Predicting potassium fertilizer requirement using exchange isotherm approach in relation to soil characteristics

Abstract

Determining potassium (K) fertilizer requirement using sorption isotherms is considered more accurate than conventional soil K tests. A total of 59 surface soil samples were used to establish K exchange isotherm. To evaluate K requirement sorption test, a glasshouse experiment using perennial ryegrass (Lolium perenne, cv. Roper) was carried out on 10 soil samples. The experiment was laid out as a completely randomized design with four replications and four K levels (K₀, K₂₀, K₄₀, K₈₀). Concentrations of K in solution established by adding K in the pots estimated from the sorption curve ranged from 20 to 80 mg K l⁻¹ including check treatment (no K). Dry matter yield of ryegrass in most soils approached maximum as adjusted K levels were increased to 20 mg K l⁻¹. The amounts of K required to bring the soils to 20 mg l⁻¹ in soil solution varied among soils and ranged from 99 to 399 mg kg⁻¹, on average 205 mg kg⁻¹ soil. It was found that a useful regression model for the prediction of standard K requirement (K₂₀) included the combination of plant available K extracted by NH₄OAc (Av-K) and clay content: K₂₀ = −41 − 0.63 Av-K + 9.0 Clay (R² = 0.61, p < 0.001, n = 59).

Keywords:

• prediction
• sorption
• sorption isotherms
• potassium requirement sorption test
• NH₄OAc-extractble K
• alkaline-calcareous soil
• soil test

Acknowledgements

Appreciation is expressed to the Iran National Science Foundation (INSF) for the financial support of this research. The assistance of Mr M. Barin, Mr B. Dovlati and Mr Hasirchi – the staff and technicians of the Soil Science Department, Urmia University – is gratefully acknowledged.