Spatiotemporal changes in the content of mobile potassium in arable soil

Abstract

The paper substantiates the spatio-temporal variability of exchangeable potassium content in the arable layer (0–30 cm) of chernozems in the Left-Bank Forest-Steppe of Ukraine on two experimental sites during 2018–2020. Site No. 1 was a 48.7 ha field with a heterogeneous soil cover dominated by podzolized chernozem (Luvic Chernic Phaeozem) and dark-gray podzolized soil (Luvic Greyzemic Phaeozem), including eroded and hydromorphic variants; Site No. 2 was a 24 ha field consisting mainly of typical chernozem (Haplic Chernozem) with contours of eroded and excessively moistened variants. Fieldwork involved regular (spring and autumn) collection of composite soil samples from 24 stationary sampling points on each field. Exchangeable potassium content determined by two methods: Chirikov’s method (for podzolized soils) and Machigin’s method (for neutral and slightly alkaline chernozems). The results revealed high spatial and temporal variability. The highest concentrations of exchangeable potassium were recorded in spring (120–185 mg/kg by Chirikov and 211–347 mg/kg by Machigin), the lowest during the period of intensive vegetation (80–150 mg/kg). After harvest, partial recovery occurred, but not to the spring level. A stable spatial structure of potassium distribution was established: maximum values observed in micro-depressions and hydromorphic contours, minimum values on eroded elevations and slightly xeromorphic variants. The difference between extreme values reached one full fertility class of potassium supply. A statistically significant relationship found between soil acidity (salt pH) and exchangeable potassium content: an increase in pH toward neutral accompanied by an increase in available potassium concentration, whereas in more acidic areas (dark-gray podzolized soils and moist depressions) potassium more strongly fixed. Both analytical methods showed high correlation of results and stability of the spatial pattern over the three years, indicating the dominant influence of soil-forming factors (relief, moisture regime, soil type) over annual weather fluctuations and agricultural practices. The obtained data confirm the need for precision (site-specific) application of potassium fertilizers in the Left-Bank Forest-Steppe, since the average level of potassium supply is high, but local zones of deficiency and excess coexist within the same field. Using only field-average values can lead either to yield losses or to irrational fertilizer expenditure.