Survival studies with Rhizobium trifolii on seed of Trifolium incarnatum L. inoculated for aerial sowing

Abstract

Several methods of clover seed inoculation were compared on a field scale, sowings of Trifolium incarnatum L. being made on the surface of tussock grass hill country. A three-step method of inoculation was significantly superior in respect of numbers of Rhizobium trifolii found in the seedling rhizosphere, nodulation of seedlings, and growth of plants.

In studies conducted with this clover on commercial aerial sowings of seeds inoculated by the three-step method, 5 weeks elapsed before germinating rains fell.

The findings were :

(i) Under exceptionally dry conditions following aerial sowing, the inoculum and seed-coating material remained on the seeds; where the seeds were largely sheltered by grass tussocks the persistence of inoculum remained satisfactory (100,000 Rh. trifolii per seed) for 2 weeks and moderately satisfactory for a further 2 weeks.

(ii) Where there was no rain for some weeks after sowing but the coating materials and most of the inoculum were removed by dew and frost, Rh. Trifolii of the applied type built up considerably in the adjacent soils and persisted fairly well.

(iii) Small falls of rain after sowing, although inadequate for seed germination, appeared to be beneficial in accelerating the transfer of coating materials and inoculum to the soil, where considerable development of Rh. Trifolii populations occurred. These new populations were sufficiently maintained to give good colonization of rhizospheres and nodulation of seedlings which became established following germinating rains 5 weeks after sowing. In spite of this delay and the consequent low temperature conditions, the great majority of established clover plants had nodules promptly formed by the applied bacterial strain under all conditions studied.

Application of the three-step method of inoculation to this clover demonstrated that:

(i) A delay of some days between inoculation and sowing was permissible.

(ii) A large number of Rh. Trifolii and some pellet constituents were carried into the soil at places where seeds sown from the air were likely to germinate.

(iii) A persistent inoculum remained loosely attached to the seed coats.