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Know Your Soil For Best Garden Results


I've always taken soil for granted. It was there. You put seeds into it. You put water on it. Plants grow and produce flowers, fruit or vegetables.

Gayle Weinstein, author of Xeriscape Handbook; A How-To Guide to Natural, Resource-Wise Gardening, takes a different view: “Soil . . . acts as a highway between life and death, land and atmosphere, plants and animals.”

It stores water, air and nutrients and makes it possible for an exchange of elements and chemical reactions to occur, she adds. She describes soil as being animal, vegetable and mineral combined.

Here are six tests Gayle recommends for getting to know your soil. Grab a shovel and a quart jar. Dig up two cups of dry soil two-to-six inches deep from the areas you want to test. Gather a glass of water, dish washing detergent and paper towels. A soil pH kit, a meter or litmus paper will be needed for the final test.
  1. Texture
    From your two-cup sample, take a pinch of soil and work it between your fingers. Are the particles small? Fine? Smooth? Are they coarse? Gritty? Abrasive? Does it feel powdery? Soil is usually put into one of five classes: coarse, moderately coarse, medium, moderately fine and fine.
  1. Composition
    Take the quart jar. Fill it with equal amounts of water and soil. Add a teaspoon of dishwashing detergent. Make sure there are no twigs, leaves or other debris mixed in. Twist on the lid tightly and shake the jar vigorously. Set it aside undisturbed for 36 hours.

    The components will settle out at different rates. Large particles such as sand will settle to the bottom in minutes; silt in an hour or so; clay in one to two days – or some particles may never settle.

    At the end of 36 hours measure how tall the solids are and then measure each layer. Divide each layer by the total and multiply by 100 to get the percentages of each layer. Mark the percentages of each on the U.S. Department of Agriculture soil texture triangle below. Connect the points with a line and see where they intersect. 

    Soil is made up of mineral particles such as some sand, silt and clay on the small end and gravel on the large end. The proportions of sand, silt and clay in your soil will determine how well your soil holds water and how much air it allows around plant roots. Texture also determines how well the soil can hold on to nutrients for plants to use. Finer soil has more particles in a given volume and thus more surface area for chemicals to bind to.
  1. Workability
    Gritty soils break apart and won't hold a shape. Clay soils will mold and not fall apart. The best soil – loam – is a balance of the two. Put about a quarter of a cup of soil in the palm of your hand. Drip water on to the soil slowly until it feels moist and holds together. (If the soil gets too wet, add more soil.) How well the soil will form a ball or a cigar shape, whether it feels gritty or sticky and whether it forms hard clods when it dries are all indicators of the type of soil you have. The USDA Natural Resources Conservation Service (NRCS) has a flow chart for diagnosing your soil's texture and workability by feel.

  2. Soil Profile
    With a shovel, dig down 20 to 24 inches. Look for changes of color and texture. How deep is the top soil? What is under it – sand? Clay? Soil layers that are dark brown or black indicate that there is more organic material. Lighter soils may be a sign of coarse texture, fast drainage and leaching of chemicals and high temperatures. Layers of gray or yellow may be signs of drainage problems. The USDA NRCS has an illustration of the parts of the soil's profile and describes how to make a card to profile you own soil.

  3. Soil Structure
    From the wall of the trench you dug in Exercise 4, remove a slice of soil and press lightly on it. If the soil is easy to work with, it will break into small crumbs when pressed (This is called good tilth.) Soil that won't hold together (sand, for example) or that forms hard clumps (like clay) has poor structure. Soils with poor structure cause drainage problems and make it hard for plant roots to grow.

  4. Soil pH
    This is a test of whether your soil is acid or alkaline. It can be done using a specially designed soil pH kit, using a pH meter, using litmus paper or observing what plants grow best. Litmus paper is easy and not expensive. Mix a small sample of soil with distilled water and sticking in a piece of litmus paper. The color changes on the paper indicate whether the soil is acidic or basic.
    Most plants have a balance of acid or alkaline levels that they tolerate best. Minerals such as nitrogen or potassium are available to different degrees in soils with different acidity levels.

    Regardless of your soil's texture, profile, workability or pH, it can be adjusted with soil amendments, compost tilling and gardening. But knowing your soil is a powerful guide to selecting plants, amendments and even pesticides for a healthy, productive garden.

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