The determination of the maturity of compost and other organic fertilizers is very important because of possible phytotoxic or phytostimulating effects. The maturity of compost can be assessed on the basis of chemical analyses, and a germination test with different test plants is most often used to determine phytotoxicity. In this research, the maturity of compost produced from the plant residues subsequent to the maintenance of green public areas was assessed using the results of chemical analyses. Simultaneously, a germination test was carried out with the four test plant species (cucumber, garden cress, triticale, and barley) to determine the phytotoxicity of compost extract in a ratio of 1:2.5 v/v (1:3.3 w/v) and 1:10 v/v (1:13.3 w/v) and the three ammonium N solutions (in the concentrations of 200, 400, and 600 mg/L NH₄-N). According to the chemical properties of compost (primarily the C/N, NH₄-N/NO₃-N ratios, and the NH₄-N concentration) and the germination test with cucumber and garden cress, we may conclude that the tested compost was mature and that we did not expect a phytotoxic effect. The choice of a plant is very significant because the germination test with a compost extract demonstrated an undoubted phytostimulating effect on the garden cress and cucumber, with a more pronounced phytostimulating effect of the 1:10 than that of the 1:2.5 v/v compost extract. No such effect was detected on the monocotyledonous test plants triticale and barley since the 1:10 v/v extract had no significant effect, and the 1:2.5 v/v extract had a phytotoxic effect, moderate on the triticale and high on barley. The conclusion is that garden cress and cucumber are suitable test plants for the determination of compost’s phytostimulative effect, but they are not suitable for the determination of phytotoxicity for monocotyledonous plants, especially if the cause of phytotoxicity is a non-ammonium component. Barley is the most suitable species for the determination of compost’s non-ammonium phytotoxicity and nitrogen’s ammonium-form phytostimulative or phytotoxic effect. It would be very useful to conduct a comparative germination test with the compost extracts in the ratios 1:2.5 and 1:10, whereby the 1:2.5 extract would be used as a test of phytotoxicity, and the 1:10 extract for the test of a phytostimulating effect.

Achieving a sustainable increase in the fertility of sandy soils is a major problem. The application of biochar (BC) is a relatively new method, but results for a temperate climate are scarce. This work investigates various combined doses of BC and plant growth-promoting rhizobacteria inoculants on the fertility of acidic and calcareous sandy soils, hypothesising that joint application could have a synergistic impact. The effects were followed by measuring chemical (pH, organic matter, nutrient availability) and microbiological properties of the soil and maize biomass. 30 t ha⁻¹ BC increased both pH (by 1 unit) and nitrification in acidic soil (causing a 60% drop in NH₄-N concentration), and the phosphorus and potassium availability (by 53 and 80%, respectively) in both soils. Substrate-induced respiration increased by up to 100% and 50% in acidic and calcareous soil, respectively, in treatments involving both BC and inoculant. In acidic soil a BC dose of 3 t ha⁻¹ resulted in a 70% decrease in arbuscular mycorrhizal fungal infection. Changes in maize grain yield were not significant, but the increase in above-ground biomass indicated that the combined application of BC and inoculant is more beneficial than separate application of these yield-increasing agents, primarily on acidic sand.

Iodine uptake and translocation was studied in cabbage and tomato cultivated on different soil types (sand, sandy silt, silt) by applying irrigation water containing iodine at concentrations of 0.1 and 0.5 mg/L. Iodine treatment at the concentrations applied did not significantly influence the photosynthetic efficiency and chlorophyll concentration of cabbage and tomato leaves. The growth of cabbage leaves cultivated on sand and sandy silt soil with iodine treatment was slightly stimulated, while, on silt soil, it remained unchanged; for tomato plant parts, independent of the soil-type, the dry mass values remained constant. It can be concluded that iodine treatment had no negative effect on the physiological characteristic of cabbage and tomato plants. Applying 0.5 mg/L in the irrigation water, the highest biofortification with iodine was achieved in plants cultivated in sandy soil and the iodine concentration calculated in the dry matter amounted to 10 and 3.6 mg/kg in the leaves and fruits of cabbage and tomato, respectively. Considering the iodine and moisture content of cabbage leaves and tomato fruits, the consumption of 100 g of fresh vegetable would cover about 80% and 15% of the recommended iodine intake, respectively. The presence of iodine resulted in a reduction in Fe and P concentrations in tomato fruits independent of the soil-type; however, the concentration of Mg, Cu, Mn, Zn, and B remained practically unchanged. However, for cabbage, no similar trend for Fe and P was observed.