Principles underlying the practice of determining lime requirements of acid soils by use of buffer methods

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Abstract
A buffer is generally a mixture of a weak acid and a salt of the same weak acid. Hence it can neutralize both acids and bases, and thus resists marked changes in pH of a system. Yet systematic change in pH of a buffer caused by addition of an acidic substance can be used to indicate the total acidity represented by the change in buffer pH. Since acid soil is itself a buffer, when it is added to a buffer mixture for the purpose of measuring its acidity or lime requirement (LR), the resulting double-buffer suspension (soil-buffer) is a relatively complex system. Much of the complication in interpreting the changes in buffer pH brought about by mixing soil and buffer stems from the facts that much of the acidity is pH-dependent and that quick-test methodology involves reaction of only a fraction of the total soil acidity with the buffer. Marked change in relative amounts of H ions dissociating from the soil buffer system at soil-buffer pH 6.9 and above accounts for relatively wide variations between buffer-indicated and CaCO3 incubation-measured LR of low LR soils. Similarly, decreased reactivity of H+ in high organic matter soils and increased reactivity of H+ in acid-leached soils cause errors in buffer-indicated LR. Awareness of these principles helps avoid pitfalls of existing buffer methods, and has led to incorporation of the double-buffer feature.