- 6- 
 
 
reaction and the chemical composition of the soil including the supply of

organic matter, By these,: and oth-r means, it becomes possible to determine

what soil ch~ratteristics are highly significant and what are less so. This

constitutes an attiempt. for the' first time to consult the plant in determining

the significancle of the physical, chemical and biological characteristics
of 
soils. 
 
       On the basis of the soil characteristics as obtained from the study
of 
 the soil map and report, the soils are grouped into productivity units or

 into groups in each of' which the soils have approximately uniform inherent

 productive capacity.  The groups are then arranged in the order of their
in- 
 herent productivity as determined by the consideration of all the factors

 mentioned above. Each group is then rated by giving it an index figure 
 designating its productivity for each of a series of crops. The crops select-

 ed are those that may be grown in the environment or those that are dominant.

 
       The index figure for the productivity of a given land type (usually
a 
 soil type) for any crop expresses the relative productivity of this type
of 
 land compared with that type having in the United States the highest product-

 ivity for the crop concerned. The land type best adapted to the crop is

 given a rating of 10 and the other lands are rated in proportion from 1O
down 
 to 1. 
 
       The "1natural" rating maybe supplemented in certain cases
by one ex- 
pressing special response to the use of fertilizers, and special cultural

treatment. Since the black soils generally must be rated on the basis of

their natural productivity, the fundamental conditions of a comparative rating

demand that all soils be rated on that basis. Since, however, the agriculture

over a large part of the United States is carried on by a systematic use
of 
manures, fertilizers, and lime, it has appeared desirable to give to the
soils 
where such practices prevail a supplementary rating based on actual perform-

ance.     Therefore, where fertilizers or other amendments are extensively

employed such soils are given a double rating. 
 
      The data on which the rating is being made consists in general of all

available geographic information. The most important data for details within

any one small area are those contained in the reports of the Soil Survey.

The work therefore in its essentials consists of an interpretation of the

soil map of the United States. The county soil maps are modified by group-

ing or differentiating soil units as shown on the map in accordance with
the 
interpretation of the relative productivity of the different soils. The 
acreages of each kind of land shown on such modified maps are summed up into

a table, and along with the table of ratings in detail'and the map, is 
supplemented by a written report discussing the bases on which classification

was made, the range in the character of each important class of land, the

relative results that may be expected on each of the given soils through
the 
use of fertilizers or special treatments of whatever kind, and in addition

any other matters that cannot be shown on the classification map. From the

county maps thus constructed a state map is prepared showing the distribution

of lands within certain limits of productive capacity,