m. LAND RESOURCES 



The Tongue River and Rosebud Creek drainages have 

 four major physiographic soil areas. They are ( 1 ) floodplains 

 and low terraces, (2) fans, terraces and uplands, (3) sedimen- 

 tary uplands and (4) dissected sedimentary uplands. 

 The elevation ranges from about 2,800 to 3,800 feet. The 

 average annual precipitation in the river valleys ranges from 

 10 to 14 inches. The frost free period is 105 to 130 days. A 

 more detailed description of these areas can be found in 

 Appendix B. 



A. Procedures for Land Classification 



1 . Definidon and Purpose 



Several agencies make soil surveys or land classi fications 

 to show the kinds of soil that occur in an area. After all the 

 soil characteristics are known, a land classification guide is 

 developed to categorize the soils into the appropriate land 

 class for the intended use. Several agencies have developed 

 guides for classifying soils for irrigation purposes. A brief 

 explanation of the most widely-used methods is provided in 

 Appendix C. 



Land classification of arable lands for irrigation involves 

 the systematic examination, description, appraisal and 

 grouping of soils on the basis of physical and chemical 

 characteristics affecdng suitability for sustained crop pro- 

 duction under irrigation. Selection ofland for irrigation also 

 involves prediction of the behavior of soils after develop- 

 ment and application of irrigation water. All factors for an 

 individual area are evaluated and delineations are made 

 separating the land into different land classes. 



The purpose of the land classification system developed 

 by the RWRCC Soil Scientist is to determine the extent and 

 degree of suitability of land for irrigation. Soil units are 

 grouped into interpretative classes, based upon relative ca- 

 pability for sustained crop production under irrigation. This 

 classification also provides an inventory ofland characteris- 

 tics and identifies potential problems that may occur with 

 irrigation. 



2. Land Classification 



Land class indicates the general capability ofland for 

 irrigation usein its present state. Land classes are based upon 



the rating and assessment of the soil properties and topo- 

 graphic features that affect the suitability of the land for 

 irrigation. Land within a land class is consistent, or nearly 

 consistent, in its potential to be developed and in its response 

 to a similar level of management. Land classes 1 , 2 and 3 are 

 arable and suitable for irrigation. Class 6 land is nonarable 

 and not suited for irrigation. Classes 4 and 5 are not used in 

 this report, since these lands are limited to rare or unique 

 situations requiring special studies. The limitations or haz- 

 ards become progressively greater from class 1 to class 6. 



The land classification process depends on die experi- 

 ence and judgement of soil scientists, based on observations 

 of land conditions and supported by laboratory data and 

 field studies. 



3. Land Classes 



Class 1 - Arable Land in this class is well suited for 

 irrigatedagriculturewithfewsignificant limitations. Class 1 

 land is capable of producing a high yield of a wide range of 

 climatically adapted crops. The soils are of a medium tex- 

 ture, well drained, and hold adequate available moisture. 

 Class 1 land is level to nearly level. This class is suitable for 

 irrigation by gravity and sprinkler methods. 



Class 2 -Arable: Land in this class is suited for irrigated 

 agriculture with moderate limitations. Slightly more devel- 

 opment and management may be required for Class 2 land 

 than for Class 1 land, such as growing protective cover crops, 

 contouring, and installing small drainage ditches. The land 

 can be maintained or improved with proper management 

 The soils in this class may be slowly permeable due to fine 

 texture or soil structure deterioration. The available water 

 capacity may be lower due to coarse texture or limited soil 

 depth. Drainage may be somewhat restricted. Class 2 land 

 is level to gently sloping or undulating. Land in this class is 

 suitable for irrigation by gravity or sprinkler methods. 



Class 3 - Arable: Land in this class is suited for irrigated 

 agriculture with severe limitations. The deficiencies may be 

 due to a single condition or a combination of several condi- 

 tions in soils and topographic features. The soils may be 

 limited by excess salinity, sodicity, slow permeability or low 

 water capacity. Surface or subsurface drainage may be 



