Research was carried out on hop cv. Aurora planted in hop garden in Croatia, near the village of Gregurovec (close to Križevci), during the six vegetation years (2001 – 2006, Srečec et al.2008). The soil type of examined hop garden is an eutric pseudogley or eutric podzoluvisol. Content of physiological active phosphorus and potash, analysed by the AL-method, was medium. Average content of P2O5 and K2O during the all six experimental years was 19.1 and 15.4 mg per 100 grams of soil, respectively. Content of humus in soil was very low, only 1.48%. Fertilization was provided on the basis of plant uptake for phosphorus and potash and in three splits of nitrogen (50 + 70 + 50 kg/ha N on 20th May, 10th June and 5th July) in all 6 experimental years. Meteorological data in Croatia were collected in Agro Meteorology Station in Križevci, placed five kilometres far away from the hop garden.
Hop cones were sampled from the same plants each year in the phenological phase of technological maturity.
The empiric mathematical equation achieved in Croatia was checked in year of 2012 at the same cultivar grown in completely different agro-ecological conditions in Žalec, Slovenia. P2O5 and K2O in the hop garden was 46.0 and 20.1 mg per 100 grams of soil, respectively. Content of humus in soil was 2.9%. The soil is middle heavy, young alluvial soil, poorly developed on sandy-gravelly deposits of two rivers.
Meteorological data in Slovenia for year 2012 were collected in Agro Meteorology Station at the Slovenian Institute of Hop Research and Brewing placed near by the hop garden.
Sum of effective temperatures, during the hop vegetation, for every experimental year, was calculated by the following equation (2):
(2)
where:
5°C is minimal temperature required for beginning of hop vegetation.
The content of alpha-acids was determined by the method of lead conductance value of hops, powder and pellets prescribed by Analytica - EBC 7.4 (Anon. 1998) and dry matter content by the method of moisture content of hops and hop products prescribed by Analytica - EBC 7.2 (Anon. 1998). The boarder of repeatability (r95) for method of lead conductance value is 0.2 and the boarder for reproducibility (R95) is1.
During the examined vegetation years in Croatia, samples of hop cones were handpicked and analysed from the same 35 plants, randomly chosen and marked in the first research year of 2001 (Srečec et al. 2008) in the same hop garden, which means five control plots with seven plants per each plot within the same hop garden. On the other hand, samples of hop cones in Slovenia in year of 2012 were also handpicked, but sampled randomly from different plants in the same hop garden.
Analyses of achieved analytical results were provided by software Eureqa Formulize 0.96 Beta (Nutonian, Inc.). Eureqa Formulize is a scientific data mining software package that searches for mathematical patterns hidden in data. Formulize’s user interface is organized as a set of seven tabs that correspond to the normal workflow through the program, and user guide is organized around those tabs (see: http://www.nutonian.com). In mathematical analyses of experimental data sum of effective temperatures and total rainfall were treated as independent variables (marked as x and w variables) and content of alpha-acids (%) as dependent variable (y variable).