Lime Needs in Minnesota
The importance of lime
When needed, liming materials are major inputs for crop production enterprises in Minnesota. When soils are acid, there are many benefits from liming. Liming to a pH of 6.0 to 6.5 or higher provides an ideal environment for bacteria in soils. Some of these bacteria actively participate in the' breakdown of soil organic matter. Others form nodules on the roots of legumes. With this bacterial partnership, legumes are able to utilize the nitrogen in the air and no fertilizer nitrogen is needed.
The availability of phosphorus is also affected by soil pH. So, liming to a pH of 6.0 to 6.5 also increases the supply of soil phosphorus available to plants. Most soils in Minnesota contain ample calcium (Ca) for crop growth. Liming materials are generally not used to supply Ca, but depending on source may be used to supply magnesium (Mg).
Determining the need for lime
The need for lime is not uniform across Minnesota and recommendations will vary. Analyzing a soil sample for pH and buffer pH is the only way to arrive at an accurate lime recommendation. Soils should be sampled to a depth of 6 to 8 inches for this test. The recommendations will not be accurate if other sampling depths are used.
After the soil sample reaches the laboratory and is dried, a pH reading is taken in a mixture of equal parts of soil and water. This value is referred to as the soil pH. If the soil pH reading is less than 6.0, a buffer solution is added to the soil/water mixture and another pH reading is taken. This second reading is referred to as the buffer pH and is used to determine the amount of lime to apply. The current buffer used at the University of Minnesota is called the Sikora Buffer. The pH of the buffer itself is 7.5. When it is added to an acid soil, the pH of the buffer will drop. The change in the pH of the buffer is directly related to the amount of lime needed.
The buffer pH is not determined on soils with a soil pH of 6.0 or higher. The relative error of using the buffer is too high in this soil pH range. For these soils, standard guidelines are used to raise the soil pH to 6.5.
Lime guidelines for Minnesota are summarized in Tables 1-3. The area of the state must also be considered when these suggestions are used (see map Figure 1).
Figure 1. Reference map for lime suggestions
The same pH is not required for optimum growth of all crops. Crops grown in Minnesota are divided into 3 groups. These groups are as follows:
Group I. Alfalfa, alsike clover, apple, asparagus
Annual canary seed
Group III. Potato, grass sod, blueberry, wild rice.
For mineral soils, lime should be applied to raise the soil pH to 6.5 if alfalfa, alsike clover, apples, or asparagus are the intended crops. For any crop in Group 2, lime should be applied to raise the soil pH to 6.0. The crops listed in Group 3 grow best in acid soils and no lime is needed except for potato and grass sod when the pH drops below 4.9. In these cases application of 2000 lb ENP/A is suggested. For potato, this application should be made prior to growing a rotation crop to minimize scab development.
|Table 1. Lime suggestions for mineral soils when soil pH is less than 6.0. The rates suggested should raise the pH to 6.0 or 6.5.|
|Target pH 6.0||Target pH 6.5|
|* An ENP of 1,000 lb per ton is an average value for ag lime (crushed limestone) in Minnesota.|
|Table 2. Lime suggestions for mineral soils when the SIKORA BUFFER TEST IS NOT USED (soil pH is 6.0 or greater). The rates suggested should raise the pH to 6.5.|
|Area 1||Area 2|
For organic soils, (Table 3), it is suggested to raise pH to 5.5. A pH greater than 5.5 in organic soils will cause nutrient availability problems.
|Table 3. Lime suggestions for organic soils. The rates suggested should raise the pH to 5.5.|
|Area 1||Area 2|
|4.5 or less||5000||5000|
See also Liming materials for Minnesota soils