Nanotechnology and nanomaterials, including silver nanoparticles (AgNPs), are increasingly important in modern science, economics, and agriculture. Their biological activity involves influencing plant health, physiological processes, growth, and yields, although they can also be toxic in the environment. A new fertiliser was made based on a urea solution with a relatively low content of AgNPs obtained by the reduction of silver nitrate V. Laboratory tests were used to assess the effect of a fertiliser solution containing 10 ppm AgNPs on the germination of agricultural plant seeds (barley, peas, oilseed rape) and vegetables (radish, cucumber, lettuce) and its foliar application on chlorophyll content, stomatal conductance, and seedling biomass. Field experiments were conducted to assess the effect that a foliar application of 15 ppm AgNPs in working liquid had on physiological plant parameters and yields of rape and cucumber. The AgNPs in the tested fertiliser reduced infestation of the germinating seeds by pathogens and positively affected the physiological processes, productivity, and yields of plants. Plant response depended on plant species and habitat conditions. Reduced pathogen infestation of seeds, higher germination energy, increased chlorophyll content and stomatal conductance, and higher seedling masses all occurred under the influence of AgNPs, mainly in oilseed rape and cucumber, and especially under thermal stress. The beneficial effect of AgNPs on the yield of these plants occurred in years of unfavourable weather conditions. The positive agricultural test results, especially under stress conditions, indicate that fertiliser produced with AgNPs as an ingredient may reduce the use of pesticides and highly concentrated mineral fertilisers. Such a fertiliser is fully in line with the idea of sustainable agriculture. However, research on the effects that AgNPs and fertiliser have on the environment and humans should continue.