Over the last few weeks this column has explored what acidity is, what determined the native (original) soil pH condition present, and how agricultural practices have affected soil pH over time. This week wraps this up and discusses how producers can adjust soil pH to optimize crop production. Remember for the crops common to our area the optimal pH is approximately 6.3 to 7.3 and acid soils have pH readings lower than 7 while basic soils are above 7. While soils in our area may have pH readings in the 8 range, typically they aren’t like the alkali soils more common as you move west in the state.
• If the soil pH is between 7.3 and 8.4 there is really no concern regarding plant growth and nutrient availability. These soils contain some free calcium carbonate (limestone) but low salts. While there may be some concern with the carryover of some herbicides, these soils are fine. While the pH could be lowered, the cost is prohibited on a farm field scale. The exception would be for homeowners wanting to plant acid loving plants like azalea and rhododendron. Here, for a small area, the application of elemental sulfur or products like aluminum or iron sulfate are appropriate and available in most home and garden departments.
• If the pH is higher than 8.4 but not alkaline, high in salts, a major problem could be iron deficiency. In a landscape setting, it may be practical to apply foliar iron but on a farm field scale, it is better to select crops less sensitive to iron deficiency, say wheat vs. soybean, or varieties/hybrids developed to be tolerant of this deficiency.
• If the soil has a high pH and is high in salts, you deal with the salt problem and concern yourself less with the high pH. These are termed alkaline soils and a major problem is a lack of soil structure. Here we tend to have small areas with high salt content and poor soil structure. The term used by producers is “slick spots.” If irrigation is available, it is possible to “flush” the salts deep enough in the soil profile to solve the problem, however, this can be difficult if the soils lack structure. A better solution to deal with the lack of structure and high salt content is the application of gypsum, calcium sulfate which isn’t cheap but is practical.
• For acid soils, the problem is corrected by the application of a liming material such as limestone or dolomitic lime. To determine if the soil needs lime, a standard soil pH test is conducted. To determine how much lime is needed to address the problem, a Lime Test Index is performed to determine how much total acidity needs to be corrected. The total acidity needing correction is a function of the soils organic matter and clay content where acidity is “hiding.” A sandy soil low in organic matter and a soil typical of the area north of the river (higher clay and organic matter contents) may both have a pH of 5.5. The sandy soil may need a ton per acre to fix the problem while the other soil may need well over two tons. While the amount may be higher for the heavier soil, the positive effect lasts longer. The key for producers is to determine if they need lime and if they do, apply it as far ahead of time as possible to allow time for the chemical reaction to work after incorporation into the soil. This is especially important when planting legumes such as alfalfa and soybean. While not cheap, liming is essential for optimizing inputs and production and is effective over several years meaning the cost should be spread over time.