Abstract
Humic substances (HS) extracted from various organic materials have been shown to stimulate plant growth. However, scarce information is available on the impact of different methods of application of HS on nutrient status of vegetable species, especially in soil and climate conditions of the temperate zone. The goal of the present study was to evaluate the effect of pre-plant (bulblet soaking) and foliar application of commercially-produced peat and vermicompost-derived HS preparations on nutrient status and yield of onion (Allium cepa L.) in field experiments using two farming systems - conventional and organic. Although the effect was variable between nutrients, dependent on timing, application methods and farming systems, pre-plant and foliar HS tested was hardly effective in preventing nutrient (S, Zn, B) deficiencies limiting onion yield. Moreover, HS induced changes that significantly reduced nutrient status of organically grown onion and had detrimental effect on yield. Although one-year results did not suggest a benefit from HS application, further studies are necessary to evaluate the possible effectiveness of peat and vermicompost extracts on onion production in Latvia.