A MINIRHIZOTRON METHOD FOR MEASURING ROOT-SYSTEM OF SOYBEAN PLANTS GROWING IN THE FIELD

In order to develop a simple and accurate system for measuring the roo t system of soybean plants growing in the field, a fiber-optic scope w ith a portable color video tape recorder was employed in a minirhizotr on system. A transparent acrylic tube of 60 mm outer diameter and 54 m m inner diameter as the observation tube was installed into the soil a t an angle of 30 degrees from the vertical. The accuracy and tile util ity of the system were examined. (1) The length of all tap and branch roots at the observation tube-soil interface was measured with an accu racy of 5% using this system. (2) The observation tube did not signifi cantly disturb root penetration into the soil. This was also suggested by the results that the changes in root length with the increase of s oil depth showed a reasonable tendency without an unusual increase or decrease. (3) There was a significant linear correlation between the r oot length measured with the minirhizotron system and the root length density measured by a soil-core sampling method in the plants growing in the field. However, variation in tile measurements taken with the m inirhizotron system was as large as those by the sail-core sampling me thod, indicating that the root distribution in soil was extremely non- uniform. From these results, it was possible to estimate root growth i n the held well using this system. Moreover, taking as many measuremen ts as in the soil core sampling method, root distribution in the field can be estimated quantitatively with less labor and time with the min irhizotron system than the conventional method.