Looking at basic soil test results

You’ve read it here in this blog, you’ve heard it from your neighbor, even your Aunt Bootsie told you to get a soil test for your garden.  I’ve always recommended using the soil testing lab at Michigan State University, it’s the “gold standard” of soil labs in Michigan. For most gardens a basic test will provide you with all the information you’ll need.

In past blogs I’ve described how to properly take a soil sample for testing and what to do with it once you’ve collected it, so we won’t discuss that today. Instead we’ll look at actual soil test results.

Once you’ve rounded up a bag of dirt, sent it to the lab along with your payment, the MSU soil lab will test your sample, processes the results and send you the test results either by email or by mail. That usually happens within ten days.

Let’s take a peek at an actual soil test readout I received last fall.

Results from a soil test
Results from a soil test

The first test result is the soil pH. pH describes how acid or how alkaline your soil is with “7” being neural. In this case the sample result is 7.6 which is somewhat alkaline. So right away we know we won’t have to add any lime to raise the soil pH since most garden plants grow best in a slightly acidic soil.

Next is the result for phosphorus, the “P” in NPK. The result came back at 58 ppm (parts per million) which, according to the graph is well in the optimum range.

Following P we see that is the potassium (K) is 67 ppm which we see is below optimum. Magnesium (Mg) at 202 ppm is above the optimum range.

Calcium(Ca), shown in the additional results section is 2443 ppm which helps to explain the relatively high soil pH reading since calcium will raise soil pH.

The next result is CEC (cation exchange capacity) this tells us how well the soil is able to retain soil nutrients. A reading of 14.1 tells us we can add fertilizer to this garden without having it leach out of the soil. Usually, soil types with a higher percentage of clay in their make-up have a higher CEC and therefore are inherently more fertile because of all the retained nutrients. Very sandy soils have low CEC values. It is very difficult to change the CEC of a soil. On the other hand, we can easily raise the NPK values by simply adding fertilizer.

Those cations (positive-charged ions) that are being described in the CEC reading are mostly K, Mg and Ca. There is a section in the readout providing the percentage of each of those. Phosphorus is not listed there because it exists in the soil as negatively charged anion (PO4 3-).

Nitrogen (N) is not tested for at MSU because soil nitrogen levels change with the temperature and other variables so you would never get an accurate reading.

We don’t have room here to discuss the soil science behind the results. Fortunately, the soil lab boils it all down to some simple recommendations at the bottom of the readout.

The nutrient needs are listed as actual pounds of each element per 1000 square feet. Since fertilizer is not sold as pure nitrogen, phosphorus or potassium, the precise amount of any type of fertilizer must be mathematically calculated. Back in the olden days when I was an MSU Extension Agent and the results were printed with a dot-matrix printer, I made those calculations by hand on hundreds of test results. Nowadays MSU has an online calculator that you can use to figure out how many pounds of fertilizer you would need to apply.

With the planting season rapidly approaching,I suggest you get your soil sample to the lab ASAP. Much like your income tax return; the sooner you send it in the sooner you’ll see your results.





Yellow leaves on sweet corn plants

We just got back from a relaxing week-long vacation at Bear Lake in northern Michigan.

The first thing I did, even before unloading the car, was to take a look at the garden. It’s amazing how much a garden changes in a week at this time of year.

Everything looked great except for the sweet corn; it’s looking a bit peaked. The lower leaves are turning yellow, which is a sure sign of nitrogen deficiency.

If plants can’t get enough nitrogen from the soil, they will rob it from older leaves and use it to grow new leaves — that’s what causes the discoloration.

Plants use nitrogen to make chlorophyll, the green part of a leaf responsible for photosynthesis.
Plants use nitrogen to make chlorophyll, the green part of a leaf responsible for photosynthesis.

I can trace the problem back to last season. In that spot last year, I mulched the growing vegetables with a generous covering of wheat straw. This spring, instead of raking out the old mulch, I left it in place and tilled it under.

Since then, soil microorganisms have been working overtime trying to decompose all of that straw. They require loads of nitrogen to do the work of decomposition. As a result, there is not much nitrogen left over for the sweet corn to use.

Now I’ll have to add nitrogen fertilizer to make up the difference. I have some urea fertilizer left in a fifty-pound bag that I have been dipping into for several years now, it’s finally almost empty.

Urea is an artificial fertilizer that contains forty six percent nitrogen and nothing else. That makes it a “hot” fertilizer, meaning it is very easy to burn growing plants with it if you’re not careful. I like to mix it with sand to help make it easier to spread evenly.

Other types of fertilizers, such as fish emulsion and blood meal, contain nitrogen in a different form and will provide nitrogen without the danger of plant damage. Because those types of fertilizers contain less nitrogen on a pound for pound basis as urea, you’ll have to apply more to get the same results.

Nitrogen deficiency results in weaker plants and lower yields so it’s a good idea to correct the problem early, while the plants still have time to recover.


November is a Good Time to Add Lime

The weather people are predicting a string of nice days through the weekend and into next week.  Many of us will looking for things to do out in the yard and garden.

Because November is the ideal time to apply lime,  this weekend would be a good time to check the pH of your garden soil. pH is a measure of acidity or alkalinity of something, in this case, garden soil.  If you have been fertilizing your garden regularly for several years in a row, the chances are your soil may need lime.

Lime is a calcium-based soil amendment that farmers and gardeners use to sweeten garden soil — raise the soil pH.

It’s not a good idea to just guess if your soil pH is low, your soil has to be tested. Fortunately, this is one test you can do yourself with a pH test kit from a garden center. These kits are pretty accurate. Just make sure you buy a fresh test kit because the test-chemicals will get old over time and produce an inaccurate reading.

There are several different types of lime and each type has a different application rate.  Don’t worry though, all lime containers have application instructions printed on them. The amounts are usually given in pounds per 100 square feet or 1,000 square feet depending on the size of the bag.

In many cases, a five pound bag of lime will be enough to treat 100 square feet of garden soil.

Keep in mind, that sandy soils need liming more frequently than loam or clay soils.

Lawns too, will benefit from a fall application of lime if the soil pH tests low.

So, there’s your excuse to head out to the garden center, pick up some supplies and get some productive work done in your garden.