Nutrient potential and capacity. II. Relationship between potassium potential and buffering capacity and the supply of potassium to plants

Abstract

The potassium status of22 soils with low reserves of non-exchangeable potassium was assessed by measuring: (a) the potassium potential (the free energy of exchange of potassium for calcium plus magnesium); (b) the soil's capacity to resist a change in potential ("buffering capacity"); (c) the amount of exchangeable potassium and its percentage saturation.The ability of these measures to predict the supply of potassium was tested by growing subterranean clover (Trifolium subterraneum L.) in pots and determining its potassium content at successive harvests.The soil's buffering capacity was found to affect the supply of potassium even when uptake was small. It is suggested that this was partly because buffering capacity affects the rate of supply by diffusion to plant roots.As the uptake of potassium became progressively larger, the potential became increasingly unsuitable as a single index of availability, but a multiple regression containing terms for potential and for buffering capacity continued to account for a large proportion (89%) of the variation. The relationship obtained can be used to indicate the amount of potassium required to bring a deficient soil to a desirable status.

Availability of magnesium in soils

“…Although the importance of buffering capacity has been recognized, little use appears to have been made of it in assessing nutrient availability. Barrow (1966) found that inclusion of a term for buffering capacity in the regression equation gave a better indication of K availability in the soil than did the activity ratio fflK/V°an-Mg alone. However, the improvement in precision did not relate to dry matter yield, but only to K concentration in the plant.…”

“…However, the ratio, V a Mg/V a ca+M g > might equally well have been expressed as the ratio of the activities themselves, since regression analyses showed that both expressions accounted for the same proportion of the variation in plant Mg concentration. There was no advantage in using the logarithmic form of the ratio, |pMg -£p(Ca + Mg), although this form may be more satisfactory where a greater range of Mg availabilities is being considered, as suggested by the results obtained by Barrow (1966) and Arnold, Tunney & Hunter (1968) for K.…”

“…They give a measure of the total pool a wide variety of soils (Beckett, 1964; McConaghy of labile Mg in the soil, but give no indication of & Smillie, 1965;Acquaye & MacLean, 1966; the relative availability of Mg within this pool. Barrow, 1966). While such methods may be useful and have found Salmon (1964) found that the Mg taken up by applicability in comparison of soils with similar ryegrass from a range of uncultivated soils was properties, they are often unsatisfactory when more closely related to ion activity ratios than to soils of different types are considered.…”

A Study of the Quantity/Intensity Relationships of Potassium in Some Calcareous Soils of Iran

“…One approach to describe the status of K þ in soil that have been used by researchers (Barrow, 1966;Sparks and Liebhardt, 1981;Evangelou et al, 1994) is the Q=I relationship, first introduced by Beckett (1964). The Q=I approach was known to be useful in understanding, characterizing, and evaluating the K þ fertility status of soils (Wang and Scott, 2001).…”

Use of the quantity/potential relationship to provide a scale of the ability of extractants to remove soil potassium

“…Beckett, Craig, Nafady & Watson (1966) and Beckett & Nafady (1967) related change in exchangeable K to the activity ratio, aic/V a oa+Mg' These relationships can be used for comparing amounts of exchangeable K in a soil given different K manuring (Beckett et al 1966;Beckett & Nafady, 1967;Addiscott, 1970) but do not give a precise measure of available K, although Beckett et al (1966Beckett et al ( , 1967 have used the intercept on the AK axis of the extrapolated linear section of the quantity/activity ratio curve as an approximate measure of available K and MacConaghy & Smillie (1965) found this to be a useful measure of the total K taken up by four cuts of ryegrass. Barrow (1966) showed that the potassium buffer capacity, dQjd(AR), (or PBC) was increasingly important in accounting for K uptake by clover as uptake increased. Acquaye, MacLean & Rice (1967) correlated PBC with the ability of some Ghanaian soils to fix K and with their reserves of non-exchangeable K as measured by cropping with oats and by HN0 3 extraction.…”