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  MG Manual Reference
Ch. 2, pp. 18 - 21
[Soils: soils | properties | classes | caliche | depth | components | pH ]

Range of pH


A pH (potential Hydrogen) is a reading taken from a scale that measures the hydrogen (acid-forming) ion activity of soil or growth media. The reading expresses the degree of acidity or alkalinity in terms of pH values, very much like heat and cold are expressed in degrees Celsius or Fahrenheit. The Celsius temperature scale is centered around zero degrees or the freezing point of water, and thermometers are used to measure intensities of heat and cold above and below this point. The scale of measuring acidity or alkalinity contains 14 divisions known as pH units. It is centered around pH 7 which is neutral. Values below 7 constitute the acid range of the scale and values above 7 make up the alkaline range.
The measurement scale is not a linear scale but a logarithmic scale. That is, a soil with a pH of 8.5 is ten times more alkaline than a soil with a ph of 7.5 and a soil with a pH of 6.5 is a hundred times more acid than a soil with a pH of 8.5.
The pH condition of soil is one of a number of environmental conditions that affect the quality of plant growth. A near-neutral or slightly acidic soil is generally considered ideal for most plants. Some types of plant growth can occur anywhere in a 3.5 to 10.0 range. With some notable exceptions, a soil pH of 6.0 to 7.0 requires no special cultural practices to improve plant growth. Most soils in Arizona are alkaline and have a pH of between 7 and 8.5.
The Effect of PH on Plant Nutrient Availability
The major impact that pH extremes have on plant growth is the availability of plant nutrients and concentration of plant-toxic minerals. In highly alkaline soils, micronutrients such as iron, zinc, copper and manganese become chemically tied up and are sparingly available for plant use. In highly acidic soils, calcium, phosphorous, and magnesium become tied up and unavailable, and manganese and aluminum can reach toxic levels. The application of certain materials to the soil can be made to adjust the soil pH value.
Reclamation of highly alkaline soils which are also high in sodium content can be accomplished using gypsum or other soil amendments. Once excess sodium is leached away the soil pH can decline, but not below about 7.5. Applications of acids or elemental sulfur to lower soil pH are usually not effective. This is because most Arizona soils contain the mineral calcium carbonate (free lime). This mineral buffers the soil pH at about 7.5 to 8. Nearly all of the calcium carbonate would have to be neutralized with a strong acid to even begin to drop the soil pH appreciably. In some parts of northern and southeastern Arizona acid soils do occur. To reduce acidity, apply a material that contains some form of lime. Ground agricultural limestone is the most frequently used but may be difficult to find in stores. Wood ashes should never be applied to garden soils unless the soil has been found to have a soil pH below 6.0. Adding wood ashes to an alkaline soil will increase soil alkalinity even further.
Most of the soils in central and southwestern Arizona are alkaline and require a certain degree of management if ornamentals, fruits and vegetables are to be successfully grown.
There are some basic principles that need to be understood and worked with if we want to garden with any degree of success. Probably the most important principle to be understood is that soluble salts in soil are transported by water. Therefore, salinity can, to some degree, be managed by irrigation practices providing the water is of acceptable quality and the flow of water through the soil can be controlled.
The concentration of soluble salts in the soil profile is increased as water is removed from the soil via evaporation and transpiration. Simply explained, soil surface desiccation by evaporation and transpiration creates a suction gradient that produces an appreciable upward movement of water and salt. In soils where the water table is near the soil surface this process can become exaggerated.
Soluble salts increase or decrease in the plant root zone, depending upon whether or not the net downward movement of salt is less or greater than the net salt input from the irrigation water source. The salt balance in the soil as affected by the quantity and quality of the irrigation water and the effectiveness of drainage and leaching are the key to managing saline or alkaline soils with irrigation practices.
Another consideration is organic matter. Most western soils are low in organic matter under virgin conditions but it commonly increases appreciably with the application of irrigation water and cultivation. The most important factor regarding organic matter is that it has very favorable effects on the physical properties of soil. Particularly, organic matter tends to counteract the unfavorable effects of exchangeable sodium on soils. Simply put, heavy applications of organic mulches will increase water holding capacity, infiltration rate, tilth and soil aggregation. It has also been proven that organic matter will prevent deterioration of the physical properties of the soil by serving as an energy source (i.e. food) for microorganisms which promote stable aggregation of the soil particles.
Use caution when applying manures as a source of organic matter because they typically contain appreciable amounts of soluble salts and are usually somewhat high in pH as well. If the soil pH is too high, elemental sulfur or commercial gypsum can be added to the soil to rid the soil of excess sodium and indirectly reduce alkalinity.
Most ornamental plants require slightly to strongly acidic soil. These kinds of exotic (imported) plants are often very difficult to grow in our alkaline soils. These species frequently develop iron chlorosis when grown in soils in the alkaline range. Iron chlorosis can be confused with nitrogen deficiency since the symptoms (a definite yellowing of the leaves) are similar. The two nutrient deficiencies can be distinguished by observing where on the plant the deficiency symptoms appear. With iron deficiency, the symptoms appear at the shoot tips and on the newest growth. With nitrogen deficiency it is the older, lower leaves that are most affected. Foliar sprays of iron can alleuicte iorn deficiency. Also, iron chlorosis can be corrected by applying a chelated iron product to the soil to add plant available iron. be beneficial.
The term chelate comes from the Greek word for claw. Chelates are chemical claws that help hold metal ions, such as iron, in solution, so that the plant can absorb them. Different chemicals can act as chelates, from a relatively simple natural chelate like citrate to more complex, manufactured chemicals. When a chelated metal is added to the soil, the nutrient held by the chelate will remain available to the plant for a much longer time than with non-cheated fertilizers. Most nutrients do not require the addition of a chelate to help absorption. Only a few of the metals, such as iron, benefit from the addition of chelate. The types of chelate used will depend on the nutrient needed and the soil pH.
Soil TestingTop
Soil testing is done to determine nutrient status, pH, electrical conductivity (EC, a measure of salt content), soil structure, and sodium absorption ratio (SAR), or exchangeable sodium percentage (ESP). A basic soil test should include nitrogen, phosphorus, and potassium status, pH, and EC. Also, a SAR or ESP should be done for alkaline or sodic soils. Testing for secondary and micro nutrients is usually not needed and is expensive to conduct. A current list of testing facilities can be requested from the Cooperative Extension office. Home soil testing kits are not as accurate as lab testing. Some kits are very poor. More expensive kits are not necessarily more accurate.

How to Take a Soil Sample
Diag. 1

Diag. 2

Diag. 3
  1. Obtain cartons and information sheets from your county agent, the state soil testing laboratory, or other sources.
  2. Map the different areas within a field - such as hilltops, midslopes, bottomlands, or known areas of different productivity. With a sampling tube take 10 to 15 cores, spaced an equal distance apart. Sample to tillage depth. Place cores from each sampling area in a clean bucket. Mix this composite sample well and fill the soil sample carton (about 1 pint). Repeat this process for each area in the field.
  3. A field that is extremely variable, or one where little is known about the variability, requires many samples. Once a field has been intensively sampled and a soil fertility map made, select sites in representative low-, medium-, and high-fertility areas of the field that can be resampled every two to three years. Periodic resampling will show if the general soil fertility level in each area is improving or getting worse.
  4. It is best to use a sampling tube if that is possible. If you use a spade or shovel, throw away the first shovelful. Then take a 1-inch slice from the back side of the hole (to proper sampling depth) and trim away sides of slice, leaving a 1-inch center core. Place core in a clean bucket, following procedure given in item 2. A garden trowel can be used in place of a spade or shovel.

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