Acid and Alkaline Soils for Vegetables

Posted on 08 April 2011

Acid soils are common in areas that get lots of rainfall. In chemists’ terms, acid soils give a pit reading of less than 7. (p” measures the hydrogen ion concentration on a relative scale from 1 to 14: p” 7 is neutral, pure water; any pi’ less than 7 is acid; any p” greater than 7 is alkaline.) The soil becomes progressively more acid as calcium and magnesium ions are removed and hydrogen ions replace them. This happens naturally by leaching, plant growth, and the weathering of some rocks. If the soil is excessively acid, valuable micro- nutrients become soluble and can leach away.

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Alkaline soils are more common in arid regions. They give a pH reading higher than 7. Soils turn more alkaline as calcium, manganese, and sodium ions accumulate and replace hydrogen ions. Natural causes of this include low rainfall, poor drainage, and native limestone deposits.
Often alkaline soils are also too salty. In extreme cases, heavy white or brown salt deposits are left on the soil surface by evaporating water. Salt problems are made worse when softened or brackish water is used for irrigation and when fertilizers with a high salt content, such as manure, are spread.

How can you tell if your soil is acid or alkaline? A quick and dependable method is to buy an inexpensive soil test kit (available at most garden supply stores or through mail order catalogs) and follow the easy directions.
The ideal vegetable garden soil should be slightly acid to neutral in reaction (p” 6 to 7). Don’t be alarmed if your test reading is slightly alkaline (p” 7 to 8). You can grow excellent vegetables on slightly alkaline soil. With the more elaborate soil testing kits, you can test the soil for the presence of major nutrients. In some states you can consult the Agricultural Extension Division of your state university for information about other sources of soil testing services.

The soil should be thoroughly tested before you add any chemicals that can alter the soil p”, such as lime, dolomite, marl, agricultural sulfur, or aluminum sulfate (alum). That way, you’ll know the right material to add and how much is needed.
1.Correcting acid soils
Ground limestone is effective in counteracting acidity— the calcium in the limestone neutralizes acids. It’s usually necessary to reapply limestone every two to three years, but don’t add any lime unless your soil test reading is below pH 5.5 to 6.
If you can find it, use dolomitic lime, which contains both calcium and magnesium. Avoid hydrated or burned lime; their caustic action can easily burn your skin, and they leach away rapidly.
2.Correcting alkaline soils
Because soils usually turn alkaline for more than one reason, you may have to do several things to correct the problem. To reduce alkalinity on well-drained land, flood the soil for 24 to 48 hours to wash excess mineral salts down below the root zone. Counterbalance any slight remaining alkalinity by feeding plants with an acid-type fertilizer. However, if moderate alkalinity remains after leaching, add substantial amounts of acidic amendments, such as peat moss, ground bark, or sawdust to the soil:
On poorly drained land where alkalinity is often most severe (ph reading of more than 8.5), treatments are complex and costly. To avoid the problem entirely, garden in raised beds or containers filled with good garden soil brought in from another area. If you can’t do that, improve the drainage by drilling holes down through impermeable layers to porous soil below. Add to the soil sulfur-bearing improvers, such as gypsum, soil sulfur, or aluminum sulfate, according to manufacturer’s directions. Water deeply to leach out salts.

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