Manganese toxicity in tropical forage legumes

Abstract

Two greenhouse experiments were carried out to study manganese toxicity in the establishment, nitrogen fixation and growth of six tropical forage legumes (Centrosema pubescens, Pueraria javanica, Glycine javanica var. SP-1, Glycine javanica var. Tinaroo, Stylozanthes gracilis, and Phaseolus atropurpureus, in two soils of Rio de Janeiro State. In the first experiment symptomatology and development were studied with the application of manganese sulphate at four levels (0, 50, 100 and 200 ppm Mn on soil basis). Germination of the seeds was not affected by these manganese levels. Twenty-two days after sowing however manganese toxicity was already pronounced. C. pubescens seemed to be the most tolerant showing at the same time fastest root growth. C.pubescens, P. javanica and Ph. atropurpureus at that time showed about 50% of plants nodulated while nodulation of the three other species was insignificant. Two months after planting these differences were still more pronounced. The interaction species x manganese was highly significant for nodule weight, plant weight, nitrogen content and total nitrogen in plants indicating varying tolerance of the species. Again C. pubescens, P. javanica and Ph. atropurpureus were the only species to develop abundant nodulation in this soil, 50 ppm of manganese reducing considerably nodulation and nitrogen fixation even of these species. The variety SP-1 of G. javanica prooved to be more tolerant to manganese toxicity than the variety Tinaroo. In a second experiment the effect of liming with and without phosphorus were studied on the two varieties of G. javanica in two manganese toxicity soils. Twenty days after sowing the affect of liming was already apparent on nodulation and plant growth, but root growth retarded. Liming reduced absorption of manganese by the pant but in one of the soils the application of superphosphate increased manganese absorption. Two months after planting liming together with the application of superphosphate increased 10 to 30 times nodulation, nitrogen fixation and forage production demonstrating high manganese toxicity besides phosphorus deficiency in the two soils. The variety SP-1 as in the first experiment was more tolerant to manganese toxicity, being specially its Rhizobium symbiosis less affected.