Every agricultural system is dynamic as it functions within changing demographic, socio-economic and physical environments. The understanding of agricultural change is important because of its strong relationship to socio-economic, environmental and political systems. This paper discusses the field allocation to crops technique for analysis of changes in crop composition in farming systems. The technique is suitable at local scale and in environments where farmers have multiplicity of fields in different locations in order to benefit from micro climates variations. The paper discusses the relevance of studying changes in crop composition for understanding agricultural change and the technique’s strengths and weaknesses. The technique is demonstrated using data from a farming systems survey in the Uporoto Highlands, Mbeya region, Tanzania. Through capturing gain and loss of fields by crops, the technique unveils changes in the relative importance of crops over time. The technique is relevant for understanding agricultural change with respect to crop composition, and for identification of patterns of changes for better informed decision making.