On Tue, 06 Sep 2016 15:23:55 -0500, Moe DeLoughan wrote:

That's easy. Have a sample of the soil tested. Most universities will
do it for a small fee. Contact your county's agricultural extension
office and ask about getting a soil sample tested. They usually have
the instructions and submission form available, plus the address to
mail or drop off the sample. They may have a listing of additional
places that you can send your sample to for testing, besides the
university.

In general, a soil test report will report on the amount of organic
matter present, indicate the type of soil, test for pH, phosphorus and
potassium levels. Testing for nitrogen levels usually isn't done
because nitrogen generally is not persistent in the soil. They'll
recommend how much nitrogen to apply based on what you tell them
you're intending to grow in it.

Or you could bypass all that and just start gardening. You could make
it a science project for the kids - one bed or container with the soil
as-is; another bed or container with the soil augmented with a small
amount of fertilizer. You could further tweak the experiment by having
a third bed, where you apply half the fertilizer at planting time, and
the other half about 6-8 weeks later. Have the kids keep records as to
how well the plants grow in each situation, and which produces the
most of each particular vegetable.

This is along the lines that I was thinking also.
Having the soil tested is good because it would be a concrete proof of
whatever it is, while doing the experiment will back that up.

The closest school is UC Santa Cruz, but their soil lab number is odd in
that it either displays busy or not in service: 831-459-4089

I'll look around for another number...
http://eps.ucsc.edu/about/contact-visitors.html