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Potassium Boosts Soybean Isoflavones
News! Results of these and other studies have been published in the Journal of Agricultural and Food Chemistry, 29 May 2002, Volume 50(12):3501-3506. Can be obtained online by subscribers; others can request a reprint.
by Tom Bruulsema, PhD, PPI/PPIC Director. Posted 28 August 2000.
Functional food components in soybeans are thought to have important beneficial effects on human health. These components include isoflavones, such as genistein, daidzein and glycitein. They are thought to be responsible for the reduced rates of cancer, heart disease, menopausal symptoms, and osteoporosis observed in people who regularly consume soybeans and soy foods. There have even been preliminary observations of benefits in swine nutrition.
Isoflavone levels in soybeans vary by more than threefold depending on variety and growing conditions. Our objective was to determine whether soil potassium (K) fertility contributes to this variation.
At a site in Paris, Ontario we compared soybeans with and without muriate of potash fertilizer applied in bands 15” apart and about 3” deep. This trial was conducted in both 1998 and 1999. The soil test K (ammonium acetate extractable) in the top 6” was about 35 ppm.
Soybean yield and all three of the major isoflavone types responded positively to added K (Table 1). It is not known whether K plays specific role in isoflavone synthesis, but it may, since it is an important enzyme co-factor for many plant metabolic reactions. It is also possible that added K boosted isoflavones only because of the increased yield.
| Table 1. Band-applied potassium (K) fertilizer boosted soybean yield and isoflavone content in a field near Paris, Ontario. Mean of two years, 1998-99. |
K Fertilizer | Isoflavones (ppm) | Yield | Leaf K | Seed K |
(90 lb/A) | Genistein | Daidzein | Glycitin | Total | bu/A | % | % |
Banded K | 688 | 579 | 122 | 1389 | 37 | 2.1 | 1.7 |
No K | 537 | 499 | 109 | 1145 | 32 | 1.6 | 1.4 |
difference | 28% | 16% | 12% | 21% | 15% | 33% | 19% |
At another site in 1999, K also boosted yield and isoflavones (see Table 2). The soil at this site was much more fertile, with a soil test K above 120 ppm. It is possible that the heavier texture of this soil, combined with the no-till situation, reduced the availability of the soil K.
| Table 2. Potassium application increased isoflavones and yield in a no-till field near Lambton, Ontario in 1999. |
Potassium | Isoflavones | Protein | Yield |
application method | (ppm) | (%) | (bu/A) |
Spring banded K | 3074 | 40.3 | 49 |
Fall surface K | 2878 | 40.1 | 48 |
No K | 2535 | 41.0 | 41 |
Soybeans grown at various levels of fertility in eight Ontario field trials were analyzed for total isoflavone content. Two of these trials involved a K fertilizer variable, and the others included various levels of lime and other nutrients added to the soil. Sites ranged from low to very high (35 to 190 ppm) in soil test K, and medium to very high (13 to 60 ppm) in Olsen soil test phosphorus (P). Isoflavone levels were correlated to seed K concentrations, as shown in Figure 1.
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| Figure 1. Soybean isoflavone concentrations were positively related to potassium levels, but the relationship changed from 1998 to 1999. The growing season in 1998 was not favorable for high isoflavone concentrations. |
There was also substantial variability between different sites and varieties, but since each variety was grown at a separate site, the effect of location could not be separated from the effect of variety.
The positive correlation indicates that K could be one of the important factors controlling isoflavone levels. This may be an important factor to consider for producing specialty beans.
For more information on isoflavones, see these sites:
Soy Protein and Risk for Coronary Heart Disease
ISU Study Finds Soybean Component Builds Swine Muscles.
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