IRRIGATION SCHEDULING FOR SWEET PEPPER - Management, Performance, and Applications of Micro Irrigation Systems - page 172

14.1 INTRODUCTION
A study was conducted to evaluate the influence of agricultural lime (CaCO 3 ) on the
movement and uptake of inorganic nitrogen for a sweet pepper crop ( Capsicum an-
nuum ) grown on an Oxisol soil (Coto clay) in north-west Puerto Rico. The Coto clay
soil, which contains the 1:1 kaolinite mineral, has a low pH (4 to 4.5). The 1:1 type
clays are known to possess a net positive charge at low pH, resulting in the adsorp-
tion of negatively charged ions such as nitrate. From an environmental standpoint
this characteristic of the 1:1 clay is favorable, since nitrate leaching, a major cause
of groundwater pollution in many areas, is reduced relative to soils with net negative
charge. However, agricultural plants, such as sweet peppers, favor a higher soil pH
(approximately 6.5), which can be obtained by the application of agricultural lime.
This, however, may have the negative effect of increasing the potential for nitrate
leaching, as the net charge on the soil particles becomes negative with increasing pH.
This chapter describes the results of a nitrogen leaching analysis for two sweet
pepper crop seasons. The analysis was based on multiplying the daily percolation fl ux
through the soil profi le by the measured concentration of nitrogen below the root zone.
Irrigations were scheduled using the pan evaporation method for estimating crop water
requirements. No signifi cant difference in nitrogen leaching was observed for the lime
and no-lime treatments. This was attributed to the low nitrate retention capacity of this
soil, even at low pH. The average percent of nitrogen leached during the 1st and 2nd
season, relative to the amounts applied, were 26% and 15%, respectively. Leaching
events were associated with large rainstorms, suggesting that leaching of N would
have occurred regardless of the irrigation scheduling method used.
14.2 TECHNICALAPPROACH
14.2.1. EXPERIMENTAL SITE
Sweet pepper crops were planted at the UPR Experiment Station at Isabela in north-
west PR (Fig. 1) during March 2002, and January 2003. Harmsen et al. [1] provided a
detailed description of the experimental layout of the field site. The soil at the Isabela
Experiment Station belongs to the Coto series. It is a very fine kaolinitic, isohyperther-
mic Typic Eutrustox. These are very deep, well drained, moderately permeable soils
formed in sediments weathered from limestone. The available water capacity is mod-
erate, and the reaction is strongly acidic throughout the whole profile. Consistence is
slightly sticky and slightly plastic in the Oxic horizons. A strong, stable granular struc-
ture provides these soils with a very rapid drainage, despite their high clay content [2].
Average values of hydraulic properties published for the Coto clay soil near the study
area are as follows: air dry bulk density1.39 g/cm3, porosity 48%, field capacity 30%,
wilting point 23%, available water holding capacity (AWHC) 9% [12]. The AWHC
of this soil is low for clay. Typical values for clay are 15 to 20% [3]. A small value
of AWHC means that there is a greater potential for leaching since the soil moisture
content associated with the field capacity is more easily exceeded.