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Tuesday - February 19, 2008

From: Bennington, NE
Region: Midwest
Topic: Seed and Plant Sources, Soils
Title: Difference in natural soil and potting soil.
Answered by: Nan Hampton


If you buy store bought soil is it different than soil from the ground?


Soil from the ground, or natural soil, is composed of finely ground rock (mineral component) and humus (organic material). One way to classify soils is by their texture. The basic texture of soil depends on the different sizes of ground rock in its composition. There are, essentially, three different sizes of ground rock particles—sand has the largest particles, silt particles are smaller, and clay has the smallest particles. The size of the particles determines how porous the soil is; that is, how quickly water and nutrients move through it. Porosity is important also for gas exchange—roots need oxygen from the air. Sand with its large particles is the most porous and clay with its very small particles is the least porous. Water moves very quickly through sand and moves through clay very slowly. Natural soils are classified by the mix of the different particle sizes and the organic humus. You can read descriptions of the twelve major textural classes of soils as defined by the United States Department of Agriculture (USDA).

Different parts of the country have different types of soil. In Austin, Texas, where Mr. Smarty Plants lives, the soils have a high clay content and, thus, are not very porous. We have native plants here that are well-adapted to our clay soil; but, if we want to grow something that isn't native to our area, we usually have to loosen up our soil by adding sand and/or humus to it. You can read about your Nebraska State Soil which is more porous than ours here in Texas. Your soil will grow plants that our Texas soil won't grow. The bottom line is that different soils will support different plants.

Although texture is the major way to classify soils, there are other components of the soil that are important, too. The acidity or alkalinity (the pH) of the soil is important and also the nutrients (for example, potassium (K), nitrogen (N), and phosphorus (P) which are the most important).

Interestingly, however, most commercial potting soils don't contain "soil" at all. They are made up of peat moss, finely shredded barks, compost, and perlite (puffy volcanic glass to increase porosity). Potting soils, or store-bought soil, is made to support house plants although there are different mixes for different types of plants. The ideal potting soil is porous enough so that water is able to flow through it and deliver nutrients but not so quickly that the water is immediately lost. There are different mixes for flowering plants, orchids, and cactuses, to mention a few. For instance, potting mix for orchids is composed almost completely of bark. You can read how to make your own potting soil mixes,

Since I believe, from your e-mail address, that you are an elementary school student, you can find more information about soils geared to kids from University of Illinois Extension Service.




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