Root Distribution of Soybean Plants in No-tillage Fields in No-tillage Fields in Yguazú District of Paraguay

Abstract

Since the late 1980's, soybean yields in Paraguay had steadily increased with the expansion of no-tillage fields. The average yield has been maintained at about 3t/ha for nearly 10 years. However, the increase of yield has shown a limitation and recently the yield has even tended to decrease. Although the causes of this phenomenon have not been elucidated yet, it is considered that long-term no-tillage practice adversely affects the soil conditions, resulting in an inhibition of soybean root development. The characteristics of the soybean root distribution in 11 fields depending on the differences in the duration of application of the no-tillage practice, differences in the crops preceding the cultivation of soybean and soil types were examined. In the majority of the no-tillage fields, more than 50% of the soybean tap roots did not elongate vertically in the deep soil profile, and lateral roots in surface soil grew vigorously near the soil surface, which resulted in a concentrated root distribution only in the uppermost soil layers. On the other hand, soybean tap roots in the year immediately after reclamation and in plowed fields showed a relatively deeper penetration with improved vertical root distribution. The occurrence of soybean plants whose tap roots did not penetrate into deep soil layers increased with the soil hardness and phosphorus accumulation in the uppermost layers. These results suggest that in the no-tillage fields the adverse soil conditions associated with the increase of soil hardness and phosphorus accumulation in the uppermost soil layers inhibited soybean root development in the deeper soil layers, and resulted in the recent decrease of soybean yields in Yguazu.