If you are new to gardening one of the areas of key importance is your soil. The soil is the anchorage for the plants root system and provides the elemental nutrients needed for plant development. Soil science is a very large field of study and what I am covering here is only a small part of a scientific venue that we as gardeners need to have a cursory understanding of. I hope you have already noticed that I did not call it dirt, after a brief exposure here I know you will appreciate or at least have a better understanding of the soil that we daily walk upon, build upon and grow in. Did you know only 11% of the earth’s total ice-free land is arable land ie. land capable of being tilled and growing crops. The remaining 89% of the earth’s land is either too cold, too steep, too dry or too infertile to support crop growth. Of the 11% arable land only a small fraction can be described as highly productive.
Soil is an end-product of rock weathering into finely divided material mixed with organic matter that can support vegetation. There are three forms of rock weathering: physical weathering, chemical weathering and biological weathering. Soil is then formed in stages. The first stage is the accumulation of weathered rock fragments. The second stage is the formation of layers, called horizons. The layers run parallel to the grounds surface and are made up of like material, having similar structure, particle size, texture and porosity. The horizons differ from the layers above and below each other. The third stage is called humification. That is the process of growth and death of lichens, fungi and plants along with bacterial action to add organic content to the soil. The upper horizon is called topsoil and as gardeners this is the area we will be dealing with the most. Soils that have the three fully developed horizons are known as mature soils. Different soil characteristics are developed by rainfall, profusion of plants, drainage, temperature, topography and the mineral contents of the rocks that originally formed the soil.
The inorganic or mineral elements of soils are particles of varying size. From the largest to the smallest, soil components are divided into three types: sand, silt and clay. The four main classes of soil texture are loam, clay, peat and alluvium.
* Loam is a texture with sand, silt and clay in an even balance, with a high percentage of organic material.
* Clay is a texture with at least 20% or greater clay particles.
* Peat is a deposit of partially decomposed organic matter that has been preserved by the lack of oxygen through water saturation.
* Alluvium is fine grained soil of silt and mud particles deposited by streams or rivers.
Experienced soil testers can tell what type and texture of soil you have by hand. They see if the soil will ball up in their hand and they rub the soil in the palm of their hands to further identify type and texture. Of course this is just their observations that back up the lab analysis. Soil structure affects the ability of soil to hold air and moisture uniformly and the ability of root structures to develop.
Soil nutrients are essential for plant growth and development. The mineral nutrients are present in solution in the soil water, where it is taken in and up by the plants root system. The plant nutrients are divided into two main categories: macronutrients and micronutrients. The macronutrients are needed in large quantities and are as follows: carbon, oxygen, hydrogen, nitrogen (chemical symbol N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Carbon, oxygen and hydrogen are found in the air and water, with all the others found in mineral form. Calcium (Ca), magnesium (Mg), and sulfur (S) are also known as Secondary Nutrients. Plants need these three secondary nutrients in the same amounts as the others but most soils are not deficient in these macronutrients. The three we will be dealing with the most are nitrogen (N), phosphorus (P) and potassium (K), with nitrogen being in inadequate amounts in most garden soils. The micronutrients are ; iron (Fe), zinc (Zn), manganese (Mn), boron, copper, molybdenum, and chlorine. The micronutrients are also known as the trace elements since they are only needed in small quantities. Excess amounts of trace elements can be toxic to plants.
Soil acidity or alkalinity is indicated by the soil pH value. Soil ranges from acid to neutral to alkaline. The pH scale ranges from zero to 14 with 7 being neutral, zero being the highest acid reading with 14 being the most alkaline. If the pH analysis is high in either direction, key nutrients can be chemically “tied up” or insoluble to the soils water, thus not available for take up by the plants root system. Most plants like a slightly acidic soil. The pH range preferred by most garden plants is about 6.0 pH to 7.2 pH. Acid soil is common in heavy rainfall areas and is frequently associated with sandy soils and soils high in organic material. Adding lime (calcium carbonate) is usually recommended to raise pH if your soil is acidic. Alkaline soil is usually found in areas of low rainfall and high calcium carbonate. Sulfur can be added to lower pH or you can amend your soil over time with compost or aged manure. Be careful if you are using fresh or green manure as this can lead to excessive salt in the soil which will pull water from the plants roots.
A soil analysis will determine your nutrient levels and your soils pH. Many DIY soil testing kits are available through Ace hardware, Lowe’s or your local gardening center. Rapitest is a brand that is widely used and is readily available online at Park Seeds. The costs vary and you want to make sure what your kit does or does not test for. Most kits do not include the pH