Exploration of potassium dynamics in coarse textured soils and saline hydromorphic soils of Kerala

Abstract

Soil, being a dynamic entity of heterogeneous nature formed from the weathering of rocks and minerals varies in its physical, chemical as well as biological properties spatially and temporally. Potassium, the third major plant nutrient is present in soil mainly in mineral form such as feldspar and mica. Solution K, exchangeable K, non-exchangeable K and total K are the four different pools of potassium that are present in soil and are in constant equilibrium with one another. The study entitled "Exploration of potassium dynamics in two selected soil types of Kerala" investigated K dynamics of K-deficient coarse-textured northern coastal soils from AEU 2 and K-rich saline hydromorphic soils of Kaipad from AEU 7 in Kerala. The research also focused on the role of potassium solubilizing bacteria (KSB) in facilitating K release from minerals (feldspar and mica) and its impact on growth and yield of cowpea. Soil samples were collected from 10 locations in each agro-ecological unit (AEU) at depths of 0-20 cm and 20-40 cm, followed by an incubation experiment and pot culture study. Saline hydromorphic soils showed higher moisture retention, nutrient content, and microbial activity compared to northern coastal soils. Both soil types showed decreasing pH, EC, and biological activity with depth. In soils of AEU 2, available K at 0-20 cm ranged from 41.44 to 142.35 kg/ha , while at 20-40 cm it ranged from 39.02 to 137.93 kg/ha. Water-soluble K was observed to be very low, ranging from 1.68 to 45.65 kg/ha at 0-20 cm, and 5.61 to 48.17 kg/ha at 20-40 cm. Exchangeable K was also low, ranging from 9.85 to 120.38 kg/ha at 0-20 cm and decreased at 20-40 cm. Acid-soluble K, representing non-exchangeable reserves, varied from 62.16 to 182.56 kg/ha at 0-20 cm and increased to 78.07 to 194.47 kg/ha at 20-40 cm. Conversely, soils of AEU 7 showed far greater K availability with available K at 0-20 cm ranging from 644 to 1603 kg/ha and 704 to 1684 kg/ha at 20-40 cm. Water-soluble K levels were also higher, ranging from 415.18 to 927.14 kg/ha at 0-20 cm and 373.24 to 857.25 kg/ha at 20-40 cm. Exchangeable K was highest in saline soils, reaching up to 695.8 kg/ha, while acid-soluble K peaked at 1286.88 kg/ha, indicating much higher nutrient reserves and fertility compared to northern coastal soils. Soils of AEU 2 had lower nitrogen, phosphorus, and sulfur with nutrient levels decreasing with depth. In contrast, soils of AEU 7 exhibited higher nitrogen, phosphorus, and sulfur maintaining or increasing with depth. Calcium and magnesium levels were also significantly higher in saline soils, reflecting greater fertility compared to the nutrient-poor coastal soils. The incubation study focused on the effect of potassium solubilizing bacteria (KSB) on K release from feldspar and mica in northern coastal and saline hydromorphic soils over a period of 100 days. In northern coastal soils, the highest available K was observed in the feldspar + KSB treatment (T3), increasing from 221.87 kg/ha on day 25 to 498.96 kg/ha by day 100. Water-soluble K peaked in T4 (mica + KSB), reaching 63.54 kg/ha by day 100. In saline hydromorphic soils, KSB with feldspar (T3) showed the highest available K, increasing from 3399.76 kg/ha on day 25 to 4331.04 kg/ha by day 100. Water-soluble K in these soils reached 1027.15 kg/ha in T3. Overall, KSB significantly enhanced potassium release, with feldspar proving more effective than mica over time, particularly in potassium-rich saline hydromorphic soils. By conducting pot culture experiment it was identified that in northern coastal soils, feldspar treatment (T3) resulted in the highest pod yield and seed yield. In saline hydromorphic soils, the mica treatment (T4) produced the highest pod yield. Post-harvest soil analysis showed that available potassium was highest in the feldspar treatment (T3) in both AEU 2 (103.48 kg/ha) and AEU 7 (4331.04 kg/ha). Significant increases in nutrient uptake by plants, microbial biomass carbon and dehydrogenase activity were observed in KSB- treated soils, indicating enhanced microbial activity and nutrient cycling. The study could illustrate the spatial as well as depth wise variation of various physical, chemical and biological parameters in different agro-ecological units. The successive incubation and pot culture experiments could demonstrate that KSB can play crucial role in enhancing potassium release from mineral sources, especially feldspar, which is more effective than mica in both K-deficient and K-rich soils. In K-deficient northern coastal soils, KSB combined with feldspar significantly increased potassium availability, crop growth, and yield. Saline hydromorphic soils, with naturally higher potassium levels, also benefited from KSB treatments leading to improved nutrient availability and plant growth. The findings suggest that integrating KSB with mineral potassium sources can be an effective strategy to enhance soil fertility and crop productivity in varying soil conditions.