Clover rhizobia in soils: assessment of effectiveness using the plant infection count method

Abstract

Visual assessments were made of the effectiveness in nitrogen fixation of nodules formed on white clover plants inoculated with increasing dilutions of soil suspensions. Partially effective nodulation of plants inoculated with high dilutions was confirmed by tests on pure culture nodule isolates. However, in two cases partially effective rhizobia formed all or most of the nodules on plants incoulated with low dilutions of the soil suspensions, even though nodulation was assessed visually as being effective. Growth of plants inoculated with low dilutions was apparently stimulated by nutrients extracted from the soil which masked any effects attributable to the rhizobia nodulating the plants. The effectiveness of the nodule isolates was compared on white and suckling clovers.