of the physical
nature of erosion losses. J. Am Soc. Agric. 28: 337-351.
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Introduction
(;astelJanos-Navarrete, A., an<i Govaerts, B. Jik.. . .
Castellanos-Navarrete, A, and Govaerts, B.2008. Soil microaggregation. In: Govaerts, B. (Ed.), How to evaluate cropping management
practices; A cook book, OMMIT, Mexico, D .F., Mexico.
Protocol: Soil Microaggregation
Soil aggregates can be divided into macroaggregates (250 µm - 8 mm), microaggregates (53 - 250
µm) and free silt+clay panicles ( < 53 µm). In the last decades there has been increasing
awareness of the imponant role in soil carbon dynamics played by soil microaggregates.
Microaggregates are formed within macroaggregates through the decomposition of
macroaggregates binding agents into fragments (paniculate organic matter or PO:tv1) which
coated with bacterial and fungi mucilgaes become encrusted with clays (Oades, 1984). 1bis has
been reflected in labelled C studies where initial C accumulation in soil macroaggregates as coarse
( > 250 µm) intra-aggregate paniculate organic carbon (iPOM q is redistributed into
microaggregates as fine iPOMC (Anger et al., 1997; Six et aL, 2000a) while macroaggregates break
down. Microaggregates are also formed during the gut transit of earthworms and other soil
macrofauna, where organic materials are intimately mixed and become encrusted with mucus to
create nuclei for microaggregate inception (Shipitalo and Protz, 1988; Barois et aL, 1993; Six et aL,
2004). It is occluded iPOM C in soil microaggregates which constitute the main mechanism for
long-term soil C sequestration in agricultural soils (Six et aL, 2004). Microaggregates constitute
relatively stable and secluded habitats for microorganisms, when compared to microaggregate
outer surfaces or macroaggregates as a whole (Mummey et aL, 2004). Low levels of microbial
activity are explained by the inaccessibility due to pore size exclusion and related to water-filled
porosity (Killham et aL, 1993) as well as reduced oxygen diffusion into these fractions (Sexstone et
al., 1995; Sollins et aL, 1996; cited by Six et al., 2004). However, enhanced C stabilization within
microaggregates within macroaggregates is not only related to the amount of microaggregates
(and the amount of macroaggregates that contain them) but also to their dynamic behaviour
(turnover) (Denef et aL, 2007). Increased macroaggregate turnover (due to disturbances such as
tillage) leads to loss of Grich macroaggregates and a decrease of microaggregate formation
(which would be then mainly constituted by Cdepleted microaggregates) (Elliott, 1986; Six et ed.,
2000a; Six et aL, 2000b). For example, Lichter et aL (2008) found that microaggregates within
macroaggregates contained significantly more C under permanent raised beds with full residue
retention (i.e. 19.35 g C kg-1) compared to conventionally tilled raised beds with residue
incorporated (i.e. 15.25 g C kg-1) (Lichter, 2008).
Materials
• Shovel • Large rectangular sampling boxes • 8 mm sieve for sample preparation • Microaggregate isolator • 50 1 deionized water tank • Plastic tubes • Spatula • Sieves with openings 2 mm and 53 µm
• Buckets • Brush • Plate/ dishes of known weight • Scale • Stopwatch
Procedure
A Field sampling
Field sampling for soil structure studies must be done carefully to avoid structure disruptions
since this can diston results. When different management practices are compared, all samples
should be collected the same day (i.e. similar water content of soils with variability only caused by
management). Samples are taken with a shovel to avoid compression and distmbance of the
sample (i.e. as would be caused by an auger) and ensuring minimum wall surface area to volume
ratio to decrease compaction risk Avoid operations such as hammering the shovel which could
result in sample disruption. Samples are then located on rigid, large sampling boxes (avoiding two
or more layers of samples that would cause compression of soil). Then, prior to analysis, samples
are stored in locations with constant conditions (i.e. temperature and humidicy1.
B. Sample preparation
After field sampling, samples are air-dried at room temperature for few hours and big clods ( > 5
cm) are gently broken along natural planes of weakness into natural aggregates These samples are
air-dried for two weeks and passed through an 8 mm sieve. C.oarse plant residues, roots and any
stones >B mm were removed_ To calculate the quantity of microaggregates within
macroaggregates, a sub-sample of 15 g (i.e. always > 5 g) consisting of macroaggregates ( > 250
µm) is taken for further analysis. When interested in free microaggregates, a sub-sample of 15 g
(i.e. always > 5 g) of soil retrieved from the 53 µm sieve is taken for further analysis.
C. Measurement
Record the exact weight of the soil sample and place in a glass flask Add 50 ml of deionized
water and wait for 20 minutes to break down large macroaggregates. The microaggregate isolator
(developed by Six et aL, 2000a) needs to have a glass funnel with 50 glass pellets over a 250 µm
mesh screen (Figure 1). Connect the deionized water tank to the microaggregate isolator and be
sure flux is constant. The exit plastic tube must be directed over a 53 µm sieve (Figure 2).
T----
1---- -·+ i -;
I
I I Figure 1. Glass funnel with glass pellets and 250 µm : Figure 2. Plastic tube directed to a 53 µm sieve. I mesh screen at the bottom
i
!
+-·······-·· .... ··- ···-······-···········-··-··················· -···---·· ·-··-···············-·····-···-··· -······ -· ... --··· -······-···-·-·······- ·······--·········-····--·-··-···· ···········-··---·····-···-·-······-·-··-·-·-·- ·-·····-········-· ····-··········--···--····· . ----Add the soil to the glass funnel and close it with its lid. Start up the microaggregate isolator (150
cycles min-1) for 5 minutes (until water going out is clear and all aggregates over the mesh screen
are already dissolved). To ensure that microaggregates are not exposed to funher disruption by
slaking, water flows continuously through the device and microaggregates are flushed
immediately through the 250 µm sieve onto a 53 µm sieve. Check with a spatula that all
macroaggregates are dissolved. If time for dissolution exceeds 5 minutes, record the new time. Be
sure all material in the 53 µ.m sieve is drained (by moving the sieve in bucket with water 50 times
in 2 minutes with an up-and-down movement of about 3 cm and with a small angle). Check sieve
sides and surfaces for material and be sure it remains in the water.
All material > 53 µm has to be located on a small pre-weighed dish and oven-dried at 105 °C for
one night. Weight all fractions and store them in glass flasks on the following day. 1his operation
would provide the quantity of microaggregates within the sub-sample (which usually are taken for
further analysis, such as carbon and nitrogen contents). While particles < 53 µm also have to be
placed in a pre-weighed dish (bigger size) and oven-dried as well for one night. If there is too
much water, take a representative subsample (shake the water to be sure soil is suspended
uniformlJ'?, but in such case the volume remaining solution has to be recorded. Sand and coarse
particulate organic matter (cPOM) are retained on the 250 µm mesh screen after breaking up in
the macroaggregates. 1his fraction is also placed in a pre-weighed dish, oven-dried and weighed.
References
Angers, D.A, Recous, S., and Aita, C, 1997. Fate of carbon and nitrogen in water-stable
aggregates during decomposition of 13C15N-labelled wheat straw in situ. Eur. J. Soil Sci.
48: 295-300.
Barois, I, Villemin, G., Lavelle, P., and Toutain, F., 1993. Transformation of the soil structure
through Pontoscolex corethurus (Oligochaeta) intestinal tract. Geoderma 56: 57 - 66.
Denef, K., Six, J., Merckx, R, Paustian, K., 2004. Carbon Sequestration in Microaggregates of
No-Tillage Soils with Different Oay Mineralogy. Soil Sci. Soc. Am. J. 68:1935-1944.
Denef, K., Zotarellia, L., Boddey, RM, Six, J., 2007. Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two
Oxisols. Soil Biol. Biochem 39:1165-1172.
Elliott, E.T., 1986. Aggregate structure and carbon, nitrogen, and phosphorus in native and
cultivated soils. Soil Sci. Soc. Am. J. 50: 627-633.
Killham, K., Amato, M, and Ladd, J.N., 1993. Effect of substrate location in soil and soil pore-
water regime in carbon turnover. Soil Biol. Biochem 25: 57-62.
Lichter, K., Govaerts, B., Six, J., Sayre, K.D., Deckers, J. and Dendooven, L., 2008. Aggregation
and C and N contents of soil organic matter fractions in a permanent raised-bed planting
system in the Highlands of Central Mexico. Plant Soil 305: 237-252.
Mummey, D.L., and Stahl, P.D., 2004. Analysis of soil whole- and inner-microaggregate bacterial
communities. Microb. Ecol. 48: 41-50.
Oades, J.M, 1984. Soil organic matter and structural stability: Mechanisms and implications for
management. Plant Soil 76: 319-337.
Shipitalo, MJ., and Protz, R, 1988. Factors influencing the dispersibilityof clay in worm casts. Soil Sci. Soc. Am. J. 52: 764-769.
Six, J., Elliott, E.T., and Paustian, K., 2000a. Soil macroaggregate turnover and microaggregate formation: A mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem 32: 2099-2103.
Six, J., Paustian, K., Elliott, E.T., and Combrink, C, 2000b. Soil structure and soil organic matter: I. Distribution of aggregate size classes and aggregate associated carbon. Soil Sci. Soc. Am J. 64: 681-689.
Six, J., Bossut, H, Degryze, S., and Denef, .K., 2004. A history of research on the link between
(micro)aggregates, soil biota, and soil organic matter dynamics. Soil Till. Res. 79: 7-31.
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Niels, J., Castellanos-Navarrete, A, and Govaerts, B.2008. Soil moisture. In: Govaerts, B. (Ed.), How to evaluate cropping management
practices; A cook book, O.MMIT, Mexico, D.F., Mexico.
Protocol: Soil Moisture
Introduction
Water content of soil is a complementary and necessary aspect of many soil analyses. Soil water
content has been traditionally expressed as the ratio of the mass of water present in a sample to
the mass of the sample after it has been dried to constant weight, or as the volume of water
present in unit volume of the sample. Computations of water content on a volume basis require a
correct measure of bulk density (Gardner, 1986). Given spatial and temporal variability of soil
moisture levels within soil, the use of a high number of replicate measures is highly
recommended.
Materials
• Metal rings ( 100 cm3
volume and 5 cm diameter)
• Plastic bags
• Trowel
• Flat-bladed knife
• Scale
• Plates
Procedure
A. Field sampling
Carefully drive the ring straight
into the soil (to avoid compaction)
(Figure 2). Once the soil is located
Soil depth (cm)
5
10
15
20
25
30
1. Soil moisture sampling at the topsoil.
A
B
V'l 0
::; 0 .... ;:::;-0 ::I V>
within the ring, dig around it and, with the trowel underneath it, carefully lift out to prevent any
loss of soil (Figure 3). Remove excess soil from the sample with a flat-bladed knife (if losses
include soil within the ring, repeat the procedure at a new sampling point). Place the sample in a
bag (touching the sample as little as possible) and label.
B. Measurement
Weight the wet soil sample in its bag (i.e. the weight of the bags must be known) and record data.
Oven-dry for 48 h at 105 DC in an open metal can. Once dried, weigh the sample (i.e. weight of
the metal can must be known). The soil samples are weighed when wet to calculate water mass
present in the soil (g).
Figure 2. Soil bulk density sample extraction. : Figure 3. Example of incomplete soil bulk density sample.
C. Calculus
Soil bulk density (SBD) is calculated as follows:
SBD = (swet -Sdry) Vol
with, SBD =soil bulk density (g/cm3)
Sctry = oven-dried soil sample (g)
Vol= volume metal ring (cm3)
Gravimetric moisture (GM) is calculated as follows:
(s -s ) GM = wet dry xl 00
sdry
with, GM= gravimetric moisture(%)
Swet = wet soil sample (g)
Sctry = oven-dried soil sample (g)
Volumetric moisture (VM) is calculated as follows:
(swet -Sdry) VM=----
Vol
with, VM = volumetric moisture (g cm-3)
Swet = wet soil sample (g)
Sctry = oven-dried soil sample (g)
Vol= volume metal ring (cm3)
References
Gardner, W.H, 1986. Water content. p. 493-544. In: Klute, A, Campbell, G.S., Jacson, RD.,
Mortland, MM, and Nielsen, D.R (Eds.) Methods of Soil Analysis. Part I, ASA and
SSSA, Madison, Wl, USA, 1986. pp. 363-375.
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Introduction
Verhulst, N., Castellanos-Navarrete, A, and Govaerts, B.2008. Direct sutface infiltration. In: Govaens, B. (Ed.), How to evaluate cropping management
practices; A cook book, OMMIT, Mexico, D.F., Mexico.
Protocol: Direct Sutface Infiltration
Bouwer (1986) indicated that infiltration rates based on cylinder infiltrometer measures are
fraught with errors and uncertainties. Measurement errors can occur due to soil disturbance by
the insenion of the cylinder into the soil. In soils with a surface crust, or other restricting layers,
at or near the surface, cylinder infiltrometers can disrupt such restricting layers, resulting in
drastic increases in infiltration rates. Also, clays and other fine particles brought temporarily in
suspension in the water inside the cylinder can settle out on the soil again during the test creating
a restricting layer on the surface. The time-to-pond methodology was proposed by Govaens et aL
(2005) to overcome these errors and provide a fast, reliable and simple (thus, potentially useful
for on-farm research) method to directly measure surface infiltration. In this methodology, a
circle of metal wire is placed over the soil to avoid soil structure disruption. Since water is not
impeded to flow out of the area marked by the wire, this methodology also provides an indirect
measure of runoff (not considered in cylinder infiltrometer measures). Direct measurements of
runoff are often time and labor consuming (Banhes and Roose, 2002; Hellin, 2006). Soil erosion
is a direct consequence of runoff which depends on surface physical soil quality (i.e. topsoil
aggregate stability? (Barthes and Roose, 2002) and management (i.e. presence of crop residue
cover which avoids direct raindrop impact into the soil) (Neave and Rayburg, 2007). Time-to
pond provides a measure of infiltration versus runoff in which both physical soil quality and
management are taken into account. Thus, measurements do not only permit comparisons of
infiltration between soils but also comparisons of the effects of management practices on
infiltration and runoff. Methodologies have often ignored the effects of management factors, but
farmers' management practices can strongly modify water infiltration rates into the soil (Hellin,
2006) .. For example Anderson and Ingram (1993) recommended removing surface litter prior to
infiltration measurements. Govaens et al. (2006) found with the time-to-pond method that zero
tillage combined with residue retention led to higher infiltration rates with lower runoff values
than conventional tillage and zero tillage without residue retention in the volcanic highlands of
Central Mexico. Given the variability generally encountered in soil physical quality, especially at
the soil surface, and variable residue levels as a result of decomposition and management
practices, multiple spatial and temporal measures of time-to-pond are highly recommended.
Materials and staff
• 3 people • Watering can (smaller for measures in beds) • Water (5 liters per sample) • Rule (min. 30 cm) • White adhesive tape • Metal wire ring ( 53 cm diameter; 50 cm for measurements in beds) • Stopwatch • Gardening scissors or other cutting device.
Procedure
A. Measurement
The wire ring is located over the soil (planting row within wire diameter) without pressing it
down, since this would impede water flowing out of the area (Figure 1). When mulch (crop
residues) is present, it is advisable to cut it out of the wire area to properly monitor runoff water
flows. The amount of water (initial water leve~ in the watering can is recorded Water is then
sprinkled into the center of the ring through a "rose" from 75 cm of height, maintaining a
constant angle of the watering can for as long as possible (flux must be kept as constant as
possible) (Figure 2). The time from the start of water application until water starts to run out of
the ring is recorded with a stop-watch. As soon as water runs out of the ring, sprinkling is
stopped and the amount of water remaining in the can is recorded We find that this process can
be done most efficiently with three people: one prepares the space for the placement of the ring,
a second does the water application and the third records the data.
B. C.alculus
Water flux is verified to be constant and statistical outliers eliminated:
Fl v before - v afier
ux = -------'--t
with, Flux= speed of flux (l s- 1)
Vbefo re =water volume in watering can before measurement (l)
Vafter = water volume in watering can after measurement (I)
t =time to pond (s)
Time-to-pond measures which are characterized by comparable fluxes are valid further statistical
analysis.
Ii .: - . ;
I 1 ~ I Figure 2. Wire placed over the soil prior. . Figure 3. Pouring water at the center of the ring and at 75
: cmheight. 1 --------- ----~
References
Bathes, B., and Roose, E., 2002. Aggregate stability as an indicator of soil susceptibility to runoff
and erosion; validation at several levels. Catena 47: 133-149.
Bouwer, H, 1986. Intake Rate: Cylinder Infiltrometer. p. 825-843. In: Klute, A, Campbell, G.S.,
Jacson, RD., Monland, MM, and Nielsen, D.R (Eds.) Methods of Soil Analysis. Pan I,
ASA and SSSA, Madison, WI, USA, 1986. pp. 363-375.
Govaens, B., Sayre, K.D., and Deckers, J., 2006. A minimum data set for soil quality assessment
of wheat and maize cropping in the highlands of Mexico. Soil Till. Res. 87: 163-174.
Bellin, J., 2006. Better Land Husbandry: From Soil Omservation to Holistic Land Management.
Science Publishers, Enfield and Plymouth, 325 p.
Neave, M, and Rayburg, S., 2007. A field investigation into the effects of progressive rainfall
induced soil seal and crust development on runoff and erosion rates: The impact of
surface cover. Geomorphology 87: 378-390.
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