From nutrient balances towards soil organic matter dynamics

The loss of soil fertility is a slow but important form of land degradation that puts the future productivity of many agricultural systems at risk. This paper looks back at the use of soil nutrient balances as an indicator for soil fertility loss. Since the early 1990s, soil fertility losses have been quantified by balancing the net inputs of soil nutrients against the outputs of the reserves of soil nutrients. Though these nutrient balances have been useful in quantifying the extent of the problem, they have been less successful in finding solutions. Nutrient balances do not consider the interactions between various nutrient flows. It is therefore difficult to use them to evaluate potential interventions. At the same time, it is difficult to predict the long-term changes in soil fertility from a current rate of change. In this paper, two alternative approaches to deal with these problems are proposed, for further development in the future. Both take the calculations in the Nutmon toolbox for nutrient monitoring as a starting point. One, called Dyn-Nutmon, includes some of the key interactions necessary to evaluate alternative management practices. The other approach, SOM-Nutmon, deals with the dynamics of the soil organic matter pool. Both systems are similar in complexity to the calculations in Nutmon, in order to keep data requirements within the limits of typical data availability. In this paper, a Nutmon dataset from Embu district in Kenya is used to illustrate the various approaches. The variation in cropping conditions makes the dataset eminently suitable to illustrate and discuss the pros and the cons of the two systems. © Wageningen Academic Publishers. The-Netherlands, 2007.