soil pH

Soil pH – The Master Variable

Posted on by

The UConn Soil Nutrient Analysis Lab tests for and analyzes multiple soil parameters; but none as critical, and as often overlooked, as pH. Soil pH plays a crucial role in the growth of vegetation planted, as well as ground water quality. Before we start talking about soil pH, I think it is a good idea to try to define what exactly pH is, and how it is determined.

When most of us think of pH, a pool probably comes to mind. I remember growing up, watching my mother apply different chemicals to our pool, and impatiently wondering why I had to wait to go swimming. She would tell me that she was adjusting the pH of the water to ensure it was safe to swim in. The basic understanding is that pH is tells us how acidic, neutral, or alkaline something is. To get a little more technical, pH is the measurement of the activity of Hydrogen Ions (H+) in an aqueous solution. The equation for determining and quantifying pH is:

pH = -log10 (aH+)

(aH+= Hydrogen Ion Activity in Moles/L)

We express pH on a logarithmic scale of 0-14, where 0-6 is considered “acidic”, 7 is “neutral”, and 8-14 is “basic”.

soil pH scale
Image from: http://www.edu.pe.ca/gulfshore/Archives/ACIDSBAS/scipage.htm

Mineral soil pH values generally range from 3.0 – 10.0. There are numerous factors that determine soil pH including climate, parent material, weathering, relief, and time. Texture and organic matter content also influence soil pH. Most Connecticut soils are naturally acidic. Nutrient availability is directly influenced by pH with most plants (with some exceptions) thriving at pH values between 6 and 7. A majority of nutrients are available within this range.

Our lab measures pH using an 1:1 soil-to-DI water ratio. The saturated soil paste is mixed, then is analyzed using a glass electrode and a pH meter. We calibrate our meter using 2 solutions with known pH values, 4 and 7. We use these values because we expect most Connecticut soils to fall within this range. Once the initial pH value is obtained, a buffering agent is added. In our lab we use the Modified Mehlich Buffer. A second pH reading is obtained, and from these two values plus crop information, we are able to make limestone and/or sulfur recommendations.

The Buffering Capacity of a soil is the resistance it has to change in pH. Soil buffering is controlled by its Cation-Exchange-Capacity, Aluminum content (in acidic soils), organic matter content, and texture. A soil with a lot of organic matter and clay will have a higher buffering capacity than one with little organic matter that is mostly sandy.

If the soil pH is lower than the target range for a particular plant, limestone would be recommended. Whether you use pelletized, ground or granular limestone, the application rate would be the same. Once the target pH is reached, a maintenance application of 50 lbs/1000 sq ft would be applied every other year to maintain it.

If the soil pH is higher than desired, sulfur recommendations are made. Typically only powdered sulfur is available locally but granular sulfur could be mail ordered. Aluminum sulfate can be substituted for sulfur and used at a higher rate. Check out this listof preferred pH ranges for many common plants.

Monitoring your soil pH is essential to ensure that it is falling within the range best suited for the vegetation you are growing. The Standard Nutrient Analysis performed at our lab gives you a pH value, a buffer pH value, a lime/sulfur recommendation, available micro & macro nutrient levels, and a fertilizer recommendation. For more information on pH, you can contact Dawn or myself (Joe) at the UConn Soil Nutrient Analysis Lab (www.soiltest.uconn.edu). Test, don’t guess!

By Joe C.

Soil Testing for Lawns and Gardens

Posted on by

By Dawn Pettinelli for UConn Extension

soilSoil testing is an inexpensive, yet valuable, tool for assessing the fertility of lawn and garden areas. Test results indicate the soil’s pH level, the amounts of available plant nutrients, and the existence of nutrient imbalances, excesses or deficiencies.

WHY SHOULD I HAVE MY SOIL TESTED?

Soil testing eliminates the guesswork many gardeners face when deciding the kinds and amounts of fertilizers or soil amendments they should purchase and apply. Each soil test report contains recommended amounts of limestone and/or fertilizer needed for optimum plant growth. Knowing how much to apply saves time and money.

It is a smart decision to test the soil every few years. Further- more, it is particularly important in new garden bed installations or in established plantings that are not performing as well as expected. Regardless of whether you garden organically or use synthetic fertilizers, you will find that plants grow best when their nutritional requirements are met. This is achieved not only by the addition of nutrients such as nitrogen, phosphorus and potassium but also by sometimes modifying the soil’s pH through the incorporation of limestone or sulfur.

Soil pH is a measurement of the acidity of the soil. A pH of 7.0 is neutral, below 7.0 is acidic, and above 7.0 is alkaline. Native soils tend to be acidic. It may be necessary to raise the pH by adding limestone. Plant species vary in their soil pH preference. Blueberries and broad-leaved evergreens, such as rhododendrons, may develop iron deficiencies if the soil pH is too high. Lack of calcium from low soil pH may contribute to the physiological condition known as blossom end rot, which affects tomatoes and summer squash.

Applying the proper amounts of limestone and fertilizer pro- motes healthy, productive plants. In addition, it minimizes the potential for water pollution from over-application of nutrients, especially nitrogen and phosphorus.

WHAT CAN SOIL TESTS DETERMINE?

The standard nutrient analysis will provide the soil sample’s pH and the available amounts of phosphorus, potassium, calcium, magnesium, micronutrients and an estimate of total lead. Site-specific fertilizer recommendations are provided based on the soil test results. Recommendations for modifying the soil pH with limestone or sulfur are made if necessary.

Separate analyses offered by the lab include: percent organic matter, particle size analysis (the relative amounts of sand, silt and clay), micronutrients, soilless media and soluble salts. Commercial agronomic or vegetable growers may be interested in our presidedress soil nitrate test.

WHAT CAN SOIL TESTS NOT DETERMINE?

The soil fertility test performed at UConn cannot detect the presence of contaminants such as pesticides or petroleum products. A listing of state approved environmental laboratories which can perform these analyses is available at the Connecticut Department of Public Health’s website, www.state.ct.us/dph

Our soil tests also cannot identify problems due to insects, diseases, poor or excessive drainage, environmental stresses such as drought or winter injury, or improper cultural techniques.

WHEN IS THE BEST TIME OF YEAR TO HAVE MY SOIL TESTED?

A soil sample can be collected any time the ground is not frozen. The lab performs soil analyses year round. Fall is an optimal time for sampling because added amendments can begin to react with the soil over the winter. When submitting samples in the springtime, try to send them in early enough to give yourself time to prepare your beds before planting. Generally, the turnover time is 3 to 5 days in the lab but may be longer in April and May.

A soil test every 3 to 5 years is adequate for most situations. An exception to this would be sites requiring large nutrient additions or pH adjustments. In this case, it would be advisable to test one year after the recommendations for limestone and/or fertilizer were followed to monitor their effect. Whenever comparisons of results are desired, take samples at the same time of year.

For more information on having your soil tested please visit: http://www.soiltest.uconn.edu or call 860-486-4274.

Liming Soils

Posted on by

by Dawn Pettinelli, UConn Home & Garden Education Center

Grd limestoneAn incredible number of chemical, biochemical and biological reactions occur in our soils. Through these reactions, nutrients, whether already present in the soil or added by fertilizers, are changed into forms that can be taken up by plant roots. The pH of the soil affects all these reactions thereby determining the availability of nutrients essential for plant growth.

Soil pH is a measure of the acidity of the soil. A pH of 7.0 is neutral with measurements below this number reflecting acidity and those above indicating alkalinity. Many native soils have a pH in the range of 4.5 to 5.5 while most of our vegetables, flowers and turf grasses prefer it to be between 6.0 and 6.8. Some notable exceptions are blueberries and broad-leaved evergreens, like rhododendrons, which require acid soils.

When the soil pH falls below 6.0, nutrients like phosphorus, nitrogen and potassium become less available to plants. Acid soils are also typically deficient in magnesium and calcium, two important plant nutrients. Another problem with acid soils is that elements like aluminum are much more soluble and may be taken up in quantities that can harm plants. On the other hand, a pH that is greater than 7.5 can also render nutrients unavailable.

Limestone is the material of choice to raise a soil’s pH. It neutralizes soil acidity while also adding necessary calcium and magnesium. Dolomitic limestone, which contains both of these elements, is most widely available and usually recommended. If the magnesium level of your soil is above optimum, a calcitic limestone which is composed mostly of calcium compounds is called for.

Limestone can be purchased in several forms with ground, pelletized and hydrated being the most common. Economically, ground limestone is your best buy but some do not like the dusty mess encountered when applying. Pelletized limestone consists of pulverized limestone that is formed into little pellets. Both take about the same amount of time to react in the soil, anywhere from 3 to 9 months depending on conditions. Hydrated lime is fast-acting but quite caustic and only warranted under specialized circumstances. Its effects on pH, however, are short-lived. Wood ashes can also be used as a liming agent at one and a half times the rate of the recommended limestone application.

How much limestone to add depends on your soil’s present pH, the desired pH, as well as the amounts of clay and organic matter in your soil. A soil test can best determine recommended amounts. Very acidic soils may require several applications to bring the pH up to a suitable level. As a guideline, for every 100 square feet apply no more than 5 to 7 pounds of limestone to the surface or 10 pounds tilled to a depth of 6 inches at one time.  Once you’ve attained a desired pH, 5 pounds of limestone per 100 square feet every other year usually will maintain that level.

For those that don’t want to guess how much limestone to apply, consider a soil test. Fall is the perfect time to test because any limestone recommended and applied will have time to start affecting the soil pH before spring planting season and, you’ll be avoiding the spring rush!  For information on soil testing, liming soils or any other home and garden question, feel free to call the UConn Home and Garden Education Center, toll-free, at 877.486.6271, visit us on the web at http://www.ladybug.uconn.edu/ or contact your local UConn Extension Office.