I can’t think of a more important topic for growers to know about than how soil pH affects plant growth and development. What is soil pH? If you remember back to your high school chemistry days, pH stands for parts hydrogen. Hydrogen ions exist in the soil and are there as a result of plant decomposition, causing the soil to be acidic, as in the eastern U.S. In the Midwest, the soil is made up of decomposing limestone, or calcium carbonate, causing the soil to be basic or alkaline.
The pH scale runs from 1 to 14, with the middle of the scale, or 7, being neutral. A soil with a pH of 7 is said to be neither acidic nor basic. A soil pH of less than 7 is considered acidic; more than 7 is considered basic.
Most crops do well at a pH between 6.5 and 7.0 (slightly acidic to neutral). Why is this? At a pH between 6.5 and 7.0, most of the major and minor plant elements (nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, iron, manganese, boron, copper, zinc and molybdenum) are available to plant roots. At a soil pH of less than 5.0 or greater than 7.0, many minor plant elements become unavailable. This is why it is so important to be sure that soil pH is within the 6.5 – 7.0 range. If it isn’t, adding fertilizer will not improve plant uptake of nutrients, since the roots will not be able to take it into the plant.
How do we adjust the pH of the soil? If it is too acidic, calcitic limestone or calcium carbonate is added. The calcium ions replace the hydrogen ions, which cause an increase in soil pH, making the soil basic. If the soil is deficient in magnesium (a minor element which aids in photosynthesis), then dolomitic limestone is added, which contains both calcium and magnesium.
In the Midwest and western states, the parent material of the soil is made up of calcium carbonate or limestone. Those soils are naturally basic. Many of the minor elements which plants require are not available at a high soil pH so sulfur is added to these soils to replace the calcium with sulfate ions. The sulfate ions will bind to hydrogen ions, which will lower the soil pH.
Certain plants, such as blueberries, thrive in an acidic soil, with a pH of 4.5. They are heavy feeders of iron, which is very readily available to the plant at this pH. At a high soil pH, blueberry leaves will become chlorotic (yellow in color) and be deficient in iron.
However, calcium is a very important element which provides the plant with a rigid cell wall and makes firm fruits. Since limestone (calcium carbonate) should never be added to blueberries because of raising the soil pH, how can calcium be introduced to the blueberry plant? Calcium sulfate (gypsum) is added. It will not raise or lower the soil pH but will provide a source of calcium for the developing fruits. The sulfur will also help to keep the soil pH acidic.
Growers need to take soil samples regularly to ensure crops receive the maximum growth benefit with an ideal soil pH. Soil sampling should occur in autumn so the necessary lime and fertilizer can be added, tilled into the soil so it can react with the soil and be available to plants the following spring. If a soil sample is not taken in autumn, then the next best thing is to sample in the spring and till additives into the soil before planting.
Soil samples are best taken with a soil probe that can easily be pushed into the soil and a core of soil, about six to eight inches deep, can be pulled up. Take many random samples in the field in an X pattern, going from the northwest to the southeast and then from the northeast to the southwest. Make sure to sample all areas of the field so that a true composite sample is obtained.
Mix all the samples together in a clean plastic pail and then remove about a pint of soil and send it to a soil laboratory to be tested. Most states have a soil testing lab at the state agricultural university; a list of reputable labs can be obtained by contacting your local County Cooperative Extension office.
Certain plants will readily show deficiency symptoms if a certain element is lacking. Some elements will be mobile throughout the plant, meaning that if the element is deficient, it will move from the old leaves into the new leaves, with the deficiency showing up in the older leaves.
Some elements will be immobile, with the deficiency showing up in the new leaves due to the element not being able to move from the old leaves into the new leaves.
Here are some examples of plant nutrient deficiencies:
Mobile Elements (Deficiencies will show up in old leaves)
Nitrogen – Upper new leaves are light green, where lower older leaves are yellow and shriveled
Phosphorus – Leaves are darker than normal and will have leaf drop
Potassium – Leaf yellowing at the tips and edges (marginal necrosis)
Magnesium – Lower leaves turn yellow from outer periphery of the plant into the center of the plant; veins remain green
Immobile Elements (Deficiencies will show up in new leaves)
Manganese – Yellow spots and/or elongated holes between veins
Calcium – New leaves are misshapen or stunted; older leaves remain green
Iron – Young leaves are yellow and white with green veins (interveinal chlorosis); older leaves are normal
There are many reference books and websites that contain good color photographs to help you diagnose plant nutrient deficiencies. Remember that waiting until you have a nutrient deficiency is usually too late to correct it to have a good yield. The best way to avoid deficiencies is to soil test regularly and apply what is needed to the soil. Do all you can to maintain the proper soil pH for the plants you are growing. No other practice will benefit the growth and development of plants more and assure that plant nutrients are always available to them.