Influence of increasing doses of nitrogen fertilizers on the productivity of field crops and the potassium regime of sod-podzolic soil

In a long-term experiment launched in 1972 on sod-podzolic heavy loamy soil, the effect of increasing doses of nitrogen fertilizers on the productivity of field crops and the potash regime of the soil was studied. The scheme of the experiment included the following options: without fertilizers, P60K60 (background), background + N30, background + N60, background + N90, background + N120. The analysis of crop yields in the 6th rotation of the crop rotation (2013–2019) showed that the most effective application for spring crops (wheat, barley, oats) was N30P60K60, for potatoes – N60P60K60. The highest yield of clover hay was noted in the P60K60 and N30P60K60 variants. The maximum productivity of an 8-field crop rotation (4.19 tons of grain/ha/year) and a payback of 1 kg of mineral fertilizers (8.7 kg of grain) was obtained by applying N30P60K60. Studies of the total potassium content, its easily exchangeable, mobile and non-exchangeable compounds in the soil were carried out at the end of the 6th rotation of the crop rotation in a layer of 0–60 cm. It was revealed that the long-term use of nitrogen fertilizers led to an increase in the mobility of potassium compounds in the soil. A significant increase in the content of easily exchangeable and mobile potassium compounds was found by 1.2–1.9 times relative to the background in layers 0–20 and 40–60 cm of soil when N60–120 was applied. With prolonged use of the N30 dose, only trends towards an increase in the content of these forms of potassium were noted, which could be the result of using a low dose or due to the maximum potassium removal by plants per rotation in this variant. The application of the maximum dose of fertilizers (N120P60K60) led to an increase in non-exchangeable potassium compounds (by 1.1 times), which may have been due to the destruction of the mineral part of the soil. A comparison of the P60K60 and N60P60K60 variants showed that the maintenance of the content of mobile potassium compounds at the initial level (1972) with the use of a complete mineral fertilizer was largely due to the influence of nitrogen fertilizers on the solubility and availability of potassium compounds. The effect of prolonged use of phosphorus-potassium fertilizers P60K60 and increasing doses of nitrogen fertilizers on the total content of potassium in sod-podzolic heavy loamy soil has not been revealed.