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The effect of different densities of Italian ryegrass (Lolium italicum L.: G1- 5 kg/ha seeds, 60 cm row spacing; G2- 5 kg/ha seeds, 20 cm row spacing; G3- 20 kg/ha seeds, 60 cm row spacing, and G4- 20 kg/ha seeds, 20 cm row spacing) and different rates of applied nitrogen fertilizer (N1- 0 kg N/ha; N2 - 50 kg N/ha; N3- 100 kg N/ha and N4-150 kg N/ha) on SPAD values under interaction between the crop and cleavers (Galium aparine L) was investigated. A tri-factorial trial (4 sowing rates x 4 fertilization rates x 4 sampling times) with three replications was set up in a random block design with plot size 10 m2 on a anthropogenious soil in the Šabac area in 2005/2006. SPAD was measured at 30 random plants of L. italicum and G. aparine on each plot four times during vegetation (V1- tillering; V2 - stem extension; V3- flagleaf, and V4-earing), and nitrogen content in plants was then computed. Relative nitrogen content in plant material was used as a parameter of competitive interaction between the cultivated and weed species. Total nitrogen in plant material was also analyzed. The data acquired showed that SPAD values were normally higher in L. italicum than in G. aparine under almost all crop densities and nitrogen rates applied, and maximum values were recorded in the third growing stage (flagleaf) at G3 density and treatment rate of 100 kg N/ha. The highest biomass yield (6 033.0 kg/ha) and seed yield (1 129.0 kg/ha) of Italian ryegrass were achieved in the same variants, i.e. at crop density of 5 kg/ha seeds, 60 cm row spacing and 100 kg N/ha fertilizer, and the crop developed in competition with the weed species G. aparine at an average population density of 15-25 plants/m2.

The researches were performed on brown forest soil used within stationary experiment with a certain fertilizing system for more than 40 years. Researches were made on experiment variants where the increasing dosage of nitrogen fertilizer was applied. The aim of the research was to establish which plant and soil parameters (pots and/or field) might be considered reliable to evaluate values of applied chemical methods (hot water method and easily hydrolyzing nitrogen), being methods used to evaluate potentially mineralized nitrogen in soil. We also wanted to establish the most favorable time to evaluate the values of applied methods (March, July and October). On the basis of established correlation dependences, the plant and soil parameters, either from pots or fields should be used to evaluate the reliability of the hot water method and the reliability of mentioned methods can be estimated in March, July and October. As for the evaluation of easily hydrolyzing nitrogen, we can use both plant and soil parameters in controlled conditions and in the field and the most suitable time is in spring (March) and in fall (October).

In this experiment we used brown forest soil that has been used in long-term experiments, for 30 years, with increasing doses of introduced nitrogen fertilizer in variants. In order to determine the quantities of nitrogen fertilizer required for wheat, a calculation model has been applied. This calculation requires not only a reliable method for determining soil nitrogen availability (both the total and easily hydrolyzed nitrogen method were used), but also a reliable method for determining the coefficient of nitrogen utilization from soil as well as data on the coefficient of nitrogen utilization from fertilizer (Nani) for the added quantities of nitrogen fertilizer.

Our research has been made on brown forest soil that is used in long-term experiments. The soil we used had a certain crop rotation (wheat-corn) and had been fertilized according to a certain system for 30 years. In the experiment, quantities of nitrogen fertilizers were gradually increased after which samples were taken from O to 30 cm depths in order to establish plant and soil parameters for assessing the applied methods. Two experiments were made: one in the field and one in a controlled environment. On grounds of the established correlation interdependence between the methods applied (the total and easily hydrolyzed nitrogen) and the plant and soil parameters, in both field and pots, one can conclude that the methods of total and easily hydrolyzed nitrogen, from the standpoint of nitrogen availability in soil, are reliable. In the method of total nitrogen assessment, one should rely on the parameters regarding plants and soil in the field. In the method of easily hydrolyzed nitrogen, the parameters regarding plants and soil, in both the field and pots, are the same.

In this paper investigations were carried out on two soil types (vertisol and brown forest soil) with different doses of applied N-fertilizer: diameter, N60 N90; N120 and N250. The potential ammonification in soil was obtained by arginine method. The following properties of soil were determined: pH value organic C, available NH4-N and mobile-Al. The pH value in vertisol was 3.75-4.07; mobile-Al was 0.67-4.90 mg/100g; % organic C 1.38-1.46 and the content of available nitrogen was 4.4-11.2 ppm. The amount of released NH4-N by arginine ammonification in this soil type was very low [(-0.12)-0.27mg/g-1h-1]. Correlation coefficients between released NH4-N from arginine and soil pH were (-0.96*), mobile Al - (-0.99**), applied fertilizer doses - (-0.95*). In brown forest soil the amount of released NH4-N by arginine ammonification was greater than in vertisol, ranging from 3.16 to 7.11mg/g-1h-1. Correlation coefficients between soil properties and released NH4-N from arginine were not statistically significant.