Soil pH is an important, but sometimes an overlooked piece to the soil health puzzle. Soil pH is a measurement indicating the acidity or alkalinity of a soil on a scale of 0 to 14, with 7 being neutral. As noted in a University of Missouri study sampling soils from 1996-2006, any of Missouri’s soils lean on the acidic side with a large portion of those having a pH lower than 5.3.
Why is soil pH important?
When a soil is acidic, with a pH less than 7, many nutrients become less available to plants. Growth may improve even without fertilizer applications when pH is raised to the optimal range of 6.0 to 6.5 for most row crop plant growth. Liming acidic soils can bring an increase in yield as well as other benefits. MU Extension shows an increase of 15 percent on soybeans by raising pH from 4.5 to 6.0. Other benefits from liming acidic soils could include:
- Increased availability of nutrients including phosphorus, potassium, sulphur, calcium, and magnesium.
- Increased efficiency of fertilizers applied to the soil.
- Increase in microbial soil activity.
- Improved soil structure and tilth.
- Increased stand longevity in legumes such as alfalfa and clover.
- Improved herbicide activity, resulting enhanced weed control.
What causes soil to become acidic?
Soil acidity naturally develops over time due to:
- The natural breakdown of organic matter, which releases hydrogen, causing an increase in hydrogen ions and an increase in acidity.
- Movement of dissolved nutrients below the root zone because of rainfall.
- Removal of calcium and magnesium from the soil by crops, which are then replaced with hydrogen.
- Acid formation in the soil from nitrogen fertilizers.
How can soil pH be increased?
Adding limestone, also known as aglime or lime, to acidic soils is the most cost-effective method for raising the pH in Missouri soils. When limestone, a material with high pH, reacts with the soil and removes hydrogen ions, the acidity is reduced. The hydrogen, no longer attached and in the soil solution, reacts with the carbonate from the limestone to form carbonic acid. The resulting carbonic acid breaks down to carbon dioxide gas and water, leaving the soil less acidic.
Lime is an effective treatment for increasing soil pH as it contains the necessary calcium as well as the carbonate. Without the carbonate, the soil would retain the hydrogen ions and the pH would not change.
Once lime is applied, pH will begin to increase within a few weeks. However, the full effects of liming soil will take six months or longer. If this if the first time a specific plot has been limed or if it been an extended period of time since it was last limed, the soil may need 1-2 tons of lime per acre or more. In this case, it will be beneficial to spread the application of lime over two or more growing seasons.
How does soil acidity impact crop growth?
Once soil pH falls below 5.5, nearly all crops are negatively impacted due to the increase in hydrogen ions. Plants require 17 essential elements necessary for growth. The three primary nutrients are nitrogen, phosphorus, and potassium. With pH balanced, these nutrients can work more effectively to increase plant health and the crop’s yield.
Nitrogen is frequently the limiting factor for plant growth. Nitrogen fertilizer is needed in the greatest quantities for non-legume crops. When soil pH allows for adequate use of nitrogen by plants and the nitrogen is available for uptake, plants will have increased growth and higher protein levels among other benefits. Plants with a nitrogen deficiency will likely show:
- Slow or stunted growth.
- Yellow-green colored leaves.
- “Firing” of tips and margins of leaves with yellowing beginning in mature leaves.
Phosphorous is essential for plant reproduction and photosynthesis. It also enhances root growth, leading to increased plant stability. Plants deficient in phosphorous may exhibit characteristics such as:
- Slow or stunted growth.
- Purple or red coloration on foliage.
- Dark green coloration with the tips of leaves dying.
- Poor seed development.
Potassium is used by plants for increased root growth, stem strength, disease resistance, and increased size of fruit/grain. It also supports photosynthesis and regulates the uptake of nitrogen and water. Plants deficient in potassium may show symptoms such as:
- “Burning” on tips and margin of mature leaves with lower leaving turning yellow.
- Weak stalks and easily dislodged plants.
- Small or shriveled seeds.
- Slow plant growth.
The first step to take in maintaining the pH in your soils is to call your ag retailer or Certified Crop Adviser to begin soil testing, with grid sampling and mapping being the best option. With the information from soil sampling, you can work with adviser to develop a plan that will best meet the needs of your soils and the goals of your farming operation.
Resources:
https://extension.missouri.edu/publications/g9102
http://aes.missouri.edu/pfcs/research/prop907a.pdf