Effect of Inoculum Rate on Competitive Nodulation of Glycine max L. Merrill. II. Field Studies¹

Abstract

To provide a scientific basis for predicting successful competitive nodulation of soybeans (Glycine max L. Merr.) by Rhizobium japonicum, the quantitative relationship between soil and inoculum rhizobia, in nodule formation, was determined in the field. Soybean seeds were planted at a rate of 1 seed/2.5 cm of row and were inoculated with up to 3.3 ✕ 1038 R. japonicum cells/2.5 cm of row. Field plots were grown on six soils in Iowa having a range of soil rhizobia populations of 1.2 ✕ 101 to 2.3 ✕ 105/g of soil. Numbers of nodules on tap and lateral roots of soybeans grown on soils containing fewer than 12 rhizobia/g were increased when inoculation rates exceeded 3.3 ✕ 104 cells/seed. Neither numbers of nodules nor total nodule mass was increased by inoculation of soybeans grown on soils containing more than 1 ✕ 103 rhizobia/g. When an inoculation rate of 3.3 ✕ 106 rhizobia/seed was used on soybeans grown in soils with fewer than 12 rhizobia/g and in soils with approximately 1 ✕ 103 rhizobia/g, the inoculum strain produced, respectively, 65% and 35% of the nodules. From our results it may be predicted that if the inoculum rhizobia are to form 50% or more of the nodules an inoculum rate of at least 1,000 times the soil population (per g soil) must be used. Commercial inoculants in the United States probably are not supplying adequate numbers of rhizobia for successful competitive nodulation of soybeans grown on land previously cropped to soybeans.

Inoculantes e fertilizantes foliares na soja em área de populações estabelecidas de Bradyrhizobium sob sistema plantio direto

“…A ausência de resposta em solos com população estabelecida confirma os resultados obtidos por Oliveira & Vidor (1984a,b) e por Weaver & Frederick (1974). Esses autores não verificaram aumento de nodulação em plantas de soja em solos com contagem de rizóbios superior a 10 3 células g -1 de solo.…”

Nitrogen nutrition of soybean in Brazil: Contributions of biological N2 fixation and N fertilizer to grain yield

“…Several reports worldwide show inconsistent responses to inoculant application in areas with established populations of rhizobia, and frequently attribute this to previously applied rhizobial strains, to the enrichment of indigenous populations due to prior cropping of legumes, or to a combination of both factors (Weaver and Frederick 1974;Singleton and Tavares 1986;Thies et al 1991aThies et al ,b, 1995. Indeed, in the United States, there are studies showing absence in the response of soybean to inoculation when the numbers of indigenous bradyrhizobia are as low as 10 to 20 cells g -1 of soil (Thies et al 1991a,b).…”

Eficiência da fixação biológica de N2 por estirpes de Bradyrhizobium na soja em plantio direto

“…Um dos principais fatores de limitação à resposta à inoculação reside na competição por sítios de infecção nodular que, normalmente, ocorre entre as estirpes utilizadas no inoculante e as estirpes existentes no solo (Weaver & Frederick, 1974;Oliveira & Vidor, 1984a,b). Neste estudo, constatouse, tanto na safra de 1994/95 como na de 1996/97, que a inoculação conseguiu incrementar, em geral significativamente, a ocupação dos nódulos pelas estirpes carregadas no inoculante, exceto pela SEMIA 5019 no tratamento SEMIA 587 + SEMIA 5019, nas duas safras, pela SEMIA 5079 em 1996/97 e pelas estirpes do sorogrupo SEMIA 566 (CPAC 40 e CPAC 42), em 1996/97 (Quadro 7).…”