A field experiment was conducted with potato in hilly zone (Nilgiris) soils of Tamil Nadu, India to study the native nutrient supplying capacity of acid soils of Nilgiri hills. A field experiment was conducted at Nanjanad farm, Nanjanad, Ooty during March to June 2002. The soil of the experimental site is sandy clay loam belonging to Ooty series. The soil was sandy clay loam in texture, non-saline and acidic (pH 5.0) with a CEC of 29.87 cmol (p+) kg-1. The fertility status of the soil was high in organic carbon (21.0 g kg-1), medium in available (KMnO4) N (360 kg ha-1) and high in available (Bray) P (178 kg ha-1) and available (NH4OAc) potassium (280 kg ha-1). The secondary and micronutrients were above the critical level, but exchangeable Mg level (0.5 cmol (p+) kg-1) was below the critical level. The treatment combinations (20) included N, P2O5, K2O and S at 120 : 240 : 120 : 80, 180 : 240 : 180 : 80, 240 : 240 : 240 : 80 kg ha-1 with 0, 6, 12, 24, 48 kg Mg ha-1, recommended fertilizer dose of 120 : 240 : 120 : 60 kg N, P2O5, K2O and Mg SO4 ha-1 and an absolute control. The twenty treatments were replicated thrice in a randomized block design. An absolute control without any fertilizer application enabled to study the native nutrient supply capacity of the soils.

Treatment combinations involving non-supply of a particular nutrient (N, K, Mg) when other nutrients were applied enabled assessment of the uptake of the nutrient from the soil as influenced by the growth of other nutrients. Treatments with omission of N, P2O5 and K2O were also tried with higher levels of other nutrient combinations. Entire quantity of phosphorus fertilizers were applied as basal along with 50 per cent dose of nitrogen, potassium, magnesium and sulphur. The remaining 50 per cent of nitrogen, potassium, magnesium and sulphur were applied at 50 days after planting (DAP). In farmers practice (state recommended level) treatment, the entire quantity of magnesium was applied as basal. The sources and the amount of fertilizers added are furnished in Table 4. Among the fertilizer sources used, Sul-Po-Mag, a naturally occurring crystalline material, which is mined together with potassium chloride, and sodium chloride, (separated, washed and screened) was also used as a source of fertilizer, supplying K2O (22%), S (22%) and Mg (11%). The magnesium dose as per the treatment was applied through Sul-Po-Mag and the balance of potassium and sulphur were supplemented through muriate of potash and elemental sulphur. To supply the higher level of Mg (48 kg ha-1) in addition to Sul-Po-Mag required MgCl2 was applied. The treatment structure in the present study enabled observations on the native supply of nutrients when particular nutrient was omitted but all other nutrients were applied at optimum levels (240 N: 240 P2O5: 240 K2O: 48 Mg : 80 S ha-1). The uptake of N, P, K and Mg when no fertilizer (absolute control) was applied were 32.9, 10, 20.6, and 16 kg ha-1 respectively. The uptake of N when N was omitted (but P, K, Mg, S applied) was 78.4 kg ha-1 indicating that there was 13 per cent increase in N uptake from the soil due to the application of other nutrients. Similarly there was increase in native P supply by 265 per cent, K supply by 263 per cent and Mg supply by 740 per cent. Removal of nutrients like Ca, S, Fe Cu, Zn and Mn are also influenced by the application and non application of nutrients (N, P, K and Mg) showing the influence of these nutrients on the uptake of other essential nutrients viz., Ca, S, Zn Cu Fe and Mn. The increased native nutrient supply would have been due to the increased root biomass due to fertilizer application and better uptake by the potato crop.