05 September 2012

How To Get a Soil Test

One thing that everyone should do before planting their garden, or even landscaping their yard is have the soil tested.  I have to admit that I had never done this - until last year.  Here's how you do it.

You need to get a representative sample of the area that you want tested, which means you should collect a dozen samples and combine it into one composite sample.  Soil samples should be collected from the surface to 6-8 inches deep.  Using a soil tube or auger is pretty straightforward, but if you are using a spade or trowel the technique is a bit different.  Dig a v-shaped hole about 6-8 inches deep and use your trowel or spade to take a thin slice from the side of the hole.  Combine all the soil samples in a clean bucket and mix them thoroughly.  You'll need at least two cups of soil per sample.  You can either take your sample to your local Clemson Extension office or purchase a soil sample mailer.

 About a week after I dropped off my soil samples at the Clemson Extension office, I received an email with a link to the results of my soil tests.  I had two different soil tests done - one for the vegetable garden and one for the beds in the front yard.  When you fill out the forms, it helps to let them know what you plan to plant and whether you want organic recommendations as well.  I thought I asked for both, but I only got inorganic recommendations.  Here is what the chemical analysis of my soil looks like:

The soil test found low levels of potassium; phosphorus and calcium levels were very high, which is common for this area.  To correct the potassium deficiency, it was recommended that I use muriate of potash.  Since I didn't get organic recommendations, I tried to learn more about these fertilizers and amendments, to understand what they were and what the organic alternatives were.  In general you can tell what an amendment is by the fertilizer grade.  I knew muriate of potash (MOP) was solely to correct the potassium deficiency, since the third number was so high, but I decided to read more.  MOP is actually potassium chloride and there are a lot of people out there that think it's harmful to soil biology, but others say it leaches out of the soil fairly quickly - and supposedly chloride increases yield by improving disease resistance.  Since I wasn't a fan of chlorine, I decided to see what my options were - it turns out that there are few.  Sulfate of potash is a common substitute, but it tends to be more expensive and it has less potassium (0-0-50), so you need more of it.  It is used for plants that are sensitive to chlorides.  An organic alternative to these are wood ashes.  They may contain around 6% potash, but they also contain lime which raises the pH of the soil, making it more alkaline.  There's no easy way of knowing how much potash is in it and, if you don't need lime, you shouldn't use it.

The other fertilizers are pretty straightforward.  10-10-10 is what it seems like and it turns out 15-0-15 is a common fertilizer for centipede grass.  I haven't looked into organic alternatives to these, but it seems easy enough.  Maybe it won't be in one product, but I'm sure there are things out there that are good substitutes for this.  That will have to be a future blog post.  For more information about soil tests, click here:

HGIC 1652 Soil Testing


Jim 2-10 said...

Helpful post because I have to do this fairly soon for a new garden and I have only ever used a tube in the past. However, I don't have a tube now so the method you describe is useful for me. Now, can you tell me how to find the Clemson Extension office in Pennsylvania? ;-)

Jenion said...

Find your state's ag school. They probably run a county extension service that would do soil testing. Like Clemson or NC State.