324

THE CONCEPT OF COMPACTION

K.H. HARTGE

University of Hannover, Institute of Soll Science, Herrenh~user Strasse 2, 3000 Hannover 21 (Fed. Rep. of Germany)

To judge the degree of compaction of tilled soil, the state of compaction of virgin soil should be evaluated. This state is characterized by the effect of permanently active forces of gravity and shearing properties of the soil. However, shearing parameters change with soil development, moisture status and previous moisture regime. Animal and root activity counteract compaction discontinuously. Therefore, the linear relationship "void ratio (VR)/log surcharge" observed in the laboratory with homogeneous samples, is not necessarily to be expected in virgin soil profiles. Evaluation of I00 profiles of soils from Europe, North America and North Asia show that down to about 100 cm depth in most cases the relationship is linear if the material is of the same geological origin and human activity is absent. The slope of the regression line is steepest in forest soils because here A-horizons frequently differ from the rest of the profile in that they have a steeper slope. Less steep slopes, on a higher VR-level with a high degree of linearity, are observed in chernosems and other soil types of the steppe zone. Human activities make regression lines bend downward on the left-hand side. Thus, the depth of the effect of all anthropogenic activity can be assessed. It frequently extends down to 50 cm depth. The linearity found here was never observed in soils where waterlogging occurred within the investigated depth of a profile.

EFFECT OF WHEEL TRAFFIC ON SOIL PHYSICAL PROPERTIES AND ALFALFA GROWTH

B.D. MEEK, L.M. CARTER, E.R. RECHEL, W.R. DETAR and R.H. GARBER

USDA-ARS, U.S. Cotton Research Station, 17053 N. Shafter Avenue, Shafter, CA 93263 (U.S.A.)

The objective of the study was to determine the influence of wheel compaction on soil properties and their effect on alfalfa growth. Four wheel traffic treatments were imposed on alfalfa during a 30-month period: I) none-treat = no compaction before planting and no traffic at harvest; 2) preplant-treat = full-wldth compaction before planting and no wheel traffic at harvest; 3) repeated-treat = full-width compaction before planting and at harvest, and 4) grower-treat = non-uniform compaction by wheel traffic before and after planting, similar to that done by a grower. The soil is a sandy loam which is easily compacted. A wide-tractlve frame which spans I0 m allows all cultural operations to be conducted without applying wheel traffic to the research plots (8 x 30 m). After 2 years it appeared that wheel traffic_~t harvest had increased soil bul~ density at the 15-cm depth_~o 1.80 Mg m or higher, compared to 1.59 Mg m-- (none-treat) and 1.67 Mg m - (preplant-treat). Water intake rates gradually increased with time and after 2 years the rates at 2 h were 2.4 (repea~ed-treat), 3.2 (grower-treat), 4.1 (none-treat) and 4.9 (preplant-treat) cm h- . In the second year, alfalfa yields were 31 (none[treat), 31 (preplant-treat), 27 (grower-treat) and 25 (repeated-treat) t ha- (15% moisture). The yield decreases in the plots which received harvest traffic were mainly the result of crown damage and not of soil compaction. Contrary to expectations, the alfalfa stands declined at a similar rate for all treatments.