Studies on soil potassium. III. The intensity of soil potassium following exhaustion by different plants

Scottish soils were cropped with two sowings of perennial ryegrass (Lolium perenne. cv, Dutch Barenza) in the glasshouse, without addition of potassium, until growth virtually ceased. Potassium uptake and the corresponding changes in soil K properties w e n examined.Uptake of K correlated well over the entire cropping period with the initially labile K predicted from the Q/I isotherms of the soils, although K uptake was at least a factor of 1.9 greater than the initially labile K.The Q/I isotherms were similar for soils from the same series, and the form of the lower part remained virtually unchanged during intensive cropping. The equilibrium activity ratio, AR,, of soil K measured in 0.01 M calcium chloride was reduced in all soils to a narrow range close to a mean d u e of 3 x 10-4 M ' . Dryingad-wetting fresh soil samples severely depleted of ryegrass-available K raised their K status. The increases were negatively correlated with the percentage K saturation of the fresh depleted soils, and a K saturation of 1.4 per cent must apparently be left in the fresh cropped soils for no K to be released. Freezing-and-thawing did not affect the K status of these soils.

Section: Changes In a R And K During Intensive Croppingsupporting

confidence: 86%

Section: Changes In a R And K During Intensive Croppingmentioning

confidence: 87%

Availability of Potassium to Ryegrass From Scottish Soils I.

Sinclair¹

1979

“…The extrapolation of the response curves back to zero yield gives more definitive values of AG,,h than the more direct methods involving exhaustive cropping (e.g. Addiscott and Johnston, 1975;Martin and Fergus, 1973;Sinclair, 1979). The initial yield response to increasing K potential probably increases exponentially, whereas the extrapolated values of dG,,h define the AGK,c~+ M~ values where responses become significant, providing a more reliable index to compare the tolerance of crops to low K levels than when crop yield is negligible.…”

Section: Discussionmentioning

confidence: 99%

“…Critical K potentials are measures of the ability of the plant to take up K and should be independent of soil type. However, previous work has suggested that yield responses of some crops to soil K potential vary with soil type (Fergus el al., 1972;Martin and Fergus, 1973). This paper describes experiments in a constant environment to obtain yield response curves of perennial ryegrass (Lofium perenne) and creeping red fescue (Festuca rubra genuina), with increasing soil K potential, from which critical K potentials can be derived.…”

Section: Critical Nutrient Potentials For Cropsmentioning

confidence: 99%

Critical potassium potentials for crops. I. The effect of soil type on the growth of ryegrass and creeping red fescue

Page

Talibudeen

1982

The quantity : potential relationships for Ca+K exchange in six soils were evaluated, where potential is defined by ~G K , c~+ M~. Using the percentage K saturation of the CEC as the index of quantity, the Worcester Series soil, rich in hydrous micas, was shown to have the highest concentration of K selective sites, and Newport Series soil, with mainly kaolinitic clay, the lowest. The other soils, containing mainly smectites, had intermediate K selectivities.An algebraic transformation of this relationship to separate the effects of exchangeable K and CEC showed that 0.01 M C~C I , released more K than M ammonium acetate. From the exchangeable K : AG relationship, two regions of K buffering were observed for all but the Newport soil, the transition occurring at a mean A G K , c~+ M~ value of -20.7 kJ mol-1, signifying the K concentration below which K from 'perlpheral' regions of micaceous minerals is released. This may explain why the percentage K saturation of the CEC of a soil cropped exhaustively (without K manuring) in the field does not drop below a minimum value.Based on pot experiments, exhaustion and optimum K potentials (dG,,h and AGop,) were derived from second-degree polynomials fitted to the response curves of plant dry matter yield against ~G K , c~+ M~ for five soils, the Worcester soil showing little response. dG,,h was inversely related to the 2 : 1 layer silicate content of the soil (r*=0.98 and 0.94 for ryegrass and fescue respectively), and similarly, AGO,,, to their CEC values (r2=0.74 and 0.77). Potassium uptake was more closely correlated with exchangeable K than with AGO,,. Introduclion Soil K potentialThe thermodynamic potential of K in soil is taken as its partial molar free energy referred to that of calcium+magnesium, and is expressed as the free energy of K -(Ca + Mg) exchange (Woodruff, 1955a, b; Arnold, 1962):It is measured as the difference between the chemical potentials of K+ and (Ca2+ + Mg2+) in a dilute solution in equilibrium with the soil, i.e.

“…Quantity/Intensity (Q/I) isotherms which relate the amount (Q) of labile K that is in exchange equilibrium with (Ca,Mg) to the intensity (I), defined here as the activity ratio, ARK = a&=) can be used to estimate K,, the amount of K released during crop growth from sources not initially in exchange equilibrium during the measurement of the Q/I isotherms (Arnold, 1970;Beckett, 1971;Islam and Bolton, 1970;Beckett and Clement, 1973;Martin and Fergus, 1973), because the form of the Q/I relationship remainsvirtually unaltered during cropping (Sinclair, 1979).…”

Section: Introductionmentioning

confidence: 99%

Availability of Potassium to Ryegrass From Scottish Soils Ii. Uptake of Initially Non‐exchangeable Potassium

Sinclair¹

1979

Summary The stable desorption parts of soil Quantity/Intensity isotherms were used to determine the contributions of initially exchangeable and non‐exchangeable potassium to plant uptake from ten soils. The activity ratio, ARK= aK/√aCa, Mg at which K was first taken up from non‐exchangeable sources varied from 3 × 10−3 to 8 × 10−3 M1/2 depending on the soil. Uptake rates of two categories of initially non‐exchangeable K were linearly related to √times;. The first category appeared to be close to equilibrium with the initially exchangeable K, and gave effective diffusion coefficients of 10−7 cm2 s−1 for four soils. The second category gave diffusion coefficients from 10−20 to 10−22 cm2 s−1, probably came from internal surfaces of micaceous clays, and began to be released at activity ratios below 3 × 10−4 to 6 × 10−4 M1/2 depending on soil type. The soils fell into three groups, broadly consistent with soil series, on their ability to release the second category of potassium.