Soil is the foundation which is ultimately responsible for a healthy garden or lawn. Soil is different from “dirt” because it contains a complex mixture of mineral matter, organic matter and living organisms. It is capable of sustaining life.
It is therefore not surprising the single most important task gardeners can perform is to improve the soil quality. No amount of fertilizer, water, or other means of regulation will have as profound effect on healthy plant growth as healthy soil.
This article will answer questions such as: “What kind of soil do I need ?”, “How can I get good garden soil?”, “Do I need to replace the soil?”.
What plants need
Besides simply anchoring the plant, soil provides certain chemical and physical properties. Plant roots must extract water and dissolved fertilizer nutrients from the soil. In addition to this, plant roots require oxygen to be present in the soil to allow them to breathe.
The optimum conditions vary by plant but generally loose, friable, water retentive soil is near ideal. It allows oxygen and water to co-exist while allowing plant roots to grow deep.
What is Soil?
The main ingredients in soil are inorganic mineral particles of clay, sand, silt, and rocks, which has been deposited on the surface over a period of many years. In the cavities formed between these particles, water and oxygen reside.
Soil is typically described based on the relative combination of the 3 particles: clay, sand and silt.
- Clay particles are very small, flat, sticky particles about 1/17,000 inch in size, e.g. they are very small, right?
- Silt is rounded particles about 10-15 times larger than clay particles.
- Sand is also rounded particle but 50-1000 times larger than clay particles (depending on the grade of sand).
- Loam is a term used about a mixture of all 3 particles in equal proportions (1/3 each).
- Since soil does not always contain the same volume of each, different “grades” of loam have been established. Examples include sandy loam which contain relatively more sand than clay and silt, whereas loamy clay contains more clay than sand and silt. There are other variations possible, too.
Each ‘loam combination’ has its own specific advantages and disadvantages.
The sand particles allow plenty of oxygen in the soil due to the relatively large spaces between them, but water drains too quickly below the root zone and washes out fertilizer nutrients, also.
Clay particles on the other hand tend to pack down leaving only very small spaces which fill with water but excludes air. The sticky clay particles can make it difficult for roots to grow deep. Clay is very good at holding on to fertilizer nutrients through their large combined surface area, but sometimes even too good. The chemical properties of clay soil can result in fertilizer being tied up; the clay particles bond chemically with the nutrients making them unavailable to the plants.
Silt retains properties of both clay and sand, including their advantages and disadvantages.
In most cases, a combination of the 3 types of minerals (loam) gives the best growing conditions. This is because the combination of small, medium and (relatively) large particles provide for uneven cavities which allow for a good combination of aeration, drainage and nutrient holding capability. Some plants do better with relatively more clay in the soil, but most plants in general prefer a loam or sandy loam soil.
What truly distinguishes soil from “dirt”, however, is the presence of numerous living bacteria, fungi, worms, and insects which inhabit the soil. Many of these organisms help maintain fertility of the soil by decomposing and recycling plant and animal residues. Soils rich in these residues, “Organic Matter”, are commonly termed “living soil”.
During the decomposition process, fungi binds small particles into larger aggregates which further improve the structure, aeration and water infiltration (see drawing above). Organic matter can help reshape the large cavities in sandy soils and thus aid in the retention of water and fertilizer nutrients. In clay soils, the sticky particles are separated by organic matter allow roots passages through the soil. Organic Matter is also capable of holding fertilizer nutrients in a (chelated) form which is available for the roots.
Changing the Structure of Soil.
The structure of our garden soils are typically determined by the general area in which we reside, e.g. the sandy soils of Florida or red clay of the Southwest. In some instances, builders will have purchased “top soil” but unfortunately it is very seldom high quality loam.
Trying to change your soil profile into loam by adding clay, sand, or silt can be difficult, and expensive, and it does not solve the goal of achieving “living soil”. For example, adding sand to clay soil can sometimes “set” the clay into concrete-like structures unless substantial amounts of sand are added. This happens when the small clay particles pack tightly around the larger sand particles which then become similar to steel rebar in concrete.
Instead, the addition of organic matter is the simplest and most effective way to improve any type of soil for almost any type of garden purpose. In a sense you can consider it the “universal ingredient” which is used to transform dirt into living garden soil irrespective the profile of your existing soil. No guessing or soil testing is necessary when you add organic matter.
Examples of Organic Matter.
Many sources of organic matter are directly available in the yard already:
- Plant trimmings
- Grass cuttings
- Shredded fall leaves
- Household compost (from the compost bin)
- Aged/composted manure (from farms)
Generally, composted products are safer to use. If non-composted materials are used, such as shredded leaves in the fall, additional nitrogen fertilizer should be added (a light layer of lawn fertilizer added to the shredded leaves is fine). Manure should ideally be composted to avoid burning tender roots, or the soil prepared 6 months in advance of planting. Grass clippings should be composted or spread in a thin layer on top of the soil.
The Process of Amending Soil.
The process is relatively simple:
- Dig the flower bed to be improved by turning over the top 8-10” of soil (one spade depth).
- Spread a 3-4” layer of organic matter, such as compost, and mix it thoroughly into the top 10-12” of soil. This result in 30-35% organic matter which is recommended.
- Plant flowers and shrubs in the amended soil. Water well once to settle the soil around the roots.
- Every fall, and spring if possible, add another 1-2” of compost to the top of the soil. Organic matter slowly decomposes in soil and adding more every year will ensure the continued improvement of your soil.
Composted forest products and manures are available in one or two cubic feet bags at garden centers. These are also commonly sold as “humus”. If you purchase amendments by bulk, retailers usually charge per (cubic) yard. You need approximately 1 cubic yard (27 cubic feet) per 100 sq feet garden area.
Obviously, the best compost you can make at home in your own compost pile, and don’t forget to check your local county or city recycling center. In spring many centers offer free compost if you pick it up yourself. Check the phone book.
If you need your garden maintenance by professional experts, check out Portland TT’s services.