Severe fluctuations in the international price of cotton fibre, a decline in the CFA Franc vs. the US $, associated with the increasing costs of fertilisers, resulted in a net reduction in the amounts of fertiliser applied by farmers to cotton. Until the early 1980s, farmers applied an average - 3 years moving average - of 235±25 kg ha-1 year-1 of urea and NPK compound fertilisers, yielding an average of 1320±17 kg ha-1 seed-cotton. As from 1987, discontinuation of governmental policies for the sector led to a decline (i) in fertiliser rates down to 150±7 kg ha' year-1 and (ii) in seed- cotton yield down to 1246±32 kg ha-1. New promotional policies from 1998 helped increasing average fertiliser use to 166±8 kg ha-1 year-1, however seed-cotton yields remain at the same level i.e. 1182±50 kg ha-1 year-1. Yields did not improve with respect to the previous period (1987-1997), and the downward trend of yields in spite of the increase of fertiliser use during the 1998-2005 period, poses the question of a "carry over" effect of low rates of fertiliser use on subsequent soil ability to respond. This paper refers to a farmer's field survey conducted in 1990 in the major cotton production area of Garoua, aimed at establishing the relationships between soil fertility and cotton response to increasing amounts of fertiliser. The ability of such a "research referential" to make a diagnosis of cotton crop productivity trends at a regional level and to assess the impacts of fertilisation practises on soil fertility understood as soil organic matter content, are the central issues dealt with in this paper.