NUMBER OF SAMPLES REQUIRED FOR THE ESTIMATION OF RESIDUAL SOIL NITRATE-NITROGEN - A RISK-BASED ANALYSIS

A field experiment was conducted to determine the number of soil sampl es required to estimate the average residual soil nitrate (NO3-) in a given field under no-till and conventional tillage conditions. Four so il sampling devices (a 20.3-cm power earth auger, a 5.1-cm hand earth auger, a 3.2-cm soil probe, and a 1.9-cm soil probe) were used to coll ect soil samples from 35 locations each within the conventionally till ed and no-till fields. Soil samples were analyzed for soil water conte nts and NO3- concentrations in the soil for various depths. Simple gra phs and tables were constructed which could be used by farmers and oth er professionals for estimating the average residual soil NO3- content s at a given confidence level or with a certain degree of risk. The co nfidence interval was taken as the difference between the highest and lowest value of the quartile range of the observed data. The results o f this study indicated that the number of soil samples required to est imate the average residual soil NO3- contents increased as the degree of risk decreased. This study also indicated that the number of soil s amples required for making a reasonable estimate of the residual soil NO3- contents were greater for a no-till field compared with the conve ntional tillage field.