The research objective was to determine the suitability of compost and vermicompost for use as a substrate component for the cultivation of seedlings of four different plant species (garden cress — Lapidium sativum L., cucumber — Cucumis sativus L., barley — Hordeum vulgare L., and triticale —x Triticosecale Wittmack). An additional goal was to determine whether the addition of different concentrations of ammonium nitrogen (400 and 600 mg/L NH4-N) would achieve a phytostimulative or a phytotoxic effect on the researched test plants. A basic hypothesis was that there was a difference in the suitability of the cultivation of different seedlings on compost, vermicompost, and their mixtures with a commercial substrate and that there was a difference in the suitability of the test plants for the evaluation of compost quality. The researched compost and vermicompost were produced from the yard waste—that is, from the plant residues subsequent to the maintenance and landscaping of the green public areas. The analyzed compost and vermicompost were stable and mature organic fertilizers with relatively high pH values and a specific density. A decrease in the pH value and a specific density can be obtained by a mixture with an acidic peat substrate. Significant differences were detected between the test plants, whereby barley was dominant in terms of height and above-ground mass, while cucumber had a slightly longer root. On the other hand, garden cress was significantly inferior in terms of the aforementioned properties. Thus, the researched compost and vermicompost can be successfully used as a component for a mixture with a commercial substrate in a 1:1 ratio —namely, as a replacement of 50% of the commercial substrate. Watering seedlings with the ammonium-nitrogen solutions (i.e., with the ammonium-carbonate solutions) increased the above-ground mass and the height of barley and ryegrass, while the effect on the mass and height of cucumber and garden cress was not consistent. It had an opposite effect on the root length, with a reduction of cucumber and triticale roots, and no significant impact on barley and garden-cress roots. In the biological tests, barley and garden cress should be used, whereas cucumber may be used in some cases; however, it is unnecessary to use triticale if barley is used as a test plant. Barley is the most suitable test plant for the determination of a positive effect of the ammonia solution, and cucumber is most suitable for testing the low (i.e., insufficient) amounts of the ammonium form of nitrogen. Garden cress is the most appropriate plant species for the determination of substrate’s phytotoxicity.