When is the best time to take a soil sample? The best answer that I have is to ask what you want to see from the soil. If you take it after harvest, the soil will be at its lowest levels because the plant have depleted nutrients. If you wait until spring, the levels will come back up and will be at their highest. The other thing you will notice is that the soil acidity will be lowest after harvest. This is because all throughout the growing season, the plants have been exuding acids into the soil. By spring, the soil will be less acidic.
If you are sampling every year or every other year, just remember to sample at the same time of year. I prefer to sample in the fall and sample every year. The changing numbers from spring to fall can give you some erroneous data and you might make some errors in application rates.