Soil Nutrients and pH Affect Plant Growth
The Availability of Minerals and Soil pH Influence Growth in Plants
Jul 10, 2009
Internally, various hormones choreograph the regulation and coordination required for a plant’s normal growth and development. However, external environmental factors can play a role as well. Among the external factors known to affect plant growth are the availability of minerals in the soil and the pH of the soil.
Availability of Minerals
Although green plants exhibit a bewildering complexity of appearance, they are all surprisingly similar in chemical composition. The synthesis of plant food through photosynthesis and the utilization of these foods require not only chemical elements contained in air and water but also a number of others obtained from the soil. Since these elements are necessary for normal growth and development, they are termed essential elements.
These essential elements may be divided into three groups. The first group includes those elements derived from the air, soil, and water – carbon (C), hydrogen (H), and oxygen (O). The second group includes nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg). These elements are obtained by plants only from the soil. Since the first two groups of elements are used in such large amounts, they are termed macronutrients
The elements of the third group include iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), molybdenum (Mo), boron (B), and chlorine (Cl). The elements in this group are required in very small amounts. For example, the concentration of boron required for normal growth and development is usually not more than one part (atom) per million parts (molecules) of water. Because such tiny amounts are required, this third group of elements is termed trace elements or micronutrients.
Plants with deficiencies of certain minerals show characteristic symptoms and stunted growth. Severe mineral deficiencies can actually kill a plant. However, excess amounts of some minerals can also be toxic to plants.
The macro and micronutrients in soil are replenished naturally by the process of decay or artificially when humans add them to the soil in synthetic chemical compounds known as fertilizer. It is a common misconception that fertilizer is food for plants. Plants use carbon dioxide, water, and minerals in the process of photosynthesis to make sugars (food). Fertilizer merely provides the mineral component required by plants to make their own food.
Soil pH
A large percentage of the soils of the more humid areas of the earth are classed as acid, or sour. This does not mean that they are acid enough to taste sour, as a lemon, but only that they are more acid than pure water. The soils of drier areas are often said to be alkaline, basic, or sweet. Again, not to imply that they would taste sweet like sugar water, but that they are more basic than pure water. What produces the acidity or alkalinity in soils? It has to do with water. Water molecules (H2O) tend to come apart into what are termed ions. The H+ portion of the molecule is called a hydrogen ion and is considered chemically to be acidic. Therefore, something acidic has a high concentration of H+ ions.
The other half of the water molecule that separates becomes a hydroxyl ion OH-. The hydroxyl ion is considered chemically to be basic or alkaline. Thus, acid soils contain more hydrogen ions and less hydroxyl ions while alkaline or basic soils contain more hydroxyl ions and less hydrogen ions.
Scientists use a logarithmic scale to express acidity or alkalinity of substances. This scale is called the pH scale and runs from 1 to 14. A pH of 7 is considered neutral. Substances with a pH of 8 to 14 are considered basic and those with a pH of 5 to 1 acidic. Because pH units are logarithms, pH 6 is 10 times more acid than pure water, pH 5 is 100 times more acid than pure water and so on.
Soil becomes acidic when certain basic elements, especially calcium are removed by the percolation of rain water (calcium is very soluble in water) carrying the dissolved calcium down deep into the soil and by crops and plants removing the calcium as they grow and develop.
In drier areas the calcium is not dissolved and carried deep into the soil. As the calcium and some other elements build up, the soil becomes basic or alkaline.
The best pH for growing plants is around 6.4. Agricultural fields that are much more acid or basic than this may require the application of chemical amendments to adjust the pH. For example, acidic soils are lacking in calcium so lime (calcium carbonate or CaCO3) is often added to “sweeten” such soils.
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