A measurement system has been built for the examination of soil acidity and estimation of soil lime requirement. This is based on active feedback to a computer that keeps the soil suspension pH at a constant value by base addition. Titration at a given pH level was achieved with an equipment, a PC and a software combination built for this purpose. The pH of continuously stirred soil suspension is regularly measured with a pH-selective electrode that is connected to a computer. The software on the computer compares the incoming pH values in preset time intervals with a predefined pH limit value. If the measured pH value is lower than the limit pH value, the computer sends a signal to an automatic burette, which adds 0.0025 ml of basic solution to the soil suspension. After each lapse of the time interval the software saves the time, pH and added quantity data on the disk. Saved data are evaluated automatically using nonlinear regression analysis. Average time of measurements was 8 hours. The volume of added base solution grows almost like an exponential associate function (Czinkota et al. 2002). Soil acidity and lime requirement are calculated from the base consumption values. The aim was to find the best titration settings for measurement of soil acidity and based on this an exact lime requirement calculation. The following titration settings were applied: - suspension: 1:40 soil/1 M KCl solution; added base: 0.1 M NaOH or KOH; target pH: 8.2; soil suspension is in connection with air or closed in N₂ atmosphere. - suspension: 1:40 soil/0.01 M CaCl₂ solution; added base: 0.1 M NaOH or KOH; target pH: 6.5; soil suspension is in connection with air. The pH 8.2 was chosen because this is the pH around the dissolving lime particles in soil and this is the pH of the extractant that is used for the measurement of hydrolytic acidity (Kappen, 1929), which is the basis of lime requirement calculations in Hungary and some other countries. The pH 6.5 is the practical target value of liming on field. Application of N₂ atmosphere can exclude the effect of dissolving CO₂, which increases the amount of added base solution. Salt solutions were used for keeping constant ionic strength. The 1 M KCl is widely accepted as a conductor salt for solution pH measurement, or as a cation exchanger in case of soils. The 0.01 M CaCl₂ is an extractant solution for measurement of plant available cations, suggested by Houba et al. The results of the experiment can be summarized as follows: Comparing the results of the titrations carried out in air and N₂ atmosphere, one can state that the CO₂ from air dissolving into the suspension causes great error. The sum of the titrated acidity was 1.5 times higher without N₂ than with it. The error manifests mostly in case of slower process. Thus, titration with NaOH at this pH gives exact results only in N₂ atmosphere. Titrations to pH 6.5 required half as much alkali as titrations to pH 8.2. The base consumption of slower process decreased in higher proportion than the faster process. At pH 6.5 the dissolving CO₂ caused less error than at higher pH. This error can be corrected by the base consumption of a control titration of a CaCl₂ solution without soil. The base consumption depends on the concentration and quality (K or Ca) of background solution of the suspension. For lime requirement calculations the base consumption of titration to pH 6.5 is assumed to be suitable. The additional alkali consumption (compared to titration to pH 6.5) of titration to pH 8.2 may result an unnecessary lime overdose. After all the most adequate settings will be chosen in the knowledge of plant tests. The acidity and relevant lime requirement values will be tested in field trials in the future.