Iron, nitrogen and phosphorus interactions in two cultivars of soybeans grown in a calcareous soil

Abstract
Hawkeye (resistant to iron chlorosis) and PI54619–5–1 (susceptible to iron chlorosis) soybeans (Glycine max L.) were grown in calcareous Hacienda loam soil (fine‐loam, mixed, thermic Aquic Natrargid) for 26 days in a glasshouse. Objectives of the study included an evaluation of iron chlorosis as phosphorus and nitrogen were varied. The 2×2×2 factorial study with Fe, P and N (with and without each) provided the possibility for a look at three deficiencies simultaneously with some assessment of the use of the Colby (1968) technique for multiple stresses acting simultaneously. Iron addition (2 μg/g soil as FeEDDHA) overcame iron deficiency and increased yields. Nitrogen (200 μg N/g soil as NH4NO3) increased yields only when both iron and phosphorus were also applied with one cultivar (Hawkeye); nitrogen increased yield in combination with iron or with iron plus phosphorus with the second cultivar (PI54619–5–1). Phosphorus (100 μg P/g soil as CaH2PO4), on the average, had no effect on yields of either cultivar but, with both iron and nitrogen also added, it increased yields for both cultivars. With the PI54619–5–1 cultivar phosphorus increased yield also when iron but not nitrogen was added. Without added iron, nitrogen alone depressed plant yields to the plants especially with the PI54619–5–1 cultivar and nitrogen plus phosphorus without iron was even more depressive of yields. The effect of the nitrogen, however, was expressed as iron chlorosis. The experiment produced bonafide phosphorus‐induced iron chlorosis. Phosphorus alone was much less effective than both nitrogen and phosphorus in inducing the chlorosis expressed as yellow leaf chlorosis. Relative yields expressed by the Colby (1968) procedure indicated that with plants reasonably free of interactions, additive effects were obtained. For example, the nitrogen treatment with Hawkeye at 113% of the untreated and the iron‐treatment at 144% were essentially equal to the nitrogen plus iron treatment of 161% vs the 163% calculated (1.13 × 1.44 = 1.63). The iron plus phosphorus treatment at 141% and the iron plus nitrogen at 161% were close to the treatment with all three variables added with 213% for actual and 227% calculated (1.41 × 1.61 = 2.27). These effects have implications on the law of the minimum. Many interactions were expressed in the mineral analyses as well as in the yields. Some of these were related to dilution. Interactions on composition for mineral elements were present with one or more pairs of the added elements or for all three for phosphorus, iron, manganese, zinc, potassium, calcium, and some other elements. The PI54619–5–1 withmore iron chlorosis had more interactions than did the Hawkeye soybean.

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