Soil is one of the most important factors influencing the success of your garden.  Both the texture and fertility of the soil make a critical difference to the growth and development of any plant so your knowledge of your soil’s characteristics is essential.  The best way to find out the characteristics of your soil and the means of making it better is a soil test.  The most reliable way to do this is to send soil samples to your state agricultural agency.  Contact your county agricultural agent and he/she will be able to advise you how to do this. (You can find your county agent at your local Cooperative Extension Office. Click this link for a U.S. Department of Agriculture list of Cooperative Extension County Offices throughout the U.S.). In North Carolina the service is free and the county agent supplies you with instructions and boxes for collecting soil samples.  The form you fill out lets you indicate the use to which you intend to put the soil and the soil test results will come back to you with suggestions for improving the soil for that use.  There are some simple and crude tests to determine texture and pH but they will not tell you how to improve the situation.  Why not use the service that your tax dollars are paying for and get a top quality product?  One more note on this, if you have a big garden or many different types of gardens, don’t hesitate to take at least one sample from each.  One year I had a disaster with tomatoes in part of my garden and not in other areas so I had soil tests for each.  Come to find out the soil was too acid for tomatoes in some areas and a good dose of lime solved the problem the following year. Also, fall is the best time to take soil tests…your ahead of the rush of farmers who usually wait till spring, and your results will come back well in time for you to add lime and/or fertilizer so they have time to work into the soil before spring planning.

If you want to know more about soil or why these soil-testing procedures are important, read on.

Texture refers to the relative the size of the mineral particles making up the soil.  Three major types of particles make up most soils:  sand, silt, and clay. Sand is the largest particle and makes the soil feel gritty.  Silt is a medium sized particle and gives a silky feel to the soil while clay is the smallest particle and gives a sticky feel. To give you an idea of the relative size of each particle picture the sand as barrel, the silt particle would be the size of a plate, and the clay the size of a coin.  The sand in a soil provides looseness because of the air pockets formed between the particles and both water and roots can move easy through sand.  The small clay particles, on the other hand, pack together and block the movement of water and roots.  Soil texture is classified by the relative amounts of each type of particle in the soil.  For example, soil may be classified as silty clay or sandy clay.  The term loam is used to indicate a soil that is roughly equal in clay, sand, and silt.  An ideal soil has enough looseness that the roots and water can move through it but enough small particles to retain water for good plant growth.

Fertility refers to the essential nutrients and pH of a soil.  The major nutrients are nitrogen, phosphorous, and potassium. Nitrogen is essential for good leaf and stem grown, phosphorus aids good root and flower grown, and potassium is responsible for overall good health of the plant.  Both nitrogen and potassium are water soluble and easily washed out of the soil and so must be replaced.  In addition to these three major nutrients, soils should contain small amounts of trace elements, like calcium, zinc, and magnesium.

pH refers to alkalinity or acidity.  It is measured on a logarithmic scale of 1 to 14 with 7 being neutral,1 being very acid and 14 being very basic.  Soil pH is important because it determines whether or not a plant is able to take up nutrients.  An azalea growing in an alkaline soil rich in nitrogen, phosphorous and potassium can not properly absorb these nutrients and will not grow well.  On the other hand lilacs and clematis can take up nutrients well in alkaline soil but would not be able to do so in an acid soil.  The pH of a soil can be changed but only slowly and will eventually revert to its original pH so yearly vigilance is necessary for good plant growth. For more detailed information on pH see KarensGardenTips Soil pH: Understanding, Testing, Changing, and Plant Requirements.

Garden basics pointer

By Karen

3 thoughts on “Soil fertility and texture: how and why to test your soil for garden success”
  1. Great information on soil. I’m a New Jersey gardner and before I began taking annual soil tests, I always used more fertilizer than was needed…I followed the “if a little is good, more must be better” method of determing application rates. Now my plants are better than ever, I’m spending less money, and know my garden is polluting less.
    I see you garden in North Carolina, have you had to do anything differently working with the heavy red clay soil?

  2. Hey Ray,
    Thanks for your comment on soils. Yes, the Carolina clay is really something to be reckoned with and other than dig it out and replace it with good soil (an expensive undertaking), you will have to be content with slowly improving it. Under no circumstances add sand as it will turn your clay to concrete. The amount of sand you would have to add to it to make a loam (20% clay, 40% silt, 40% sand) is inhibitive. An application of gypsum improves the texture of many soils and is not expensive. A copious amount of organic matter such as compost and manure is the real solution and this should be done on a yearly basis as long as you garden in the soil. Dig in a 2-3 inch layer of organic mater every spring; any time you add plants, add as much organic matter as you can; mulch with an organic mulch. Always remember, however, that the clay acts as a vast piece of plastic under the good soil restricting the movement of water downward so you want to make the good soil as deep as possible.
    For a source of leaf mold check the group that does your city/count/area leaf pick up in the Fall as they may give away or sell it inexpensively. Alternatively, start a compost pile of your own.

    Good luck with your garden.


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