The aim of this study has been to determine the indirect and direct effects of farmyard manure and sewage sludge composts, produced in the north-eastern part of Poland, on the content of 1 mol HCl⋅dm-3 soluble forms of zinc and copper in soil. In 2004-2007, a field experiment was carried out at the Experimental Station in Bałcyny near Ostróda. The experiment was established on proper grey-brown podzolic soil originating from light boulder clay, which was rich in P, moderately abundant in K and low in Mg. It comprised a four-field crop rotation system (potato, spring barley, winter oilseed rape and winter wheat. The design of the experiment, set up according to the random block method, involved 8 objects (2 x 4): 1) farmyard manure, 2) compost (sewage sludge + straw), 3) dried and granulated sewage sludge, 4) composted sewage sludge. The composts and FYM were introduced to soil once (in 2004) at a rate of 10 t d.m.⋅ha-1 or 2 × 5 d.m.⋅ha-1 (under potato and winter oilseed rape). In 2004, nitrogen in the soil enriched with natural fertilizers was balanced to 150 kg⋅ha-1 according to the N-total content. In 2005, soil cropped with spring barley received only mineral fertilization, whereas winter oilseed rape received the second rate of organic fertilizers (in the series consisting of 2 × 5 d.m.⋅ha-1) and nitrogen was balanced to 120 kg⋅ha-1. In 2006, soil under winter wheat received only mineral fertilization. Prior to the establishment of the experiment, soil, manure and compost samples were taken. Having been averaged, the samples were subjected to determination of their content of Cu and Zn in 1 mol HCl dm-3. The soil, whose reaction was 5.04 in 1 mol HCl dm-3, was moderately abundant in available zinc and low in copper. After four years of the trials, the levels of available forms of copper and zinc in the soil fertilized with sewage sludge composts, compared to the soil enriched with FYM, were higher. In the first and fourth year of the experiment, the content of both elements was found to increase significantly in the objects fertilized with dry, granulated and composted sludge. Sludge composted with straw significantly raised the content of Zn and Cu in the second and third year of the experiment. However, fertilization of grey-brown podzolic soil with sewage sludge did not change its classification according to the abundance of available forms of copper and zinc.
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