Yield, vitamin and mineral contents of organically and conventionally grown potatoes and sweet corn

An experiment was conducted for three years in a Pugwash sandy loam near Truro, NS. Five replicates of two treatments (organic and conventional) were established annually in different plot areas for potatoes (Solanum tuberosum L. Superior) and sweet corn (Zen mays L. var. saccharata 'Sunnyvee' or 'Pride and Joy'. The addition of pesticides, lime and NPK fertilizer to the conventional plots followed a soil test and provincial recommendations; lime, composted manure and insect control applications to the organic plots were according to the guidelines of the OCIA [Organic Crop Improvement Association, 1990. OCIA Certification Standards. OCIA, Bellefontaine, OH, 11 pp.] The compost was analysed for total N and applied to provide 260 kg N/ha for potatoes and 200 kg N/ha for sweet corn, which assumed 50% availability of the total N. Marketable yields were determined, and potato leaves and tubers, as well as sweet corn kernels and ear leaves were digested and analysed for 12 macro- and micronutrients. In addition, the vitamin C content of the tubers and the vitamin C and E contents of the kernels were analysed. Soil samples were also taken at harvest and analysed for essential nutrients and available N. Analysis of the three years of data showed that the yield and vitamin C content of the potatoes was not affected by treatments. However, the conventionally grown treatment outproduced the organically grown treatment for Pride and Joy (cv.) corn, but there was no difference between treatments in the yield of Sunnyvee (cv.) corn or the vitamin C or E contents of the kernels in any year. At p < 0.11, four elements in potato tubers (P, Mg, Na, Mn) and four elements in potato leaves (N, Mg, Fe, B) were influenced by treatments, but only leaf Cu was affected in the sweet corn. Correspondingly, extractable P, Ca, Mg and Cu were higher in organically fertilized potato plots; only extractable Mg was affected by treatments in the sweet corn, with the Mg content higher in the organic plots. Only leaf P and K were significantly positively correlated with extractable P (r = 0.70) and K (r = 0.73) in the potato plots, while leaf Cu and kernel S were positively correlated with extractable Cu (r = 0.56) and S (r = 0.62) in the sweet corn plots. (C) 1998 Elsevier Science B.V.