Modelling Salvia sclarea L. yields depending on plants spacing, mineral fertilisers and depth of ploughing in the irrigated conditions of cold Steppe zone

Ukraine has favourable natural conditions for the cultivation of medicinal and aromatic plants, but there is a lack of scientific knowledge and practical guidelines for their cultivation. Clary sage is a valuable and prospective crop for the South of Ukraine and deserves special attention. This study is aimed to investigate theoretical mechanisms of clary sage productivity formation on the irrigated lands of the Southern Ukraine. The experimental basis for the study is the data on crop inflorescence yields, collected during field investigation of clary sage cultivation technology in the dripirrigated lands of the Kherson Oblast during 2013-2018. Theoretical study on the yield formation depending on the cultivation technology elements was performed through rank correlation and heteroscedasticity analyses, while the mathematical model for the crop yield prediction was developed using multiple regression. As a result, the null hypothesis about the influence of the factors studied on crop yields was denied according to the Breusch-Pagan and Glejser tests, although the results of the analysis of variances revealed no significant effect of the depth of the plough and the spacing of the crop on the yields. Rank correlation analysis revealed that the highest influence on crop yield is attributed to phosphorus fertilisers. In addition, information on the influence of each agrotechnological factor on the yield of clary sage was determined, namely: 1 kg of nitrogen fertilisers applied per ha reduces the yield by 6.34 kg; 1 kg of phosphorus fertilisers applied per ha increases the yield by 156.44 kg; 1 extra cm of row spacing reduces the yield by 3.33 kg; 1 cm deeper ploughing improves the yield by 56.56 kg. The model has moderate fitting quality (correlation coefficient 0.5885) and reasonably good yield prediction accuracy (mean absolute percentage error 24.12%). The study has no analogues in Ukraine or in the world and provides novel theoretical and practical insights on the formation of clary sage productivity in the cold Steppe zone