IRRIGATION SCHEDULING FOR SWEET PEPPER - Management, Performance, and Applications of Micro Irrigation Systems - page 180

lower than nitrate leaching (Table 3) except in the case of treatment NLF1 in 2002,
in which 21 kg /ha ammonium was leached as compared to 47 kg /ha nitrate. The fact
that no clear difference was observed between nitrogen leaching for the two lime treat-
ments is consistent with laboratory studies currently being conducted on the Coto clay
soil at the University of Puerto Rico Mayaguez Campus, which indicates that the pH at
which this soil will possess a net positive charge (<4) is below the native pH measured
in the fi eld (around 4.3).
14.4 METHODLIMITATIONS
There are several sources of uncertainty in the estimates of nitrogen leaching, which
include:
• Between sampling dates, soil nitrogen concentrations were derived by linear in-
terpolation. Nitrogen concentrations were measured every two weeks. In some
cases, the average nitrate concentration was observed to change as much as 15
mg/kg in the 60-80 cm depth interval. The estimated nitrogen leaching would
be in error if these concentrations did not change linearly between sampling
dates.
• The method of estimating percolation in this study does not account for the
leaching that can potentially occur by unsaturated flow. All leaching was as-
sumed to occur when the moisture content of the soil exceeded the soil field
capacity. However, significant downward gradients can exist which would re-
sult in unsaturated flow. Although not presented in this paper, continuous soil
pressure data obtained from vertically spaced tensiometers indicated downward
hydraulic gradients throughout most of the season.
14.5 SUMMARY
This chapter described the results of a nitrogen leaching analysis for two sweet pep-
per crop seasons. The study was conducted on an Oxisol soil in NW Puerto Rico. The
analysis was based on multiplying the daily percolation flux through the soil pro-
file by the measured concentration of nitrogen below the root zone. Irrigations were
scheduled using the pan evaporation method for estimating crop water requirements.
Estimated percolation in 2002 was three times greater than occurred in 2003, whereas
the nitrogen leached during 2002 was only slightly greater than two times the nitrogen
leached during 2003.
No clear difference in nitrogen leaching was observed for the lime and no-lime
treatments. This result is consistent with on-going studies of the Coto clay, which
indicate that this soil has little to no capacity to retain nitrate. The average percent of
nitrogen (nitrate plus ammonium) leached during the 1st and 2nd season, relative to
the amounts applied, were 26% and 15%, respectively. Leaching events were associ-
ated with large rainstorms, suggesting that leaching of N would have occurred regard-
less of the irrigation scheduling method used. During the fi rst and second seasons,
respectively, 34% and 60% of the total N lost by leaching occurred during a single day
(April 6 in 2002 and April 10 in 2003) when fl ooding was observed in the study areas.