USA
November 24, 2010
An expansion of soybean production into areas where soybean has seldom, if ever, been grown presents a steep learning curve for growers as well as both public and private researchers and university extension agents. This is especially true if the area of expansion includes soils having high pH values and large amounts of calcium and/or magnesium carbonate—soil conditions that promote iron deficiency.
Iron deficiency in the North Central U.S. is estimated to reduce soybean production by about 12.5 million bu each year. New areas of production also usually lack sufficient soil populations of native rhizobia (needed for nodule initiation and development) and may require one or two years of prior inoculation (wheat or soybean)to establish adequate numbers of effective rhizobia. Although controversial, the high N requirement of soybeans (approximately twice as much as wheat and 3.5 times as much as corn) is not completely satisfied by N2 fixation; soil or fertilizer N may be needed for maximum yields. The three legs holding up this maximum yield stool are (1) high populations of effective rhizobia, (2) some N from the soil (mineralization or fertilizer), and (3) the “correct” variety.
Adding N fertilizer to high-pH, highly calcareous soils almost guarantees that iron deficiency will be a serious concern, especially if a variety susceptible to iron deficiency chlorosis
(IDC) is grown. John Wiersma, a research scientist at the University of Minnesota Northwest Research and Outreach Center at Crookston, has studied the importance of including IDC resistance in the “correct” variety determination where producers anticipate adding N fertilizer to compensate for poor nodulation or seasonal rates of N mineralization are commonly high (60 to 100 lb ac–1 year–1). He reports his findings in the November–December 2010 issue of Agronomy Journal.
Six varieties (two iron-efficient, two moderately iron-efficient, and two iron-inefficient) and six rates of fertilizer N (0, 30, 60, 90, 120, and 150 lb ac–1) were evaluated during 2003,
2004, and 2005 on soils where soybean has historically exhibited mild to severe iron deficiency. Seed was inoculated at twice the recommended rate, and the amount of extractable iron in soils of the experimental areas was measured each year. Growing season temperature and rain also were recorded each year and compared with each other and with the 30-year average for Crookston.
Growing conditions in 2004 were colder and wetter than in either 2003 or 2005, whereas DTPA-extractable iron was more than twice as great in 2004. Extractable iron did not necessarily reflect available iron as relative chlorophyll concentrations (SPAD readings), seed number and weight, and grain yield were all notably less in 2004. According to Wiersma, this result highlights a common problem associated with IDC screening nurseries: iron deficiency and IDC scores vary with time, space, and environmental conditions. He says producers must keep this in mind when selecting next year’s soil and variety, as their experience may be different than that reported or published.
Somewhat in contrast, but as expected, nodulation decreased consistently and dramatically in response to added N for all varieties, regardless of their iron efficiency characterization
or yearly growing conditions. Wiersma says producers must keep this in mind when deciding whether or not to add N fertilizer: fertilizer N will decrease nodulation and the
contribution of N2 fixation to final yield. As important, do IDC resistant, moderately resistant, and susceptible varieties respond the same as N rate is increased? “Yes and no,” Wiersma says.
Nitrogen fertilizer decreased nodulation of all varieties, and similarly, it had little influence on SPAD values of any variety. Plant height, seed number, and grain yield all decreased linearly in response to increasing N rates for iron inefficient varieties, whereas these responses in iron efficient and moderately efficient varieties changed little as N rates increased. Wiersma says the take-home message is that additional N definitely should not be applied when iron-inefficient varieties are grown on chlorosis-prone soils, and there is little support for adding N to iron efficient or moderately efficient varieties.
Adapted from Wiersma, J.V. 2010. Nitrate-induced iron deficiency in soybean varieties with varying iron-stress responses. Agron. J. 102:1738–1744. View online at www.agronomy.org/publications/aj/tocs/102/6
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