Does Your Soil Make the Grade?
Soil Tests Reveal A Lot About Your Organic Garden's Soil

Most soil testing labs have special boxes and instructions for you to use and follow when taking your soil sample, but the general procedure is for you to take some soil from various locations in your garden, mix all the soil together and then get some this "homogenized soil" to the testing lab, either by mail or by bringing your sample in person. Soil testing labs usually require about 1 cup of soil in order to perform their analysis.

Once your sample arrives at the lab, teams of dedicated soil scientists will catalogue it, oven dry it and then get busy learning all about your soil so that they can give you valuable information about it. Here are some of the things that a typical soil testing lab assesses:

Soil pH -- soil pH indicates how acidic (or alkaline) your soil is. This is important to know because all plants have pH ranges in which they grow best. If the pH of your soil isn't right for the type of plants that you are trying to grow, you will have a real uphill battle on your hands.

Buffering Capacity-- This measurement indicates how resistant your soil will be to attempts to change its pH. For example, the pH of soil with a low buffering capacity can be increased with a lot less lime (a soil amendment used to decrease soil acidity) than a soil with a high buffering capacity.

Soil cation exchange capacity (CEC)-- Most of the nutrients that are essential to plant growth exist in the soil in the form of positively-charged ions, which are known as cations. Sodium, potassium, Calcium, Magnesium are only a few examples of the plant-essential cations that are present in your soil. CEC is simply a measure of how many of these important cations your soil can hold. The higher your soil's CEC, the better.

Base Saturation-- The base saturation number on your soil test report tells you what percentage of your soil's CEC is used up by Calcium, Magnesium and other basic ions. Base saturation is generally directly linked to soil pH: when base saturation is high, pH goes up, when it is low, pH goes down (becomes more acidic). Your base saturation can help you pinpoint the source of soil structure or nutrient availability problems in your organic garden's soil.

Percentage of Organic Matter-- Organic matter is one of the most important ingredients that goes into making a healthy soil. Organic matter promotes good soil structure, which leads to good water holding capacity. Organic matter can hold lots of nutrients, which increases your garden soil's CEC. It also encourages beneficial creatures such as earthworms to make your organic garden their home. In general, the more organic matter you have in your soil, the better. The percent organic matter measurement tells you how much of your soil is made up of organic matter. Ideally, this number would be at least 5%, although some soils are naturally much higher in organic matter than this and others (sandy soils, in particular) often contain much less than 5% organic matter.

As you can see, a soil test can explain a lot about your soil. It will let you know what you should do in order to make your soil healthier: most soil test results also include customized recommendations for how to help you grow the crops of your choice. It also will remind you just how amazing and complex your garden's soil is and, if for no other reason than that, it is worth getting one!

For More Information:

New Mexico State’s guide to interpreting soil test results. It is particularly useful if you live in the Southwest US, but also has information that applies to soils everywhere.
http://cahe.nmsu.edu/pubs/_a/a-122.html

This links to a list of alternative soil testing labs: if you want your soil tested for things that aren’t tested for at your state’s lab, such as microbial biodiversity, or if you want your fertilizer recommendations given to you based on application rates for organic fertilizer, send you soil to one of the labs on this list.
http://attra.ncat.org/attra-pub/soil-lab.html