Taking a Soil Sample for Analysis

A soil test will provide a quantitative analysis of your soils chemical components. This test can inform you as to amendments you should make to place your soil in the optimum condition for growing food. Crops removed from your garden required nutrient inputs to grow, and unless you are composting wastes back into the field, there is a net loss of nutrients. The sustainable farmer wants to replenish nutrients to hold the soil at the optimum levels. Before we can amend, we need to know where we are starting. You would not just pour gas in your car, not knowing if there was room in the tank or a need for it. Our goal is to provide balanced, healthy soil for crops. Chemical balance is where pH, and mineral quantities are optimum. Some nutrients is too high a concentration could adversely affect production, so there is a precise level of each nutrient to be achieved.

In this article, I am dealing with chemical balance, but I want to always acknowledge that this is one of three areas defining healthy soil. There is also physical characteristics, and biological components that are also crucial. Organic matter, both fresh and decomposed, are needed for all three of these factors to yield a healthy soil.

How much of a mineral is needed? Well, it is not as straight forward as you would think to answer this question. Various plant types may require unique nutrient characteristics. For example, blueberries like a more acidic soil than most other plants, and their plot should be amended with this in mind.

Plants utilize nutrients via their roots, and various plants will reach different depths of soil. There is a fascinating set of root diagrams for various garden plants in the book: Gardening When It Counts by Steve Solomon. Even through some plants may stretch roots deeper, we usually amend soil to the depth that we can till, since we need to be able to mix the amendments to a uniform spread. I suggest a depth of 12 inches. This gives a good bank of minerals for current and future crops.

Rather than a non-calibrated home test, I suggest sending your samples to a high volume lab, where their technique would be proven, and their tests are routinely calibrated, and therefore highly consistent. If you are interested in a recommendation, I use A&L Labs, and this company has regional centers around the country. Some A&L Labs offer the "S3" test as a complete workup, others you have to specify "S1 and S3". In 2011, a "S1 + S3" costs under $20 per sample. Either way make sure that you get the complete test, as clarified below.

Aspects of the quantitative analysis you want:

S1: Organic matter, available phosphorus, exchangeable potassium, magnesium, calcium, soil pH, buffer pH, Cation Exchange Capacity, Percent Base Saturation of cation elements.

S3: Sulfur, Zink, Manganese, Iron, Copper, Boron

Note that other labs may call these same tests under different test identifiers, but these are the mineral tests that you want.

To take a soil sample, you need to extract a representative average of soil to the depth you want to amend. You can do this by digging a hole, and getting a clean sample with a stainless steel spoon. Take care that your sample does not become contaminated by the shovel used to dig the hole. I suggest removing a column of soil in the hole with the spoon, and then take a second clean sample in the same area. Draw the spoon from bottom to top, and collect a uniform sample through the depth you want to analyse. Place the soil directly into a soil collection bag. (The soil collection bags can be ordered from A&L free of charge. You can also request the transmittal forms at the same time.) You should have several samples from around your field to get a good average. You could test a field of under 2 acres by just taking several representative samples from around the field, and mix them into one submission bag. Large fields of 2+ acres could be sampled by grid or zone. If you are going to amend your fields separately, then you need to take separate tests to be able to specify the needs of each more specifically. If there is some natural feature of the land that makes you think it has a different soil type or characteristic, you may want to sample that area separately, taking a sample for the various zones of your field.

Some farmers take the top 6 inches and the bottom 6 inches from several samples in the field and they submit the lower samples in one bag, and the upper samples into a different bag. There are soil probes which make the process of collecting a pure sample easier. You just press the probe into the soil, and pull it out. The core sample can be scooped out of the open cylinder, and deposited into a sample bag. If like me you want to sample 12 inches deep, I suggest a soil probe with a foot pedal to assist in penetrating the soil.

With the test results in hand, you will be able to come up with a plan on how to exactly amend your field. The lab may offer recommendations of treatment, but the formulas I will present to you may or may not agree with the Labs amendment suggestions. You can get the results for comparison, but I would not consider the suggestions as authoritative. In an upcoming article, I will present the desired quantities for your soils.

It is suggested that you add lime to your soils in the fall or early winter (as long as the ground could still be worked). You do not really want to place your amendments at the time of planting. Lime takes several weeks to months to react with your soils to impact pH. 

Now, let's get some soil samples!