Soybean Root Development as Influenced by Compacted Layers in No-tillage Fields of Paraguay

Abstract

In Paraguay, soybean is widely grown without tillage in heavy clay soil called“Terra Roxa”. As a result of the longterm no-tillage practice, a compacted layer tends to develop at a 5-15cm depth in this soil, which impedes the root development of soybean plants. Little is known about the relationships between the root development and soil layer compacted in the Terra Roxa fields. An experiment was conducted in a field with 3 levels of soil hardness in the compacted layer, which were similar to those of farmers' fields under the long-term no-tillage practice. Different compacted soil layers were made by the passage of tractor wheels 0, 4 and 8 times after rotary tillage at a 15cm depth. At the time of emergence, the tap root length was 9.0cm in the absence of tractor traffic, while the length was 2.9 and 1.6cm by passage of the tractor 4 and 8 times, respectively. In both tractor passage treatments, the soil hardness of the compacted layer induced bends and thickness of the tap root tips of many soybean plants at emergence and resulted in shallow root distribution in the latter growth period. Within a few fine days after rainfall, the soil hardness of the compacted layers increased rapidly and reached a high level resulting in the impairment of tap root elongation. These results indicate that the high degree of soil hardness during a short period of time centering on the period of emergence impeded tap root elongation, and that the shallow poorly developed root system did not improve in the latter growth period. It is suggested that the recent decrease of soybean yield recorded in 1998 and 1999 associated high drought was due to soil compactness in the no-tillage fields.