Nutrient balance
“Best bet” Land-use Systems
Country reports
Alternatives to Slash-and-Burn in Brazil
Agronomic Sustainability
Unique id: IDA0LSCE
Source file: D:\Projects\ASB\ASB Country and Thematic reports\Brazil country report\ASB Brazil Summary Report.xml
Authors: S. Vosti, C. L. Carpentier, J. Witcover, . Carvalho dos Santos, E. Muñoz Braz, J. Ferreira Valentim, S. J. de Magalhães de Oliveira, C. Palm, F. de Souza Moreira, A. Cattaneo, A. Gillison, A. Mansur Mendes, V. Rodrigues, T. C. de Araújo Gomes, M. V. Neves d’Oliveira, E. do Amaral, S. Fujisaka, C. Castilla, T. Tomich, D. Bignell, D. Gonçalves Cordeiro, A. Hermes Vieira, R.S. Correira da Costa, M. Faminow, M. Locatelli, M. Swift, S. Weise, M. van Noordwijk, N. Sampaio, I. L. Franke, H. J. Borges de Araujo, L. M. Rossi, E. Barros, B. Feigl, S.P. Huang, J. Cares, C. Pinho de Sá, . Carneiro, P. Woomer
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As nutrients are removed from a piece of land through the harvested product, it is important to assess if these nutrients are adequately replenished through internal processes and/or external inputs, such as fertilizers. Internal processes that make nutrients available to plants include the mineralization of soil organic matter, releases from the soil matrix, and the biological fixation of atmospheric nitrogen. At the same time, nutrients can be lost from the system through processes such as leaching, lateral flow, soil erosion and denitrification. It is not easy to measure many of these processes rapidly. Simplified nutrient balances are, therefore, often used as a first indication of the nutrient dynamics of a system. The working group limited its calculations to the three major plant macronutrients: nitrogen (N), phosphorus (P) and potassium (K).
Nutrient
export, balance and depletion
Nutrient exports are easily determined if the quantity of harvested products, including any crop residues removed from the field, and their nutrient content are known.
Nutrient export = (nutrient
content x harvest offtake) summed across all products over system cycle/length
of system cycle (kg/ha/yr).
A more precise measure is:
Simple nutrient balance = nutrient import less
nutrient export (kg/ha/yr).
Nutrient imports include fertilizers and N fixed through legumes. Fertilizer inputs are corrected for use efficiencies, i.e. 25% of N, 20% of P and 30% of K fertilizers are assumed to be effectively taken up by crops. Negative balances indicate greater exports than imports.
It may be desirable to calculate an NPK index that combines the three
macronutrients, as a basis for investigating the tradeoffs between nutrient
balance and other parameters of a system (e.g. biodiversity or profitability):
NPK index = sum of N, P and K ranks / 3;
where LUS are ranked in terms of the simple nutrient balance (with the highest value receiving a 1, the second receiving a 2, the third a 3, etc.). The NPK index is valid for within-country comparisons only.
Fertilizers play an important role in replacing the nutrients exported through harvested products. However, if their cost is too high relative to the value of the products, farmers will hesitate to apply them, even if they are available. The lower the ratio of the fertilizer cost to the farm-gate value of the crop, the more likely the farmers will be to consider using fertilizers and thus avoid nutrient mining (van Noordwijk et al, 1997):
Nutrient replacement value (NRV) = sum of cost of fertilizers required to replace all exported NPK nutrients / value of all products produced by the LUS.
The fertilizer requirement is, however, corrected for nutrient recovery, i.e. only 25% of N, 20% of P and 30% of K fertilizers are assumed to be recovered by the crops. Nitrogen provided through fixation by legumes is deducted from N exports before calculating N fertilizer replacement requirements. Low NRVs indicate that the output of the LUS is high in value relative to the cost of nutrient replacement through fertilizers. Generally, NRV is calculated only for a specific crop and year. This calculation works well for monocrop systems.
As discussed in the previous section, mycorrhizae and rhizobia play important roles in agricultural productivity through their influence on nitrogen fixation and on the symbiotic relationships between soil and roots.