Vol.16. No. 9, 1995

The Effect of Organic Matter on Maximum Compactability of Soil

G.W. Thomas, G.R. Haszler, and R.L. Blevins


   Bulk density is the weight of a given volume of soil
expressed by soil scientists as grams per cubic centimeter
(g/cm3).  The higher the bulk density, the lower the volume
of soil occupied by pore space, that volume of soil in which air
and water reside.  Because of this, there is much interest on the
degree to which soil can be compacted.
     The Proctor test is a means of determining the maximum bulk
density that can be attained in a soil sample.  This "maximum
compactibility" is widely used on highways and building
foundations but has had little use in agricultural soils.  In the
fall of 1994 we began to determine maximum compactability on
samples from plots and fields with variable land use history in
Kentucky.  This is a report of the preliminary results.

Materials and Methods

     Soils were sampled from 0 to 2 inch depth at the following
1.  Maury silt loam was sampled in the long-term (25 yrs.) no-
tillage plots at Lexington .  Four treatments were sampled: no
tillage, 0 and 300 lb nitrogen (N)/A rates and conventional
tillage, 0 and 300 lb N/A rates on each of four replications for
a total of 16 samples.  In addition, one sod sample from
alongside the plots was taken.
2.  Lonewood loam was sampled at eight farm locations in Russell
County, Kentucky.  Treatments included sod, no tillage, and
conventionally tilled fields of long duration.
3.  Pembroke silt loam was sampled in four fields, all located
close together in Logan County.  The treatments were fescue sod,
no tillage soybeans, conventional corn, and alfalfa recently
planted on a soil where conventional tillage had long been
4.  Grenada silt loam was sampled on several farms in Hickman
County.  Treatments were sod, various years of no tillage and one
conventionally-tilled site.

     All soils were crushed by hand while still slightly moist
and passed through a 2 mm sieve.  Soil organic carbon was
determined on each sample using a Leco CR-12 Carbon Analyzer.
This is a dry combustion analysis for organic carbon.  Duplicate
samples were used for all determinations.
     Maximum compactability (the maximum bulk density obtainable)
was determined using the standard method with the following
details.  The mold or compaction chamber was filled and compacted
in four layers, each layer receiving 25 blows from a standard
falling hammer, for a total of 100 blows.  Water content was
varied in each case from the dry side of maximum bulk density to
the wet side.  A minimum of four and occasionally five individual
moisture contents was used to approximate the curve.
     After the weight of wet soil in the compaction chamber was
determined, three soil moisture samples were taken so that soil
dry weight and moisture content could be determined.  These
samples were weighed, dried in the oven at 110oC for 24 hours and
weighed again.  We observed practically no variation in soil
water content between the three subsamples.
     Maximum bulk density for each sample was estimated by
linearly extrapolating the "dry" leg and the "wet" leg of the
samples to a point of intersection.  This point also gives the
moisture content at which the maximum bulk density is attained.

Results and Discussion

     Table 1 shows the land use and management of sites sampled,
the soil series, the maximum bulk density, and the percent
organic carbon.  The range in both bulk density and organic
carbon is relatively large, 1.4 to 1.8 g/cm3 and 1.0 to 3.5%,
respectively.  These data were separated by soil series.  Fig. 1
shows the bulk densities of samples of Maury silt loam plotted
against % organic carbon.  The data gave an r2 of 0.922 and
a slope of -0.15x.  The latter indicates a change of 0.15 g/cm3
in bulk density for each one percent change in organic carbon.
The samples do not appear to separate according to tillage
itself but instead are closely related to organic carbon which,
in these experimental plots, is a result of 25 years of
continuous corn production.
     Fig. 2 shows the relationship between maximum bulk density
and % organic carbon for Lonewood soils.  Again, regardless of
treatment, the relationship is affected by the organic carbon
content of the samples.  The slope of the curve is - 0.23 g/cm3
per percent organic carbon, somewhat steeper than that found in
the Maury silt loam.  The r2 value is 0.92, about the same fit as
in the case of the Maury samples.
     The intercept and slope of the Pembroke and Maury soils were
exactly equal, 1.88 and - 0.15, respectively, indicating that
texture and perhaps other soil characteristics are very similar
in the two soils.
     The effect of organic carbon was considerably less in the
very silty Grenada soils, but still lowered maximum bulk density
by - 0.103 per one percent increase in organic carbon.
Interestingly, the Grenada soil even without organic carbon, has
an intercept of only 1.72 g/cm3 so that it is a much less
compactable soil than any of the others used.  The r2 is also
slightly lower at 0.85.


     This work has far-reaching, practical consequences.  Of all
the doubts about continuous no tillage as a viable practice, the
fear of compaction probably looms largest.  This work shows
clearly that with an adequate organic carbon content (probably
around 2.5%) in the surface soil, the fear of compaction is
essentially groundless.  A prime role of organic matter, in
addition to many other favorable effects, is resistance to
compaction.  Exactly how organic matter accomplishes this is open
to question.  We may speculate on two possible causes: First,
organic matter aggregates the soil so that it resists breakdown
when it is compacted by hammering.  Second, organic matter binds
the particles so that vibration induced sorting does not occur
during the compaction process.  Both of these mechanisms would
result in less compaction.  It should also be pointed out that
the direct effect of lower density organic matter on bulk
density is neglible.
     Whatever the mechanisms, we have shown that one percent
organic carbon lowers the maximum bulk density in four Kentucky
soils by 0.10 g/cm3 to 0.23 g/cm3, which is a very significant
effect.  Together with the other favorable effects of organic
matter on soil properties, this certainly suggests that the
maintenance and increasing of soil organic matter contents is a
worthy goal.  The use of no tillage is a practical and efficient
means to attain that goal.

Table 1. Soil, county, land use, maximum bulk density, and %
organic carbon in soils used in the experiments.

Soil/County   Land Use History  Rep   Maximum   % Organic %H20@
              and site location     Bulk Density  Carbon  Maximum
                                       g/cm3              Density

Maury       Conventional Corn,   I      1.70       1.23    18.3
 silt loam    25 years, 0        II     1.69       1.36    18.8
 (Fayette Co)  Nitrogen          III    1.67       1.44    19.3
                                 IV     1.66       1.47    19.8
Maruy       Conventional Corn,   I      1.65       1.44    19.2
              25 years,          II     1.67       1.69    18.3
             300 lbs N A-1       III    1.62       1.64    19.2
                                 IV     1.64       1.98    20.0
Maury       No-Tillage Corn,     I      1.53       2.15    25.2
         25 years, 0 Nitrogen    II     1.49       2.18    26.3
                                 III    1.55       2.16    23.8
                                 IV     1.52       2.15    25.2
Maury      No-Tillage Corn,      I      1.40       3.41    27.1
            25 years,            II     1.39       3.47    28.0
           300 lbs N A-1         III    1.44       2.75    24.0
                                 IV     1.39       3.10    29.0
Maury     Permanent bluegrass-fescue    1.50       2.40    25.5
          sod, next to corn plots
Lonewood  Sod near fence line, Hwy 379  1.50       2.40    24.0
 loam     No-Tillage soybeans, Hwy 279  1.58       1.99    21.5
(Russell  No-Tillage soybeans, hog      1.54       2.19    21.4
  Co.)          manure applied
          No-Tillage corn for silage    1.70       1.33    18.0
          Conventional pepper field     1.82       1.27    14.2
          No-Tillage soybeans           1.58       2.12    21.5
          No-Tillage soybeans, half     1.65       1.69    18.8
          Conventional corn, John St.   1.77       1.10    14.7
Pembroke  Permanant fescue sod, Hwy 663 1.46       2.85    25.0
silt loam Conventional Corn, Hwy 663    1.72       1.24    18.3
(Logan    New alfafla-formerly          1.68       1.30    18.5
 Co.)  conventionally tilled, Hwy 663
          No-tillage for soybeans       1.65       1.45    19.8
Grenada   Long-time conventional        1.62       0.82    18.0
 silt     tillage, no-tilled since 1990
 loam     No-tillage for several years  1.59       1.16    19.3
(Hickman  Long-time no-tillage soybeans 1.54       1.63    20.7
 Co.)     Conventionally-tilled corn    1.60       1.34    18.2
          No-tillage corn-wheat-        1.56       1.57    18.7
          soybeans & chicken manure
          No-tillage for more than      1.50       1.97    22.3
           20 years
          Fescue and broomsedge sod     1.59       1.46    18.0