The influence of 14 years application of mineral fertilizers on the organic carbon content of the Agege sandy soil was appraised. The fertilizer treatments in comparison to unfertilized plots led to higher as well as lower organic carbon. The sole applications of N. P, and K were compared, while N alone increased the organic carbon content of the soil, P alone tended to decrease it, but K alone especially decreased the soil organic carbon. Increased P application, however, could compensate this decrease, whereas increased K-accelerated the decrease. N P and N K combinations showed similar trends to decrease the organic carbon but this decrease was more with N K than NP. But P K fertilizer combinations ill contrast to the effect of each of these nutrients alone influenced the organic carbon positively, whereby higher P contents showed again favourable effects than higher K. The fertilizer combinations of the three nutrients (NPK) showed in comparison to unfertilized variants not much higher carbon contents. The complete fertilizer decreased the organic carbon as against unfertilized variants. It was therefore concluded that not always and everywhere can harmonious fertilizer lead to higher organic carbon in the soil, this depends on the type of soil and kinds of cations available in the soil concerned.
Ammonium sulphate and urea influenced the content of organic carbon in the soil differently, owing to their different influences on P and K in the soil. Ammonium sulphate in contrast to urea led to mobility of organic carbon ill the soil, therefore for better results split application of ammonium sulphate will minimize this effect. For better results of the organic carbon ill the soil, mineral fertilizer should not be applied alone but ill combination with organic manure. It is also very essential not only to assess the quantity but also qualitative aspect of organic matter as influenced by 14 years of applications.