Soil Testing

Immediately after breaking ground for a new garden you should test the soil. You can buy kits to do this, or buy a soil sample pouch at a garden center and send a sample of your soil to a state laboratory for testing. Within a few weeks the lab will send back a detailed report on the soil’s current nutrient levels, with a recommendation for fertilizers. Unfortunately, most soil labs ignore organic materials and offer their advice solely in terms of chemical fertilizers. If you will be using bagged organic fertilizers, you will find their N-P-K levels printed on the bag. If you will be using “raw materials,” however, you’ll need to know the nutrient content of various organic materials like manure, leaves, blood meal, seaweed, and rock powders.

Table 4–1 lists N-P-K levels of these materials based on their weight (see page 000). Ideally, they should be composted first, and the compost used for enriching the soil. But this is not always convenient, or even possible. Very bulky materials like hay, straw, seaweed, or leaves can be used as a mulch first, and allowed to break down partial before being turned under to complete their decomposition. Fresh animal manures can be spread and turned under immediately so their nutrients are caught in the soil rather than being lost to the air while the pile waits for other materials to arrive; just don’t plant crops for at least a month so the manure has a chance to break down. Materials like rock phosphate, greensand, wood ash, or blood and bone meals can be added at any time and mixed into the top few inches of the soil. They can also be used to fortify a compost pile.

To make sure that your test sample represents actual conditions at the plants’ root level, take three or four samples from around the whole garden plot. Don’t take a sample from any place that was recently fertilized or limed; it will distort the results. To get a clean sample from root level, take a shovelful of soil out of the ground and set it aside, then slice another section, only an inch or so thick, from the side of the hole. With a penknife or scrap of wood, scrape away the top inch or so, and take your sample from a one- or two-inch-wide vertical section of what remains. Mix that small bit with the other samples from around the area to be tested; all roots, leaves, rocks, and other material should be removed, and the test sample should be dry and fully pulverized before mailing.

One of the most critical aspects of a soil test is the pH report, which tells you if your soil is overly acid or alkaline. This is important, because all nutrients are more or less available depending on the pH balance of the soil. On a scale of 0 to14, each whole number represents a tenfold difference from the next whole number. Thus, taking the number 7 as neutral (which it is on the pH scale), a pH of 6 indicates that the soil is ten times as acidic, while a pH of 8 indicates it is ten times as alkaline. The soil report will usually include a recommendation of how much lime (to raise the pH) or sulfur (to lower the pH) should be added, and in what form. Keep in mind: if the pH of your soil is more than two points away from neutral, you should break the application of lime or sulfur into two or more applications to avoid shocking the resident soil life with too radical a change.

Once an ideal pH of 6.0 to 6.8 (at which the widest range of nutrients is optimally available to most plants) has been established, an ongoing program of manure and compost applications will remove the need for any further attention to soil pH. Except for special conditions, the latest research backs up this belief. Only if your soil is of the most extreme acid or alkaline nature, or your garden is subject to serious acid rain and snowfall, should an ongoing program of pH balancing be necessary.

ShepherdSoil Testing

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