Soil Science News & Views Vol. 16, No. 10, 1995
K.L. Wells
A knowledge of the nature andaracteristics of soils on a
farm is basic to farm planning so as to properly match crops and
soils and minimize the effect of soil variation on crop
production and economic return. This information also provides a
realistic basis for estimating crop yields on a field-by-field
basis, a planning step necessary in estimating anticipated costs
and returns. Detailed soils maps which have been prepared by
soil scientists identify the various soil series which occur on
the landscape, and show the slope of the landscape on which they
occur, as well as the amount of topsoil (degree of erosion)
present. Soil survey reports containing these maps also include
detailed descriptions of soil profiles, which can be used as a
basis for determining crop production capability for the
different soil mapping units.
Source of Soils Maps
The best source of soils maps is from published county soil
survey reports. The USDA-Natural Resource Conservation Service
(NRCS) in cooperation with the Kentucky Natural Resources and
Environmental Protection Cabinet and the Kentucky Agricultural
Experiment Station is responsible for Kentucky's soil survey
program and publishes a comprehensive report for each county
which has been mapped. Currently, there are reports for 95
counties in Kentucky. These reports are the best and most
comprehensive published information available on the
identification, occurrence, physical and chemical properties, and
use interpretation of soils in those counties.
In counties without published soil survey reports, a
generalized soil association map of the county is available which
can provide some information about the more widely occurring
soils in the county. Also, soils maps of individual farms are
quite often available. Many farmers or previous landowners
currently are or have been cooperators with NRCS. If so, more
than likely an NRCS soil scientist has surveyed the farm tract
and prepared a soils map for it. This map is part of the
conservation plan which NRCS provides to individual cooperators,
and along with the interpretative information the conservation
plan contains, is the next best source of detailed soils
information.
Using a Soils Map
For farm management purposes, the use of a soils map should
be (1) to determine what specific soils occur on a land tract and
in each individual field, (2) to determine the suitability of
those specific soils for production of the crop intended, and (3)
to determine best suited cultural and management practices for
the intended crop on specific soils. In using a published soil
survey report, the land tract in question first needs to be
located. This is not difficult to do if the number of the map
sheet it exists on is first determined by referring to the index
of map sheets. Once the correct map sheet is identified, the
specific field, farm, or land tract can be located. Since the
soil maps are drawn on aerial photographs of the landscape, this
shouldn't be difficult if the location on the map sheet is
correctly oriented with respect to highways, streams, and other
landmark features identified on the map. Quite often,
particularly if the survey report has been published within the
past several years, field boundaries can be readily identified.
Once located, the correct field or farm boundary should be
outlined with a pen to facilitate easy future reference.
Soils have been identified on the landscape by soil
scientists and are delineated on the map as soil mapping units.
Symbols are shown on the map for each mapping unit delineated,
and can be keyed-out on the legend to determine the name of the
soil series, texture of the surface horizon, slope of the
landscape, and degree of erosion. The descriptive information
contained about each mapping unit can be used to determine its
physical and chemical characteristics and how these
characteristics are likely to affect productivity of and
management practices best suited for specific crops. The survey
report contains an index of soil mapping units, referencing the
specific descriptions for each mapping unit. These descriptions
indicate the major physical and chemical characteristics that
would influence plant growth. Additionally, the more recent
reports include land use suitability interpretations in these
descriptions. Survey reports also include detailed soil profile
descriptions of each soil series in the county, and tables which
show percolation rate, available water holding capacity, texture,
and many other useful agronomic and engineering properties of the
soils.
Accounting for Different Soils in the Same Field
Fields rarely occur which contain only one soil mapping
unit. There are usually two or more different mapping units
present. Because of this, characteristics of the profile of each
mapping unit should be examined to determine how well crop roots
can grow. Based on this information, a judgment should then be
made about the rooting volume of the soilspresent in the field.
Although fertility and acidity of the plow layer can be adjusted,
little can be done about naturally occurring restrictions in the
soil profile below plow depth. Preferably, all soils in the
field can be managed alike (most producers do this anyway,
regardless of soils differences).This is practical if subsurface
profile features of the different soils are not greatly
different. If they are, changing field boundaries may be
justified. Soils with root growth restrictions in the profile,
such as a fragipan within 24-28 inches of the surface, a high
water table, bedrock within 36 inches of the surface, or low
water holding capacity, will not perform as well as SS News &
Views soils with no root growth limitations, even though they may
all have highly fertile plow layers.
Developing Economically Efficient Crop Production Systems
Land is initially the most limiting resource to consider in
setting up a farm production system aimed at maximizing returns.
This is because soil, its topographical features, and its
physical and chemical properties are largely fixed. Little can
be done about them except to manage soil fertility and erosion
control. Over the long run, crop production will be directly
influenced by the nature and character of the soils which occur
in each specific land tract. For this reason, the most basic
step in initiating or redesigning a farming operation is to
evaluate the soil characteristics which affect crop growth. By
following the steps out lined below, a farm plan can be developed
which will make the land resource as least limiting as possible
on ultimate economic returns.
(1)Inventory the land tract - Use a soils map to determine what
soils are present and which of their characteristics influence
crop growth. Rooting depth will likely be the most important.
If there are 36 to 42 inches of depth without any root-
restricting barrier, the soil has the potential to produce nearly
any climatically adapted crop. Profile features which limit
volume of soil from which plant roots can extract nutrients and
water are such things as bedrock, water table, surface layers too
compact for roots to grow through (claypans, plowpans
or traffic pans, or fragipans), or subsurface layers with adverse
chemical properties (strongly acid subsoil). Soil texture is
also of great importance because of its influence on plant
available water holding capacity.
The objective of this step is to identify the soil types in a
land tract and evaluate their characteristics which affect crop
growth as a basis for establishing field boundaries.
(2)Lay out field boundaries - This step is necessary for
potential maximum returns because: (a) the field SS News & Views
is the basic management unit of a farm, and (b) fields should be
laid out to include soil mapping units which can be managed
somewhat alike. The nature of individual fields is what will
determine cropping systems and levels of agronomic production of
the whole farm operation. Slope of the landscape is probably the
single most SS News & Views important factor in establishing
field boundaries. Establishment of field boundaries should
represent a practical compromise between the effect of slope and
differences among soil types. The item of ultimate importance is
that the basic management units (fields) of the farming operation
are being established.
(3)Benchmark soil fertility of fields - After field boundaries
are established, a thorough job of soil sampling should be done
to determine acidity level and content of plant-available
phoshorus and potassium. Knowing the results of reliable,
representative soil samples from each field on the farm is
necessary each year for top production. A field record book is a
great help for tabulating costs and returns from each field.
After a few years of keeping field records, more precise
decisions can be made on allocating money available for
production costs each year. This is one of the best ways to
determine where to spend production money in such a way as to
maximumize profits.
(4)Design a cropping system to best utilize land - After laying
out boundaries, fields should be categorized according to Page 5
their suitability for use, such as (a) continuous row-crop land,
(b) rotation land (short-term and/or long-term rotations between
hay and row crops), and (c) permanent pasture land. Crops are
then allocated to these fields on the basis of the highest value
crop to the best land (continuous row-crop land) and
progressively down to the lowest value crops to land which can be
used the least intensively (long-term rotation land or permanent
pasture land).
Summary
Use of soils maps makes it possible to best utilize the soil
resource to its natural productive capability, and make land as
least limiting as possible on whole farm productive potential.
Logically planning agricultural land use involves the following
steps: (a)Taking inventory of the land resource. This involves
use of soils maps to determine what soils are present, where they
occur, and their physical and chemical characteristics which
affect plant growth.b) Establishing field boundaries. Inclusion
of soils which can be managed somewhat similarly within the same
field boundary minimizes the effect of adverse naturally
occurring soil characteristics on field productivity.(c) Soil
testing of each field. Annual expenses for lime and fertilizer
are often the largest out-of-pocket production costs. Knowing
soil test levels within each field provides the basis for using
lime and fertilizer in the most economical manner.(d) Matching
crops to soils. Development of a cropping system to match high
value crops to fields with the greatest production potential
provides for maximum economic crop production.