Fallow versus wheat cropping of unamended and manure-amended soils relatedto mycorrhizal colonization, yield, and plant nutrition of dry bean and sweet corn

Previous studies reported manure application to eroded Portneuf silt loam s oil (Coarse-silty, mixed, superactive, Durinodic Xeric Haplocalcid) improve d dry bean (Phaseolus vulgaris L., cv. Viva) yield to levels of topsoil. Th ese yield increases only correlated with whole-plant zinc (Zn) concentratio n and soil organic matter. This might be related to enhanced arbuscular myc orrhizal (AM) colonization stimulated by manure application. A greenhouse s tudy with dry bean suggested a relationship between manure application, inc reased AM colonization, and whole-plant Zn uptake, while field studies with wheat (Triticum aestivum L.) and sweet corn (Zea mays L.) did not. To clar ify the apparent contradiction of manure application on AM relationships, t he present field study with dry bean and sweet corn was conducted in subsoi ls on the same experimental site established in 1991 and used in previous s tudies. The existing rotation also allowed the study of the effects of prev iously fallowed versus wheat cropped subsoils on yield, AM colonization and nutrition of dry bean and sweet corn. Average mycorrhizal root colonizatio n in dry bean was greater on unamended than on manure-amended soils but was not related to increases in yield, Zn concentration. or Zn uptake. Average colonization of sweet corn roots was generally greater in unamended than m anure-amended soils, but yields were greater in manure-amended soils. Colon ization of sweet corn roots measured over time was consistently greater in subsoils previously cropped to wheat than fallowed, but yields were similar . Previous wheat-cropping resulted in leveling off of colonization beginnin g 7 July (second sampling) in dry bean, while previous fallow resulted in c ontinuously increasing colonization throughout the five sampling periods. B ean yields were greater on subsoils previously cropped than fallowed; thus yields were generally not related to AM colonization. Results of our study confirm other field results where AM colonization was greater in unamended than manure-amended soils and in cropped than in fallowed soils. Any yield increases observed were not closely related to AM colonization.