dependent use of manure and crop residues
Dept. Agronomy, Wageningen Agricultural University, The Netherlands
For the quantification of the ecological sustainability of agricultural systems nutrients should be involved as one of the parameters. Very useful is the approach through nutrient balances. They can be made at different scales: field, farm, region, country (Smaling and Fresco, 1993). Up- and downscaling seems feasible, but is not always straightforward, due to scale effects. For some of the nutrient input and output factors data can be obtained irrespective the scale.
With data on soils, rainfall and topography -usually available in the data base of a GIS- the use of pedotransfer functions (Bouma and Van Lanen, 1987) may yield reasonable data -at farm or at regional level- for output factors like leaching, erosion, and denitrification. For input factors like deposition and biological fixation, data usually are derived from literature; no difference is made between the farm or regional level. For appropriate geo-referenced data on output of nutrients through yield, a scale dependent source is required: the farm, a local database or a national database; data on nutrient content usually are derived from literature. Data on use of inorganic fertilizers at country level can be obtained from national or supra-national databases, whether or not weighed per region (Stoorvogel and Smaling, 1990). However very local differences in use of inorganic fertilizers can be observed, which can partly be explained by distance to the market. This makes this input factor scale dependent. The same holds for the use of manure and crop residues - for which usually no direct data are available. The amount of crop residues is obtained from yield data, assuming a given relation between useful product and total biomass ("harvest index"); estimation of the amount of manure is hardly possible without farm data. Nutrient contents in manure and crop residues usually are derived from literature, although large differences exist in reported data. Main bottleneck is the flow of crop residues and manure; where for most of the input and output factors hold that they enter or leave the system, it is for the factors crop residues and manure farm dependent whether they are removed from, or returned to the same component of the system. It is also dependent on the scale used: a regular one way flow of crop residues or manure from one component to an other, will cause an undesirable disbalance between components, but will not change the outcome for the whole system. At regional and higher levels the factors crop residues and manure are ignored, although at farm level these factors show a high variation, and make up a relatively high proportion of the nutrient balance. Patterns of farm layout and location may explain part of this variation, e.g. areas of grassland mixed with arable land may indicate use of manure; if no grazing land is present, but statistics indicate the presence of cattle, use of crop residues as fodder is likely; use of manure is less likely in urban or remote areas where no grazing fields are found. Differences in physiography, urbanisation, or distance to main infrastructure - relatively easily to be detected with remote sensing techniques, and stored in a GIS- can be used to explain differences in the use of manure and crop residues. More attention should be given to such "pattern transfer functions" to facilitate an assessment of nutrient flows, an important tool in sustainable land management.
Bouma J and HAJ van Lanen (1987) Transfer functions and threshold values; from soil characteristics to land qualities. ITC publication 6: 106-110
Smaling EMA and LO Fresco (1993) A decision-support model for monitoring nutrient balances under agricultural land use (NUTMON). Geoderma 60: 235-256
Stoorvogel JJ and EMA Smaling (1990) Assessment of soil nutrient depletion in Sub-Saharan Africa: 1983-2000. Report 28, The Winand Staring Centre, Wageningen