Soil potassium effects influencing growth, nodulation, N₂ fixation, and ureide transformations of soybeans grown on a Typic Eutrustox

Abstract

Soybean (Glycine max (L.) Merril) production within Brazil is a major source of oil and meal exports for world markets. Vast underutilized areas within Brazil have favorable climates with potential for increased production. However, nonfavorable soil fertility factors and erosion losses are restraints for high yield soybean cropping within most neotropical regions. The objective of these studies was to determine influence of basic soil cations Ca, Mg and K with P amendment governing growth, nodulation, nitrogenase activity C₂H₂ red.), ureide and associated nodule components of Forrest cultivar soybeans grown on a Typic Eutrustox (dark red latosol) of Jaiba, Minas Gerais, Brazil.

Available soil P, K and Ca limited soybean growth, nodulation, ureide levels and nitrogenase activity at anthesis. Potassium levels necessary to maximize parameters associated with biological nitrogen fixation and assimilation into amino acids were higher than levels required for maximum soybean top growth. The interaction PCa was observed to strongly influence aspartate amino‐transterase (AST), glutamine synthetase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) activity. Magnesium fertilization, however, depressed most parameters measured in these studies.

Fertilization with a given element always increased the nodule cytosol concentration of that element, and influenced nitrogenase activity. Ca addition to the soil increased cytosol P and K, but diminished Mg concentration. Phosphorus fertilization, however, inhibited Ca uptake. The deleterious effect of Na on nitrogenase activity was ammeliorated by P and K amendment.

Highest ureide composition of xylem exudate occurred during the pod‐filling stage at 76–81 day age germination (DAG) with lowest levels at mature seed stage 91–96 DAG. In most instances, ureide components were present at higher levels for the fertilized treatments as compared to the check.

Key words:

• Rhizobium Japonicum (Kirchner) Buchanan
• symbiotic N₂ fixation
• Glycine max (L.) Merril
• nodule cytosol

Notes

J.F.M.Gomes, Coordinator, Soil Conservation Service, Brasilia DF Brazil. A.A.C. Purcino, Agronomist, Belo Horizonte MG Brazil. J.Q. Lynd, Professor, Soil Microbiology, Oklahoma State University, Stillwater, OK, 74078.