Abstract: Field studies were carried out in 2 different soils (Alfisol and Ultisol) occupying Ajibode in Ibadan between latitude 07°0¹ and longitude 03°53¹E and Ikorodu in Lagos between latitude 6° and 7¹N and longitude 3 and 4°E, respectively in Southwestern Nigeria. To estimate NPK in zero-tillage soils after harvesting 2 croppings of soybean (Glycine max (L) Merr). With the aims of investigating this leguminous crop its potential to either increase or decrease these nutrients in different soils when cropped without inorganic fertilizer applications. The zero-tillage concept was introduced because it is assumed to encourage soil conservation in these soils suspected to be erosion prone. The NPK were used as test nutrients because of their significant to crop yields and to validate the zero-tillage practice, which was expected to reduce the depletion of NPK that are easily leached in these soils. The initial routine soil analysis before soybean croppings showed that both soils had low NPK content, when compared to critical levels for Southwestern Nigeria. The estimated NPK of both soils after harvesting the first and second soybean croppings, respectively showed increases of these nutrients when compared to initial values before the croppings. However, the Alfisol had higher values than the Ultisol. This might be related to a probable susceptibility of these nutrients to be leached in the Ultisol more than in the Alfisol due to variabilities of their ecological zones. The location (Ikorodu in Lagos) having Ultisol experiences more rainfall (annual rainfall of about 1029 mm, with a range of 1016-1270 mm) than the Ajibode in Ibadan with 1100, with a range of 980-1200 mm. Increased NPK in both soils might also have been encouraged by remains of soybean roots parts (debris) within the soil after harvest. These act as compost of plant origin, which upon decomposition by soil microbes probably increased organic matter and through mineralization helped to increase these nutrients. Similarly, it is expected that the soybean in association with the bacteria rhizobium japonicum fixes nitrogen. This study therefore, inferred that in these soils under zero-tillage after harvesting soybean croppings there were increased NPK in both soils with a reduction in erosion. Without which the soil NPK would had been reduced due to leaching.