Plants in nutrient‐deficient soils often benefit when colonized by vesicular‐arbuscular mycorrhizal fungi (VAMF). Plants vary, however, in responsiveness or mycorrhizal dependency (MD). The objective of this work was to evaluate MD within corn and soybean cultivars. Three improved and three unimproved corn (Zea mays L.) and three improved and two unimproved soybean [Glycine soja Siebold & Zucc. and Glycine max (L.) Merr.] cultivars were evaluated for growth response, nutrient uptake (N, P, K, Ca, Mg, and Zn), and root phosphatase activity. Greenhouse studies were conducted with VAMF (Gigaspora margarita or Glomus intraradices) and without inoculation in a Iow‐P soil. The VAMF colonization of roots with inoculation ranged from 62 to 87% for soybean and 49 to 68% for corn. Soybean had a higher MD than corn, but considerable variation occurred within soybean cultivars. Relative growth of the two unimproved soybean cultivars was significantly greater with VAMF colonization than without (Soja, >1900%; Mandarin, >400%). Among improved cultivars, relative growth was less enhanced with colonization (BSR 201, Richland, and Swift cultivars averaged ≈ 200% greater growth with VAMF than without VAMF colonization). An unimproved corn cultivar, Reid Yellow Dent, was unresponsive to mycorrhizal colonization, whereas another unimproved cultivar, Argentine Pop, increased growth 400% with colonization. Total uptakes of N, P, K, Ca, Mg, and Zn were significantly greater (P < 0.001) in mycorrhizal plants, but the concentrations (mg g−1) of N, Mg, and Ca were lower in mycorrhizal plants than in nonmycorrhizal plants. These results suggest that considerable variability exists in MD of corn and soybean cultivars when grown in P‐deficient soils, and the variability extends across both improved and unimproved cultivars.

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Mycorrhizae, a mutualistic association between fungi of the family Endogonaceae and higher plants, have been studied extensively in the greenhouse, but much less is known about the natural ecology of these fungal‐plant associations in field soils. A better understanding of the mycorrhizae of agronomic crops is needed because of their potential involvement in systems of sustainable agriculture. This study reports on the extent of root colonization by mycorrhizal fungi, the distribution of mycorrhizal fungal spores in the rhizosphere, and the mycorrhizal fungal genera associated with soybean [Glycine max (L.) Merr.] in 15 Iowa soils. Results indicated that soybean roots from most soils, in spite of some soils having very high soil‐test P levels, were extensively colonized (60–100%) by vesicular‐arbuscular mycorrhizal (VAM) fungi. Colonization was determined by evaluating the percentage of root segments with VAM fungi. The average colonization of plant roots from all soils was 89%, and the average Bray P1 soil‐test value was 60.3 mg kg−1 (values greater than 30.5 are considered by the Iowa State Soil Testing Laboratory to be very high). Abundant colonization was unexpected in these high‐P fertility soils because most authors report extensive colonization to occur mainly in plants growing in soils of low fertility. The VAM fungal spores were quite common in all rhizosphere soils sampled and ranged from 66 to 998 spores 100 g−1 soil. Total spore counts were significantly different among soil series (P < 0.001) and within soil series (P < 0.01). A negative correlation (P < 0.05) was found between soil organic matter, P, and VAM fungal colonization. For VAM fungal genera (Glomus, Gigaspora, Acaulospora, and Scutellospora) were found associated with soybean rhizosphere soil. Among these, Glomus was the most abundant.

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