Soil Aggregate Characteristics and Organic Carbon

Biological Agriculture and Horticulture, 2008, Vol. 25, pp. 223–233 0144-8765/08 $10 © 2008 A B Academic Publishers Printed in Great Britain

Soil Aggregate Characteristics and Organic Carbon Concentration after 45 Annual Applications of Manure and Inorganic Fertilizer J.O. Ogunwole* Department of Soil Science, Faculty of Agriculture, Ahmadu Bello University, Zaria, Nigeria

ABSTRACT Soil management practices that increase soil carbon sequestration can improve soil quality and reduce agricultural contribution to carbon dioxide (CO2) emissions. The long-term (45 years) effect of applications of manure (FYM) and inorganic (NPK) fertilizers on soil aggregate characteristics and soil organic carbon (SOC) concentration was studied in surface soil of a Typic Haplustalf sandy loam. Soils from four treatments: inorganic fertilizer in combination with farmyard manure (FYM + NPK), fertilizer only (NPK), farmyard manure only (FYM) and a control receiving neither NPK nor FYM were studied. Results indicate that long-term application of manure with or without NPK improved aggregate stability by increasing mean weight diameter (MWD) of dry aggregate and, fraction of dry aggregate > 2.0 mm. Higher values of MWD were also recorded in the water-stable aggregate size distributions for soils amended with either FYM or NPK. Soils amended with FYM + NPK sequestered more SOC than all other treatments. There was a low, but significant positive correlation between MWD of dry aggregates and SOC. Highest values of SOC were recorded in the > 2.0 mm aggregate fractions. This tendency towards greater SOC concentration for > 2.0 mm dry aggregate is suggestive of the effectiveness of this aggregate size range to sequester SOC in savanna Alfisols. These results demonstrate that continuous application of soil amendments like FYM and NPK over a long-term promotes soil aggregate stability and long-term SOC sequestration. Hence, for sustainable production in savanna Alfisols, there is a need to design policies that will promote soil aggregate stability and long-term carbon sequestration under continuous cultivation. Such policies must give attention to soil management practices that maintain higher level of SOC. Here, FYM with or without NPK was essential to increase stability of dry aggregates and SOC sequestration. However, the NPK amended soils possess higher fraction of water-stable aggregates.

*Corresponding author – [email protected] 223

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INTRODUCTION In the Savanna agroecosystem, bush fallowing has been the traditional method of restoring and maintaining soil fertility. This method is stable and biologically efficient (Jones, 1971). However, population and economic changes have caused widespread disappearance of this soil fertility restoration practice (Mrabet, 2002). Presently, continuous cultivation has inevitably replaced bush fallow which resulted in accelerated soil erosion and soil nutrient depletion (Lal, 1995; Bationo et al., 2003). This is because continuous cultivation alters soil structure and increases the loss of soil organic matter (SOM). Several studies have shown that SOC and soil aggregate size and stability decline when lands are brought under continuous cultivation (Unger, 1997a; Mrabet, 2002). Soils of the savanna are generally weakly aggregated with high soil bulk density, low organic matter content and poor buffering capacity (Jones & Wild, 1975). These features pose a major threat to soil productivity and thus to sustainable agricultural development. Application of FYM has long been considered a valuable source of organic matter and plant nutrients (Farage et al., 2003). One of the principal importance of manure application is that it promotes the formation and stabilization of soil macroaggregates (Whalen & Chang, 2002) and facilitates SOC sequestration (Farage et al., 2003). Consequently, it improves soil quality and reduces the contribution of agriculture to CO2 emission. Soil scientists agree that improved aggregation and SOC sequestration are suitable indicators for evaluating changes in soil quality (Beare et al., 1994; Unger, 1997a). Soil aggregation is important for soils to remain productive since it maintains the surface integrity of the soil thus facilitating infiltration rather than runoff (Franzluebbers et al., 2000). On the other hand, SOC affects soil quality by positively influencing water retention, aeration and porosity, and also serving as a major repository and source of plant nutrients (Bandaranayake et al., 2003). Although reports on the effect of manure application on soil physical and chemical properties are available, data on the long-term effects of manure on soil quality, i.e. aggregation and SOC concentration, in the African Savanna are scanty. The objectives of this study were, therefore, to determine the effect of 45 years application of FYM and NPK on (i) dry- and waterstable aggregate size distribution and (ii) SOC concentration of the various dry aggregate size fractions, and (iii) to establish relationships among soil aggregates and SOC concentrations. MATERIALS AND METHODS This experiment was conducted on the long-term D (FYM) NPK trial field of the Institute for Agricultural Research, Samaru (11º 11’N, 07º 38’E. 686 m altitude) in the Northern Guinea Savanna ecology of Nigeria. The soil is

SOIL AGGREGATES AND ORGANIC CARBON

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a leached tropical Ferruginous soil classified as Typic Haplustalf according to USDA soil taxonomy (Ogunwole et al., 2001). The long-term DNPK trial at Samaru is about the oldest manure and fertilizer experiment in West Africa. The field plots consist of 81 treatment combinations randomly arranged into nine plots of 220 m2 size. Each plot has been fertilized with either FYM, N, P, and K or their combinations, and the field cropped since 1950. Summary of the cultural practices and crops grown is presented in Table 1. The four treatments selected for this study include: T1: a control plot receiving neither FYM nor NPK amendment for 45 years T2: FYM amended plot with 5 t FYM ha–1 year–1 for 45 years. T3: NPK amended plot with 48–135 kg N ha–1 year–1, 18–54 kg P ha–1 year–1 and 29–58 kg K ha–1 year–1 for 45 years. T4: FYM + NPK amended plot with FYM and NPK at rates specified above for T2 and T3. TABLE 1 General management practices adopted on the long term DNPK trial at Samaru. Year 1950–1968 1967 1981 1950–1991 1992–1996 1997–2001

Management practice

Crop grown

Ammonium sulphate was the N-fertilizer source, which is usually not applied whenever groundnut monocropping is practised

Cotton in rotation with groundnut and sorghum

Lime was applied on plot by plot basis depending on the actual lime requirement for each plot. Micronutrients (Zn, Mo, B and Cu) were also sprayed on crops growing on the field

Cotton

Each plot was divided into four subplots, which were Maize either subjected to subsoiling or left as control Single superphosphate and muriate of potash were the sources of P and K respectively, while calcium ammonium nitrate became N-source from 1969. As a management practice, all crop residues and weeds etc. are removed after the crop is harvested and burnt in nearby trench

Cotton, maize, groundnut, sorghum, cowpea

Urea became the source of N from 1992. Single superphosphate and muriate of potash still remain the sources of P and K, respectively

Maize, cowpea

The trial was under natural fallow, no treatment went into any plot and no crop was cultivated. Soil sampling for this study was done at this time

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Each of these plots were divided into three homogenous subplot of 72 m2 each and, soil samples were collected from three sites in each subplot, bulked and considered a replicate. For dry aggregate size distribution, about 5–6 kg soil samples were obtained with a traditional hoe at the Ap horizon (0–150 mm) and stored in air-tight polythene bags. The soil samples were air-dried, then sieved with a mechanical Shaker (EFL 2 MK3 Test sieve shaker) with different sieve opening sizes of 0.1, 0.25, 0.5, 2.0 and 5.0 mm square. The weight of aggregates in each size range (0.1, 0.1–0.25, 0.25–0.5, 0.5–2.0 and 2.0–5.0 mm) was determined as a fraction of the initial air-dry sample weight and the mean weight diameter (MWD) of both wet and dry soil was calculated as: n

MWD = å x i Wi i=l

where, x i = wi = n =

mean diameter of each size fraction (mm) proportion of the total sample weight occurring in the size fraction i. number of size fractions.

In determining water-stable aggregate distribution, separate duplicate subsamples were wetted by rapid immersion in water and then sieved using the same sieve sizes in water with the aid of the mechanical shaker. The SOC concentration of the bulk soil samples and of the different dry aggregate size fractions was determined using the dichromate oxidation method of Walkley and Black (Nelson & Sommers, 1982). Analysis of variance was performed using the procedures of a completely randomized design using the GENSTAT statistical program (Genstat V Committee, 1993). The least significant difference was employed to compare treatment effects on MWD, SOC concentration and aggregate size fractions. Simple and polynomial regression analyses were used to establish relationships between and/or among soil dry- and water stable-aggregates, MWD and SOC concentration. RESULTS Results indicate that greater mean dry aggregate fractions were recorded for the > 2.0 mm while, the lowest dry aggregate fractions were recorded for the 0.1–0.25 mm range. Differences in means aggregate fractions between the four soil amendments were slight (Figure 1). Soils amended with FYM + NPK had significantly (p £ 0.05) more dry-sieved aggregates > 2.0 mm than the


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2.0 1.8

a

Aggregate fraction

1.6 ab

1.4

b

b

1.2 Unamended FYM + NPK FYM NPK

1.0 0.8

Aggregate fraction

0.6 a

0.4 b

0.2

ab

b

0.0 < 0.1

0.1 - 0.25

0.25 - 0.5

0.5 - 2.0

> 2.0

MWD

Aggregate size range (mm) Aggregate size range (mm)

FIGURE 1. Size distribution and mean weight diameter (mm) of dry aggregates as influenced by Figure 1: Size distribution and mean weight diameter (mm) of dry aggregates as influenced 45 annual applications of FYM and mineral fertilizer at Samaru, Nigeria. by 45 annual applications of FYM and mineral fertilizer at Samaru, Nigeria

unamended and NPK amended soils. The dry-sieved aggregate fractions at risk to wind erosion from this study include a portion of the 0.5–2.0 mm fraction and the < 0.5 mm fraction. The risk of accelerated wind erosion is greater in unamended and NPK amended soils than in soils amended with FYM with or without inorganic fertilizer. The two FYM amended soils in this study resulted in the most > 2.0 mm dry aggregate fraction and the highest MWD of the dry aggregates (Figure 1). The MWD of unamended and NPK amended soils were similar. The mean water-stable aggregate fraction was highest for the > 2.0 mm size range, next was < 0.1 mm and lowest was for 0.5–2.0 mm size range. In the > 2.0 mm size range significantly (p £ 0.05) higher water-stable aggregate fraction was recorded with FYM amended soil than all the other amended soils (Figure 2). In the 0.5–2.0 mm size range significantly greater waterstable aggregate fraction was recorded with NPK amended soil than the other treatments. At the < 0.1 mm size range the FYM + NPK amended soil had significantly (p £ 0.05) higher aggregate fraction than the other treatments. The MWD of the water-stable aggregates was significantly lower for unamended soil and soils amended with FYM + NPK. While, MWD values for the FYM and NPK amended soils were similar (Figure 2). The FYM + NPK amended soil recorded significantly the highest bulk SOC concentration followed by FYM and NPK amended soils, whose SOC concentrations were similar to but higher than that of the unamended soil (Figure 3). The SOC concentration of the different sizes of dry aggregate show that the FYM + NPK amended soil was highest in virtually all the aggregate sizes particularly, in the large aggregate size range.

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J.O. OGUNWOLE 0.8 a a

Aggregate fraction

Aggregate fraction

0.6

0.4

a a b

0.2

b

Unamended FYM + NPK FYM NPK a

b b

b

b

a

a b

b

b

0.0 < 0.1

0.1 - 0.25

0.25 - 0.5

0.5 - 2.0

> 2.0

MWD

Aggregate size range (mm)

Aggregate size range (mm)

)

Organic carbon concentration (g kg–1)

FIGURE 2. Size distribution and mean weight diameter (mm) of water-stable aggregates as Figureby 2: 45 Size distribution and mean weight diameter (mm) ofatwater-stable aggregates influenced annual applications of FYM and mineral fertilizer Samaru, Nigeria. as influenced by 45 annual applications of FYM and mineral fertilizer at Samaru, Nigeria

70 a

60

-1

b

50 40

b

c

Unamended FYM + NPK FYM NPK

30 20

b

10

a

g ab b

b

b

ab

Organic carbon concentration (gkg

0 Bulk soil

< 0.1

0.1 - 0.25

0.25 - 0.5

0.5 - 2.0

> 2.0

Aggregate Aggregatesize sizerange range(mm) (mm)

–1 -1 FIGUREFigure 3. Organic carbon concentration (g kg ) of and ofofdrydry aggregates 3: Organic carbon concentration (gkg ) ofbulk bulk soil soil and soilsoil aggregates of of sizes obtained dry sieving differentdifferent sizes obtained by dry by sieving.


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In the dry aggregate fractions, the > 2.0 mm fraction was positively a function of the bulk SOC concentration as indicated in Table 2. Thus suggesting that increased fraction of large macroaggregates (> 2.0 mm) be the result primarily of the SOC content of the bulk soil. There was however, no significant relationship between SOC of bulk soil and the other dry aggregate fractions. An indication that SOC may not be the only cementing agent in the aggregation of soil particles of those sizes. Furthermore, the MWD significantly increased with an increase in the mass proportion of aggregates > 2.0 mm (r = 0.99, p £ 0.0001) (Table 3). In the case of the water-stable aggregate, the bulk SOC followed a negative correlation with aggregates of 0.5–2.0 mm fraction. This is an indication that most of the dry aggregates > 2.0 mm disintegrated into the lower fraction (i.e. 0.5–2.0 mm) upon wet sieving. The MWD of the water-stable aggregates followed a positive power model (r = 0.89, p < 0.001) in its association with the 0.5–2.0 mm fraction. The rest size range was not significant in their association with MWD.

TABLE 2 Relationships between SOC concentration (gkg–1) of bulk soil and various dry and wet stable aggregates along with their MWDs. Variables Dependent Independent Dry aggregate > 2.0 mm SOC 0.5–2.0 mm SOC 0.25–0.5 mm SOC 0.1–0.25 mm SOC < 0.1 mm SOC MWD (mm)d SOC Water-State Aggregates > 2.0 mm SOC 0.5–2.0 mm SOC 0.25–0.5 mm SOC 0.1–0.25 mm SOC < 0.1 mm SOC MWD (mm) SOC

Equation

b

p levelb

Y Y Y Y Y Y

= = = = = =

5.68 – 0.35x + 0.007x2 – 0.00005x3 1.16 – 0.06x + 0.0012x2 – 0.00001x3 0.51 + 0.03x – 0.004x2 + 0.000001x3 0.098 + 0.0089x – 0.00032x2 + 0.0000029x3 4.5 + 0.3x – 0.006x2 + 0.00004x3 20.7 – 1.26x + 0.03x2 + 0.0002x3

0.60 0.08 0.06 0.17 0.34 0.59

0.04 NSc NS NS NS 0.04

Y Y Y Y Y Y

= = = = = =

-0.104 + 0.005x – 0.00005x2 0.3 – 0.003x – 0.24 + 0.016x – 0.0002x2 –1.51 + 0.113x – 0.0025x2 + 0.00002x3 0.71 – 0.022x + 0.0003x2 – 0.13 + 0.024x – 0.0003x2

0.06 0.57 0.07 0.46 0.43 0.48

NS 0.05 NS NS NS NS

Coefficient of correlation Level of significance of coefficient of correlation c Not significant d MWD, mean weight diameter. a

ra

230

J.O. OGUNWOLE TABLE 3 Relationships between the various aggregates and their mean weight.

Variables Dependent Independent Dry aggregate MWDc > 2.0 mm MWD 0.5 – 2.0 mm MWD 0.25 – 0.5 mm MWD 0.1 – 0.25 mm MWD < 0.1 mm Water-State Aggregates MWD > 2.0 mm MWD 0.5 – 2.0 mm MWD 0.25 – 0.5 mm MWD 0.1 – 0.25 mm MWD < 0.1 mm

Equation

ra

p levelb

Y Y Y Y Y

= = = = =

0.28 + 3.47x 1.61 – 0.84x 3.7 – 58.3x + 421.94x2 – 850.15x3 2.58 – 7.5x + 9.38x2 0.98 + 1.67x

0.99 0.0001 0.09 NSd 0.40 NS 0.48 NS 0.45 NS

Y Y Y Y Y

= = = = =

0.32 + 3.02x – 0.1 + 3.31x + 3.58x2 – 32x3 – 0.72 + 12.9x – 36.8x2 0.45 – 0.48x 0.57 – 0.83x

0.39 NS 0.89 0.0004 0.38 NS 0.32 NS 0.85 0.0004

Coefficient of correlation Level of significance of coefficient of correlation c Mean weight diameter d Not significant. a

b

DISCUSSION AND CONCLUSIONS The data presented highlights three significant issues. The first is the significant effect of the various FYM amendments on aggregation. The second is the distribution of SOC within the various dry aggregates and the third is relationship between aggregates, their MWD and SOC. The effect of 45 annual applications of FYM with or without inorganic fertilizer increased the MWD of dry aggregates and increased the fraction of dry aggregate > 2.0 mm. There was a tendency for soils receiving FYM treatments to have more aggregate > 0.84 mm. The wind erodible fraction of soil is considered to be the unstable aggregates with diameter of less than 0.84 mm (Unger, 1997a; Whalen & Chang, 2002). The dry sieved aggregate fractions that are at risk of wind erosion from this study will include a portion of the 0.5–2.0 mm and the < 0.5 mm fractions, since no effort was made to separate the 0.5–2.0 mm fractions into aggregates larger than 0.84 mm, it will be appropriate to refer to aggregate fractions < 0.5 mm to be at risk of wind erosion. This limitation will definitely result in a slight underestimation of the soil mass that are liable to wind erosion. The FYM + NPK amended soil converted 28% of the wind erodible fraction into non-erodible aggregates while, application of FYM converted only 9%. About 3% of the wind erodible fraction was aggregated by the application of NPK. The complementary application of FYM and NPK favoured aggregation of dry soil in the Ap


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horizon. The result of MWD of dry aggregates is an indication that manure amended soil produced more structurally stable soil than soil receiving mineral fertilizers. The use of FYM + NPK increased MWD of soil by 47%, while increments of 23 and 2% were observed for FYM and NPK-amended soils, respectively. For the water-stable aggregates, the MWD of soils treated with FYM + NPK recorded the lowest value. A probable reason was that most of the large macroaggregates (> 2.0 mm) fraction in the dry soil degraded when rapidly immersed in water, a situation consistent with natural wetting by intense rains (Unger, 1997a). The presence of large quantities of ions present as a result of complementary application of manure and inorganic fertilizers as in FYM + NPK amended soil could destabilize the soil by dispersing larger soil aggregates. The MWDs of the FYM and NPK amended soils were significantly higher than those of the unamended and FYM + NPK amended soils. Their higher MWD values may probably be due to the much lower quantities of ions, particularly, monovalent and certain divalent cations in them relative to the FYM + NPK amended soil. Al-ani & Dudas (1988) demonstrated that MWD of water stable aggregates increased with addition of calcium carbonate from 0 to 4%. However, further increase in calcium carbonate from 4% decreased MWD. It is expected that FYM + NPK amended soil will have higher quantity of cations than either the FYM or NPK amended soils. Highest water-stable aggregate fraction in the FYM + NPK amended soil was in the < 0.1 mm size range. This must be contributions from larger aggregate fractions. Aggregates and primary particles in this size range have been associated with surface seal formation in soils (Loch, 1989 cited by Unger, 1997a). The high SOC in FYM + NPK and FYM amended soils were not unexpected. Application of manure to soil increase SOC content and FYM + NPK influences root growth thereby, adding more organic materials to soil upon decomposition. Most of the SOC concentrations were retained in the macroaggregate fractions (> 0.25 mm). This agrees with the hierarchical model of aggregate organization postulated by Tisdall & Oades (1982). The model states that microaggregates flocculate to form macroaggregates. It is therefore, expected that the largest macroaggregates should sequester more SOC than smaller aggregates. The simple and polynomial regression equations presented in Table 2 showed SOC as a determinant of MWD of dry savanna soils in the Ap horizon. This result differs from that of Unger (1997b), where no significant relationship exists between bulk soil and MWD of dry aggregates. One reason may be because Unger used soil materials from clods resulting from tillage rather than aggregates. The SOC content explains 60% of the variation in dry aggregates > 2.0 mm. This agrees with Unger (1997b) who reported significant relationship between SOC and aggregate > 2.0 mm. This result has further confirmed the role of SOM in facilitating aggregation in semi-arid- and subhumid savanna soils. There was a negative relationship between water-stable

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aggregates in the 0.5–2.0 mm size range and SOC. This must be due to the fact that differences in treatments in this size range were small. Duiker & Lal (1999) showed that where the range of SOC concentration was limited, they would cause no significant difference in the fraction of water-stable aggregates. In this study, SOC concentrations for the various amendments were similar within this size range (Figure 3). These results show that application of FYM with or without NPK as soil amendment improves the soil structure by increasing the proportion of dry macroaggregate fraction and SOC concentration. The consequence is increased soil resilience to wind erosion. These results unambiguously show that application of either FYM or NPK increased soil stability to water erosion than when FYM + NPK is applied. Aggregates > 2.0 mm explains 99% of the differences in MWD of dry aggregates while, in the water-stable aggregates the 0.5–2.0 mm size range explains 89% of differences in MWD. For improvement of soil structural properties of the West African Savanna, adopting technologies that incorporate soil amendments like FYM into soils under continuous cultivation, will go a long way to enhance the quality status of these soils. Acknowledgement I thank the Director, Institute for Agricultural Research, Ahmadu Bello University for kind permission to publish this work and Dr. I.Y. Amapu for providing important help with the field layout and cropping history. Professor B.A. Raji and the anonymous reviewers provided very insightful comments on earlier drafts of this manuscript. References Al-ani, A.N. & Dudas, M.J. (1988). Influence of calcium carbonate on mean weight diameter of soil. Soil Tillage Research, 11, 19–26. Bandaranayake, W., Qian, Y.L., Parton, W.J., Ojima, D.S. & Follett, R.F. (2003). Estimation of soil organic carbon changes in Turfgrass systems using the CENTURY model. Agronomy Journal, 95, 558–563. Bationo, A., Mokwunye, U., Vlek, P.L.G., Koata, S. & Shapiro, B.I. (2003). Soil fertility management for sustainable land use in the West African Sudano-Sahelian zone. In Soil Fertility Management in Africa: A Regional Perspective (M.P. Gichuru, A. Bationo, M.A. Bekunda, H.C. Goma, P.L. Mafongoya, D.N. Mugendi, H.M. Murwira, S.M. Nandwa, P. Nyathi & M.J. Swift., eds.), pp. 255–292. Tropical Soil Biology & Fertility Institute of CIAT; Nairobi, Kenya. Beare, M.H., Hendrix, P.F. & Coleman, D.C. (1994). Water-stable aggregate and organic matter fractions in conventional- and no-tillage soils. Soil Science Society of American Journal, 58, 777–786. Duiker, S.W. & Lal, R. (1999). Crop residue and tillage effects on carbon sequestration in a Luvisol in Central Ohio. Soil Tillage Research, 52, 73–81.


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Farage, P., Pretty, J. & Ball, A. (2003). Biophysical aspects of carbon sequestration in drylands. Seminar paper presented at University of Essex, U.K., pp. 1–24. Franzluebbers, A.J., Wright, S.F. & Stuedemann, J.A. (2000). Soil aggregation and Glomalin under pastures in the southern Piedmont USA. Soil Science Society of American Journal, 64, 1018–1026. Genstat V Committee (1993). Genstat Release Reference Manual. Oxford University Press, Oxford, U.K. Jones, M.J. (1971). The maintenance of soil organic matter under continuous cultivation at Samaru, Nigeria. Journal of Agricultural Science (Cambridge), 77, 473–482. Jones, M.J. & Wild, A. (1975). Soils of the West African Savanna. Technical Communication No. 55. Commonwealth Bureau of Soils; Harpenden, U.K. Lal, R. (1995). Erosion-crop productivity relationships for soils of Africa. Soil Science Society of American Journal, 59, 661–667. Mrabet, R. (2002). Stratification of soil aggregation and organic matter under conservation tillage systems in Africa. Soil Tillage Research, 66, 119–128. Nelson, D.W. & Sommers, L.E. (1982). Total carbon, organic carbon and organic matter. In Methods of Soil Analysis Part 2 Chemical and Microbiological Properties. 2nd edn. (A.L. Page, R.H. Miller & D.R. Keeney, eds), pp. 539–580. American Society of Agronomy; Madison, U.S.A. Ogunwole, J.O., Babalola, O.A., Oyinlola, E.Y. & Raji, B.A. (2001). A pedological characterization of soils in the Samaru area of Nigeria. Samaru Journal of Agricultural Research, 17, 71–77. Tisdall, J.M. & Oades, J.M. (1982). Organic matter and water-stable aggregates. Journal of Soil Science, 33, 141–163. Unger, P.W. (1997a). Management induced aggregation and organic carbon concentration in the surface layer of a Torrerttic Paleustoll. Soil Tillage Research, 42, 185–208. Unger, P.W. (1997b). Aggregate and organic carbon concentration interrelationships of a Torrerttic Paleustoll. Soil Tillage Research, 42, 95–113. Whalen, J.K. & Chang, C. (2002). Macroaggregate characteristics in cultivated soils after 25 annual manure applications. Soil Science Society of American Journal, 66, 1637–1647. (Received 12 September 2005; accepted 22 October 2007)