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Improved Soybean Growth in Urea with pH Buffered by a Carboxy Resin¹

An improved nutrient culture technique has been devised for producing vigorous soybean [Glycine max (L.) Merr.] plants with urea as the only source of N. Of the sources of reduced-N tested, (urea, amides, and ammonium salts of citrate, malate, fumarate, succinate, and sulfate), urea caused the least amount of plant injury. However, seedlings grew less when cultured with urea than with nitrate. The rapid decline of solution pH was likely responsible for the limited plant growth with the reduced N sources. Adjusting the solution pH daily did not alter the injurious effects. Addition of the Ca form of a carbpxy resin (Amberlite IR.C-50, 50 g/liter) to the urea nutrient medium stabilized the pH and permitted plant growth equivalent to that of nitrate grown seedlings. In the presence of resin, plant growth with urea was superior to that with ammonium salts [e.g., (NH₁₂SO₄].

Growth of soybeans and wheat (Triticum aestivum L.)was equivalent with urea (in the presence of resin) and nitrate while growth of peas (Pisium sativum) and velvet leaf (Abutilon theophrasti) was less with urea than with nitrate. All species grown on urea contained low amounts of nitrate (about 10% of nitrate grown controls) indicating that some interconversion of urea to nitrate occurs, although no nitrate was detectable in the nutrient solutions. In vivo nitrate reductase activity was low [less than 0.21 /µmoles NO₂ (g fresh wt-hour)^–1] with all species while in vitro activity ranged from 68% (soybean) to 11% (peas) of the activity of respective nitrate-grown controls.

Key Words: Glycine max (L.) Merr. • Nutrient culture • Ammonium nutrition • Amberlite IRC-50 • Nitrate reductase • Nitrate

² Research associate, Dep. of Agronomy; Plant physiologist, ARS-USDA; and professor, Dep. of Agronomy, respectively, Univ. of Illinois, Urbana, IL 61801. Present address of E. Lahav: Division of Subtropical Horticulture, Agricultural Research Organization, The Volcani Center, P. O. B. 6, Bet-Dagan, Israel

Received for publication July 21, 1975.