The aim of this study was to compare the most commonly used methods for the determination of organic carbon (OC) and organic matter (SOM), and evaluate the capability of LOI (loss on ignition method) to estimate OC, using reference soils, compost and sludge as standards. The use of a titrimetric endpoint (adding phosphoric acid prior to titration) is more suitable for assessing the level of organic carbon in soils with low, medium, high and very high content. The use of a photometric endpoint is more appropriate for samples of medium and high carbon content. LOI 400 and LOI 450 may also be used for the determination of samples with a high organic content whereas LOI 450 - in the determination of samples with an average organic content. LOI 450 can be used to evaluate SOC in a wide range of organic matter concentrations (3-55%). Indirect assessment of the organic carbon content or organic matter using a correction coefficient is not very accurate for samples with average organic carbon content. Keywords: methods, organic carbon, organic matter, certified samples

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The aim of international research project of European Union “DOMINO” was to develop and examine an innovative management strategy in intensive organic fruit orchards: 1) dynamic “living mulches” in the row, such as cash crops with vegetative organs and the sod with legumes between the rows for weed control; 2) fertilization by recycled soil amendments and leguminous crops to increase resource use efficiency and improve ecosystem services, and 3) partly closed covering net systems to support non chemical pest and diseases control. The content of NH+4 during the second season – 2020 was an increase for on row and inter row forage pea and for inter row white clover variants, because of the leguminous crops sown there. The data for phosphorus for 2020 and 2021 show a clear increasing trend for Golden B variety, which is clearly visible in 2021. Values of K2O for 2020 and for 2021 increase significantly about 2–3 times which meets the requirements for good soil availability. Calcium leads Golden B for inter row white clover variant and for magnesium content for the same variant. The two leguminous crops provided the trees with Mg very well. The highest content of iron in the forage pea variant on row for Golden B and for Granny Smith varieties.

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A study of local and introduced walnut cultivars was carried out at the Fruit-Growing Institute – Plovdiv in the period 2018 – 2020. Four introduced walnut cultivars – two Romanian (‘Valmit’ and ‘Valeris’) and two Turkish (‘Şebin’ and ‘Yalova 1’) – and the standard Bulgarian cultivars ‘Izvor 10’ and ‘Silistrenski’ were the object of the study. Their vegetative habits were investigated and biometric measurements of the nuts were performed. Based on the results obtained the pomological characteristics of the studied cultivars were evaluated. When grown in the conditions of South Bulgaria, the cultivars introduced from Romania – ‘Valmit’ and ‘Valeris’ and from Turkey – ‘Yalova 1’, are characterized by very large nuts. ‘Izvor 10’ and ‘Şebin’ have a high kernel ratio.