Information relating to the microbiological implications of additional manure dressings being applied to growing crops is exceptionally scarce. The main concern appears to be that the additional manure may contain human pathogens. If manure is spread to land prior to planting/drilling, there is a chance that any pathogens present will decline in number over the growing season of the crop. However, once a crop is actively growing; there will be insufficient time for significant decline in the numbers of any human pathogens present. Although the decline of human pathogens in soil is well described, the consequences of pathogens being present in any additional manure dressings have never been determined. The FSA guidance for safe manure usage takes a sensible precautionary approach and advises against the practice.
Goss et al (2013) review the use of manures as crop fertilisers and specifically list good and bad practices to prevent the spread of microbiological contamination from the material. Goss and colleagues also advise against an application of manure to a growing crop. Goss et al also advise the good practice of having grass buffer strips as preventions agains the spread of microorganisms from manures.
One potential aspect of applying human pathogens to growing crops is the role of plant exudates (Brandl, 2006). Crops such as carrot can exude sugars and other nutrients into soils. Pathogenic bacteria can be attracted by these nutrients and thus become concentrated in the soil around and on the surface of root crops. The higher the numbers of human pathogens, the more likely it is that an infection will occur. Thus even small numbers of pathogens present in additional manure dressings can become a problem if they become concentrated by exudate chemotaxis.
Goss, M.J., Tubeileh, A. and Goorahoo, D. (2013) A review of the use of organic amendments and the risk to human health. In Advances in Agronomy, Vol 120 ed. Sparks, D.L. pp.275-379. (a book chapter that is not available electronically)