Sandy Soil and Soil Compaction
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"oLCsS'^Y Sandy Soil and Soil Compaction ABSTRACT A brief review of soil and plant response to compac- Plant growth characteristics indicating soil compac- tion is presented. Management practices which mini- mize compaction and additional reference material are tion problems were observed on sandy, irrigated soils. listed. Soil bulk density, roots per cubic centimeter, and above-ground plant growth were measured. Compac- tion, as measured by bulk density, extended to more than 19 inches deep with maximum compaction occur- SOIL AND PLANT INFORMATION RELATING TO SOIL ring from 3 to 14 inches. Volume weights of 1.76 grams COMPACTION per cubic centimeter and higher occurred where ma- chinery had traveled. Root length in the compacted The Normal Soil soil in the 6- to 12-inch depth was only about one-third A soil favorable for plant growth consists of ap- that in loosened soil. Above-ground plant dry weight in proximately one-half solid mineral particles and one- the compacted areas also was only about one-third that half pores, voids, and cracks between the particles. of areas where soil was loosened by harvest equip- Pores, voids, and cracks are of irregular size and ment. Subsoiling has been the most practical method shape and are filled with air and water.'•A mixture of of loosening compacted soil. large and small pores is desirable. Large and small pores as used in this report are relative to the thick- INTRODUCTION ness of a root. For wheat, root thickness varies from 1/64- to 1/16-inch. Roots enter large pores and cracks Considerable acreage of native land adjacent to freely or with little mechanical resistance. Large pores the Columbia River recently has been brought into irri- allow rapid air movement and water infiltration, gated agricultural production. Most of the soil is sandy whereas small pores are better able to hold water textured and is classified in the Quincy, Adkins, Sage- against the pull of gravity. Sandy soils have larger and hill, Koehler, Burbank, or Winchester series. Progres- fewer pores than finer-textured soils; also, pores of sive agricultural operators on these soils must use a sand are rigid and not easily enlarged by roots, making high degree of mechanization. Large tillage, planting, penetration difficult. and harvesting implements are pulled by equally large, Soil mineral particles are incompressible. Change powerful tractors. Trucks capable of hauling heavy in soil volume during compaction is a change in vol- loads are used to remove the harvest. Pressures ex- ume of pores, voids, and cracks. Forces causing the erted on and below the soil surface by these machines compaction slide, roll, and pack the soil particles into are causing soil compaction. The trend toward heavier a smaller volume. and more powerful equipment is anticipated to con- Particle size distribution of the sandy soils along tinue, thus equal or even more soil compacting forces the Columbia River is nearly ideal for maximum com- can be expected. paction. These soils contain 60 to 85 percent sand, The information presented in this publication deals much of it fine sand, 10 to 25 percent silt, and less than with a soil management-compaction problem that is 5 percent clay. Pores formed by sand are large enough being encountered on Quincy sandy soils. The work to hold clay and small silt particles. Many of the larger was initiated when poor growth of winter wheat along pores, when rearranged and reduced in size during wheel track patterns left from potato digging opera- compaction, are filled with smaller particles. The re- tions indicated that adverse effects of compaction sulting soil has smaller and fewer pores, a higher bulk might exist. Specific data from the study site on vol- density (volume weight), and greater soil strength. All ume weight (bulk density) and winter wheat growth are these features are detrimental to good plant growth. reported. Soil can be compacted most easily when the mois- Authors: F. V. Pumphrey, professor of agronomy, Oregon ture content is near optimum for plant growth (field ca- State University, Pendleton; B. L. Klepper, plant physiologist, pacity). Many of the field operations necessary with ir- USDA, SEA, AR, Pendleton; R. W. Rickman, soil physicist, rigated agriculture on sandy land are performed when USDA, SEA, AR, Pendleton; D. C. Hane, research assistant, the soil is at or near field capacity and, therefore, most Oregon State University, Hermiston. susceptible to compaction. Circular of Information 687 October 1980 Agricultural Experiment Station Oregon State University, Corvallis Adverse Effects purposes, the volume weight of water is one g/cm3. Restricted depth of penetration of roots is the pri- Normal coarser-textured (sandy) soils have a slightly mary adverse effect of compaction on these sandy higher volume weight than medium- and fine-textured lands. The plants utilize less volume of soil for mois- soils. No definite volume weight designates a soil as ture and nutrient absorption. A reduced rate of water being ideal for rooting or being too dense for root infiltration, slower exchange of air between the atmos- penetration. Generally, root penetration difficulties phere and the soil, and slower air movement within the increase as the volume weight of a soil increases. One soil are other possible problems occurring from this method of measuring the effects of crops, tillage, and condition. implements on soil compaction is to determine changes Some of the adverse effects of soil compaction ob- in the volume weight of the soil. served with finer-textured soils are not apparent with Compaction increases soil strength (the ease with the sandy soils. Increased clod formation, which may which the soil can be penetrated). However, soil interfere with seedbed preparation, tillage, and har- strength increases rapidly as the moisture content of vesting in finer-textured soils, does not occur. Internal the soil decreases. A penetrometer measures pene- soil drainage is not so slow that water ponding or tration resistance and indicates soil strength. Root water logging occurs for days or even hours. Surface growth decreases as penetration resistance increases crusting is much less of a problem with sandy soils and stops when penetration resistance approaches than with finer-textured soils. 350 to 400 pounds per square inch. Causes Soil compaction considered in this report has de- TILLAGE EFFECTS ON VOLUME WEIGHT AND veloped from pressure exerted on the soil by imple- WINTER WHEAT GROWTH ments used in agricultural production. Wheels, whether Plant and soil samples were taken in December on tractors, planting and harvesting equipment, or from a winter wheat field showing uniformly spaced trucks, apply pressure to the soil surface. Tillage equip- streaks of distinct plant growth differences. The better- ment such as rototillers that are improperly designed looking wheat occurred where the soil had been or operated and discs exert considerable pressure on loosened by potato harvesters. The poor wheat was the soil below the surface. Wheel slippage and soil growing where tractor wheels had compacted the fur- vibration when under pressure compact soil. Compac- rows between potato rows, or where trucks hauling tion from pressure exerted on or near the soil surface potatoes during harvest had been driven. The poor- by equipment may extend as deep as 20 or more looking wheat was small, had few tillers, and appeared inches. deficient in nutrients. Some common sources of soil compaction are of Soil in this field is classified as Quincy fine sand. minor importance. Most of these soils do not have a Quincy soil is coarse textured and shows little change naturally formed compacted layer. Shrinking and swell- in texture, horizons, and color with depth. Darker, ing of clay particles does not contribute to the com- coarser sand may occur at 30 to 40 inches. The dry paction problem for two reasons: (1) These soils have a soil is loose, nonsticky, and nonplastic. These charac- low clay content—5 percent or less. (2) The clay par- teristics impart a single grain or a weak massive struc- ticles are of a type which exhibit little shrinking and ture if sufficiently compacted. Soil texture at this par- swelling. Extended livestock grazing when the soil is ticular location was more than 85 percent sand, less wet compacts these sandy soils to a depth of several than 10 percent silt, and less than 5 percent clay. inches, but not to the depth or intensity that results Soil samples for volume weight determinations from use of machines and trucks. were taken from non-farmed soil growing native vege- Natural Forces Reducing Compaction tation, from areas having "normal" wheat growth, and from areas having poor wheat growth. Root and plant Several natural-occurring forces are reported to samples were taken where wheat growth was "normal" reduce soil compaction. Freezing and thawing can re- and poor. duce the volume weight and the strength of com- pacted soil. The beneficial effects of freezing are usu- Volume Weight ally observed in climates where the annual depth of The volume weight of the surface few inches of the freezing is a foot or more. In this area of the Pacific native soil was 1.51 to 1.58 g/cm3 (Figure 1). Volume Northwest, soil freezing to this depth does not occur weight increased slightly with depth to the coarser annually. sand at 30 inches. The coarser sand had a volume A moderate amount of soil shrinking and swelling weight of 1.75 and higher. reduces compaction. As mentioned previously, local Volume weight of the soil loosened by the potato soils contain only a very small amount of clay parti- harvester gradually increased from 1.52 at the sur- cles that exhibit a capacity for shrinking and swelling. face to 1.60 at 7 inches (Figure 1).