Improving the soil is done for the purpose of making a more habitable home for roots and plants, teaming with beneficial microbes. To accomplish this, an improvement in the physical, biological and chemical components are needed, affecting organic content and soil structure.
Practice No. 3. Adding Organic Amendments
Our Panhandle soils are naturally deficient in organic matter. Quantities present can range from below a half percent to up to 3%, most soils will contain about .5% to 1.5%. Five percent organic matter is a good percentage to work towards for vegetable gardens and other high feeding, high water-use plants. If you've had a soil analysis performed, you'll know your starting point.
Adding organic matter to the soil brings many positive benefits. Organic matter:
- Increases growth promoting fungi which helps control diseases, root rot and damping off fungi
- Promotes growth of mycorrhizal fungi, the beneficial fungi that grow in decomposing organic matter, destroys root-knot nematodes and other disease causing organisms
- Promotes nitrogen fixing organisms
- Enables long-term nutrient release
- Unlocks minerals for better absorption
- Helps balance the pH of the soil
- Promotes better soil structure
- Promotes good soil tilth, increases moisture retention, aeration and drainage ability of soil
- Recycles plant waste products
- Prevents water erosion
- Acts as buffer to chemicals
- Reduces soil toxicity
- When ample minerals and nutrients are readily available, plants require less water for their uptake
- Less maintenance (fewer herbicides/pesticides/fungicides are required)
- Increases plant's ability to survive when under stressful weather/climate conditions
- Enables more beautiful gardens.
Many different beneficial micro and macro biological life feed on the organic matter to produce the above mentioned benefits. The structure of the soil itself is very important to the ability of plants to grow and survive. Organic matter has a big effect on soil structure. Soil structure is different from texture. (Texture refers to the size of the particles, whether the particles are sand, silt or clay sized.) Soil particles don't exist as single particles, but as aggregates. Soil structure refers to the way the particles are grouped together, the form and shape of the soil aggregates. It is not practical or feasible to add enough organic matter to change the soil texture. However, adding organic matter improves soil structure.
Through the process of breaking down organic matter, microbial life and other soil fauna decompose it producing exudates, fecal pellets and casts packed with plant nutrition, lignin and humus. Humus is not readily decomposed, but is important in binding tiny soil aggregates and improves water and nutrient holding capacity. Many different microbes produce different exudates (biochemical compounds). A diversity of each type of microbial life is necessary for an active, healthy soil community.
Organic matter contains carbon. Adding organic matter is the key, rather than adding nitrogen, one of the elements essential to plant growth. Carbon is the chief food of soil biotic life and needs to be present in a balanced ratio of 23-30:1 with nitrogen (C:N ratio). Organic matter is broken down by soil fauna into many different compounds, in the process becoming depleted. Your soil needs to be replenished with organic matter from time to time, depending on the plants needs.
But there is more to it than that, as researchers, particularly Dr. Elaine Ingham (The Soil Food Web: It's Importance in Ecosystem Health, and www.soilfoodweb.com) are finding out. Dr. Ingham's work centers around the rhizosphere, the interactions of soil and root, how nutrients are made available and how a plant feeds. Anywhere from 60 – 80% of a plants energy is first directed to establishing the root system. After that, half of the roots' energy produces root exudates. Root exudates are mixtures of simple sugars, carbohydrates and proteins whose purpose is to feed compatible microorganisms. Different plants exude different mixtures to attract and grow particular microorganisms for different stages of growth. The science of the biological component of soils has grown to the point where an analysis of the soil's microbes can identify a particular plant species or cultivar, and the time of the year the soil sample was taken! (Organic Soil Fertility Management, Steve Gilman, Chelsea Green Publishing Co., 2002.)
Gardeners need not obtain a biological analysis each time they feed the soil, unless their goal is maximization of crop yield. For the average home gardener, adding composted organic matter and organic matter to the soil produces pretty good results. Using a variety of organic materials is better than using just one kind of composted material. Sir Albert Howard, in Soil and Health, a Study in Organic Agriculture, discovered that composted organic material from both plant and animal (manures) sources are better than from just one or the other. The microbes and plants will work together and draw from the soil the nutrients they need, as they need it. Over applying compost or organic matter is usually not an issue with medium and high feeders. Some drought tolerant, low water-use and low feeders do not require much organic matter (and certainly not chemical fertilizers either) to thrive. In fact, with some few drought-requiring plants and plant groups, organic amounts in the medium and high categories are detrimental and cause plants to die. They prefer their soil lean.
Applying manures has been brought into question lately. Doesn't manures add salts and increased phosphorus as well? I think one needs to be careful as to the source of manures one uses, whether one uses raw or composted manures and for what crops or plants. If one wishes to be as organic or ecologically-friendly as possible, one would use composted manures only from certified organic operations. Everything an animal ingests or is injected doesn't always metabolize – some of the substance may be passed along into the manure, which then would go into your soil. This includes weed seeds as well as any harmful or unwanted substances. A complete discussion on raw manures and composted manures can be found on the ATTRA website, The National Sustainable Agriculture Information Service at (download fee may be applicable).
To summarize, the best manure is a composted manure from dairy cows (because of cows four stomachs), which will have less salts and weed seeds (from an organic certified dairy). One should be cautioned against using manures for animals that are meat eaters, as well as raw manure, unless one is knowledgeable in its correct use. The manure should be composted using the hot, rather than passive method. Most home gardeners are not careful enough in their composting practices to insure the compost heats up to specific temperatures and that temperatures are sustained, according to the compost method. Do not spread manures on vegetable beds just before planting.
Composted manures spread on ornamentals should not normally pose a problem. Other composted manures besides cow manures are OK too. Just be mindful that weed seeds, pathogens or heavy metals may not be broken down, depending on the composting process. In some dairy operations, straw, sawdust or lime may be included in the raw manure before composting. Adding composted manure to soil will lower the pH over time, which is what we need for our alkaline soils. However, if the manure was laced with lime (which is great for acidic soils), this could be detrimental instead of helpful in alkaline soils. Again, this is a case of the gardener checking out his source, becoming knowledgeable about the product. Likewise composted cottonseed hulls, unless composted properly, may not have destroyed the weed seeds.
One additional factor to always keep in mind is the origin of whatever product the compost or amendments. Crop injuries of poor germination, death of young plants, twisted, cupped and elongated leaves, misshapen fruit and reduced yields can be caused by the presence of certain persistant herbicides in the manure, compost, hay or grass clippings applied to the soil. A class of persistent herbicides known as pyridine carboxylic acids, including aminopyralid, clopyralid, fluroxypyr, picloram and triclopyr, (sold under the trade names Curtail, Confront, Forefront, Lontrel, Grazon P + D, GrazonNext, Milestone, Redeem R&P, Surmount, Millennium Ultra Plus, Millennium Ultra and Ultra 2, Clopyr AG, Stinger and Imprelis; there may be others) remain active in the plant and in the manure of animals who have eaten it even after composting. The herbicides can also leach into the soil with rainfall, irrigation and dew, as well as remaining active in the treated soil for up to 3-4 years.
Degradation of the herbicides is particularly slow in piles of manure and compost. Because of the long residual effect, you may not know, nor would the farmer know, that his horse or cow had eaten hay, or that the straw was either sprayed with these herbicides, or previously contaminated manure was applied on fields. The subsequent composted manure, and composted straw from those animals would still contain the active herbicides. Cases of this hidden herbicide carryover has been discovered in Great Britain, Europe and the United States, including the Texas Panhandle (North Carolina State University Cooperative Extension, “Herbicide Carryover in Hay, Manure, Compost and Grass Clippings”). It is unfortunate that compost, one of nature's best soil amendments, now comes with numerous disclaimers because of current chemical practices in agriculture.
How Much to Add
Adding about three inches of composted organic matter is the minimum for most soils and plant requirements when initially amending the soil. Drought tolerant, low water-use plants do not require large amounts of organic matter. Adding three inches in low water-use beds is usually sufficient; adding more could be detrimental to xeric plants. Four to eight inches of composted organic matter should be added to beds for medium water-use plants, and 6 to 12 inches for high water-use beds. Work the organic matter down into the top 8 to 18 inches of soil, depending on the amount added.
Besides compost and humus, other organic substances can be used. Composted cottonseed meal, alfalfa pellets and meal, cocoa bean hulls, corn meal and corn gluten meal, dry horticultural molasses, worm castings, bat guano, fish emulsion, fish meal, kelp meal, bone and blood meal are organic amendments. Follow the directions on the bag for correct application. Compost is usually added to the soil in “inches thick”, all the rest is applied according to pounds per 100 or 1000 square feet.
It probably seems strange for a segment in Creating Organic Landscapes to be about inorganic amendments. Creating organic landscapes means gardening without using synthetic chemicals and salt-based chemical fertilizers, micro-niche gardening and gardening for the needs of the plants in a more sustainable manner.
When growing drought-tolerant plants that are regionally appropriate in our compacted clay, caliche and sandy soils, we don't need copious amounts of organic matter. Many xeric plants require good to very good drainage. This is problematic, in particular, for compacted clay and caliche soils.
Nearly a decade ago I attended a lecture entitled Western Organics in Albuquerque, NM at the Water Conservation Conference and Expo. Hoping to hear about organic soil amendments for the West, I was surprised when the presenter began talking about several different types of inorganic amendments. These products were relatively new and were being promoted as a way to loosen up tight soils for drought tolerant plants where little organic matter is needed and provide nutrient and water retention. This was a breakthrough for me and gardeners throughout the western half of the United States. Previously I had used coarse sand along with compost, but worried about the amounts of compost needed. Now I use expanded blue shale or calcined clay when digging or re-working beds, only adding about 3 inches of compost at initial bed preparation.
Using Inorganic Amendments
Another way to affect soil structure is by the addition of inorganic amendments. If you have tight, compacted clay and caliche soil, a mix of organic matter with one of the newer inorganic amendments should be added. I recommend adding an inorganic amendment for heavy clay soil, such as Turface® (calcined clay), Tru-Grow® (expanded blue shale), Soil Mender® Expanded Shale, Ecolite® (Zeolite) or Axis® (diatomaceous earth) and Profile® (calcined clay) for sandy soil. These are a few of the better inorganic amendments that retain water and nutrients, in addition to improving the drainage ability of the soil.
These are longer lasting amendments to the soil (report from Texas AgriLife Extension on expanded shale). It is claimed by some that expanded shale will breakdown over a period of years through compaction, that claim is refuted by the manufacturers and others. Diatomaceous earth will not hold it's shape in the soil as long as expanded shale, calcined clay and Zeolite.
Expanded shale, Soil Mender® brand, and diatomaceous earth are available at most nurseries, Turface® and Profile® are available at Pro Chem at 900 Ross Street and Landscape Supply at 6652 Canyon Drive on 1-27 in Amarillo. They may be available at other locations. I haven't yet seen Ecolite®, the Zeolite product sold locally. There may be similar products sold under different brands or names here and in other areas of the country. It is recommended to add 1-6 inches of expanded blue shale to the garden bed area, along with the compost, mixing it to the top 9 to 12 inches.
Other inorganic amendments with much more limited ability to retain water and nutrients are crushed granite, granite and lava sand, greensand, glass sand, and finally, regular sand. If the clay soil develops cracks during summer drought, please consider adding the inorganic as well as organic amendments into the cracked earth. Be careful in adding inorganic amendments with low water and nutrient holding capacities to soils with low organic content, such as fine grain and glass sand. You could end up with worse soil, mimicking brick.
Both organic and inorganic amendments improve soil drainage – the rate and extent of water movement in the soil, either across the surface or downward. This includes both the ability of the soil to retain water and nutrients and enables air to circulate. As an example, clay soil has good water retention qualities, but poor drainage; sandy soil has good drainage, but poor retentive qualities.
It may be that your soil has become so hard it is unworkable. In that case, aerate the area and topdress the soil with compost (3 inches is a good beginning) and water this in. Let this sit for a month or two, or over winter, moistening the soil periodically every 3-4 weeks if moisture is not received. The compost will slowly seep into the soil and microbes will begin to multiply and work. After this, you'll be able to begin digging and amending the soil.