Lab. of Soil Science and Geology, Wageningen Agricultural University

P.O. Box 37, 6700 AA Wageningen, The Netherlands

E-mail: jetse.stoorvogel@bodlan.beng.wau.nl

Guidelines for estimating the soil nutrient balance at the continental, regional, and national level are presented. Indicators for soil nutrient depletion are evaluated and procedures for their quantification are discussed.

In many cases soil nutrient balances are not in equilibrium. Increasingly, farming systems in the tropics experience production losses as a result of decreasing soil fertility, whereas nutrient leaching as a result of nutrient accumulation is increasingly experienced in western, high input agriculture. Although the process of nutrient depletion is generally slow and no abrupt changes in soil fertility occur, the loss of agricultural production can be significant. On the other hand, the risk for nutrient leaching may increase when nutrients accumulate in the soil. How best to address the issues of nutrient depletion and accumulation is not an easy task. Do we need to restore soil nutrient stocks to a specific fertility level, or can we stabilize the process of soil nutrient depletion or accumulation? In case of the restoration of soil fertility, what is the preferred fertility target, and how should this be reached?

Many agronomic practices are available that permit farmers to manage their soil nutrient balance. At the same time, but at another scale level, policy makers have access to a wide range of tools to influence land allocation decisions as well as soil management decisions of farmers. These policy measures range from the intensive margin where management is controlled through regulations to the extensive margin where soil management is influenced indirectly through e.g. incentives and price regulations. Before identifying the preferred intervention(s), it is essential that the rate of soil nutrient depletion or accumulation is quantified through a soil nutrient balance indicator. Only then the required nutrient management strategies can be defined.

The accuracy with which we would like to quantify soil nutrient balances depends on the scale and objectives of the questions being resolved. At continental scales, the objective may be to create awareness of the extent of soil nutrient depletion. Broad, qualitative classes as used by Stoorvogel and Smaling (1990; High, Intermediate and Low) may be sufficient. At the country level, a land quality indicator for soil nutrient depletion can be used for different objectives. For planning purposes, it is necessary to identify of problem areas to focus research and target specific action. This screening process can be based on a number of general classes and well-defined threshold values (see for example in Smaling et al., 1992). Also at the national level, the depletion of natural resources (Solorzano et al., 1991) can be accounted for in the national financial budget. Economists have identified the importance of environmental accounting in the use of natural resources, but operational indicators such as soil nutrient depletion are required.

At more detailed scales, e.g. community level and farm level, planning to maintain soil nutrient stocks, and avoiding either depletion or overfertilization and associated environmental damage is an important decision in maintaining farm level sustainability (see e.g. van de Pol, 1993). At this level, alternative agricultural technologies, such as integrated nutrient mangement and various types of soil husbandry and other agricultural practices need to be screened, and their impacts on soil nutrient stocks and flows determined. These decisions clearly require quantitative estimates of soil nutrient depletion, and broad qualitative classifications are not adequate. Soil testing results are often helpful at this level. Figure 1 summarizes the different scale levels and illustrates some of the objectives for the estimation of soil nutrient depletion.

Considerations for Calculating Soil Nutrient Balance

Land quality indicators are required at different scale levels. Although aggregation procedures can be used for certain scales of indicators, policy makers often require estimates at specific scales, and do not want to be confused with too much details at underlying scales. A generic procedure based on an accounting exercise of the different inputs and outputs (stocks and flows) of soil nutrients is recommended for calculating soil nutrient balance (Stoorvogel and Smaling (1991). The methodology involves separate assessments of major inputs and outputs of nutrients (Table 1).

A minimum data set is required for different levels of detail, but the basic requirements are a soil map, production figures and some basic characteristics of land use systems. If more data are available, the procedure will perform the calculations with a higher degree of detail. An essential addendum is that the procedure needs to include an estimation of the accuracy, and that it adapts automatically to the appropriate level of detail in the outputs of the indicator.

The loss of nutrients as calculated by the net balance of nutrient inputs and outputs is not a good indicator for the sustainability of agricultural systems, in many cases. In Figure 2 shows the effect of similar rates of soil nutrient depletion on a fertile (soil A) and an infertile soil (Soil B). The shaded area represents soil nutrient stocks with concentrations that are limiting for plant growth. Although both soils have similar soil nutrient depletion rates, Soil B will have a nutrient limiting situations much sooner than soil A.

Table 2 presents a qualitative approach towards the classification of soil nutrient balances on the basis of the balance as well as the soil nutrient stocks, the adsoption capacity of the soil and the crop requierments.

However, other, more quantitative indicators can be developed using, for example, the relative change in soil nutrient stocks. If the study aims at the long-term effects nutrient depletion may be compared with total nutrient stocks including the not readily available fraction of the nutrients. In the case of shorter time horizons, however, weathering and mineralization can not keep up with nutrient losses and the stocks of readily available nutrients can be used.

Other indicators for nutrient depletion include actual land use and critical levels at which a certain nutrient becomes limiting for current land use (). They indicate the rate at which the nutrient surplus, i.e. the amount of nutrients above the critical level, is depleted. :

If is larger than 40%, for example, it is likely that the nutrient stock will become limiting within a few years. If is small, it can be concluded that under current management no nutrient limiting situation will occur in the near future.