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
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
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
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 IMPORTANCE OF SOIL pH
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