In Chapter 5 of The Chrysalids, what is the mark or measure of a good season among Waknuk farmers?

In Waknuk, in John Wyndham's novel The Chrysalids, a good growing season among the farmers is one in which the ideal of purity is upheld. After the holocaust that destroyed as it had been, Waknuk became pledged to protecting purity without blemish, deviation or mutation. For example, corn that showed deviation in the alignment of rows of kernels would result in whole corn-bearing fields being burned down to expunge the mutations, blemishes and deviations from Waknuk.

Therefore, a good growing season is one in which the Purity Record is upheld and few or no fields have to be burned down. In Chapter 5, Waknuk was celebrating because the growing season had been relatively free of deviation and, in fact, they had broken the standing Purity Record thus setting a new higher Purity Record to strive to match in future (which will result in a greater proportion of deviation purges).