Some soilshave a pH above 7.5. What factors cause this pH to occur?
Calcareous soils
Calcareous soils are those that have free calcium carbonate in the profile. Some soils that develop from calcareous parent materials will be calcareous throughout their profile. This will generally occur in the arid west where precipitation is less than 15 inches per year. Soils with excess illuvial carbonates in the profile have a "k" subscript to denote the CaCO3.
In eastern Minnesota for soils that developed from calcareous glacial tills, the calcium carbonate has been leached from the upper A and B horizons. The C horizons will be calcareous. In western Minnesota this is not the case, and the soils may be calcareous to the surface. The Buse (Calciudoll) soil is calcareous throughout, and the white horizons beginning in the Bk have nodules of calcium carbonate (CaCO3).
An easy way to check for calcareous soils is to add hydrochloric acid to the soil. If free carbonates are present, the acid will produce bubbling due to the evolving of CO2 gas.
Calcareous soils have 100% base saturation, and the exchange complex is dominated by calcium. The pH of calcareous soils is from 7.0 to 8.2 due to the hydrolysis of calcium carbonate which produces the strong base, calcium hydroxide, and the weak acid, carbonic acid. You will have a chance to test for calcareous soils in the laboratory.
Saline Soils
Saline soils are those that have accumulated soluble salts. The conductivity of the saturation extract is greater than 4 mmhos/cm, the exchangeable-sodium percentage is less than 15, and the pH is usually less than 8.5. They have sufficient soluble salts to impair plant growth, mainly by increasing the osmotic pressure of the soil solution which will restrict water uptake.
Soluble salts may accumulate naturally in soils in arid regions. In some cases the lack of rainfall reduces leaching, and salts can build up in the profile. In this slide, the white areas are accumulations of soluble salts such as NaCl, CaCl2, and KCl.
In other cases the development of saline soils is related to the flow of
groundwater. See information from the University of North Dakota on
Saline
Soil Development.
Under saturated groundwater flow conditions excess groundwater builds up in a
recharge area to the point where lateral groundwater flow occurs. As the
groundwater flows between particles of rock and soil, it dissolves and
transports soluble salts. The groundwater emerges at the soil surface in a
discharge area. When the water evaporates, the salts are left behind on the
soil surface or if the water remains the wicking of the water to the surface
will carry the salts with it. Over time, the salts accumulate in the soils of
the discharge area or nearby soils and eventually the salt concentration
becomes so high the soil becomes saline.
Irrigation water can also be the supplier of soluble salts to soils. Where drip irrigation is used, the salts move to the edge of the moist soil, and when the water evaporates, they leave a circular ring, as seen here. House plants are also susceptible to a build-up of soluble salts if the soils are not flushed periodically.
Soluble salt injury can be caused by adding fertilizers to soils, since the use of water-soluble fertilizers will produce soluble salts. The symptoms of salt injury in the plant are generally a chlorosis or burning of the leaf edge, as seen here on geraniums. On some plants, wilting will also occur due to the lack of water absorption. Seed germination can also be reduced in saline soils. Soils that are saline will have a neutral to slightly alkaline pH.
Soluble salts can be measured by using a conductivity meter. This instrument measures the electrical conductivity in millimhos per centimeter (mmhos/cm) when the conductivity is greater than 2 mmhos/cm and the amount of sodium is less than 15% of the exchange sites the soil is considered to be Saline.
Crops that are tolerant of a slightly saline environment are barley, cotton, and alfalfa. Saline soils can be reclaimed by leaching the soils to remove the soluble salts.
Sodic Soils
In areas where sodium is a major component of the surrounding rocks and minerals, it can become a major component on the cation exchange sites. Where this occurs and exchangeable sodium is greater than 15%, we have a sodic or alkali soil. The pH values range between 8.5 and 10. This will be toxic to most plants.
The exchangeable sodium also results in the defloculation of the colloids, a breaking down of the structural aggregates. This puddled conditioned impedes irrigation or rain water and thus limits the ability to reclaim these acres for agricultural production. These alkali flats of sodic soils in the California deserts are frequently devoid of vegetation.
Sodium affected soils can be reclaimed for agriculture by leaching, with an important additional step. The sodium on the soil exchange complex must be replaced with calcium. One way to accomplish this is by applying gypsum to the soil. The calcium in the gypsum replaces the sodium on the soil exchange complex forming sodium sulfate which can be leached from the rooting zone. See Sodic Soils in North Dakota (on-line) for more information on managing this problem.
Saline, sodic and saline-sodic soils (soil with both saline and sodic conditions) are not common in Minnesota, but in low spots in the western half of the state these conditions can be initiated. In eastern Minnesota, it is more common to be concerned with raising soil pH, which is an important practice discussed in Chapter 5 of this unit.
Chapter 5 Significance of Soil pH
Soil pH Chapters
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