Nov 23, 2015

Managing soil fertility: taking a ‘good’ soil sample

Now is often the time of year that we think about soil sampling and getting a handle on the fertility needs of individual fields.  This is a great time to collect soil samples if soils are not too wet and still not frozen.   One of the keys to getting good information from soil samples is to take a good sample.  Almost any lab (or soil fertility instructor) will tell you that your results are only as good as the quality of the sample you take.  Here are a few recommendations on how to take a “good” sample:

1. Choose sample areas based off of previous management, crop history, slope, and soil type.  Fields can vary in nutrients levels due to management practices that occurred many years before.  Some examples of things to consider would be old field borders, band applied fertilizers (especially Lime, P, & K), or any differences you have observed in crop growth.  If you do not sample specific areas separately, the results from one area might dilute or inflate nutrient estimates, not giving you an accurate assessment of the field.
2. Size of each sample area.  Generally I would recommend a maximum sampling area size of no more than 5 acres (2.5 acres preferred) for fruit and vegetable producers, at least until you have a baseline understanding of the field.  However, for many specialty crop growers with smaller field sizes, you may want a sample for a plot as small as ½ acre or even less.  Again, use management history instead of a set number of acres to determine how much area to represent with one sample.
3. Pull 6 to 10 cores (6 to 7 inches deep) and mix them together to form each sample. This gives you a good representation of the variability of an area, and this number of samples helps to control for any small-scale variability in the field.  Mix well and fill to the designated level on the soil sample bag, which is usually no more than a pint.  You can discard of  any extra soil; no need in paying for extra shipping, and most soil labs already have an abundance of extra soil so they probably would prefer not have any more in excess.
4. Use a probe or spade to get a sample that is representative of the top 6 -7 inches of the soil … we typically consider this zone in the soil to be the primary rooting zone, and your sample should equally reflect that profile. You want to have just as much soil from 6 inches down as the top inch so you are not biasing the sample for possible increased fertility.  A soil probe is the easiest tool to use for this.
5. Keep good records.  Record the sampling areas and how you designated them. This will help you when you take samples in the future (at least every 3-4 years) to be able to compare back so you can see how your management practice have changed the soil fertility levels.  Also, if you are comparing from year to year, it is recommended to always sample in the same time of year for consistency.

For soil labs that will analyze your samples, you can find a listing here or from the Illinois Soil Testing Association here.

 – Nathan Johanning, University of Illinois.