Agronomic Zones: A Link to Applying Research

Chapter 10 – Economics and Application, No. 5, Summer 1986

Don Wysocki

Comments such as “will this research apply in my situation” or “that practice won’t work here because our conditions are different” are frequently heard in discussions of STEEP research. These comments are justified because STEEP research is conducted at numerous locations under a variety of soil and environmental conditions. It is proper and necessary that research be conducted under a variety of conditions, but the results of research must then be interpreted and applied. To assist in the interpretation and application of STEEP research, Clyde Douglas (USDA-ARS soil scientist, Pendleton) and co-workers are developing a system of land classification for the Pacific Northwest.

Developing Agronomic Zones

The need to interpret and apply research requires a land classification system that is defined on factors that are important to crop growth. Douglas and others are developing their system, which they call “Agronomic Zones for the Pacific Northwest,” on the basis of ecological, climatological and soil conditions. They have selected these characteristics to delineate areas where similar cropping and management systems can be used. They anticipate that no more than 10 zones will be required to classify land in the dryland cropping regions of the PNW. Specifically they define each Agronomic Zone on the basis of (1) annual precipitation, (2) growing degree days and (3) soil depth.

Annual Precipitation

The amount of annual precipitation is an obvious environmental factor that influences cropping and management practices. Precipitation affects such items as potential yield, amount of erosion, diseases and types of crops that can be grown. Annual precipitation has been used to help define zones where annual cropping and crop-fallow farming systems can be practiced.

Growing Degree Days (GDD)

Growing degree days are a measure of the amount of heat that is received. Research at the Columbia Basin Conservation Research Center at Pendleton has shown that development of wheat plants is directly related to the amount of GDD. The number of GDD is a concept very similar to the length of growing season but is much more quantitative and is a measure of biological time. GDD are determined by averaging the maximum and minimum daily temperature in degrees Celsius and summing positive values into a cumulative record. The Agronomic Zones classification system uses GDD accumulated between January 1 and May 31 to help define management units based on probability of winter damage, evaporative water loss and high temperatures.

Soil Depth

Depth of the soil profile is one of the important characteristics that determines the amount of water that can be stored. Shallow soils provide less volume for moisture storage and root exploration and should be managed differently than deep soils. In areas of shallow soils the moisture storage capacity can be exceeded on an annual basis even if rainfall is limited. Under these conditions annual cropping can be practiced. If the soil profile is full every year there is no advantage to summer fallow for additional moisture. In the Agronomic Zones classification system soils are divided into two depth categories using the Soil Conservation Service depth criteria of deep (greater than 40 inches) and moderately deep (less than 40 inches). This division provides a guideline for practicing annual cropping in areas of low rainfall.

Agronomic Zones for Eastern Oregon

Using information on rainfall, growing degree days and soil depth, Douglas and others have developed an Agronomic Zones land classification system for a five-county area in eastern Oregon. The zones they have developed and the criteria they used are shown in Table 1. These zones have been defined to delineate areas that will respond similarly to a given set of management and cropping practices. A map of the five-county area and the agronomic zones is shown in Fig. 1.

Future Plans and Objectives

Expansion of the Agronomic Zones concept into eastern Washington and northern Idaho, and ultimately into the entire Pacific Northwest, is planned. It is expected that no more than 10 zones will be necessary to classify land in all the dryland farming areas.

The Agronomic Zones land classification system will provide a method of transferring and applying the results of STEEP research to users. It will allow growers, ranchers and others to more easily interpret research results and judge the applicability for their situation. It should assist Extension Service and Soil Conservation Service personnel in the transfer of conservation farming technologies into areas with contrasting environmental, climatic and soil conditions.

Table 1. Agronomic zones for Gilliam, Morrow, Sherman, Umatilla and Wasco counties, eastern Oregon.