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Tests You Can Do For Soil Health

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This article comes from the NOFA/Massachusetts 2017 November Issue Newsletter

by Jack Kittredge

Do you ever wonder whether you are building your land’s best possible soil health? Have you thought about the relationship between your farm or garden soil and the excess carbon in Earth’s atmosphere?

Beyond the typical mineral analysis of soil there is the question of soil life and vitality— which is a very good “proxy” for soil carbon. In most cases if you have lots of soil carbon you are going to have lots of soil life, and vice versa.

As a part of our soil carbon program at NOFA/Mass we have developed a number of tests adapted from agricultural institutions like NRCS and Cornell, which are simple and hands-on and which can be done in current or planned farms and gardens to determine soil quality, vitality, and carbon sequestration capability. The best procedure is to run these tests annually on the same ground and note changes since the last time – is your biodiversity growing, is there greater vitality, are your changes in management giving you what you want?

We are currently able to run these tests for a small number of growers at a reduced price (it costs about $250 to send a person to your farm to run all the tests, and with outside funding we offer them at $50 to the grower). However, we also are hoping growers will run these tests themselves. Most do not require much in the way of expensive materials or equipment—those that do, we can help with. This and subsequent articles will describe the tests.

Test #3: Soil bulk Density (based on work by NRCS)

Summary: The density of a material is its mass divided by its volume. In the case of soil, density often means compaction – the space between soil particles for air, water, plant roots, etc. has been compressed. Healthier soils, including those with higher carbon, support more life and tend to have lower density.

This protocol measures density at several layers in your soil. Exact soil and cylinder measurements for this protocol are best done using the metric system, as the resulting value will be easy to compare to other soil density figures, and to that of water (which is equal to 1 in the metric system).

(If soil is very rocky or gravelly, there is an alternate method you can use to measure bulk density)

Procedure: Find or make a cylinder roughly 3” in diameter and 4” long which is sharp enough to drive into the ground. Measure the cylinder’s volume (π * r2 * length) where r = half the diameter, π=3.14159

Now dig a hole roughly 15” deep, placing the soil on a tarp to use to refill hole later. Leave one edge of the hole straight down, undisturbed by the shovel. Three samples will be taken from this edge.

Using a trowel, horizontally slice 1” under plants/roots atop that edge, leaving a 6”x6” soil surface area.

Using a mallet and block of wood, place the cylinder on the cleared area and drive it ¾ of the way into the soil.

Using the ruler, measure the average distance from the top of cylinder to the top of the soil in it.

Subtracting this length from that of the whole cylinder, find the volume of soil in cylinder (π * r2 * length).

Record this volume on the empty plastic bag along with the depth of sample, date, and location.

Using the  trowel, dig carefully around the cylinder on 3 sides to loosen it.

Using a putty knife, carefully lift the cylinder out while preventing any loss of contained soil.

Using a serrated knife, remove excess soil from the outside and bottom of the cylinder, making the bottom flush.

Place the sample into labeled, re-sealable plastic bag and seal it.

Using the trowel, remove soil to the depth of the next sample.

Repeat process for medium and deep samples, placing them in separate labeled bags.

Take samples to drying chamber or oven and dry them.

Weigh dried soil samples and record weights.

Calculate and record bulk density of samples, bulk density = weight (grams) divided by volume (cc).





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