Effect of Liming and Fertigation Frequences on Nitrogen Retention on Coto Clay

Nitrate leaching through the soil profile can result in a significant loss of fertilizer N, and may be a source of nitrate concentration in groundwater. Highly weathered soils may exhibit substantial anion exchange capacity (AEC), which retards the movement of anions such as NO3" through the profile. However, AEC decreases as soil pH increases with agronomic practices such as liming. A study was conducted to determine the effect of lime application and frequency of Ν fertigation on NH4+ and NO3" concentrations on Coto clay. A field experiment was established at the UPR Experiment Station at Isabela, PR, using peppers as the test crop. Two lime treatments (lime or no lime) and two fertigation frequencies (weekly or bi-weekly applications) were evaluated. Soil samples were collected every two weeks at four depths and analyzed for NH4+ and NO3" by steam distillation. During the first year, probably due to spatial variabil ity and buffering capacity of the Coto clay, the limed plots did not reach the expected pH level. Consequently, no significant differences were observed between lime treatments. No significant differences between fertigation treatments were observed either. During the second year, lime application was doubled, and pH in the limed plots increased from an average of 4.6 to approximately 6.4. Nitrogen analyses of soil and plant material from the second year is currently under way. Results are also presented from laboratory studies conducted to evaluate NO3" adsorption characteristics of Coto clay. Adsorption isotherms were conducted under three pH levels and nine NO3" solution concentrations.