The proper use of potassium fertilizer can stimulate a significant yield increase. However, the application of excessively high rates of potassium can reduce the availability of soil calcium for apple trees. The potassium fertigation rate must meet the apple tree’s requirements so that the applied fertilizers can be absorbed by the roots as much as possible. Crop load in apple orchards sometimes varies significantly in different years. The potassium content in apple fruits is relatively high, and the maximum requirement for this nutrient occurs when fruits grow and ripen. Different crop loads at that time mean the various demands of trees and need for changing application rates for this nutrient. The investigation was carried out in the experimental orchard of I.V. Michurin Federal Scientific Centre (Michurinsk, Russia) in 2016 and 2017 (52.885131, 40.465613). We studied seasonal changes of potassium and calcium contents in soil, fruits, and leaves and their relationship with yield during the research. We paid much attention to the potassium rate shift on its content in leaves and fruits and cultivars “Lobo” and “Zhigulevskoye” yield. If the potassium application rate changes according to the actual crop load, it stimulates the yield growth or (if the crop load was relatively low) the reduction of the rate did not lower the productivity. Moreover, we studied the relationship between potassium and calcium nutrition. The decrease in potassium fertigation rate increased the availability of soil calcium. It was the reason for fruit calcium concentration enlargement and mitigation of the K/Ca ratio. We also specified some parameters for soil–leaf diagnosis for potassium nutrition during the growing season.

中文翻译：

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生长期俄罗斯中部苹果树（Malus domestica Borkh。）钾肥施用量的修正

钾肥的合理使用可以大大提高产量。但是，钾的施用量过高会降低苹果树土壤钙的利用率。钾肥的施肥速度必须满足苹果树的要求，这样施用的肥料才能尽可能地被根部吸收。苹果园的农作物载重量有时在不同年份会有很大差异。苹果果实中的钾含量相对较高，并且在果实生长和成熟时才需要这种营养素。那时不同的农作物负荷意味着树木的各种需求以及改变这种养分的施用量的需求。该调查于2016年和2017年在IV Michurin联邦科学中心（俄罗斯Michurinsk）的试验果园中进行（52.885131，40.465613）。在研究过程中，我们研究了土壤，水果和树叶中钾和钙含量的季节性变化及其与产量的关系。我们非常关注叶片，果实和品种“ Lobo”和“ Zhigulevskoye”产量中钾含量的变化。如果钾肥施用量根据实际农作物负荷而变化，则会刺激产量增长，或者（如果农作物负荷相对较低）降低钾肥施用量不会降低生产力。此外，我们研究了钾和钙营养之间的关系。钾肥施用量的减少增加了土壤钙的利用率。这是水果钙浓度增加和K / Ca比降低的原因。