An investigation into the supply of potassium in soils of the Narli plain irrigation area, Kahramanmaras, Turkey

This research was conducted to determine relationship between forms of potassium and clay mineralogy in the Narli plain, Kahramanmaras. Understanding potassium behaviour in the soil is very important for plant nutrition, because there is a balance among potassium forms in the soils. Values for available potassium ranged from moderately low at 59 mg kg⁻¹ to very high at 989 mg kg⁻¹ in the soils that are dominantly either clay loam or clay in texture. The results show that while available potassium in the topsoil is generally in very good supply, it decreases rapidly with depth, and in 22% of the measurements, it is in low or very low supply. Available potassium correlated well with slowly available potassium. Slowly available potassium is about 2 times greater than available potassium in top soils whereas this increases to about 3 times for depths 20–40 and 40–60 cm. While there is an evidence of weathering, potassium release and utilisation, there is no evidence of potassium depletion in the top-soils. The spatial distribution of the results across the irrigation area shows trends relating to clay content, but not duration of agricultural use. The clay mineralogy was a mixed assemblage dominated by smectite and palygorskite, followed by kaolinite, illite and vermiculite. The high illite contents were related to high potassium levels. Clay mineralogy suggests that small variations in illite play a key role in the supply of available potassium and slowly available potassium. The amounts of illite ranged from a minimum of 10 to a maximum of 21% of the clay fraction. The results show the illite content of soils can be used as a criterion to determine the potassium availability status of soils.