There are many levels of soil testing out there. You have to choose the
one that's most important for your particular situation. Of course, the
more things you test for, the higher the price.

The home soil test kits can provide a good estimate of pH. (By a good
estimate, I mean that it can tell you if the pH is 5 or 6 or 7. The
laboratory test will get you down to a couple tenths of a pH point.
However, for most home gardens this level of detail is not really
important.) Phosphorous and Potassium are probably so-so. Nitrogen is
probably not worth the effort.

The soil structure can be estimated by observing the soil and its
response to large doses of water. High sand components drain quickly.
High clay components don't drain. Assuming the soil is not really high
in clay or sand, the loam content can be estimated by taking a handful
of moist, but not wet, soil and squeezing it into a ball. If it forms a
crumbly ball, it's probably moderate loam and sand. If it forms a hard
ball there's probably heavier in loam and clay. This all takes some
experience, but then knowing what to do about your soil structure also
takes some experience. Observing your garden over a couple of years can
give you some insight into this.

Of course, having a professional do the tests also allows you to get the
advice on what to do with the results from the professional.

To do a soil test:

Take a small sample of soil from several different locations in your
garden and mix them thoroughly. Take samples from the surface to at
least 6" deep, and if you plant deep rooted crops, a foot wouldn't hurt.
Don't be tempted to buy a soil testing probe for a home garden. They
cost like $50. You can make your own. A 10' piece of 1" EMT conduit from
Home Depot (about $3). Cut a reasonable length off so you can handle it.
Jam it into the soil. You will get a core sample. You can get it out by
shoving something through the conduit. If you know an electrician, (s)he
might have a scrap piece. 3/4" conduit will work.

Dry them out. The testing lab needs them dry for the test and you don't
need to pay the postage for the water that they are going to get rid of
(not to mention that it increases their work load if they have to dry
all the samples.) You can dry them out by just spreading a pile of soil
on an old newspaper in the garage (fairly thin) for a day or two. Don't
dry them with a lot of heat. That will alter the organic composition of
the sample. The testing lab will let you know how much they need (It
will depend on the number of tests), but it'll probably be on the order
of a sandwich bag or so.

Tell the lab what you are planning to grow. This will enable them to
make specific recommendations on addition of lime, fertilizer, other
amendments.



David Ross wrote:
It's obvious from several of the replies in this thread that many
do not know the purpose of soil testing. It's not merely to
determine the pH. It's to determine the overall quality of the
soil. Beyond pH, this includes the relative abundance of
nutrients, not only the primary NPK (nitrogen, phosphorus,
potassium) but also trace elements such as iron, zinc, magnesium,
etc. A good soil test will also determine salinity, which is
important in the western U.S. where both soils and water supplies
might contain excess sodium. Finally, a soil test should determine
the structure (i.e., relative proportions of sand, silt, clay, and
humus).

Since trees, shrubs, and even some perennials can send their roots
well below the level where you have tilled amendments and
nutrients, you should test the native soil before doing any
improvements. (In deep soil, tomatoes can send their roots down 10
feet.) For example, adding gypsum before testing will definitely
skew the results and prevent you from learning what the roots will
eventually find. The results might indicate certain improvements
are not necessary while others are indeed needed. Why spend money
and effort before you know what needs to be done?

If you live in an area where government agricultural agencies do
not do testing, there are likely to be commercial testing labs.
Even in the U.S., not every county has an agricultural extension
office; and in those that do, not every agricultural extension
office does soil testing.