What is Soil pH?


Basically soil pH is a measure of how acidic or alkaline your soil is. Soil pH is measured on a scale of 1 to 14. If your soil has a pH value of less than 7 then you have acidic soil. On the other hand if your soil has a pH value of greater than 7 then you have alkaline soil. A pH value of 7 is neutral, meaning you have neither acidic nor alkaline soil.

The Effect of Soil pH on Plants:

Each plant has its own recommended soil pH value range. The reason for this is that soil pH affects the availability of nutrients within the soil and plants have different nutrient needs. For example the nutrient nitrogen, a very important plant nutrient, is readily available in soil when the pH value is above 5.5. Similarly the nutrient phosphorous is available when the pH value is between 6 and 7. If a plant is placed into the wrong kind of soil it will be lacking in nutrients that it needs which will promote disease. In general the best pH value range for soil is approximately 6 or 7 as this is the range in which most nutrients can be readily available.

Finding Out pH of Soil:

Finding out the pH of soil is usually easy with kits available at most good garden centers. Usually a pH testing kit will include a small container / test tube, testing solution and a color chart. A sample of soil is taken from your garden, placed into the container / test tube and a few drops of testing solution are added. The container is then shaken and left for a certain period of time. The color of the sample in the container is then compared against the color chart to determine the pH value of the soil. You can also, in most areas take sample to your County Extension agent for testing.
How to Make Soil More Alkaline (Increase pH)

How to Make Soil More Alkaline (Increase pH):

You can make your soil more alkaline (increase its pH value) by adding a form of lime. Lime is a compound of calcium or calcium and magnesium. It is usually applied in the form of ground agricultural limestone, burnt lime or hydrated lime (slaked lime). The smaller the limestone particles then the quicker your soil will become more alkaline. For this reason hydrated lime will offer the quickest performance because it is slightly soluble in water so it can permeate the soil quicker and reduce acidity faster.

Increasing the pH of your soil is not an overnight process and it is best to allow 2-3 months to allow the lime to neutralize the acidity of the soil acidity.

How to Make Soil More Acidic (Decrease pH):

Some ornamental plants and fruit plants like blueberries require an acidic soil. To make your soil more acidic (decrease its pH value) you can use either ammonium sulfate or sulfur. Ammonium sulfates a fertilizer and with continued use it will increase the acidity as it dissolves into the soil. The downsides are though that its effects can be short term and it is possible to over-apply it.

The more recommended but slower way to increase your soil pH is to use sulfur. Sulfur converts to sulfuric acid with the help of bacteria in the soil but this takes time depending on factors like the presence of bacteria, texture of the soil and moisture levels. This could take months if conditions are not ideal.


The relationship of trace elements and soil biology to soil pH:


This chart shows the effect of pH on trace elements and  live biology in the soil. As you can see in the lower ranges some of the biology does not function.




pH Affects Nutrients, Minerals and Growth:

The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.

Phosphorus is never readily soluble in the soil but is most available in soil with a pH range centered on 6.5. Extremely and strongly acid soils (pH 4.0-5.0) can have high concentrations of soluble aluminum, iron and manganese which may be toxic to the growth of some plants. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients.

But some plants, such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees, tolerate strong acid soils and grow well. Also, some plants do well only in slightly acid to moderately alkaline soils. However, a slightly alkaline (pH 7.4-7.8) or higher pH soil can cause a problem with the availability of iron to pin oak and a few other trees in Central New York causing chlorosis (insufficient chlorophyll) of the leaves which will put the tree under stress leading to tree decline and eventual mortality.

The soil pH can also influence plant growth by its effect on activity of beneficial microorganisms Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.

Changes in Soil pH:

Soils tend to become acidic as a result of: (1) rainwater leaching away basic ions (calcium, magnesium, potassium and sodium); (2) carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid; (3) formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids.

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.


The pH of soil or more precisely the pH of the soil solution is very important because soil solution carries in it nutrients such as Nitrogen (N), Potassium (K), and Phosphorus (P) that plants need in specific amounts to grow, thrive, and fight off diseases.

If the pH of the soil solution is increased above 5.5, Nitrogen (in the form of nitrate) is made available to plants. Phosphorus, on the other hand, is available to plants when soil pH is between 6.0 and 7.0.

Certain bacteria help plants obtain N by converting atmospheric Nitrogen into a form of N that plants can use.These bacteria live in root nodules of legumes (like alfalfa and soybeans) and function best when the pH of the plant they live in is growing in soil within an acceptable pH range.

For instance, alfalfa grows best in soils having a pH of 6.2 - 7.8, while soybean grows best in soils with a pH between 6.0 and 7.0. Peanuts grow best in soils that have a pH of 5.3 to 6.6. Many other crops, vegetables, flowers and shrubs, trees, weeds and fruit are pH dependent and rely on the soil solution to obtain nutrients.

If the soil solution is too acidic plants cannot utilize N, P, K and other nutrients they need. In acidic soils, plants are more likely to take up toxic metals and some plants eventually die of toxicity (poisoning).

Herbicides, pesticides, fungicides and other chemicals are used on and around plants to fight off plant diseases and get rid of bugs that feed on plants and kill plants. Knowing whether the soil pH is acidic or basic is important because if the soil is too acidic the applied pesticides, herbicides, and fungicides will not be absorbed (held in the soil ) and they will end up in garden water and rain water runoff, where they eventually become pollutants in our streams, rivers, lakes, and ground water.

Summery:  You need to match up soil pH with the plants you are planning on growing. Plants vary in what soil pH they grow in best so keep this in mind as you select plants.

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