Soil pH and why it's important for deer food plots
By Matt Tarr, Whitetail Stewards, Inc.
What is soil pH?
Soil pH is simply a measure of how acidic or basic your soil is. pH is measured on a scale ranging from 0 to 14. Soils with a pH less than 7.0 are considered to be acidic. Soils with a pH greater than 7.0 are considered to be basic (or alkaline). A pH of 7.0 is considered neutral.
pH measured on logarithmic scale
It is important to realize that the pH scale is logarithmic; this means that a full number change on the scale is actually a ten-fold change in soil pH. So, a pH of 5 is 10 times more acidic than a pH of 6, and 100 times more acidic than a pH of 7. A pH of 4 is 1000 times more acidic than a pH of 7.
Why is it called "pH"?
The term "pH" stands for "potential hydrogen" and it is actually a measurement of hydrogen ions (H +) in your soil. An acid is something that releases hydrogen ions; therefore, soils with a greater amount of hydrogen ions are more acidic than soils with fewer hydrogen ions.
The pH of your soil determines how well your forages will be able to absorb nutrients from the soil. As pH rises above 7.5, some nutrients will bind to soil particles and become difficult for your plants to absorb. When soils are too acidic, nutrients also bind to soil particles making them unavailable, but elements such as iron, magnesium, and aluminum can become so easy to absorb that they become toxic to your plants. Forages grown on soils of the proper pH will absorb soil nutrients properly. This not only allows your forages to grow to their biggest yields, but will make them more attractive and nutritious to animals.
Proper soil pH makes forages absorb fertilizer properly
If you are spending time and money fertilizing your crops, proper soil pH will also allow your plants to absorb fertilizer more efficiently. If you are growing legumes such as clover, alfalfa or soybeans, nitrogen fixation by these crops will be most efficient when the soil pH is correct.
How do you know what pH is right for your forages?
In general, most forage crops grow best when soil pH is between 5.8 and 7.0. However, each forage crop has its own required range of soil pH within which growth is greatest. If you are planting a mixture of forage species in the same plot, be sure to choose forages with similar soil pH requirements. Sometimes the forages you purchase will be labeled with instructions indicating required soil pH. If not, the best way to know the correct pH for your crops is to consult directly with your seed supplier, or with your local Cooperative Extension agricultural agent.
How do you measure soil pH?
While you can purchase hand-held pH meters to test your soil, these instruments often require the user have some experience to get consistent, accurate results. (Purchase a pH meter) Therefore, the most accurate way for you to test your soil pH is to send a soil sample of your soil to your local University or Cooperative Extension agency. A complete test for soil pH and fertilizer needs will usually cost less than $20. This is a minor investment when you consider the cost of quality seeds and fertilizer, as well as the time involved in planting and maintaining your food plots. The results of your soil test will tell you the current pH of the soil and exactly how much lime and fertilizer is needed to make your soils optimal for the forages you are planting. How do I collect a soil sample?
How often should you test soil pH?
Be sure to follow the sampling and shipping instructions of the testing agency carefully to be sure your soil test is accurate. Food plots planted to perennial forages should have a soil test at least once every two or three years. Plots that are planted to annual forages should be tested each year. Soil testing should be done several months prior to planting, especially if you suspect soil pH must be raised.
How do you change soil pH?
In rare instances when soil pH needs to be decreased sulfur can be added to the soil to make it more acidic. However, in most cases, the soils you will be working with will require the pH to be raised closer to neutral, if any change is needed at all. The most common and practical way to raise soil pH is by adding some form of lime.
What are the different kinds of lime and which is right for you?
There are many different kinds of liming materials that you can use to increase soil pH. These materials include, pure lime (calcium carbonate), dolomitic lime, calcitic lime, oyster shells and wood ash. Some of the basic differences between these materials include their neutralizing value, particle size, and any additional elements.
What is the neutralizing value of lime?
The neutralizing value compares the liming material to pure calcium carbonate which has a set neutralizing value of 100%. When other materials have neutralizing values near 100% they are comparable to pure lime.
Why is particle size of lime important?
Particle size of the lime is important because finer particles react quicker and more completely with the soil than do larger particles. The cost of the liming material will increase as particle size decreases.
Other elements in lime
Some lime materials such as dolomitic lime contain other elements such as magnesium. If your soil test indicates that magnesium levels were low, dolomitic lime might be appropriate for your application.
Consult with your local Cooperative Extension agent or agricultural supply store to determine which liming material is right for your application.
How do you know how much lime to add?
Your soil test is the best way to know exactly how much lime you will need to raise your soil pH. Lime application rates are usually given on a tons per acre basis. In some areas, especially in the Northeastern United States where soil pH can be between 4 and 5, it is not uncommon to see application recommendations calling for two or three tons of lime per acre.
Applying lime on small acreage food plots
The type of liming material, amount of lime needed, and size of the area to be treated determines how the lime will be spread. You can purchase pelletized lime in 50lb bags and spread this with a broadcast spreader pulled behind either a tractor or 4-wheeler. Some companies make attachments specifically designed for pulling behind a 4-wheeler. Such attachments are especially useful when working on small acreage food plots or on plots that can't be accessed with larger vehicles. Be sure to consult with your lime supplier to be sure the liming material will work in your spreader and to learn about any safety precautions required for working with the liming material.
If you decide to spread the lime yourself you will have to calibrate your lime spreader to be sure that your are applying the correct amount of lime. How do I calibrate my lime or fertilizer spreader?
Applying lime on large acreage food plots
If you are spreading a large area or large volumes of lime you may find it is too expensive and difficult to handle many bags of pelletized lime. For these larger jobs, you may find it cheaper and more convenient to contract with your local agricultural supply store and have them spread agricultural lime (Ag-lime) for you with a lime truck.
When do you apply lime?
Since most types of liming material move slowly through the soil it is best to apply your lime well in advance of planting. This is especially important if your soils are very acidic and soil pH must be increased substantially. While lime can be applied at any time of the year, applying it at least three to six months before planting will allow enough time for the lime to react with your soil.
Apply all the lime in one application
Large lime applications can be split into two applications if needed, the first made three to six months prior to planting and the second made at planting time. However, it is usually best to apply all the lime at once, three to six months before planting. Applying all of the lime at once will reduce the time and costs associated with multiple lime applications, and will result in your soils having the required pH at the time of planting.
Till lime into the soil
Lime moves very slowly through the soil so be sure to incorporate it well after spreading. Till the lime within the top six inches of your soil so the lime will be available uniformly within the rooting area of your forage crops. This is especially important if you are applying lime very near the time of planting.
Know the acreage of your food plots before applying lime
It is extremely important that you know the exact acreage of your food plots so you can avoid adding too much or not enough lime. The only way to know exactly how large you're your plots are is to measure them. Probably the easiest way to calculate the acreage of your plots is by using a GPS unit. If you don't have a GPS unit you can measure the plot with a 100' measuring tape. Measure your plots to the nearest one-tenth acre, remembering that: Area = Length x Width, and that there are 43,560 sq. ft. in an acre. How do I measure the size of my food plots?
Ball, D. M., C. S. Hoveland, and G. D. Lacefield. 2002. Southern Forages, 3 rd ed: Modern concepts for forage crop management. Potash and Phosphate Institute. Norcross, GA. 322pp.
Koerth, B. H., and J. C. Kroll. 1998. Food plots and supplemental feeding. Stephen F. Austin State University. Nacogdoches, TX. 162 pp.
Soil fertility manual. 1999. Potash and Phosphate Institute. Norcross, GA .
