Soil with a pH below 6.5 is considered acidic. This condition can occur in two basic ways: naturally over time, or almost immediately, through human agricultural practices. Acidic soil can cause problems for both crop production and home lawns and landscapes, leading to lower yields, sparse lawns and plantings that fail to thrive. According to Oklahoma State University, there are four main reasons for soil acidity: organic matter decay, parent material, rainfall and leaching or repeated plantings of the same crop. Making the soil more alkaline requires amending with lime.
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Organic Matter Decay
Decaying leaves, needles, bark and grass clippings take a long time to add acid to soil, but they manage to do so over a period of many years. The breakdown of organic matter releases carbon dioxide, which reacts with moisture in the soil to create trace amounts of carbonic and other weak acids. These acids slowly accumulate, raising the pH of the soil.
Parent material is what the actual dirt is made of. It's usually some type of stone, much the way that sand is pulverised rock. Shale and limestone are more alkaline than acidic, so the soil that results from these rocks tends to have a high pH. Soil that results from granite, which has an acidic chemical composition, will result in more acidic soil. This is one of the reasons that areas near rocky shorelines aren't good for growing plants or crops.
Rainfall and Leaching
When rain washes through soil, it washes away organic material. If an area receives more than 30 inches of rainfall per year, it is on its way to becoming acidic. This is a very slow process, sometimes taking more than a hundred years, but can be speeded up by particulate and chemical pollution deposited in the soil by rain. Acid rain occurs naturally, but human pollution from cars and industry contributes most of it.
Concentrated planting in neutral-to-alkaline soil depletes the lime, or alkaline, elements in the soil. The roots of plants pull this material from the dirt to nourish themselves. High-yield crops grow more intensely, and pull even more nutrition from the soil, leaving it highly acidic if fields aren't allowed to lie fallow and recover, or if they aren't amended with lime and fertilisers. This is the fastest way to cause acidic soil, and the easiest to remedy with proper growing practices.
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