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Soil pH for the control treatment increased slightly from 6.5 to 6.8 over the course of the experiment. All urine treatments showed less of an increase than the control; in particular, treatment B had a much more acidic pH and the highest potential acidity (Table 3). The low pH of fresh urine (∼6 (Udert et al. 2006)) and exchange of urine cations for protons may have led to slight soil acidification (Bouman et al. 1995). This conclusion was corroborated by the decrease in exchangeable Ca2+ and Mg2+ for treatment B, which commonly decrease in more acidic soils (Bouman et al. 1995). Soil acidification and lower divalent cation concentrations support our theory of a critical urine application, above which, deleterious effects outweigh benefits. In addition, plants in treatments A and B showed yellow precipitate and fungi in the soil upper layer. Both of these phenomena could indicate over-fertilization with urine.

Table 3

Soil physicochemical properties before and after lettuce was fertilized with urine

ParameterBefore treatmentAfter treatment
A: 12,000 L/ha neat urineB: 1,500,000 L/ha diluted urineC: 20,000 L/ha neat urineD: control (no urine)
pH 6.5 6.5 5.6 6.7 6.8 
Organic matter (g dm−397 106 108 106 114 
B (mg dm−3– 0.73 1.27 0.61 0.72 
Cu (mg dm−3– 6.5 7.8 5.9 4.6 
Fe (mg dm−3– 47 53 63 52 
Mn (mg dm−3– 
Zn (mg dm−3– 2.6 2.6 2.1 
S (mg/dm−393 11 40 13 23 
CEC (mmol/dm−3141.3 143 133 137 122 
P (mg dm−378 55 68 54 49 
K (mmol dm−30.11 0.055 0.065 0.049 0.039 
Total N (mg kg−14,200 2,320 3,480 1,930 1,540 
N (NH4+) (mg/kg−14,500 – – – – 
N (NO3) (mg/kg−12,300 – – – – 
Ca (mmol dm−3) 86 95 73 92 83 
Mg (mmol dm−331 27 22 23 21 
Potential acidity (mmol dm−313 13 27 14 13 
ParameterBefore treatmentAfter treatment
A: 12,000 L/ha neat urineB: 1,500,000 L/ha diluted urineC: 20,000 L/ha neat urineD: control (no urine)
pH 6.5 6.5 5.6 6.7 6.8 
Organic matter (g dm−397 106 108 106 114 
B (mg dm−3– 0.73 1.27 0.61 0.72 
Cu (mg dm−3– 6.5 7.8 5.9 4.6 
Fe (mg dm−3– 47 53 63 52 
Mn (mg dm−3– 
Zn (mg dm−3– 2.6 2.6 2.1 
S (mg/dm−393 11 40 13 23 
CEC (mmol/dm−3141.3 143 133 137 122 
P (mg dm−378 55 68 54 49 
K (mmol dm−30.11 0.055 0.065 0.049 0.039 
Total N (mg kg−14,200 2,320 3,480 1,930 1,540 
N (NH4+) (mg/kg−14,500 – – – – 
N (NO3) (mg/kg−12,300 – – – – 
Ca (mmol dm−3) 86 95 73 92 83 
Mg (mmol dm−331 27 22 23 21 
Potential acidity (mmol dm−313 13 27 14 13 

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