Salt thresholds for liquid manure applied to corn and soybean

Sprinkler application of liquid manure to growing crops is often a convenient, and agronomically acceptable, means of land application. However, manure salts can cause crop damage, and safe salt concentrations of liquid manure have not been determined. Research was conducted to better understand the relationship of salt concentration to phytotoxicity. Swine (Sus scrofa) manure from a below-barn storage facility was applied during the summer of 2003 to corn (Zea mays L.) and soybean [Glycine max L. (Merr.)] growing on a Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludoll) soil under field conditions. The manure had a mean electrical conductivity (EC) of 20.3 dS m(-1) and was diluted to EC levels of 11.7, 6.4, and 0.6 dS m(-1). The 20.3 dS m(-1) and the 6.4 dS m(-1) concentrations are typical of under-barn storage and anaerobic lagoons, respectively. The liquid manure was applied to crop canopies at a rate of 11.7 mm ha(-1) in three increments over a 10 min period at growth stages V7 or V14 for corn, and V3 or R1 for soybean. At the 20.3 dS m(-1) concentration, phytotoxic effects occurred for both crops, although the effects were minor for the corn with application at the V14 stage. At the 11.7 dS m(-1) concentration, liquid manure caused leaf necrosis, stunting, and reduced leaf area in soybean, while visible phytotoxic effects were much less severe in corn. Eighty-eight percent of the soybean plants were killed by the 20.3 dS m(-1) concentration when applied at the V3 stage. Soybean and corn yields were decreased for the V3 and V7 applications by 89% and 15%, respectively. With the later application of 20.3 dS m(-1) concentration, soybean yield was reduced by 45%, but corn yield was not reduced. The V3 application of solutions of 11.7 and 6.4 dS m(-1) caused an 8.5% decrease in soybean yields. Application of 11.7 dS m(-1) liquid manure at V7 reduced corn yield by 12% but increased yield by 10% when applied at V14. Application of 6.4 dS m(-1) liquid manure increased corn yield by 4% and 13% for the V7 and V14 applications, respectively, compared to the control. Under the conditions of this study, application of liquid manure of 6.4 dS m(-1) or less should be safe for all stages of soybean and corn, while 11.7 dS m(-1) should be safe for soybean after R1 and for corn after V7. Further research is needed to evaluate these results across several growing environments and to determine potential differences in plant damage threshold.