ABSTRACT
      
      This thesis studies the processes that pig slurry suffers, when instead of being treated, it is used as fertilizer in agricultural fields. The work done for this thesis can be summarized in several stages.
      The first stage consisted of an experimental montage of nine columns with variable length, soil texture and pig slurry doses. The minimum dose applied was the one established by the current Spanish legislation. The water volume applied corresponded to the daily rain information of the climatic station of Onda (Castellón).
      Laboratory work was done to obtain different parameters necessary for the calibration and the later utilization of the model PRZM.3.12.2. Among these parameters are the physicochemical characteristics of the pig slurry, the pedologic and hydraulic characteristics of the used soil, as well as the parameters related to the soil nitrogen cycle as is the ammonium adsorption isotherm. 
      The purposes of this experimental work were to highlight the importance of the diverse processes of the soil nitrogen cycle (mineralization, nitrification, denitrification and volatilization) as well as to evaluate its dependence on external factors (soil texture and depth and pig slurry dosage). Finally, to analyze the nitrogen mobility and transformations in the soil in order to obtain the information needed for the calibration of the mathematical model PRZM.3.12.2 of infiltration of the nitrogenous forms in the unsaturated soil zone.
      After this, a sensibility analysis of the model PRZM.3.12.2 was done. The constants with which the nitrogen cycle was model and the effect and trend in the leaching and stored nitrogenous forms in the soil profile were quantified, as well as in the nitrogen cycle reactions. Also the sensibility of the program upon entering variables, which values are difficult to establish in the laboratory, as are the fraction of refractory organic nitrogen pig slurry and the percentages of initial soil store of labile and solution organic nitrogen were analyzed. 
      The PRZM.3.12.2 model was calibrated from the experimental columns information obtained, emphasizing in the most sensitive model variables. Taking advantage of the experimental montage, the main parameters of the water movement and the nitrogen cycle constants were related to the present percentage of gravels in the soil columns and the humidity at the end of the experiment. The experimental and simulated results of leaching as well as the nitrogen retained in the soil were compared to analyze the adjustment by means of two statistical tools: the root mean squared error (RMSE) and the index of efficient model (IA). Finally, the model was validated using different information of the experimental essays, obtaining an acceptable error between the real values and the simulated with the calibrated model.
      With the model already calibrated and validated, simulations of application of pig slurry were realized changing the soil characteristics, the pig slurry dose and the presence or not of cultures that extract nitrogen out of the soil, to find the maximum dose possible for application in an agricultural soil without contaminate groundwater.