Why is soil normally slightly acidic

Lime is usually added to acid soils to increase soil pH. The addition of lime not only replaces hydrogen ions and raises soil pH, thereby eliminating most major problems associated with acid soils but it also provides two nutrients, calcium and magnesium to the soil.

Lime also makes phosphorus that is added to the soil more available for plant growth and increases the availability of nitrogen by hastening the decomposition of organic matter. Liming materials are relatively inexpensive, comparatively mild to handle and leave no objectionable residues in the soil. Some common liming materials are: 1 Calcic limestone which is ground limestone; 2 Dolomitic limestone from ground limestone high in magnesium; and 3 Miscellaneous sources such as wood ashes.

The amount of lime to apply to correct a soil acidity problem is affected by a number of factors, including soil pH, texture amount of sand, silt and clay , structure, and amount of organic matter. In addition to soil variables the crops or plants to be grown influence the amount of lime needed. In addition to monitoring soil pH the nutrient status of the soil should be examined. To obtain soil sampling instructions and kits along with specific recommendation contact Cornell Cooperative Extension listed in your local phone book under United States Government Offices - Agriculture Department.

Illustration by Robert Schmedicke. Apply Give Partner. Gateways for Advanced Directory. Search ESF. James F. Therefore, soils east of I tend to be acidic and those west of I, alkaline. There are many exceptions to this rule though, mostly as a result of items 4 and 5, intensive crop production and ammoniacal nitrogen application. Rainfall is most effective in causing soils to become acidic if a lot of water moves through the soil rapidly. Sandy soils are often the first to become acidic because water percolates rapidly, and sandy soils contain only a small reservoir of bases buffer capacity due to low clay and organic matter contents.

Since the effect of rainfall on acid soil development is very slow, it may take hundreds of years for new parent material to become acidic under high rainfall. Due to differences in chemical composition of parent materials, soils will become acidic after different lengths of time.

Thus, soils that developed from granite material are likely to be more acidic than soils developed from calcareous shale or limestone. The carbon dioxide CO 2 produced by decaying organic matter reacts with water in the soil to form a weak acid called carbonic acid. This is the same acid that develops when CO 2 in the atmosphere reacts with rain to form acid rain naturally. Several organic acids are also produced by decaying organic matter, but they are also weak acids.

Like rainfall, the contribution to acid soil development by decaying organic matter is generally very small, and it would only be the accumulated effects of many years that might ever be measured in a field. Harvesting of crops has its effect on soil acidity development because crops absorb the lime-like elements, as cations, for their nutrition. When these crops are harvested and the yield is removed from the field, then some of the basic material responsible for counteracting the acidity developed by other processes is lost, and the net effect is increased soil acidity.

Increasing crop yields will cause greater amounts of basic material to be removed. Grain contains less basic materials than leaves or stems. For this reason, soil acidity will develop faster under continuous wheat pasture than when grain only is harvested. High yielding forages, such as bermudagrass or alfalfa, can cause soil acidity to develop faster than with other crops.

Table 1 identifies the approximate amount of lime-like elements removed from the soil by a 30 bushel wheat crop. Note that there is almost four times as much lime material removed in the forage as the grain.

This explains why wheat pasture that is grazed out will become acidic much faster than when grain alone is produced. Using 50 percent ECCE lime, it would take about one ton every 10 years to maintain soil pH when straw or forage and grain are produced annually at the 30 bushel per acre level.

The use of fertilizers, especially those supplying nitrogen, has often been blamed as a cause of soil acidity. Degradation of the soil resource is also of wider concern and off-site impacts must be considered. Off-site impacts mainly result from reduced plant growth. Deep-rooted species required to increase water usage may not thrive, increasing the risk of salinity.

Increased run-off and subsequent erosion has detrimental impacts on streams and water quality. Increased nutrient leaching may pollute ground water. Soil acidity is measured in pH units. Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity. A soil with a pH of 4 has 10 times more acid than a soil with a pH of 5 and times more acid than a soil with a pH of 6.

Plant growth and most soil processes, including nutrient availability and microbial activity, are favoured by a soil pH range of 5. Acid soil, particularly in the subsurface, will also restrict root access to water and nutrients. When soil pH drops, aluminium becomes soluble. A small drop in pH can result in a large increase in soluble aluminium figure 1. In this form, aluminium retards root growth, restricting access to water and nutrients figure 2.

Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of inadequate water and nutrition. The effects of aluminium toxicity on crops are usually most noticeable in seasons with a dry finish as plants have restricted access to stored subsoil water for grain filling.

Figure 2: Roots of barley grown in acidic subsurface soil are shortened by aluminium toxicity. In very acid soils, all the major plant nutrients nitrogen, phosphorus, potassium, sulphur, calcium, manganese and also the trace element molybdenum may be unavailable, or only available in insufficient quantities. Plants can show deficiency symptoms despite adequate fertiliser application. Low pH in topsoils may affect microbial activity, most notably decreasing legume nodulation.

The resulting nitrogen deficiency may be indicated by reddening of stems and petioles on pasture legumes, or yellowing and death of oldest leaves on grain legumes.

Rhizobia bacteria are greatly reduced in acid soils. Some pasture legumes may fail to persist due to the inability of reduced Rhizobia populations to successfully nodulate roots and form a functioning symbiosis. Soil acidification is a natural process accelerated by agriculture. Soil acidifies because the concentration of hydrogen ions in the soil increases.

The main cause of soil acidification is inefficient use of nitrogen, followed by the export of alkalinity in produce. Ammonium based fertilisers are major contributors to soil acidification. Ammonium nitrogen is readily converted to nitrate and hydrogen ions in the soil. If nitrate is not taken-up by plants, it can leach away from the root zone leaving behind hydrogen ions thereby increasing soil acidity.