Soil Testing: Take Advantage in the Fall
 | Author:
Allison Kosto, MSU Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
Soil Testing: Take Advantage in the Fall
Allison Kosto
MSU Broadwater County Extension Agent
One of the more frequently asked questions in my office is, “what type of fertilizer should use?” Like so many things, it depends. There isn’t a “one size fits all” fertilizer. All soils are unique, and Broadwater County has a wide range of soil types. It’s important to match the fertilizer needs with the soil based on your soil texture, current nutrient status, plant community, yield goals, production type and budget. So if you ask me this question, I’ll probably respond with another question of “did you test your soil?”
A soil analysis identifies nutrient shortages and allows for the development of a fertilizer and amendment plan. Applying too much fertilizer or targeting the wrong nutrients can result in toxicities or enhance existing deficiencies. For example, gardens have a large variety of plants, and each one has different nutritional needs and sensitivities. Therefore, it’s important to avoid over-fertilization. It can also save you money from unnecessary fertilizer treatments!
Fall provides a good opportunity to collect soil samples for a few reasons. Time is an important factor. Spring always feels like a rush to make a decision to fertilize and get seeds in the ground. This is compounded by unpredictable and often uncooperative weather conditions. Fall allows for a longer window for collecting samples to match with ideal conditions. The soil should be moist, not too wet and not too dry. It also allows for a longer time to process the results and make decisions. This is especially valuable in agricultural fields where fertilizer is a costly input and requires careful evaluation.
For the most part, soil nutrients will not change significantly from spring to fall with the exception of nitrogen. Nitrogen is highly mobile in the soil so depending on the type of winter, crop residual and soil organic matter, it can fluctuate significantly. If nitrogen is a major focus and rationale behind sampling, then it might be necessary to wait or resample just nitrogen in the spring.
Collecting in the fall may also give you an additional window for application. Phosphorus and potassium can be successfully applied in the fall because they will not leach away with winter precipitation. Applying them in the fall allows these nutrients to be available immediately in the spring as plants come out of dormancy.
Nitrogen is generally not recommended in the fall because there is a larger risk of leaching over the winter. The exception to this rule is perennial cool-season pastures and lawns. If timed correctly, a late summer to early/mid fall application of nitrogen fertilizer can help with root development, winter survival and early spring green-up. Don’t wait too long in the fall, though or you won’t get enough benefit from the fertilizer to justify the cost. Also, avoid fertilizing if it is really dry or you will risk nitrate accumulation in the plant and less benefit on root and growth development.
A basic soil test usually costs $15 to $20 per sample. There are many different labs that will do soil testing. At the Extension Office, we can help you with this process if desired and will provide free recommendations when the test is returned. Home soil test kits are available. However, their accuracy is often questionable. If you go that route, I’d recommend choosing a test kit that has been specifically calibrated for our high alkaline soils of the west and keep in mind that it will only give you a ballpark measurement.
Soil sample strategies differ slightly for gardens and fields. However, the general philosophy is the same of trying to get a representative sample from the area. This means that you should sample from a variety of different areas within the field or garden. Gardens only need to be sampled 6 inches deep. It is recommended to take samples from 12 and 24 inches deep in fields, depending on the crop. However, if that is not possible, then those numbers can be estimated as long as there is a sample from the top six inches.
MSU Extension offers many resources on soils. We have a soil probe you can check out, guides on soil sampling and can even send in your sample for you. For more information, you can reach the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu.
