The selective soil covering mechanism of weed harrows on sandy soil

Improvement of intra-row mechanical weed control is important to reduce the reliance on herbicides in arable crops and vegetables. Covering weeds by soil is an important weed control mechanism of weed harrows. A shallow post-emergence harrow cultivation controls weeds but also damages the crop to some extent. This paper explores how plants get covered by soil and how a plant’s resistance against being covered is related to its height, flexibility and shape of leaves. Seedlings of two contrasting species were sown in bins filled with a sandy soil and harrowed by a small model harrow in the laboratory. Covering selectivity (percentage covered ryegrass/percentage covered garden cress) could be influenced by soil moisture content, working depth and working speed. Differences in covering were related to spatial patterns of plant downward bending and soil surface level upheaval. These patterns are associated with soil failure patterns near tines and soil flow patterns, connected with different effects of plant height and plant flexibility. This study indicates that relationships between weed control and crop covering may not only depend on weed and crop characteristics but also on soil conditions and implement settings. As less than 10% of the covered plants were buried deeper than 15 mm, covering would mainly cause growth reduction and little killing. Limited burial depth may be an important cause for limited weed control effectiveness of harrowing.