Is Fall the Best Time for Soil Sampling?

Many growers take soil samples in the fall after their crops are off. It’s a good time to sample for several reasons. In fall, it’s convenient to drive the fields and pull samples when there isn’t anything growing to get in the way, and soils tend to be drier. Fall also tends to be less busy than spring, giving more time for sampling. Knowing your soil nutrient levels in fall gives you time to put on a fall soil amendment and plan your spring fertilizer application. These are all good management reasons for soil sampling in fall, but are there scientific reasons why we should pull soil samples in the fall rather than the spring?

Seasonality of Soil Characteristics

The fact is that there are fluctuations in soil nutrient levels throughout the year, but like so many things in soils, there is no one exact answer for what those fluctuations are. Nutrient levels and seasonal fluctuations are all impacted by soil type, soil health and what inputs are applied to the soil. In general, soil P and K levels tend to all be higher in the spring and lower in the summer as crops are growing and removing nutrients. Soil pH also tends to be lower during the peak of the growing season, especially if commercial nitrogen is applied in spring. Soil pH levels will drop as that nitrogen converts to nitrates, and also as plants grow and put out acidic compounds through their roots.

P and K levels then tend to come back up again in the fall, though not necessarily to the same levels they were at in the spring. How much of the nutrients were removed and how much becomes available again in fall depends upon what crop was grown, the amount of residues left on the soil, as well as moisture levels and biological activity in the soil. Soil pH levels will also tend to come back up in the fall. As crops are removed, there are fewer root exudates, and this combined with slowing of soil biological activity, microbial respiration and nitrification all contribute to the pH rise. But these fluctuations in pH and mineral levels aren’t easily predictable, and vary quite a bit depending on the soil’s organic matter level, biological activity, if a cover crop is present and overall soil health.

Given seasonal fluctuations in pH and nutrient levels, the best strategy is to sample at the same time each year. Ideally, sampling would also be done at about the same soil moisture level, but that isn’t always possible. In order to control as many variables as possible and to get the most consistent results, try to sample at the same time each year. Given the management advantages to sampling in fall, it makes sense to take your soil samples each year after your crops are off.

Soil Sampling Support

Midwestern BioAg is ready to help you with your soil sampling. We offer both grid and non-grid soil sampling services at all our locations. Grid samples are taken from 2.5-acre grids; non-grid samples, or composite samples, are taken from 5- to 20-acre parcels. Once the soil test results are in, we provide a complete program based on the soil samples we take. Growers who sample with us get a comprehensive look at what’s in their soil, help with interpreting data and access to variable rate technology (VRT) fertilizer recommendations.

Soil sampling is a good investment. We recommend that growers sample each field every three to five years. The return on investment from soil sampling can be significant. After taking that sample, you’ll be able to determine what nutrients you need, how much to apply and where to place that fertilizer. This is a more efficient way to spend your fertilizer dollar then purchasing a standard fertilizer and applying it throughout the field. With a soil test, you will be spending money on nutrients only where they are needed, applying the correct amounts and increasing the chances of achieving higher yields, better soil mineral balance and increased soil health.