Effects of agricultural machinery traffic on pea growth and penetration resistance in the subsoil

This thesis is based on studies of the effects of machinery-induced soil compaction on soil properties and crop growth in the Province of Halland in southwestern Sweden. It is focused on interacting effects of soil compaction, high water content and Aphanomyces root rot on the growth and nutrient uptake of peas, as well as on the effects of normal agricultural machinery traffic on the penetration resistance in the subsoil. Most studies aimed at determining the extent to which soil compaction caused premature yellowing in pea fields under wet soil conditions and the degree of interaction between compaction and Aphanomyces root rot. Soil compaction resulted in inadequate drainage and, in combination with high precipitation, led to anoxia in the soil. This in turn reduced N-fixation, led to N-deficiency for the crop and reduced the uptake of most other plant nutrients. It also led to soil conditions conducive to the evolution of toxic substances, to physiological disturbances in crop growth, and, possibly to increased nitrate leaching and denitrification. The combination of soil compaction, high soil water content and Aphanomyces infection resulted in dramatically reduced growth, sometimes in a total loss of yield. Penetration resistance measurements were carried out in normal agricultural fields to determine how subsoil is affected by the use of heavy machinery in practical farming. In one investigated region (Halland) with potato and sugarbeet production, machines with high axle loads are frequently used. At 40 cm depth, penetration resistance was about 40% higher in the fields than in adjacent, untrafficked control sites. In another region (Uppsala), where no potatoes and sugarbeets had been cultivated, the corrensponding increase was only 10%. A comparison of these values with data from previous field experiments indicated that heavy machinery traffic caused a permanent average yield decrese of approx. 6% in Halland and 2% in Uppsala. These results warrant serious concern since the yield decreases, in probability, will become more severe, because of still increasing weights of agricultural vehicles. Research assessing cable tillage of a modernized type would be of utmost interest in order to prevent over-compaction of topsoil and subsoil.