The infectivity of ten commercial mycorrhizal inoculants was examined in nursery conditions. Corn plants were grown in a soil-based medium and in two different soilless substrates, a potting mix prepared with redwood bark, pine sawdust, calcined clay and sand, and the commercial Sunshine #5 mix, mainly composed of Canadian sphagnum peat moss. The percentage of mycorrhizal colonization obtained with the different mycorrhizal inoculants ranged from 0 to 50%. This variation might reflect the presence or absence of viable propagules, the difference in the recommended rates applied as well as the difference in the content and type of infective propagules of each product. However, the infectivity of each mycorrhizal inoculant was also influenced by the growing media. Two products promoted higher values of mycorrhizal colonization in the Sunshine mix, three in the nursery mix and one in the soil-based medium. Mycorrhizal colonization did not enhance plant growth. Only the plants inoculated with the products that did not promote mycorrhizal colonization increased their growth relative to the non-inoculated controls, suggesting the presence of other growth promoters in the inoculum products. Based on these results, nurseries should conduct preliminary tests to determine which inoculants will perform in their potting mixes to assure the best fit of inoculum with their particular conditions.

Our goal was to investigate the effects of mycorrhizal colonization on nitrogen (N) and phosphorus (P) leaching from plants grown in nursery containers. We compared the growth response and the content of nitrate (NO3), ammonium (NH4), and orthophosphate, in leachates collected from mycorrhizal (AM) and nonmycorrhizal (NonAM) plants of the fast-growing perennial, Encelia californica Nutt. (california sunflower), and the slow-growing woody shrub, Rhus integrifolia (Nutt.) Brewer & S. Watson (lemonade berry). Plants were grown for 8 weeks with no fertilizer or with 0.88 g (half rate) and 1.76 g (full rate) of 18N–2.6P–9.9K Osmocote (18-6-12, 6–7 month longevity at 26 °C, Osmocote® controlled-release fertilizer; Scotts Co., Marysville, OH). Mycorrhizal colonization increased the growth and nutrient uptake of E. californica and R. integrifolia but was more effective at decreasing nutrient leaching from containers with E. californica. Mycorrhizal colonization contributed to reduce the content of NO3, NH4, and orthophosphate by up to 65% in leachates from E. californica grown with half rate of Osmocote and up to 70% to 80% in those from plants grown in full rates of Osmocote. In contrast, only the leachates from AM plants of R. integrifolia grown without fertilizer had generally lower nutrient content than those from NonAM plants. Leachates collected from AM plants grown in half rates of Osmocote had less P but no less N, and there were mostly no significant differences in the leachate content of NO3, NH4, and orthophosphate from AM and NonAM plants of R. integrifolia grown in full rates of Osmocote. However, mycorrhizal colonization reduced the fertilizer requirement to achieve maximum growth in both species. AM plants of E. californica and R. integrifolia grown with half rates of Osmocote had greater dry weight than the NonAM ones grown in full rates of Osmocote. Our study shows that mycorrhizal colonization can reduce N and P leaching either by increasing nutrient uptake or by allowing the use of lower fertilizer rates.

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The effectiveness of several commercial mycorrhizal inoculants on the growth and development of Liquidambar styraciflua (sweetgum) was evaluated. Plants were grown in a nursery potting mix and were inoculated with the mycorrhizal products at the manufacturer's recommended rate. The growth response of mycorrhizal and nonmycorrhizal plants was analyzed at two harvests (8 and 14 weeks after transplanting). Significant differences were found in the growth of L. styraciflua to mycorrhizal colonization with the different commercial products. Fourteen weeks after transplanting, inoculation with products 1 (Earth Roots), 2 (MycoApply endo), and 3 (VAM 80) enhanced the growth of sweetgum relative to the nonmycorrhizal plants. However, plants inoculated with products 2 and 3 had greater leaf area, dry mass and relative growth rates than those inoculated with product 1. Plants of L. styraciflua inoculated with product 4 were less responsive to mycorrhizal colonization and only increased their leaf area relative to the non-inoculated controls. Testing both the infectivity and effectiveness of mycorrhizal fungi is recommended for the successful application of mycorrhizal technology in horticultural practices.

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