The short answer is, "When you can get a soil probe in the ground." There may be other things to consider, but if you are farming "new" land, that may be good enough.
We all know that good representative soil samples give us the opportunity to make fertilizer and liming decisions based on technical, scientifically obtained data. How do we assure that data is valid enough that we do not short change ourselves on fertilizer and lime?
- Pull soil samples in the same season every year
We usually start in Mid-March and work until crops are too tall in Mid-June. In fall we start as soon as complete fields are harvested and continue until Thanksgiving or shortly thereafter. We try to do the same customers at the same time of year, year after year. Our first customer in the spring is always our first customer in the spring. Producers tend to have similar timing in operations year after year. It is surprising that year after year, we tend to sample the same fields on nearly the same date. (yes I track that)
- Sample under similar conditions every year
This pretty much follows the seasonal reasoning. Soil test results can vary depending on soil moisture and temperature, so we are likely to see results following a predictable pattern from year to year.
We are able to cover more acreage in the spring-summer sampling season than in the fall. For the most part we can cover the land we want to cover better in the spring. Individuals doing there own sampling can time sampling on their own. In the fall, we are captives of the speed of harvest.
The graph below reflects some of the variation we see because of moisture and temperature from year to year. It gives you a good idea of why we sample yearly. If you do not have frequently collected data, you never know where you might be as far as whether the soil test is higher or lower than it "should" be.
While sampling at the same time of year is important, if it has been several years since you have sampled, and you want to switch seasons, it may be a good time to switch. Spring sampling has the added advantage that recommendations are available before harvest.
