The mineral elemental composition of soybean plants can vary considerably according to soil fertility and is affected by disequilibrium between nutrients in the soil. Under optimal conditions, however, the plants show a fairly uniform composition regardless of region. Carbon, hydrogen and oxygen from the air make up 90% of dry matter production. However, these cannot be assimilated unless the other major and minor elements are present in the soil in sufficient quantity. In decreasing order of importance, these essential elements are nitrogen, potassium, calcium, magnesium, phosphorus and sulphur. Per hectare, a soybean crop yielding 2.5 t seed removes about 125 kg nitrogen, 23 kg phosphorus, 101 kg potassium, 22 kg sulphur, 35 kg calcium, 19 kg magnesium, 192 g zinc, 866 g iron, 208 g manganese and 74 g copper from the soil (Pasricha and Tandon, 1989; Tandon, 1989).
The nutrient requirements of a crop vary according to soil and climatic conditions, cultivar, yield level, cropping system and management practices. Soybean can fix atmospheric nitrogen if the proper Bradyrhizobium bacteria are present in the soil or if the seed is properly inoculated. The plants start to fix substantial amounts of atmospheric nitrogen approximately 4 weeks after germination. Most estimates show that soybean derives between 25% and 75% of its nitrogen from fixation (Deibert et al., 1979).
Deficient, sufficient and high concentrations of nutrients for upper fully developed trifoliate of soybean prior to pod set have been compiled by Fageria et al. (1991) from Small and Ohlrogge (1973) and Rosolem (1980). The respective concentrations are 40, 45-55 and 56-70 g kg-1 for nitrogen, 1.5, 2.6-5 and 6-8 g kg-1 for phosphorus, 12.5, 17-25 and 26-28 g kg-1 for potassium, 2.0, 3.6-20 and 21-30 g kg-1 for calcium and 1.0, 2.6-10 and 11-15 g kg-1 for magnesium. Similarly deficient, sufficient and high concentrations, respectively, for micronutrients are 30, 51-350 and 351-500mg kg-1 for iron, 14, 21-100 and 101-250mg kg-1 for manganese, 10, 21-50 and 51-75 mg kg-1 for zinc, 10, 21-55 and 56-80 mg kg-1 for boron, 4, 10-30 and 31-50 mg kg-1 for copper and 0.4, 1-5 and 6-10 mg kg-1 for molybdenum. These data provide some guidelines for understanding the mineral requirements of the soybean crop.
Hanway and Weber (1971) studied the relative uptake of nitrogen, phosphorus and potassium by indeterminate soybean under field conditions. The total accumulation of nitrogen, phosphorus and potassium in the plants followed a pattern similar to that of dry matter accumulation. Accumulation was slow early in the growth stage but then became rapid, and nutrients accumulated at constant daily rates at between 54 and 100 days after emergence. Approximately 80% of the total accumulation of these nutrients occurred during the 46-day period from 54 to 100 days after emergence. The order of concentration of macronutrients in stems and leaves, with the highest first, is nitrogen, potassium, calcium, phosphorus, magnesium and sulphur, while the order of concentration in the seeds is nitrogen, potassium, phosphorus, sulphur, calcium and magnesium. When the mean nutrient uptake is compared with the mean nutrient removal in seeds, 80% of absorbed phosphorus, 78% of absorbed nitrogen and only 53% of absorbed potassium is removed with the seeds. The remaining phosphorus, nitrogen and potassium are returned to the soil as stems and leaves and are recycled when the soil organic matter is mineralized. Although micronutri-ents are required in smaller quantities than macronutrients, they are essential in soybean nutrition and removal in seeds as a percentage of nutrients in tops is also high. The order, in descending order of importance, is molybdenum, zinc, copper, chlorine, manganese, boron and iron.
Nitrogen is required in the greatest quantity of all plant nutrients absorbed from soil. It is present in all amino acids, which are the building blocks of protein, nucleic acids and chlorophyll (Jones et al., 1991). Soybean plants can use nitrogen released by mineralization, residual soil nitrogen, fertilizer nitrogen or atmospheric nitrogen, which is converted into a usable form in root nodules through a symbiotic relationship between Bradyrhizo-bium japonicum bacteria and the soybean plant. While the soil is the primary source of nitrogen for many crops, soybean obtains 65-85% of its needs through the symbiotic nitrogen fixation process. A high rate of nitrogen fertilizer suppresses nitrogen fixation and most specialists recommend either no fertilizer nitrogen or a modest application of 30-50 kg ha-1 either at sowing or just before flowering. Some researchers have noted a favourable effect of nitrogen applied at the time of sowing on nitrogen fixation, root nodule weight and activity (Eaglesham et al., 1983).
Phosphorus, although required in far lower quantities than either nitrogen or potassium, is critical to rapid growth and proper development. The most noteworthy functions of phosphorus in plants include energy storage and transfer, membrane function and genetic transfer (Marschner, 1995). Phosphorus is critical for root and plant canopy development and seed production. The phosphorus content of harvested soybean seed is approximately 0.50-0.58%. Therefore, the maintenance of available soil phosphorus requires at least this range to be returned to the soil each year. A minimal tissue phosphorus level at flowering of >0.31% has been suggested by Bell et al. (1995). A range of 0.26-0.50% for the most recently matured leaves prior to pod set is considered sufficient (Jones et al., 1991). Sammi Reddy et al. (1997) found 0.25-0.26% phosphorus in the third fully opened trifoliate leaf at 50% flowering stage to be the optimum concentration in the most commonly grown soybean variety (JS 335) in India.
The crop takes up about 125 kg ha-1 potassium. Potassium is particularly important in plant physiology - it is very mobile and is involved in the transport of assimilates, the activation of many enzymes, the water economy of the plant and photosynthesis. It favours the formation of nodules and hence nitrogen fixation. It improves disease and stress tolerance. It has a large effect on yield, increasing grain weight and protein content, although it slightly decreases the oil content.
Soybean requires a large amount of calcium, taking up 50-90 kg ha-1, but only 20% of this is removed in the grain. Calcium has a beneficial effect on nodulation, either directly or through improvement of soil pH; it is difficult to distinguish between its direct and indirect effects. The necessity of calcium for plant growth can easily be demonstrated by interrupting calcium supply to the roots. The growth rate is immediately reduced, and after some days the root tips become brown and gradually die (Mengel and Kirkby, 1987). Magnesium is the central element in the chlorophyll molecule and is a co-factor in the activation of many enzymes. It has multiple functions. It is needed for photosynthesis and CO2 assimilation is restricted when the calcium content falls too low. It plays an important part in plant symbiotic nitrogen fixation.
Sulphur is needed for the synthesis of certain amino acids and thus in the formation of proteins. It is involved in the formation of chlorophyll. Soybean plants use nearly as much sulphur as phosphorus or magnesium and the removal by seeds may be 27-66% of that absorbed by stems and leaves. The sulphur concentration of the third fully opened trifoliate leaf of soybean collected at 50% flowering has been found to range from 0.15% to
0.47% and showed positive relationship with Bray's percentage yield. Using scatter plots, a concentration of 0.34% sulphur has been established as the critical concentration (A.N. Ganeshamurthy, Bangalore, 2008, personal communication).
Micronutrients are absorbed in smaller quantities by the soybean plant than are nitrogen, phosphorus, potassium and, sometimes, calcium, magnesium and sulphur. Their role is equally as important, however, and deficiencies of micronutrients lead to severely depressed growth and yield. Zinc activates several enzymes and is involved in nitrogen metabolism in the plant and the formation of protein. Iron is an essential constituent of chlorophyll and necessary for respiration and photosynthesis. Manganese has important roles in the metabolic processes, such as activating enzymes, chlorophyll synthesis, photosynthesis and nitrate reduction. Copper plays an important role in chloroplast functioning and improving photosynthesis. Its deficiency can reduce growth and yield by reducing the rate of photosynthesis. Molybdenum is required for the activity of two important enzymes, nitrate reductase and nitrogenase, which are essential for nitrate reduction and atmospheric nitrogen fixation. It is also needed for the proper functioning of root nodules and nitrogen assimilation; deficiency in the field looks similar to a nitrogen deficiency. Boron is required in meristematic activity, and hence in the growth of shoot tips and roots and of the floral organs.
Cobalt is the other beneficial element required in addition to micronu-trients. It is essential for efficient nitrogen fixation. Cobalt has beneficial effects on nodule number and weight and on plant nitrogen content when it is supplemented through soil or foliar application.
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