As your combine churns through broken corn stalks, and some that even explode into a powder, you are probably wondering if your corn had enough nitrogen, and whether early wet soils pulled it all out of the root zone and into the field tile when the roots were trying to wrestle for it.  But maybe any fertility issues that might have hampered your crop growth were the availability of phosphate and potash.  P and K could be on the top of your to-do list this fall when the crop is out of the field, but before firing blind shots, invest a small amount of time and expense in a soil test.  They tend to pay for themselves and a whole lot more.

University of Illinois soil fertility specialist Fabian Fernandez says the best use of a soil test is to track changes over time.  Because of the way they are taken, soil moisture, how they are analyzed and other reasons, a soil test may not give a good picture of fertility levels.  But he says they can certain show trends that are important in knowing what problems may occur.

Fernandez compares this fall and last fall as similar, with an early start to harvest, dry soil, and a potentially long time to complete harvest, apply fertilizer, and conduct fall tillage.  He says 2010 recorded a lot of variability in soil test results and many farmers were uncertain of the reason, and reacted in a variety of ways, some with unnecessary expense.  He says fall soil is typically dry (2009 was an anomaly) and that makes it a challenge to interpret the results.  But he says the results of the sample collection process can also cause heartburn when the data comes in the mail.

Think about what happens when you push a shovel into the ground.  You might get it down the recommended 7 inches, but the sample will be made up of more soil that comes from the surface and only a small amount from that 7 inch depth.  That is not your fault.  That is the physics of using a shovel to collect a soil sample, compared to soil probes and other tools that do a better job.

The use of soil tests to track changes over time will require you to eliminate as many variables as possible.  Fernandez says you need to be a creature of habit when you are taking soil samples as part of your fertility management program.
1) The same location should be used, and while that seems difficult, use a GPS receiver to pinpoint that location.
2) The same depth should be used, which is the recommended 7 inch sample depth.  If you are in a no-till program and the soil has little mixing, the sampling process is critical.  If the soil is not mixed, P and K levels will be higher in the top inch where plant tissues have deteriorated and blended with the top layer of the soil.  A shallow sample gives you a reading of more P and K than actually exists throughout the 7 inch sample depth, and if the top layer falls out of the probe, your test will not be true.
3) The same time should be used in collecting the samples, whether it is in the fall or the spring or whenever, do it at the same time of year.
4) The same tool should be used in collecting samples, whether you used the recommended soil probe or the lesser recommended auger or the non-recommended shovel. 
5) The same testing lab should be used, since it will use the same methodology in testing the sample, and that reduces the variability that might change your application rate.

Fernandez says potash is particularly troublesome because of uncertainty in testing, and the wide swings of test results, depending on time of year and soil moisture when the sample is taken.  He says readings are more uniform when potash is tested in the spring because moisture is more uniform, but there are substantial differences from year to year with fall tests.  In 2010 the soil was dry and readings may have been low and extra potash may have been applied because of that.

Fernandez says leaching of K molecules from decaying plant material into the soil can also influence its availability.  That is because the potash stays as a K ion in the plant, and not part of a larger molecule like nitrogen or phosphate which become part of the plant chemistry.  The latter are slower to be released into the soil, compared to the K which leaches out of the plant material much quicker.  But last year, the lack of moisture in the fall meant the K ions remained in the undecayed plant material and soil tests may have indicated a shortage of K compared to other years with more normal moisture and biological activity to break down the corn stover.

Summary:
Soil testing can be an important part of your crop management program, but the testing process can sometimes be fraught with peril.  Use soil tests as yardsticks to measure change, more than using them to determine the exact amount of P and K in a field.  Take soil tests with minimal change in your routine from year to year to reduce variability.  Beware of the fact that potash test results depend on the time of year and the amount of soil moisture for accuracy.

Source: Farmgate blog