ROOTING DEEPER IN MINIMUM TILLAGE TO CONSERVE ENERGY
Robert B. Campbell
Conserving energy in the 1980's is more than just reducing fuel or "petrol" use. We would like to believe a little energy conservation is essential, preferably by someone else or by some governmental action
that will provide us with labor saving productivity improvements to
maintain the comforts we have become accustomed to. Scientific reality,
however, dictates that quick easy solutions will not be developed
without careful planning for the efficient use of our energy resources
and without strong efforts to find and develop new sources of energy.
Because agriculture is the primary source of our food supply, energy
must be considered in relation to the total crop production potential,
i.e. production per petrol dollar spent or production per unit of
energy input.
Reduced tillage defined No-till farming in concept is directed to lower use of energy for crop production. Unfortunately the word no-till is misleading, in fact, no-till is not no till at all. The term has been coined to refer to a system of residue management. In this system, seeds are drilled into soil with live or dead plant materials still remaining on the soil surface. Weeds are mostly controlled by the application of constant or residual grass and broad leaf herbicides. However, mechanical weed control is possible under some circumstances. This concept of residue management has been referred to as eco-fallow (2), minimum till (5), or conservation tillage ( ) 3 . These systems require higher levels of soil and crop management than conventional clean till farming methods. Advantages and problems in minimum tillage
Often claimed advantages of minimum tillage over conventional
tillage include: lower erosion, water conservation, ability to plant
earlier, planting on steeper less fertile slopes, lower fuel costs, and
lower compaction ( ) 5 . Minimum tillage methods can be used in multiple
cropping systems ( 4 ) . Even though these appear to be distinct ad
vantages, there are disadvantages or special challenges that must be
addressed to make minimum tillage successful. Because minimum tilled
land is not 'smooth and open, stands of crops are difficult to esta
blish. Birds, and rodents are more active because the residue provides
protective cover. Fungi and insects infestations are more common when
residues remain on the surface. The real question is how can these
problems be solved. Most certainly they can be solved, but only with
greater scientific input.
Robert B. Campbell is a Soil Scientist at the USDA-SEA-AR, Coastal Plains
Conservation Research Center, Florence, South Carolina 29502.
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The soil physical system and minimum tillage
Recognizing soil physical and chemical conditions is an essential
part of residue management in different parts of the country. Minimum
till farming in the Southeast has to be accomplished in deep sandy
soils or in sandy loam or loamy sand soils with genetically compact or
These soils also have low water
mechanically compacted layers (1). retentivity, consequently it is just as important to consider deep
rooting and ways of achieving deeper rooting in minimum tillage as in
conventional tillage. Without giving proper attention to these soil
physical conditions, minimum tillage practices would eventually reduce
the production base and actually increase energy use per unit of crop
production.
In view of the limitations that soil physical conditions may have on
residue management and energy use, corn rooting patterns were studied
in relation to soil strength and soil water availability to corn in a
Norfolk loamy sand soil with a compact A 2 horizon. Large acreages of
these soils occur in the Southeast. For example, in Florence County,
South Carolina alone, 58% of the tilled s o i l s have an A layer ( 1 ) .
Although these layers vary in compactness, they are easily compacted by
tillage tools and wheel traffic.
Describing soil physical parameters
Soils are never uniform in texture, structure and bulk density. Roots
are not symetrically distributed in soil, therefore, water withdrawal
can not be uniform. Consequently, a mean value and frequency distri
bution of certain properties such as bulk density are frequently used
to describe soil conditions shown in Table 1.
Table 1. Bulk density and related frequency distribution
for a Norfolk soil at Florence, SC
Relative Frequency
Bulk Density
B
5.0 5.0 5.0 20.0 30.0 20.0 5.0
1.30- 1.34 1.35- 1.39 1.40- 1.44 1.45- 1.49 1.50- 1.54 1.55- 1.59 1.60- 1.64 1.65- 1.69 1.70- 1.74 1.75- 1.79 1.80- 1.84 1.85- 1.89 1.90- 1.94
4.3 2.1 2.1 8.7 26.1 17.5 8.7 17.4 2.2 2.2 2.9 7.7 15.4 32.7 38.5 1.9 0.9 1.78 0.049 - 0.2283
Mean Std. deviation Schewness
1.57
0.155
-0.0107
1.48 0.099 - 0.7704
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The mean density bulk values for the Ap, A2 , and B horizons are 1.57, respectively. The wide distribution of the Ap 1.75, and 1.48 layer is a result of subsoiling in a minimum tillage experiment in which corn was planted into a standing rye cover crop. The subsoil tool1- produced a narrow slot 10-15-cm wide in the A layer that pene 2 trated 47 cm, about 5 cm into the B horizon. The Ap bulk denaity measurements were more normally distributed about the mean value than or B horizons. the Rooting and soil strength
Increasing bulk density increases resistance to rooting but bulk density
is not the only factor that affects rooting because decreasing soil
water content also increases the strength of soil. Therefore, root penetration is a function of bulk density, water content, and texture. We have determined that soil probes give a reliable index of roota bility in soil, and that a penetrometer index of 20 kg/cm2 represents a value beyond which few roots penetrate. In the Ap horizon at the mean bufk density of 1.57 root penetration is severely restricted at a matric potential of a little over -1000 mb. One could anticipate that
horizon because of
roots would be well distributed throughout the the wide range in the bulk density frequency distribution (see Table
In the horizon however, the matric potential at which roots were
Root
redtricted was -220 mb at a mean bulk density of 1.78 development observations in a corn field showed that rooting in the Rooting in
horizon occurred only in the subsoiled portion of the the B horizon was restricted to those roots that extended down the subsoiled soil. The B horizon had the lowest bulk density of all
Rooting observations demonstrated that
layers studied, 1.48 once a root grew through the disturbed horizon, root growth into the
B horizon was only slightly impeded. Because soil strength restricted
rooting, soil strength affects water availability. By taking -50 mb as the upper limit of water availability and the water content corresponding to as the lower limit of water availability to the plant, the amount of water storage for each layer to the 75-cm depth can be calculated. These calculated water storage values are given in Table 2. Table 2. Water storage in a 75-cm profile based on
-50 mb and the matric potential water content at 20 as the upper and lower availability water limits, respectively.
was considered)
(only the subsoiled portion of the Layer Depth 0-17 17-35 35-75 Total Storage
2.37
0.30
2.91
5.58
with an - Brown-Harden Superseederand does attached subsoil tool. Mention of
tradenames is for reference not constitute endorsement by USDA or its cooperators.
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Various assumptions were made for calculating effective soil water storage. Four examples taking various limiting factors into con sideration are presented in Table 3 . Table 3 . Calculated available water storage in a Norfolk loamy sand profile to...