From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1996, No. 3


Soil pH is a logarithmic expression of the inverse of the hydrogen ion activity. Putting that in plain English, each unit change in soil pH represents a ten-fold change in the intensity of soil acidity. For example, a pH of 6.0 is 10 times more acidic than a pH of 7.0, while a pH of 5.0 is 100 times more acidic than a pH of 7.0. Seemingly small differences in pH, especially below 5.5, can have a marked impact on crop growth and productivity.

Limestone needs should be based on a well-calibrated soil test. Many recognize that the soil test is no better than the sample collected. A single composite sample should represent no more than about 10 acres of a uniform soil area. A composite sample is comprised of 10 to 15 cores or borings, usually taken in a systematic, unaligned zig-zag pattern in the field. This unaligned pattern is important in being able to avoid patterns or "streaks" caused by past manure, lime, and/or fertilizer applications, or possibly by tillage operations in the same direction for numerous years. This sampling procedure helps remove much of the potential error in soil test results and lime recommendations.

Soil pH can be quite seasonally variable...especially in some of the younger alluvial soils deposited by rivers and streams. Older, more highly weathered soils may not experience as much variability in pH levels among the different seasons. Research has shown that soil pH can seasonally vary as much as 0.6 pH units in certain soils in the Mississippi Delta. Soil pH changes are thought to be associated with fluctuations in soil moisture, organic matter decomposition, soil microbial activity, and soluble salt levels. The lower the organic matter, and the lower the clay content, the less buffered the soil is to seasonal variations. Soil pH values measured in a 1:1 ratio of soil:water paste often tend to be lower in the drier summer and fall months, and higher than the field average in wetter springs and winters. (Soil test phosphorus and potassium levels tend to be more stable and are less affected by seasonal variability than are pH measurements.)

To avoid problems with potential pH variability, avoid taking soil samples after extended dry spells. If doing precision or site-specific sampling over time, attempt to collect samples at soil moisture and temperature levels near those of the original sampling. If the soil moisture is near ideal for planting, you will probably be able to collect a sample that represents the true mean across seasons. Collecting soil samples after a couple of fall rains is considered fairly good insurance against the effects of seasonal variation on soil pH measurement.

If the soil pH does drop as the summer growing season progresses, will the pH lower into a range that will limit root growth, moisture and nutrient uptake? Would liming be justified for the field to help prevent any temporary seasonal pH depressions that dip into the danger zone? The answer(s) probably depends on the soil manganese, aluminum, and phosphorus levels, and also the crop grown and its sensitivity to soil acidity and associated acid infertility factors (e.g. manganese toxicity, aluminum toxicity, phosphorus deficiency). Relatively long-term lime research data can help answer some of these questions, since the variability effects over time can be measured in crop yields.

While many consider fall the ideal time to lime, any time that the field can be traveled without damaging the crop, or without causing excessive compaction, is a good time to apply aglime.

While seasonal variation in soil pH is important, it should not prevent the most acidic soils from receiving aglime now. If winter does not permit time to lime, plan to apply aglime at the first opportunity in the spring. Focus first on the most acidic fields. Good quality lime applied as late as planting can provide very good returns, and will also continue to benefit successive crops for three to five years, and possibly longer.

Soil test to determine your aglime needs, apply aglime where appropriate, increase your nutrient use efficiency and enhance your profit potential.

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