TILLAGE SLOWS FECAL BACTERIA INFILTRATION THROUGH SOIL 
by
M.S. Coyne, S.W. McMurry, and E. Perfect

INTRODUCTION

     Bacterial pathogens can degrade ground water quality by infiltrating 
and eroding from land treated with poultry wastes. The potential for ground 
water contamination (as well as associated health risks and cost of water 
treatment) greatly depends on the depth of soil to the water table or 
bedrock and soil structure. Pathogens must move through the soil profile 
to contaminate ground water (although sinkholes can provide a direct channel 
from the soil surface to the water table in karst areas). Deep soils have 
less potential for contamination than shallow soils. Structureless soils 
retain fecal bacteria better than well structured soils. Research at UK 
indicates that surface-applied fecal bacteria, and other contaminants, 
travel rapidly toward ground water through soil pores in well structured, 
intact soil. Tillage disrupts pores and channels in the tilled layer, and 
increases water and bacteria contact with soil. To improve our understanding 
of bacterial movement, and of the potential for ground water contamination, 
we decided to examine whether tillage affected fecal coliform transport 
through intact soil amended with poultry wastes. We used poultry wastes 
because their disposal is an increasingly important waste management issue 
in western Kentucky. 

METHODS

    We evenly distributed undercage poultry manure from layer production 
houses over the surface of intact soil blocks (13 inches per side) to 
approximate a 5 ton/acre field application rate (about 0.2 lb. of wet 
manure/block). The soil blocks came from a Maury silt loam in Lexington 
and were either sod-covered or removed from a field that was chisel plowed 
to a depth of 5 inches and disked (the soil was too dry for deeper tillage).
The total fecal coliforms added to the top of each block ranged from 100 
million to almost 5 billion. We looked for fecal coliforms because they are 
indicator bacteria used for water quality standards in Kentucky. A  
laboratory rainfall applicator simulated rain at  0.4 inch/hour for 36 
hours, during which we sampled the leachate below the soil blocks every 
4 hours.

RESULTS

     Most of the poultry waste added to soil is broiler litter which is 
drier, less compact, and has 100 to 1000 times fewer fecal coliforms by 
weight than laying house manure. However, fecal coliforms should behave 
the same once leaching begins, regardless of whether they come from litter 
or manure. Our results should be applicable to litter as well as manure 
application to soil.
     Ten percent of the area beneath the soil blocks accounted for between 
63 and 100 % of the total drainage. This means that preferential flow 
through intact soil pores let water bypass much of the soil  mass that 
would otherwise filter pathogens and chemicals. Water movement and fecal 
coliform transport were significantly correlated. Even though we applied 
fecal coliforms uniformly to the top of the soil blocks, they leached from 
just a few areas at the bottom. On average, 10% of the area beneath the 
soil blocks accounted for 94% of the total fecal coliforms that leached 
(the range was from 77 to 100 %).
     Figure 1 shows the fecal coliform leaching patterns for representative 
sod-covered and tilled soil blocks. There were 18 million fecal coliforms/100 
ml in the drainage we collected from the sod-covered block after 4 hours 
(Figure 1).  The standard for fecal coliforms in primary contact water 
(bathing and swimming water) is only 200 CFU/100 ml. Fecal coliform 
concentrations in the three tilled blocks were similarly above water 
quality standards in the first drainage we collected.
     The fecal coliforms leached from the waste in a pulse; the more 
numerous the fecal coliforms applied, the higher the maximum concentration 
in the pulse. Figure 1 shows that the maximum fecal coliform concentrations 
appeared after about 3.1 acre-inches of rain fell on the sod-covered soil 
block. In the tilled soil block, fecal coliforms were held up in the soil 
profile, and the maximum concentration of fecal coliforms in drainage didn't 
appear until 6.2 acre-inches of rain fell. The delay was probably because 
tillage disrupted preferential flow paths in the upper 5 inches of soil. 
On average, maximum fecal coliform leaching occurred after about 2.4 
acre-inches of rain fell on sod-covered soil blocks and not until about 6 
acre-inches of rain fell on tilled soil blocks. Although tillage did not 
hinder the flow of fecal coliforms sufficiently to meet water quality 
standards for primary contact water, our results are encouraging and 
suggest that more extensive tillage does retard their movement.
     Rainfall delivering 1.6 acre-inches of water (the amount of rain 
applied in 4 hours) was sufficient to drive bacteria-contaminated water 
to a depth of at least 13 inches in the Maury soil, and presumably to as 
great a depth in similarly well drained, well structured soils. The 
potential exists for leaching to greater depths with the same rainfall 
because of preferential flow, but we can't extrapolate with confidence 
beyond the depths we sampled. Between 1990 and 1995, rain exceeded 1.6 
acre-inches 52 times (13 % of all measurable rains) at the Lexington site 
from which we removed the soil blocks. We previously showed that  fecal 
bacteria applied to a Maury soil can move at least 35 inches deep when 
less than 1.6 acre-inches of rain falls after application (Soil Science 
News and Views, Vol. 17, No. 4), so the potential for preferential flow 
to contaminate ground water with fecal bacteria at this site appears to 
be high. However, rain exceeding 3.1 acre-inches occurred only 8 times 
(2% of all measurable rains).
     Tilled soil reduces the potential for leaching the peak concentration 
of fecal coliforms from poultry manure. For the tilled soil block  in which 
fecal coliforms most rapidly leached, the maximum fecal coliform populations 
appeared after 4.6 acre-inches of water fell. That much rain only occurred 
twice in Lexington between 1990 and 1995. So, the potential for maximal 
fecal bacteria concentrations to leach through a tilled Maury soil in a 
single rain is probably negligible.

CONCLUSIONS

     Tillage made it unlikely that the maximum leachable fecal coliform 
concentrations in surface-applied poultry manure would contaminate ground 
water during a single rain. Nevertheless, fecal coliforms moved rapidly 
through soil in preferential water flow regardless of whether the soil 
was tilled or sod-covered. Their concentration in drainage water was 
thousands of times greater than water quality limits. This is a problem 
in well structured, shallow soils, if preferential flow reaches ground 
water, and inadequate dilution or treatment of ground water occurs before 
its use.


FIGURE LEGENDS

Figure 1. Fecal coliform leaching patterns for representative sod-covered 
and tilled soil blocks.