Soil nutrient contents in a long-term field experiment following the suspension of phosphorus and potassium fertilisation

The effect of mineral, organic (manure or straw + intercrop) and combined fertilisation on the development of soil nutrient contents over time and their mutual ratios was evaluated in a long-term field trial, IOSDV (established in 1984 at two sites), differing in the soil-climatic conditions. Three cropping cycles, from 2016 to 2018, 2019 to 2021, and 2022 to 2024, were studied in the following crop rotation: winter wheat-winter barley-root crop (sugar beet at Ivanovice na Hané and potatoes at Lukavec). Potassium and phosphorus in mineral fertilisers have not been applied since the year 2020 due to their high content found in soils after dry years with low yields. Consequently, their content decreased, most in the third rotation, both by the Mehlich 3 method and especially the exchange fraction extractable with NH4-acetate (Ivanovice: P 5-14%, 32-40% and K up to 12%, 9-20% determined by Mehlich 3 and NH4-acetate, respectively; Lukavec: Mehlich 3 - P increase: 5-16%, K decrease: 0-8%; NH4- acetate decrease - P: 10-13%, K 8-23%). The mutual ratio of nutrients equivalents K:Mg:Ca was lower than required values 1:2-3:10-15 at both sites and all studied treatments, however a slight increase was observed during the studied period, above all in system with only mineral fertilisation (Ivanovice: K:Mg:Ca from 1:1.2:5.6 to 1:1.4:6.8, Lukavec: from 1:1.0:7.7 to 1:1.0:9.6). A correctly balanced ratio of nutrients in the soil is important for maintaining soil fertility. In this long-term field experiment, the increase in nutrient levels in soils over reasonable levels was observed, highlighting the necessity of regular nutrient testing in agricultural soils, especially when multiple types of fertilisers are used simultaneously.