The Pacific Northwest Wheat Region

G. M. Horner, W. A. Starr, and J. K. Patterson.

Many variations exist in the amount of rainfall and snowfall, length of growing season, topography, and soils in the Pacific Northwest wheat region. Systems of farming and the problems and production potentials vary accordingly.

The amount of precipitation is the chief factor that divides the region into the annual cropping zone and the summer-fallow zone, which can be divided into the "dry" farm (or low-precipitation) area of 8 to 12 inches of annual precipitation and the area of intermediate precipitation, 12 to 18 inches.

The annual cropping system commonly is used in the zone that receives 18 to about 25 inches of precipitation annually. Thus, in the areas of lower rainfall, crops are grown under a moisture stress; in the 18- to 25-inch belt, the lack of moisture seldom is a major factor in crop growth.

Humid winters and dry summers characterize the region. About 60 percent of the annual precipitation occurs in the five months from November through March. Only 20 to 25 percent of the moisture is received during the active growing season of April, May, and June. July and August are the driest months. Moisture stored in the soil in winter therefore strongly influences crop production.

Relatively low intensities of rainfall permit the infiltration of most of the precipitation into the soil. Total precipitation of 1 inch in 24 hours is unusual. Most of the winter storms at Pullman, Wash., have intensities of less than 0.15 inch an hour. The intensities of storms in late spring and summer are about double the intensities of storms in winter.

The amount of snow is much greater at the higher elevations in the northern sections than in the southern areas, which have low elevations. This variation in snowfall influences runoff and erosion. Runoff may be increased by rapid melting of snow, especially when the soil is frozen. Runoff may be reduced when the precipitation falls as snow instead of rain. A slow rate of melting of the snow allows increased infiltration and reduces runoff.

The topography varies from nearly level to steeply sloping. The more gently sloping land usually is in the low-precipitation zone. The more steeply sloping land is in areas of higher rainfall. Some of the cropland has slopes of 50 percent or more.

The main soils are the Brown soils in the low-precipitation zone and the Chestnut, Chernozem, and Prairie soils in areas of increasing precipitation. These soils vary from rapidly permeable and well drained to slowly permeable and poorly drained. Most of the soils are deep enough to store moisture adequately. Some areas have shallow soils with inadequate storage capacity.

The soils have a very fine sandy or silty texture in the zone of low precipitation. They become finer in texture in the high-precipitation zone, where most surface soils are silt loams with silty clay loam or clay loam subsoils.

Soil organic matter also varies with the amount of precipitation and ranges from about 1 percent in the dry sections to 4 percent or more in areas of high precipitation.

Winter wheat is the chief crop. Soft white wheats, suitable for pastry flour, comprised about 90 percent of the total wheat production in 1955. The hard red wheats are grown to a limited extent in the low-precipitation areas.

Wheat is practically the only crop grown in places where precipitation is less than 18 inches annually. No satisfactory substitute for wheat has been found in this drier zone. Sod and legume crops are difficult to establish and do not produce well. Wheat is generally grown in alternate years with summer fallow.

In areas that have more than 18 inches of precipitation, several crops are grown in rotation with wheat peas, barley, oats, clovers, alfalfa, and grass. The acreage of these crops, in relation to the amount of wheat, increases in the sections of higher rainfall.

One of the more satisfactory rotations includes a sod crop of sweetclover or alfalfa plowed under as a green manure. Other cropping sequences commonly used include wheat-peas and wheat-fallow. The production of clover and grass seeds is important in the Moscow Mountain foothills and Nez Perce-Grangeville Plateau areas.

The crop second in importance is peas. They are grown primarily as dry peas in the eastern Palouse and Moscow Mountain foothills areas and as green peas in the Blue Mountain area.

Cattle and other livestock are raised on many farms, especially those that are near rangeland or grow forage crops in the cropping system.

Wheat production may be limited by moisture or plant nutrients or both.

Moisture nearly always is the determining factor in the zone that gets less than 12 inches of precipitation a year. Even with summer fallow, the supply of moisture usually is too small for wheat yields of more than 30 bushels an acre. Yields sometimes drop to about 10 bushels in years of subnormal precipitation.

Plant nutrients, especially nitrogen, have a more dominant influence on wheat yields in the higher rainfall zones. In these areas where moisture is less frequently a limiting factor, wheat yields average 30 to 40 bushels an acre. Yields of 60 to 65 bushels are common on the better managed and well-fertilized fields.