Is maize root growth affected by pig slurry application on a tropical acid soil?

The effects of field application of liquid pig (Sus scrofa) slurry (PS) on the root system of maize (Zea mays L.) were assessed during two successive cropping seasons in a tropical environment. Root development was monitored on a plot treated with pig slurry, corresponding to a total nitrogen input of 265 kg N ha-1 the first year and 185 kg N ha-1 the second year, in comparison to that recorded in an unfertilized control plot (C plot). In both treatments, the root system was determined at four phenological stages by the trench-profile method and using a model that calculates root length density from counts of numbers of living roots. The results showed that PS had a positive effect on maize aboveground biomass until harvest. Its effect on root development was very positive during the first third of the cycle (+130% for the root length density), but this beneficial impact quickly vanished. At the grain maturation stage, the root length was 18% lower on the PS plot as compared to the C plot, despite the much higher shoot biomass (+50%). It was concluded that one factor began having an adverse effect on root growth in the surface soil layer during the maize growth cycle. This factor was not fully identified, but several elements suggest that it involved soil acidification associated with the nitrification of ammonium nitrogen in PS in this already relatively acid soil. This could lead to a rhizotoxicity problem