Two field experiments were conducted during 2002 (March-June) and 2003 (April-August), at Nanjanad Farm, Ooty. The soils of the experimental field comes under Ooty series (Ultic Hapludalf). There were 19 treatments, which were replicated 3 times in randomized block design. Lime @ 4.3 t/ha and FYM @ 15 t/ha was applied as a common dose for all the plots. As per the treatment requirements, entire quantity of phosphorous fertilizers were applied as basal along with 50 per cent nitrogen, potassium, magnesium and sulphur. The remaining quantities were applied at 50 days after planting (DAP). In the farmers practice (state recommended level) treatment, the entire quantity of Mg and P was applied as basal along with 50 per cent N, K and S. The remaining 50 per cent of fertilizers were applied at 50 DAP. Planting was taken up with the potato cv. Kufri Giriraj of 120 days duration was used as a test crop. Fifteen days prior to harvest haulm drying was done using paraquat (@2.5 lit ha-1) and the tubers were harvested at 120 DAP and the yield was recorded.

Based on the results from the field experiments, on farm trials on potato were conducted in farmers field with eight treatments at four different villages in Nilgiris. District during 2003. The variety Kufri Giriraj was used as a test variety in 3 locations and variety Kufri Jothi was used as a test variety in one location. The maximum potato tuber yield of 62.11 t ha-1 was recorded with 240 N, 240 P2O5, 240 K2O, 48 Mg and 80 kg S ha-1 for which there was 43 per cent increase in yield over state recommended level (43.58 t ha-1). Enhanced level of N (240 kg ha-1) and K (360 kg ha-1) increased the tuber yield but with higher level of Mg @ 72 kg ha-1 there was a decline in tuber yield. The tuber yield reduction due to non-application of N, P, K, Mg and S was 27, 8, 12.2, 13.9 and 14.0 per cent when 240 N, 240 P2O5, 240 K2O and 48 Mg and 80 kg S ha-1 was applied. Increasing the levels of N and K by 100 and 200 per cent from the currently recommended doses of 120 kg ha-1 increased the yield at all levels of Mg tried in the study.

The results of on farm trials on potato revealed that there was an increased in yield from 36.78 to 65.98 t ha-1 for Kufri Giriraj and 12.51 to 24.31 t ha-1 for Kufri Jothi. The balanced fertilizer schedule (optimum treatment) of 240 kg N, 240 kg P2O5, 240 kg K2O, 48 kg Mg and S application at 80 kg ha-1 recorded maximum tuber yield of 65.98 t ha-1 with 79.4 per cent increase in yield over the state recommended level (36.78 t ha-1). The per cent increase in yield due to balanced fertilizer schedule of 240 kg N, 240 kg P2O5, 240 kg K2O, 48 kg Mg and 80 kg S ha-1 was 59.44 as compared to farmer's practice (41.38 t ha-1). Reduction in yield due to omission of S (-S), Mg (-Mg), Mg and S (-Mg, S) and K, Mg and S (-K, Mg, S) was 17.14, 23.45, 31.57 and 39.92 per cent when balanced fertilizer schedule of 240 kg N, 240 kg P2O5, 240 kg K2O, 48 kg Mg and 80 kg S ha-1 was followed.

The results from both the field experiments conducted during 2002 and 2003 and the on farm trials conducted during 2003 at farmer's fields at four different locations on potato have clearly indicated that the presently followed state recommended level of 120 kg N: 240 kg P2O5: 120 kg K2O with 60 kg MgSO4 ha-1 is sub optimal. Hence, there is a need for upward revision of the existing blanket recommendation to 240 kg N, 240 kg P2O5, 240 kg K2O, 48 kg Mg and 80 kg S ha-1 enhanced the potato tuber yield, uptake of nutrients, starch content and crude protein content besides maintaining soil fertility. This fertilizer schedule would ensure higher profit to potato farmers in the Hilly zone soils of Tamil Nadu. This study also indicated that omission of any single nutrient result in declining yield that emphasize the need for balanced fertilizer application. The balanced fertilizer amounts calculated on the basis of this experiment have to be promoted as the new recommendation to enhance the profit of the farmers.