Agriculture Reference
In-Depth Information
Figure 3.1. Relationship between primary samples and composite, submitted and working samples (diagram
by Sabry Elias).
THE SAMPLInG ProCESS
The importance of obtaining a sample that faithfully represents the quality of the seed lot being tested has
already been emphasized. Proper and representative sampling is only possible if the sampled seed lot is
sufi ciently uniform. A seed lot is never completely uniform. Variations in the i eld from which seed is har-
vested, as well as post-harvest operations contribute to seed lot heterogeneity. However, a seed lot must be
sufi ciently uniform if samples are to represent overall lot quality. Samples drawn from heterogeneous seed
lots do not represent the true quality of the lot; therefore, heterogeneous seed lots should not be sampled. In
cases where heterogeneity is observed or suspected, the sampler should i rst determine whether the extent
of heterogeneity is within acceptable limits. ISTA's H-value test is an objective method for determining the
degree of seed lot heterogeneity, and provides acceptable ranges for different sample sizes and seed types.
In the H-value test, the observed variation is compared to acceptable variation for either purity or germina-
tion tests. Seed lots with calculated H-values below a theoretical maximum allowable H-value are deemed
