On the basis of the field experiment carried out at the Experimental Station at Jadwisin (52^o29’ N; 21^o03’ E), Polish mid-early potato cultivar Triada was grown on sandy loam in years 2004 and 2005 in 6 treatments, which included the application of water and nitrogen: organic (cattle manure) and mineral. Water drip irrigation and nitrogen fertigation were scheduled by the Decision Support System (DSS). Diverse water supplies and organic or mineral nitrogen did not change harvest index (HI) values, which varied between 0.7 and 0.8 at the end of the growing period. Average tuber dry matter yield, about 11.0 t∙ha^-1, in both years of the investigation was reached with different HI values: 0.78 for 2004, and 0.73 for 2005. Higher percentage of biomass distributed to tubers was found in 2004, which was characterized by lower cumulative thermal time and global radiation, as well as better water balance in comparison with the growing period of 2005.

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