Soil Science News & Views
Vol. 16, No. 10, 1995

Interpreting Soils Maps

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

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

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).


     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.