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Crops are heavily influenced by the acid content of soil - Soil acidity can be caused by many different factors, including the natural mineral content of the soil itself. Often, areas with heavy rainfall tend to have more acidic soil, simply because the rain water leeches away alkaline minerals in the soil over time. Soil with naturally high levels of limestone or similar substances will be more alkaline, while soils with high levels of granite tend to be more acidic. Decaying biomaterial will also briefly raise the acid levels of soil. Humans can contribute to soil acidity by adding acidic fertilizers or through certain types of pollution.
- Soil acidity is graded on the pH scale like other types of acid/alkaline measurements. A pH level of 7.0 indicates a neutral pH, or balanced conditions. Each whole number increment indicates a tenfold increase in soil conditions, so that a pH level of 8.0 is 10 times more alkaline than 7.0, and 6.0 is ten times more acidic. When referring to plant growth, only a certain range is considered, usually from 4.0 to 9.0 on the pH scale. Outside of this range almost no plants can grow, and few natural soils reach such extreme levels.
- Soil acidity affects the way plants grow, beginning with their roots. The acidity inhibits the growth of roots and the absorption of necessary minerals through the root cell walls. This can drastically weaken plants, causing them to grow very slowly or turn yellowish in color. Some plants may have difficult absorbing nutrients at all in highly acidic soils. If acid levels remain high, the plants may eventually wither and die. Different plants can survive in different acidity ranges, and hardier plants will be able to deal with high acid levels more easily.
- The spread of plant-oriented diseases is encouraged by the weakened condition of plants in high-acidity soil environments. Beneficial insects and microorganisms in the soil can be killed or weakened by the acid levels, which will in turn further deplete the health and fertility of the soil.
- Exchangeable acidity refers to the built-up acidity in the soil that can be reversed by applying alkaline substances, such as lime. Farmers use exchangeable acidity to calculate how much lime they need to reverse the acid effects of their soil, often caused by the natural disappearance of alkaline buffers in the soil over years of crop growing. Amateur gardeners, however, do not need to be so precise. It is generally advisable to mix in a small amount of lime or an alkaline soil with new crops on a yearly basis. This should be done only if the soil is being drained of nutrients or is naturally acidic--alkaline soils can be dangerous, too! Gardeners should know what types of soil their plants prefer before treating for high acidity levels.
