If plants could squeal like hungry pigs, we gardeners would pay more attention to their fertilizer needs.
But plants do tell us when they are hungry — with poor or distorted growth and with leaf discolorations. Why wait for your plants to become so desperate? Test your soil every few years.
Testing can be done by you or by a private or state laboratory, and there are options in what to test for. At the least, test the acidity (pH), because if it is unsuitable, plants cannot absorb certain nutrients, even if those nutrients are present.
Most plants like a slightly acidic soil, with a pH about 6.5. A standard test checks levels of the so-called macronutrients — phosphorus, potassium, calcium, magnesium and sulfur. A complete checkup would also include testing for micronutrients, such as iron, manganese and zinc, which are essential but required in only minute quantities.
SAMPLING IS MOST IMPORTANT
The accuracy of any soil test depends on how you take the sample. In even a modest-size garden of 100 square feet, one cup of soil — the amount used for the test — represents only 0.001 percent of the top 6 inches of soil, so that sample must be as representative as possible of the whole area to be tested.
The test area should be relatively uniform. Areas devoted to very different kinds of plants — vegetables versus lawn, for example — require separate samples. Vegetable and flower gardens can be sampled together.
Subdivide the area where obvious differences in topography or soil exist, and stay away from walls, sites of old compost piles, etc. Even out small differences over even relatively uniform soil by taking a half-dozen samples from random spots. Sample the top 6 inches of vegetable and flower beds and the top 2 inches of lawns, first removing any surface debris such as compost, weeds or sod.
Whoa — don’t use that first trowelful of soil. It’s cone-shaped, with a greater proportion of soil from the surface layers than from lower down. Take a slice, uniformly thick from top to bottom, from along the edge of that hole you just made. Alternatively, use a soil sampling tube, homemade or bought, to get a uniform sample.
Combine all your samples from a test area into a clean plastic bucket. Thoroughly mix the composited soil to average out differences between samples, crumbling it and discarding stones, sticks, insects and other debris as you mix. Spread the soil out on a clean baking pan to air- dry for a day, then remove about a cup for testing.
RESULTS COME IN
If you are sending your sample out for testing, follow any instructions supplied by the laboratory about packing the soil. For testing at home, use a portion of that 1 cup subsample you got from the combined samples. Home testing kits involve mixing small amounts of your soil sample with various solutions and noting color changes, which you compare against standards — all detailed in the included instructions.
If you are testing more than one area, label samples from each area and make a note to yourself of the locations. A testing laboratory may also want other information, such as past fertilization history, as well as what you intend to grow. Indicate whether you wish any special tests, such as for micronutrients or toxic elements (such as lead) in the soil.
If you are sending your sample out for testing, follow any instructions supplied by the laboratory about packing the soil. If you are testing more than one area, label samples from each area and make a note to yourself of the locations. Equally important is to supply the laboratory with any information requested about past fertilization history, as well as what you intend to grow. Indicate whether you wish any special tests, such as for micronutrients or toxic elements (such as lead) in the soil.
When your soil test is complete, you will receive information about your soil’s organic matter, texture (clay, sand, etc.), acidity and levels of specific nutrients, along with a recommendation for fertilizer and lime. Fertilizer recommendations are based on what is in the soil and what kinds of plants you intend to grow. Follow fertilizer recommendations closely, because too much can be as harmful as too little, causing nutrient imbalances, even death, of plants.
Keep in mind that a soil test determines fertility and acidity, but does not address such problems as waterlogging, pests or insufficient sunlight. An observant eye over coming months is a necessary adjunct to soil testing. There’s truth in the old saying that the best fertilizer is the gardener’s shadow.