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Thursday, February 24, 2011

The Dirt On Soil

Soil is the most important component of your garden. Let me repeat that. Soil is the most important component of your garden. The large majority of plant problems, as much as 80 percent, can be attributed to the condition of a garden's soil. You can have fancy raised beds, state-of-the-art drip irrigation systems, and expensive heirloom seeds, but if your soil is poor your crop results will be too.

Soil is simply a mix of minerals, air, water, and organic material that combine to form a growing medium for plants. A few days ago I mentioned soil tilth. (In "Developing New Garden Beds" on Feb 20, 2011).  Tilth is all about the soil's ability to support a plant's root growth. Root growth directly correlates to plant growth; poor root growth equals poor plant growth.

There are three properties that define soil tilth: soil texture, soil structure, and soil fertility.

Texture refers to the size of the particles that make up the soil. Sand, silt and clay identify relative sizes of soil particles with sand being the largest particle and clay being the smallest particle. To visualize a relative comparison, think about golf balls. Imagine that one golf ball represents one grain of sand. Compared to a golf ball, a particle of clay would be smaller than a single grain of salt. Clay can be 10,000 times smaller than sand. As all those varying particles join together to form soil, the prevailing component defines it. If sand is the major component you have a sandy soil, if everything balances well you have a loamy soil, and if clay is overwhelming you have a clayey soil.

Sandy soil in one of my gardens.

Structure is how those particles of clay, silt, and sand fit together to create soil. An important part of that structure is the little air pockets that exist when two particles join together. Those air pockets are defined as pore space. Think about golf balls again. When you hold six or seven golf balls in your hand you can easily identify the big air spaces between the individual balls. Sandy soil has lots of air, lots of pore space. Now pour salt into your hand. Can you see the air pockets when the sand grains stick together? It's not as easy. Clay soil has very little air space between particles, very little pore space.

Pore space between soil particles.

Fertility refers to the capacity of the soil to supply nutrients to plants. Organic matter in the soil, microorganism activity in the soil, and chemical elements present in the soil all influence its fertility. Soils are primarily mineral based and depending which minerals are present, different elements are available to plants as nutrients. The primary nutrients are nitrogen, phosphorus, and potassium; those are the same three nutrients you see available on almost every box or bag of fertilizer. There are a total of 17 elements that plants need for growth. Oxygen, carbon, and hydrogen are supplied through water and air and all of the rest come from the soil.

16% nitrogen, 16% phosphorus, 16% potassium

Good soil, or a soil with good tilth, has good texture, good structure, and good fertility. In good soil only about 50 percent of it is solid; the remaining 50 percent is pore space. That pore space is filled with air, water, or a combination of the two. Roots grow through the pore space seeking the air and water.

If a soil has too small pore spaces, like in a high-clay (clayey) soil or in a compacted soil, roots have a hard time growing. Clay soils do well in retaining water and nutrients inside the pore spaces, but at the cost of air. The lack of large pore spaces means the water has a hard time draining out and air has a hard time seeping in. There are plants that do relatively well growing in clay soils, but many can be drowned when the soil gets wet.

If a soil has too large pore spaces, like in a high-sand (sandy) soil, the opposite problem holds. There is an over abundance of air while water and nutrients drain quickly through the pore space. Roots can easily grow through the air space, but without water retention they'll quickly dry out.

The key is to moderate the soil texture and improve the structure. This is most easily accomplished by adding soil amendments. The best soil amendment to improve structure is organic matter. Think of these soil amendments as little moist chunks of sponge as compared to the golf balls and salt grains discussed earlier.

When the large pore spaces between the sand particles are filled with little sponges, the water is soaked up as it drains through, retaining moisture and nutrients. When those same sponges are in clay, the tiny particles join together and surround it and as the organic matter decomposes it leaves behind a little pocket of air, creating new pore space.

Adding organic matter to a poor soil won't correct all of its deficiencies overnight, but it will begin to improve right away. Regular applications of amendments over a long period of time is often necessary to make good soil, and it is worth the effort. Organic material is good for all three soil tilth properties.

I'll discuss amendments and organic matter in another article soon. If you think your soil is too sandy or too clayey to grow a garden don't despair. Help is on the way with the addition of organic matter.

Link to "Developing New Garden Beds"
Soil is the most important component of your garden. Let me repeat that. Soil is the most important component of your garden. The large majority of plant problems, as much as 80 percent, can be attributed to the condition of a garden's soil. You can have fancy raised beds, state-of-the-art drip irrigation systems, and expensive heirloom seeds, but if your soil is poor your crop results will be too.

Soil is simply a mix of minerals, air, water, and organic material that combine to form a growing medium for plants. A few days ago I mentioned soil tilth. (In "Developing New Garden Beds" on Feb 20, 2011).  Tilth is all about the soil's ability to support a plant's root growth. Root growth directly correlates to plant growth; poor root growth equals poor plant growth.

There are three properties that define soil tilth: soil texture, soil structure, and soil fertility.

Texture refers to the size of the particles that make up the soil. Sand, silt and clay identify relative sizes of soil particles with sand being the largest particle and clay being the smallest particle. To visualize a relative comparison, think about golf balls. Imagine that one golf ball represents one grain of sand. Compared to a golf ball, a particle of clay would be smaller than a single grain of salt. Clay can be 10,000 times smaller than sand. As all those varying particles join together to form soil, the prevailing component defines it. If sand is the major component you have a sandy soil, if everything balances well you have a loamy soil, and if clay is overwhelming you have a clayey soil.

Sandy soil in one of my gardens.

Structure is how those particles of clay, silt, and sand fit together to create soil. An important part of that structure is the little air pockets that exist when two particles join together. Those air pockets are defined as pore space. Think about golf balls again. When you hold six or seven golf balls in your hand you can easily identify the big air spaces between the individual balls. Sandy soil has lots of air, lots of pore space. Now pour salt into your hand. Can you see the air pockets when the sand grains stick together? It's not as easy. Clay soil has very little air space between particles, very little pore space.

Pore space between soil particles.

Fertility refers to the capacity of the soil to supply nutrients to plants. Organic matter in the soil, microorganism activity in the soil, and chemical elements present in the soil all influence its fertility. Soils are primarily mineral based and depending which minerals are present, different elements are available to plants as nutrients. The primary nutrients are nitrogen, phosphorus, and potassium; those are the same three nutrients you see available on almost every box or bag of fertilizer. There are a total of 17 elements that plants need for growth. Oxygen, carbon, and hydrogen are supplied through water and air and all of the rest come from the soil.

16% nitrogen, 16% phosphorus, 16% potassium

Good soil, or a soil with good tilth, has good texture, good structure, and good fertility. In good soil only about 50 percent of it is solid; the remaining 50 percent is pore space. That pore space is filled with air, water, or a combination of the two. Roots grow through the pore space seeking the air and water.

If a soil has too small pore spaces, like in a high-clay (clayey) soil or in a compacted soil, roots have a hard time growing. Clay soils do well in retaining water and nutrients inside the pore spaces, but at the cost of air. The lack of large pore spaces means the water has a hard time draining out and air has a hard time seeping in. There are plants that do relatively well growing in clay soils, but many can be drowned when the soil gets wet.

If a soil has too large pore spaces, like in a high-sand (sandy) soil, the opposite problem holds. There is an over abundance of air while water and nutrients drain quickly through the pore space. Roots can easily grow through the air space, but without water retention they'll quickly dry out.

The key is to moderate the soil texture and improve the structure. This is most easily accomplished by adding soil amendments. The best soil amendment to improve structure is organic matter. Think of these soil amendments as little moist chunks of sponge as compared to the golf balls and salt grains discussed earlier.

When the large pore spaces between the sand particles are filled with little sponges, the water is soaked up as it drains through, retaining moisture and nutrients. When those same sponges are in clay, the tiny particles join together and surround it and as the organic matter decomposes it leaves behind a little pocket of air, creating new pore space.

Adding organic matter to a poor soil won't correct all of its deficiencies overnight, but it will begin to improve right away. Regular applications of amendments over a long period of time is often necessary to make good soil, and it is worth the effort. Organic material is good for all three soil tilth properties.

I'll discuss amendments and organic matter in another article soon. If you think your soil is too sandy or too clayey to grow a garden don't despair. Help is on the way with the addition of organic matter.

Link to "
Developing New Garden Beds"

4 comments:

  1. I just found your blog over the weekend and I have thoroughly enjoyed reading ALL of your posts! What a great wealth of information you have about gardening in this tricky area. I am just getting my garden started for the season after having gardened in Florida for the last 10 years - what a change! Keep up the great posts - we all really appreciate it!

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  2. Thanks so much, Diana. Good luck with your garden. Don't hesitate to ask if you have any questions.

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  3. Hello,
    This is my 2nd year participating in a community garden. My plot is 30'x12'. The soil is mostly clay. Last year, I amended to a spade's depth with dried leaves, green grass clippings and wood chips. Small, long stemmed fungus grew from the chips toward the end of the season so I'm leary about adding them to this season's amendment. This season, I have 2yr old compost (beautiful) and more dried leaves. I feel that I should dig as deep as last season because the soil is still very compact, few worms, etc., but others tell me that I don't need to repeat this very though process and should be able to get by with just mixing in the amendments to a depth of 6" or so. What do you think?

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  4. Sorry for the delay in a response. First, think about how deep the roots are of your plantings. If they are shallow plants, six inches may be enough. However, I recommend amending as deeply as you can so you have options for many types of plants and so you can build a good soil environment. The long fungus is most likely mycelial growth and is perfectly natural. That's one of the soil microorganisms that decompose organic matter, like wood chips. It's not harmful and is a sign of an active and potentially healthy soil. I recommend digging as deeply as you can again. Particularly with clay, it takes awhile to improve the soil structure and fertility. Compost is my favorite amendment and working it deep in the soil will improve the entire area. The only concern I have about adding dried leaves and wood chips is that they will take nitrogen from the soil as they decompose so you should plan to fertilize the plants with nitrogen to counteract that; they are good choices for clay soil because they take longer to decompose and improve the soil structure along the way. Worms are a good indicator of overall soil health. When you can put the spade in the soil, pull it out, and see worms in the hole, that's the point that you've attained a good level of amendment and can reduce some of your labor and amending. Until then, keep up the labor.

    ReplyDelete