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Soil-Building Tips for Green-Thumb Gardeners

By Skip Richter
Contributing Editor

Gardeners are dreamers. Everyone who sets out to create a garden has a vision in mind, whether it is a bountiful vegetable garden or an orchard, a garden of flowers exploding with color or a landscape with the perfect lawn framed with an array of shrubs, vines, trees and perennial beds. When I’m at the garden center picking out plants, my thoughts are on how great they will be as they put on their show in my garden and landscape.

If these dreams are to become reality, our plants must have a foundation in which to establish and thrive. Soil is the foundation of every great garden. Get the soil right and plants will thrive. Skip this step and all the fertilizers and miracle elixirs in the world won’t deliver success. By the time you put the first seed or transplant in the ground, you are 75 percent of the way to success or failure. You’ve chosen a spot with sun or shade, you’ve selected a species or variety that is or is not adapted, and you’ve either built a great soil or plopped your plants into something that will scarcely support survival, much less growth and production.

As an Extension agent I’ve heard many folks lament that they had brown thumbs when in fact a little information and soil preparation would have made all the difference in the world. This article is about turning brown thumbs green!

Think like a plant or, better still, think like a root. A plant will only be as successful as its root system. What does a root need to thrive? Moisture, oxygen, nutrients, a good microbial community and soil loose enough to give way for a proliferation of growth. Give your plants the soil they want and they will make you look good. Not all plants want the exact same soil conditions. Interview an azalea and a Texas mountain laurel and you’ll get two different stories for sure. But there are some basic principles of soil preparation that hold true for most of the plants in your landscape and gardens.

Texture, Structure
Texture describes the size of particles in the soil. Clay is extremely fine-textured particles. Silt particles are quite a bit larger and sand is like boulders compared to the other two! Clay soils hold water and nutrients very well, while on the other end of the spectrum sand allows water to run through and has comparatively little surface area for nutrients to adhere to. To give you a picture of this difference, consider the fact that about 2 cups of sand has a total surface area of 20 square feet, compared to 220 square feet for silt and 5,500 square feet for clay! Think about what that means for the soil’s ability to hold water and nutrients on the particle surfaces to support future plant growth and development.

The problem with sand is that it tends to be droughty and nutrient poor. Decomposed organic matter helps a sandy soil by improving its ability to hold onto water and nutrients for more sustained plant growth over time. Think of compost as tiny sponges holding onto moisture and increasing the soil’s cation exchange capacity — its ability to hold onto positively charged nutrient particles.

On the other hand, clay soils often tend to be poorly drained. While we can’t change the texture of our soil (other than by purchasing and bringing in more soil of a different texture), we can change the structure of our soil.

Structure is the way that soil particles group together. Some clay soils tend to have a very poor structure, with the particles tightly smashed together so that there is little room for air and water movement between them. Think of modeling clay. It’s great for making a bowl to hold water but is not inviting for plant roots that need oxygen to thrive. Rainfall or irrigation tends to run off rather than soak into such a clay soil.

A well-structured clay, on the other hand, has particles grouped together in clumps with air space between them. Think of a bowl of popcorn with the clumps of exploded kernels having air space around them. Water and air can move freely within such a soil, as can plant roots.

Sodium tends to cause some types of clay to have poor structure. That is why gypsum is recommended for sodium-laden clay soils. Gypsum will not improve the structure of all clay soils but can be helpful when sodium is causing structural problems.

The real miracle worker for clay soils is organic matter. Decomposed organic matter helps clay develop structure over time. So, if you have a heavy clay soil, compost is critical to developing better structure. A clay soil with poor structure and organic-matter content when dry will be as hard as concrete, whereas one with good structure will crumble apart in your hand when squeezed. Because compost continues to decompose into humus, it should be added whenever possible to maintain its structural benefits.

If you are building a landscape bed in a clay soil to plant perennials or shrubs, rototilling compost in every year is not an option. The best approach to improve the soil’s internal aeration and drainage is to mix in expanded shale. This material looks somewhat like fired-clay kitty litter but is heated to a much higher temperature, creating a hard, expanded, porous particle that will maintain its structural integrity over time and help open up a heavy-clay soil.

Nutrient Content, pH
Plants will grow as much as the most limiting nutrient in the soil will allow. Think of a barrel with each stave representing a single nutrient. The staves have been sawed off at various heights. How full can you fill the barrel? Only up to the height of the shortest stave. When even one nutrient is lacking, the plant’s ability to grow, bloom or produce a crop will be limited to that extent. Having extra potassium or nitrogen, for example, won’t make up for a lack of magnesium or iron.

It is much easier to amend your soil with nutrients before planting than after plants are already in the ground and starting to show nutrient-deficiency symptoms. So, have your soil tested before you plant a garden. The state soil testing lab (http://soiltesting.tamu.edu) has downloadable forms and instructions on how to take an accurate soil sample. When the results are in, they will include fertilization recommendations. You can contact your County Extension Office to discuss the best approach for your gardening plans. There are organic and synthetic sources of nutrients to help you optimize your soil’s nutrient levels. Spread the nutrients as per the soil test recommendations and spade or rototill them into the soil to a depth of about 6 inches.

Soil tests also include the pH levels. Put simply, pH ranges from acidic (low) to alkaline (high) and affects the availability of the nutrients that are present in the soil. Plants differ on their ideal pH but most perform best in the 5.5 to 7.5 range. It is difficult to significantly change the pH of most clay soils, but in sandy soils it is easier to make pH adjustments, using sulfur to lower pH or lime to raise it.

Life in the Soil
Soil is literally teeming with life. There are 100 million to a billion microbes in a kidney-bean-sized pinch of rich garden soil. In a small handful of soil from a garden, you are holding more living things than there are people on the face of the earth! In addition to the large creatures of the soil, such as earthworms and various insects and mites, there are beneficial fungi and nematodes, bacteria, actinomycetes, protozoa and algae.

Living things eat and excrete. They change their environment in various ways by their activities. Shrink down to a root level and you’ll find microbes releasing organic acids that help dissolve nutrients off of mineral particles, fixing nitrogen from the air in the soil into a form plants can use, living in symbiotic relationships with roots helping them draw water and nutrients from the soil or fend off disease organism, and a multitude of other functions.

In a very real way, microbes feed plants. They also are fed by plants as plant roots exude substances to support microbial proliferation. We can enhance microbial growth by adding organic matter, which they will proceed to turn back into soil and plant-enriching nutrients. We can also help by keeping the soil moderately moist and minimizing tillage.

Building Great Soil
So what about that patch of soil out back that you would like to turn into the next Garden of Eden? Once you’ve had the soil tested and added any needed nutrients, what now?

Take every opportunity you can to add organic matter to the soil. Here in our warm southern climate we “burn up” organic matter faster than we create it. In unimproved soil the organic matter content may be around one half of a percent. There are several ways to increase the organic matter content in the soil.

The simplest way is to mix well decomposed compost into the soil to a depth of 6” or more. If you’ve never amended your soil with compost, start by spading or rototilling in 4”. Depending upon the equipment you are using, this may be difficult to do at one time, so spread compost about 2” deep, rototill it in and then spread another 2” and rototill again.

If you have been amending your soil each year, you may only want to add a couple of inches of compost. Some gardeners apply an inch every time they transition from one crop to another.

You can also create an instant garden by purchasing a bed mix from a local compost supplier that includes a combination of soil and compost. These bed mixes vary in content and particle size depending on the supplier and the type of plants you intend to grow, but are usually at least 50 percent composted organic matter. Spread a few inches on the surface of your existing soil, mix it in to create a more blended interface between the two types of soil and then add the rest of the store-bought soil on top. While this does create a ready-to-grow garden, I’ve found that it often takes a year or two for the soil to adjust and for vegetables and flowers to thrive.

If you want to save on purchasing compost, you can rototill undecomposed organic matter into the soil surface and then give it time to start to decompose prior to planting. This technique is often referred to as “sheet composting.” Spread shredded leaves, grass clippings and/or manure a couple of inches thick, mix it in as deeply as you can, and repeat. Then water the area well and cover the soil surface with a blanket of leaves or other mulch to hold in moisture and deter weeds. Allow the area to sit undisturbed at least several weeks before planting. In our warm Texas conditions, the buried organic matter will quickly begin to decompose.

Green-Manure Crops
Another option for building the soil is to plant a cover crop to protect the soil, improve structure and crowd out weeds. When these crops are turned under to add their organic matter to the soil, we refer to them as “green manure” crops. Some common green manure crops include cereal rye (‘Elbon’ is one variety) and mustard (cool season), buckwheat (spring or fall season), or Sudex (sorghum-sudangrass hybrid for warm-season use). Note that Sudex gets quite large and is not suitable for a small garden. If you have a large garden area where you’d like to try Sudex, plan on cutting it back to about 6–12” high when it reaches about 2–3’ tall, and eventually cutting it back to near the soil line before plowing it under.

Legumes such as southern peas (warm season) or vetch and crimson clover (cool season) will provide the additional benefit of fixing nitrogen in nodules on their roots if left to grow for a sufficient amount of time. Other less common green manure crops may be worth experimental use but most anything that you can grow in a dense planting and turn under could be a potential green manure crop.

Finally, consider drainage. The farther east you live in Texas, the more it rains, and every Texan knows, “when it rains it pours” — sometimes for days on end! As a 93-year-old veteran gardener used to tell me, “You can always add water, but you can’t take it away!” He built his raised garden beds prior to planting when the soil was dry enough to work. Then when a deluge arrived, his plant’s roots were out of standing water and his gardens thrived while others drowned in soggy, anaerobic conditions. If drainage is at all in doubt, go with raised planting beds.

So if past failures at gardening have left you feeling that you have a brown thumb, take heart! Brown is just a reminder to begin with the soil if you want to enjoy the success of bountiful harvests and beautiful landscapes. Spend a dollar and a little time on your soil before you spend a dollar on some plants, and you’ll have the greenest thumb on the block!

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