SAMPLE-SIZE REQUIREMENTS TO EVALUATE SPORE GERMINATION INHIBITION BY COMPOST EXTRACTS

We examined the effect of compost sample size (50 to 5000 g) on the pr ecision of estimates of inhibition of Venturia inaequalis (Cke) Winter conidial germination induced by extracts of anaerobically-incubated c omposts. Composts were prepared from vegetable material; some were ame nded with manure. Variability due to sample size was considered as a f unction of extract efficacy (high, medium or low) and compost heteroge neity (high or low). Extract efficacy was characterized as ability to inhibit more than 75% of the conidia (high), 75 to 40% (medium), or fe wer than 40% (low). Heterogeneity was characterized as visual or tacti le presence (high) or absence (low) of parent material in the compost. To address the question of sample size-related variability we partiti oned individual extract incubations (samples) into aliquots (sub-sampl es), and aliquots into microtiter plate wells (sub-sub-samples). For t his nested design, the largest component of variation was consistently found to be that associated with wells; aliquots were a negligible so urce of variation. Sample size over the range examined was generally o f small importance for extracts of medium and high efficacy, but not l ow efficacy, independent of compost heterogeneity. For previously unte sted composts, particularly those of high heterogeneity, statistical a nalyses of our data suggest that samples of at least 500 g circumvent potentially large errors and consequent difficulties to detect differe nces among composts or effects of experimental variables. Copyright (C ) 1996 Elsevier Science Ltd