Reduction of soil tare by improved uprooting of sugar beet : a soil dynamic approach

The relative amount of soil in sugar beet lots, called soil tare, should be reduced to curtail the cost and negative aspects of soil tare. Highest soil tare occurs in beet lots harvested out of wet clay soil. The main problem is that commonly-used share lifters press the soil against the beet. Thereafter, the wet clay soil adheres strongly to the beet and is difficult to be removed. The objective of the research was to analyse and improve the uprooting process of sugar beet, in order to reduce soil tare during harvest on wet clay soil.A new characteristic, the relative soil adherence ( RSA ; 100% = all soil adheres strongly) was introduced to quantify soil adherence. The adhering-soil tare and RSA following various experimental beet extraction methods and lifting with a driven rotary-shoe lifter, were compared with conventional share lifting in field experiments, using stand-alone lifters on wet clay soil. Conventional lifting resulted in 50% ( w/w net; i.e. relative to the clean beet mass) adhering-soil tare and an RSA of 32%. Quick, small-pitch-spiral extraction, however, resulted in 8% adhering-soil tare and an RSA of 40%. The driven rotary-shoe lifter resulted in 13% adhering-soil tare and an RSA of 47%. The RSA turned out to increase naturally with decreasing adhering-soil tare. When compared at the same level of adhering-soil tare, the RSA after beet extraction was significantly lower and the RSA after shoe lifting was about equal to the RSA after conventional lifting.To provide theoretical foundation for the observed effects, the soil-beet-lifter system was modelled and the initial stage of uprooting was simulated, using PLAXIS, a geotechnical computer programme. Characteristics of the root system and of the uprooting method had a prominent effect on the stress state in the soil around the beet, and on the resulting zone of initial soil failure. The simulated behaviour of soil agreed well with effects observed in the field experiments, provided that reinforcement of the soil by rootlets was taken into account.Based on the results of this research, it is estimated that complete harvesting systems with common beet cleaning facilities, on wet clay soil, may at best reach 7 to 15% soil tare by using further-improved conventional beet lifters and 3 to 6% soil tare by using beet lifters that would induce beet rotation at the initial stage of lifting.