Root penetration through a high bulk density soil layer: differential response of a crop and weed species

Weed control remains one of the greatest problems in agro-ecological systems. An important factor controlling crop and weed competition for below ground resources is the presence of compacted soil layers or 'hard pans'. In a series of experiments, we investigated the ability of roots of soybean (Glycine max L.) and the weeds sicklepod (Senna obtusifolia L.) and Palmer amaranth (Amaranthus palmeri S. Wats) to penetrate through a compacted soil layer and acquire N from lower in the soil profile. Soil columns were constructed to simulate a compacted soil layer with different bulk densities similar to 8 em beneath the soil surface. Results indicated that roots of the two weed species penetrated high bulk density soil layers more effectively than those of four soybean lines. Root penetration was not related with growth rates among the species or soybean genotypes. Overall root and shoot growth of the weeds was sustained when downward root growth was inhibited, while both declined with soybean, even under high fertility conditions. The weeds also acquired relatively high amounts of N-15 - NO3- from buried patches beneath the high bulk density layers compared to soybean. The results indicate that the weed species would have a competitive advantage when plow pans are present, and an attempt is made to relate this advantage with competitive dynamics observed in the field.