The Dry Mild-Winter Region

D. G. Aldrich.

More than 200 commercial crops are grown in the dry mild-winter region of the Far West, which includes most of the farmlands of California, most of southwestern Arizona, the Rio Grande plain south and west of San Antonio, and the lower Rio Grande Valley.

Production estimates based on preliminary reports from the 1954 Census of Agriculture indicate that it produces more than 90 percent of the almonds, lemons, olives, dried figs and prunes, walnuts, and apricots in the United States; 50 percent of the asparagus, pears, and tomatoes; 40 percent of the peaches; 33 percent of the oranges; about 25 percent of the sugar beets, beans, hops, and rice; more than 15 percent of the grapefruit; 10 percent of the alfalfa and cotton; and 30 percent of 21 of the 27 major vegetables grown for the fresh market.

The soils, natural vegetation, and cropping practices in the northern coastal valleys of California, where the annual rainfall is 30 to 60 inches, differ from those of the southern coastal valleys, where rainfall averages 10 inches.

Similar but smaller differences exist as to soil, vegetation, and crops in the Great Central Valley of California, where annual rainfall is 20 inches in the north and 5 inches in the south, and the desert valleys of southern California and Arizona, where the annual rainfall is 3 to 12 inches.

Probably the most noteworthy variation in climate in region 11 is that California and Arizona receive rain mainly in the winter months, but rainfall in southwestern Texas is concentrated in the late spring and early fall.

Temperatures in winter are mild. There are 200 to 300 frost-free days. Seasonal rainfall is so limited that most crops require irrigation.

THE SOILS of the region may be divided into six general categories.

The valley lands consist of alluvial flood plain or alluvial fan soils that are developed in rainfall zones ranging from a high of 60 inches in the north coastal area of California to a low of less than 3 inches in the desert regions of southern California and Arizona. The upland soils, developed from loosely consolidated water-laid material and found in the Winter Garden area of the lower Rio Grande Plain, are in this category.

The valley lands of wind-modified sandy soils are in zones of intermediate and low rainfall.

There are also valley basin lands which are imperfectly drained and may be high in organic matter, salinity, or alkali.

The gently sloping to undulating terrace lands vary in reaction from acid to alkaline and are underlain by clay subsoils or iron cemented hardpans.

The rolling to steep uplands consist of acid to alkaline residual soils of different depth and are found in rainfall zones varying from high (timberland soil) to moderate (grassland) to low.

The sixth category includes unmapped soils in the desert.

The valley lands include the best agricultural soils of the region. Properly cultivated, fertilized, and irrigated, they produce most of the field crops, vegetable crops, fruits, and berries in the region. Valley soils in the high-rainfall zone are productive of berries, vegetables, and apples. Large irrigated acreages of alfalfa, potatoes, sugar beets, carrots, corn, lettuce, apricots, peaches, and walnuts are grown in the zone of intermediate rainfall. Climatic conditions permitting, citrus, avocados, grapes, and cotton are also produced extensively on these soils. The valley soils in low-rainfall zones are in the desert and semidesert areas. With adequate irrigation, they are valued highly for such crops as alfalfa, cotton, flax, sugar beets, melons, and lettuce. The wind-modified sandy soils of the valleys range from acid to calcareous, depending on rainfall. All of these soils are subject to wind erosion, are low in organic matter, have a low water-holding capacity, and are somewhat deficient in at least one plant nutrient. Properly leveled, fertilized, and irrigated, these soils produce many of the special crops, such as grapes, watermelons, dates, and grapefruit.

The valley basin lands are in the lowest parts of the valleys and are nearly flat and usually poorly drained. The peat and muck soils of the Delta area of California are among them. It is one of our largest bodies of continuous peat soils. It was formed from the decomposition of tule beds and is acid in reaction and low in bases. Most of these soils have been reclaimed and protected from floods and tidal overflows by an extensive series of levees. They are drained by open canals and pumps. Irrigation is necessary because these soils exist in an area of moderate rainfall. Managed properly, they produce great yields of asparagus, celery, onions, potatoes, and sugar beets.

The imperfectly drained valley basin soils of predominantly mineral origin generally are dark-colored clays and have a high water table or are subject to overflow. Under dry-farming conditions, these soils are used for wheat, barley, and native pastures. Supplied with adequate irrigation water, they are used extensively for irrigated pasture and rice. They are not adapted to orchard or vineyard culture because of their poor drainage, very fine texture, and susceptibility to frost damage.

The saline, alkali, or saline-alkali soils in the various basins throughout the region generally are unsuitable for cropping until they are reclaimed by leaching if they are saline or by chemical treatment and leaching if they are saline-alkali soils. Partly reclaimed soils produce such salt-tolerant crops as barley, rice, and flax. Completely reclaimed soils produce excellent yields of a wide variety of field, vegetable, and fruit crops.

The soils of the terrace land are along the edges of the valleys, usually at elevations 5 to 100 feet above the valley lands. These soils consist of older secondary deposits or old valley filling material that has moderate to dense clay subsoils or hardpan layers cemented by lime or iron. In the 15-to 40-inch rainfall area, they are like the Prairie soils and are excellent for grazing. Phosphate fertilization is generally needed to maintain good yields.