Residual nitrate nitrogen in a soil profile is influenced by agricultural practices such as fertilisation, irrigation and cultivation. In this study, the effects of different tillage methods and timing on corn yield were investigated in relation to soil penetration resistance and variation of nitrate nitrogen (NO3-N) through a vertisol soil depth. A field experiment was carried out with three different tillage times (Fall-at the end of October, Early-at the middle of May; and Late-at the end of May) and tillage methods (mouldboard, chisel and direct drilling) in the Black Sea Agricultural Research Institute in 2011. Nitrate nitrogen values in 0-20, 20-40 and 40-60 cm soil layers were measured for the six different soil sampling times using the potentiometric method. Corn yield values generally decreased when the first soil tillage time when mould board and chisel applications were delayed or were made at the end of May. The fall tillage treatment with mouldboard had the highest corn yield (61.1 Mg/ha) while the lowest yield value (30.9 Mg/ha) was found with the direct drilling treatment. Generally, late tillage timing at the end of May reduced corn yield due to changing soil structure with reducing penetration resistance, increasing macroporosity and nitrate leaching in the soil profile. In conclusion, fall season soil tillage using mouldboard in clay soils is suggested to achieve optimum plant growth soil conditions resulting in high corn yield, and because of nitrate leaching, it also results in beneficial effects on conservation of water pollution.