Soil

Soils are often described by their predominant minerals, such as sand, silt, or clay. Water drains quickly from sandy soils; they also tend to be low in nutrients. Clay soils are higher in nutrients but do not drain well and can be quite sticky. Soils that have a balanced mix of sand, silt, and clay particles are called loams. When a loam soil is mixed with 3%–5% organic matter, it has the ideal texture for cultivation of a garden because it provides adequate drainage and nutrients. Soils with less than 1% organic matter are not very fertile. Soils that are in the best condition for growing plants are said to have good tilth. The mix of minerals in the soil is referred to as the texture and can be determined by sending a sample to a soil testing laboratory. Soil structure also affects tilth. If a soil has been compacted due to heavy equipment, it will be more difficult for plant roots to spread.

Natural soils are evolving, dynamic entities that consist of minerals plus live microbes and decomposed organic matter. Organic matter is the result of decomposed plant, insect, and animal matter plus the microbes that decompose it. Organic matter has been decomposed beyond recognition of the original material. It is black and crumbly and has the ability to hold large amounts of water and nutrients. It is often called humus or compost.

If the soil has an abundance of clay, compost is added in order to improve the tilth. Adding sand to clay soil is not recommended, as this produces a material similar to cement, which as you can well imagine is not very conducive to growing plants. Likewise, if the soil is silty or sandy the addition of compost will also improve the tilth. However, many wildflowers and some culinary herbs prefer to grow in nutrient-poor, sandy soil so how you amend your soil depends upon the soil you have to start with as well as what you want to grow in it.

There are 17 nutrients that plants require in order to grow. They are supplied by the air, water, and soil. Nutrients are elements used by growing plant cells to synthesize more cells and to help fuel physiological processes. These elements are cycled through the environment in a process called biogeochemistry. Biogeochemical transformations of elements are mediated by microbes that may derive energy for growth and/or nutrients from the transformation reactions.

Some nutrients are considered macronutrients because they are required in relatively large concentrations. Hydrogen comes from water; potassium and phosphorous come from the soil; and carbon, oxygen, and nitrogen come from the air. Sulfur, magnesium, and calcium are required in lower concentrations and also come from the soil. Other soil-derived nutrients that are required in even lower concentrations are called micronutrients or trace elements and include chlorine, iron, manganese, boron, copper, zinc, molybdenum, and cobalt. If any one of these nutrients is not present in the required amount, growth of the plant will be inhibited. This is known as Liebeg’s Law of the Minimum. Too much of a good thing is not good in this case, as micronutrients can become toxic to plants when applied in excessive amounts. Silica, a major component of sand, is also believed to be an important mineral for plant growth.

Hydrogen is the most common element and is also an important macronutrient. It is a component of water that is found in soil pores. When water molecules (H₂O) split, as they often do, they generate positively charged hydrogen ions (H⁺) and negatively charged hydroxy radicals (OH⁻). A pH meter measures the number of hydrogen ions in a saturated soil and returns the value as a negative log function that ranges from 0 to 14. If there are an equal number of hydrogen ions and hydroxy radicals, the pH is neutral and has a value of 7. A pH of less than 7 is acidic; the lower the number, the more acidic it is. If the pH is greater than 7 the soil is said to be alkaline; the higher the number, the greater the alkalinity.

Most cultivated plants prefer a soil with a pH between 6.5 and 6.8 although a few tenths of a unit either way is usually tolerable. The reason for this has to do with the effect of pH on the availability of nutrients. When the soil pH is in this range, all of the nutrients are soluble and the plant can access them through its roots. When the pH is more alkaline or more acidic, some of the nutrients will form insoluble compounds that the plant cannot access even though they are present in the soil.

Minerals such as calcium, magnesium, potassium, and sodium contribute to alkalinity and are called soluble salts. They may be leached out of a soil that has formed in hot, humid conditions with high annual rainfall (tropical, subtropical). The soil becomes very acidic and may have problems with aluminum toxicity because aluminum changes to a soluble form at low pH and the plants can take it up through the roots.

Figure 4.2 The excessive use of fertilizers and irrigation in arid regions can cause overly alkaline soil. The alkalized soil forms a white crust of accumulated salts on the surface of the fields, which is toxic to most plants and detrimental to the land.

Soluble salts tend to accumulate in arid and semiarid regions that are heavily irrigated—especially in poorly drained clay soils (Figure 4.2). Water evaporates quickly in these environments and the salts become concentrated in the upper layers of the soil. These soils are not good for growing a garden and may become home to microbial communities and plants that are adapted to the high salt concentration. Soils that have accumulated very high concentrations of some salts (such as sodium) are very difficult to bring into good tilth. Some plant breeders are working on the creation of hybrid crops that can withstand high salt concentrations.

Soil pH can be increased by adding lime, which is crushed limestone mined from quarries and contains calcium carbonate. Sometimes it is mixed with magnesium and called gypsum. The addition of wood ashes, which are high in potassium, also raises the pH of the soil. This needs to be done carefully, several months prior to planting to allow the chemical reactions to take place. Too much calcium or potassium can elevate the pH beyond what is tolerable to garden plants, and it may take years for the pH to lower again to a suitable range.

Sulfur lowers the pH of the soil. Organic amendments such as pine needles and peat moss also lower the pH. The amount to use depends upon the pH of your soil, the size of the area you need to cover, and what you plan to grow. Some plants such as blueberries and azaleas prefer a soil pH of 6.0, more acidic than for most garden plants. Soil test results can indicate the appropriate rate of application of materials to alter the pH of your soil for the particular plants you intend to grow. Soil test results can also indicate whether you need to add nutrients and in what amounts.

If your compost and mineral soil do not supply enough nutrients, you may need to supplement with fertilizer. There are many fertilizers on the market. Organic and chemical fertilizers are formulated to supply a mix of the three nutrients required in abundance, which are nitrogen (N), phosphorous (P) and potassium (K). The amount of fertilizer is shown on the label as a ratio of N-P-K. Some fertilizers are formulated to supply the micronutrients as trace minerals. Chemical nitrogen fertilizers are generated by the conversion of nitrogen gas in the air to a soluble powdered form. This process requires energy because the gaseous nitrogen bonds are difficult to split. Rock phosphate is a substance mined from the ground; large supplies are found in Florida. Rock phosphate supplies high phosphorous and also adds calcium. Greensand is from ancient seabed deposits and is a source of potassium and many trace minerals. Seaweed is also high in potassium.

Organic fertilizers such as blood meal and fish meal also supply high nitrogen. They come from slaughterhouses and processing plants. If you prefer not to use animal products, legumes such as soybean meal and alfalfa meal can be used as a nitrogen fertilizer. Many growers plant alfalfa to increase the nitrogen in the soil. Home gardeners often plant peas or beans, which are also legumes.

Plants have different requirements for nutrients at different stages of growth. Nitrogen promotes vegetative growth and too much may result in poor flower and fruit formation. Phosphorous stimulates root growth and flower production and is useful to add to a new transplant and when plants begin to flower. Potassium is required in the transport of nutrients and also is involved in turgor pressure and transpiration.