Castellanos-Navarrete, A., and Govaerts, B., 2008. Soil microaggregation. In: Govaerts, B. (Ed.),...

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)


Volumetric moisture (VM) is calculated as follows:

(swet -sdry) VM=---

Vol

with, VM = volumetric moisture (g cm-3)

S wet = wet soil sample (g)



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

FOR COMMENTS ON HOW TO IMPROVE EMAIL TO: [email protected]

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.


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



result in sample disruption. Samples are then located on rigid, large sampling boxes (avoiding two


are stored in locations with constant conditions (i.e. temperature and humidity?.


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.


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



result in sample disruption. Samples are then located on rigid, large sampling boxes (avoiding two


are stored in locations with constant conditions (i.e. temperature and humidicy1.


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.


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)



Gravimetric moisture (GM) is calculated as follows:

(s -s ) GM = wet dry xl 00

sdry

with, GM= gravimetric moisture(%)

Swet = wet 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)



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.


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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