The Comparative Efficiency of Free and Combined Nitrogen for the Nutrition of the Soybean¹

Abstract

R ECENT agronomic studies on pasture mixtures supplied various fertilizers indicate that leguminous plants are apparently unable to compete successfully with nonleguminous crops in the presence of an adequate supply of fixed available nitrogen. This fact might be interpreted to mean that the products of symbiotic nitrogen fixation are better suited for the development of leguminous plants than are the products elaborated from combined sources. On the other hand, it is well established that a supply of fixed nitrogen reduces nodule formation and nitrogen fixation which suggests that leguminous plants may prefer combined nitrogen and only resort to the process of fixation when there is no fixed nitrogen available. The question is thus raised, "Is free or combined nitrogen the better source of nitrogen for leguminous plants?"Results from experiments carried out at the Wisconsin Agricultural Experiment Station during the past three summers indicate that the answer·, in the case of the soybean plant at least, is a function of the carbohydrate-nitrogen relation in the plant. This relation may be varied in a number of ways, e. g., an unfavorable pH may at one time be the cause of a low carbohydrate-nitrogen relation, while low light intensity or low temperature may, at another time, be the cause of a similar relation; but independent of the method of change, a given type of plant with reference to its carbohydrate-nitrogen relation will give a definite response to the form of nitrogen supplied. EXPERIMENTAL PROCEDUREThree series of Manchu soybeans were used in each experiment as follows: r. Inoculated.-Plants were inoculated with a mixture of Wisconsin strains 9, I 7, and sos of Rh. japonicum, but no forms of combined nitrogen were added. Nodule development was that characteristic of a good strain.2. High nitrogen.-P!ants were not inoculated, but fixed forms of nitrogen were added in excess. In the data to be discussed the fixed nitrogen was added as ammonium nitrate. In similar experiments in the previous summer, calcium nitrate was used with essentially the same results, but since the range of observation was more limited and the data incomplete it has been omitted in this consideration. In all cases the plants were periodically supplied with all the nitrogen they could assimilate--equivalent to ISO to 200 pounds per acre, or even more in some cases. No nodules were present.3· Low nitrogen.-Plants were not inoculated but furnished with ammonium nitrate in a quantity more comparable to that used in actual practice--7,1 to 'Contribution from the

Nitrogen Nutrition of Field‐grown Soybean Plants: I. Seasonal Variations in Soil Nitrogen and Nitrogen Composition of Stem Exudate1

“…T HE soybean (Glycine max L.) plant, like other legumes, has available two options in obtaining sufficient N for growth and reproduction: N fixation or absorption and assimilation of inorganic N. The extent to which each of these processes contributes to the growth and productivity of soybeans has been studied by various methods for several years (10,12,17). Experiments involving different levels of 1 5Ntagged inorganic N (12) showed that, as inorganic N concentration was increased, total N assimilated increased but the proportion assimilated via fixation decreased.…”

Produção, absorção e extração de nutrientes por linhagem de soja que nodula e que não nodula. I. Efeito do nitrogênio mineral e da inoculação

“…A soja pode rapidamente utilizar tanto o N simbiótico quanto o disponível no solo (2,6,8,11,13), mas a quantidade de N simbiótico produzida é inversamente proporcional à quantidade de N disponível (1,2,13). Quando suprido em quantidades que excedam as necessidades para o crescimento da planta, o N pode impedir a fixação simbiótica (2,12).…”