The Northern Great Plains

E. B. Norum, B. A, Krantz, and H. J. Haas.

The most compelling fact of agriculture in the cool, temperate Northern Great Plains is the irregular and generally deficient rainfall. The climate, topography, soil, and native vegetation together encourage an agriculture broadly devoted to the production of spring wheat and range livestock.

The climate of the easternmost part has been termed "dry-subhumid."

The drier central and western part may be characterized as semiarid.

The annual precipitation averages slightly more than 20 inches along the eastern border. It averages about 12 inches in north-central Montana, about 15 inches in southeastern Wyoming, and 15 to 22 inches in the Nebraska Sandhills. About three-fourths of the annual precipitation comes in the 6 months from April through September. Nearly half comes in May, June, and July.

In the more humid eastern section and in irrigated areas elsewhere, a greater diversity of crops is grown than in the less favored areas. Alfalfa, sugar beets, potatoes, flax, and corn are produced there. Moisture for crops becomes progressively more limiting to the westward, the variety of crops and the proportion of the land given to crop production are correspondingly less, and the proportion of land devoted to grassland for hay and grazing increases.

Broad zones of soils correspond to broad differences in climate and vegetation. The Chernozem, Chestnut, and Brown soil zones are alined in a generally north and south direction corresponding to climatic belts. The Chernozem soils, largely devoted to crops, occupy the most humid part. They have thick, black surface horizons and a large amount of organic matter. The Brown soils occur in the driest parts and are devoted primarily to grazing. They have thin, brown surface horizons and relatively little organic matter. The Chestnut soils, between the Chernozem and Brown soils and intermediate in these features, have thin, black or dark-brown surface horizons and moderate organic matter.

The topography generally permits cultivation. Steeply sloping land occurs in the Sandhills of Nebraska; the Black Hills in South Dakota; the deeply dissected, high, old terraces in Montana; and the river breaks in all parts.

Steep lands and sandy or thin soils usually are devoted to rangeland. Spring wheat is grown throughout the region if soil and topography are suitable over a large enough acreage. These factors and climate form a pattern that gives rise to the production of spring wheat in the northeast and range livestock in the western and southwestern parts.

In comparatively extensive areas, soil or topography, or both, assert themselves so as to warrant recognition of subregions. Thus the topography and the soils of the bed of glacial Lake Agassiz in eastern North Dakota and northwestern Minnesota the Red River Valley pose opportunities and problems that set it apart from the extensive ground moraine to the west. The flatness of the terrain and the nearly stone-free soils developed on the clayey, silty, and sandy lake deposits lend themselves especially well to extensive production of spring wheat, barley, sugar beets, and potatoes. It is the most favored with moisture of any section of the region, and a high proportion of the rainfall soaks into the nearly level land. Much of the area is affected by poor drainage, however, which in some seasons results directly in considerable crop loss and handicaps the field operations, especially on the most clayey soils.

The spring-wheat subregion extends from the Red River Valley westward and southward to the range country of Montana and South Dakota. Its eastern part includes mainly loamy and sandy soils (of the Chernozem soil region), developed on varied glacial deposits. The gently undulating ground moraine is marked by low knolls, shallow swales, and closed depressions. There are occasional dry, small lake-beds and deeper depressions, or potholes. Sandy soils, sometimes underlain by gravel, often occur on glacial lake terraces, on outwash plains, and along the larger watercourses and outflow channels of glacial melt water. The largest bodies of sandy soils are on materials deposited as deltas.

Soils within a field that consists mainly of gently undulating upland may include thin soils of knolls; thick, dark soils of lower slopes; and beds of extinct shallow glacial lakes. The closed depressions, or potholes, are a problem for crop production and make it harder to work the land with large machinery. The exposed knolls frequently are the point of inception of wind erosion.

The western part of the spring-wheat subregion includes soils of the Chestnut and Brown soil zones, which developed on loamy and sandy glacial deposits, weathered sandstones, and loamstones and ancient river deposits. An area of several counties in north-central Montana has soil and topography that are favorable to production of spring wheat, although rangelands separate it from the major spring-wheat area.

The Sandhills section of Nebraska is an extensive area of windblown sands in rounded or choppy hills and irregular ridges, and intervening draws and broad bottom lands. Large streams are few. Harold Rhoades, of the Nebraska Agricultural Experiment Station, observed that small watercourses have been obliterated at intervals by drifting sand, giving rise to lakes and marshy tracts. The sandiness of the soils presents hazards for cultivated crops, although they are good for range. The broad bottom lands are commonly wet meadow haylands.

The extensive rangeland to the west includes Brown soils developed on loamstones, sandstones, and clay shales; loess and old alluvium on high ancient river terraces; and, in northern Montana, some glacial deposits. It includes soils of the Chestnut soil zone in western South Dakota developed on clay shales and on saline and alkali loam-stones, which generally are unsuited to cropping.

The soil-management problems of the Northern Great Plains would include most of those of other sections management of fertility, conservation of moisture, control of erosion, maintenance of good soil structure, and drainage. Their relative importance and the range of treatment differ from those of the other sections, however.

EVEN IN THE YEARS when the total amount of precipitation for the growing season is adequate in the spring- wheat subregion, there may be long periods without rainfall. Crops in most years depend on available moisture stored in the soil at seeding time.

John Cole and Oscar Matthews, of the Department of Agriculture, reported results from 15 field stations in the Great Plains that showed the relationship between the yield of spring wheat and the depth to which the soil was wet at seeding time. When soil was wet to a depth of 1, 2, and 3 feet at seeding time, the average yields were 6.9, 12.6, and 19.9 bushels an acre, respectively. Under continuous cropping to spring wheat, the soil was wet to a depth of 1 foot or less at seeding time, about 30 percent of the years. It was wet to a depth of 2 feet in 44 percent of the years. It was wet to a depth of 3 feet or more in only 26 percent of the years.

John Cole found that about 8 inches of precipitation was required before spring wheat began to produce grain. Each additional inch of rain increased yields 2.2 bushels an acre. It was found in Saskatchewan that 5 or 6 inches of water stored moisture plus crop-season rainfall was needed to produce a minimum yield of 1 or 2 bushels of wheat an acre. At 10.5 inches of water, a yield of 14 bushels an acre was obtained. For higher levels of moisture, yields increased at a nearly constant rate of 6 bushels an acre for each additional inch of water.