In the Atlas the minimum nitrogen input requirement (kg N/ha) to realize a defined target maize yield is defined as the minimum amount of nitrogen needed to replace all N taken up in the aboveground crop biomass (stover and grain) for that specified target yield. Using default N concentrations of stover and grain, this is equivalent to 19 kg N per ton of maize yield; or 52 kg grain is produced per kg N uptake. Those values apply to yield levels up to ca. 60% of Yw and a moisture grain content of 15.5% (Janssen et al., 1990*; those authors use a 12% moisture content of grain; uptake requirement then becomes, 20 kg N per ton of maize yield; in other words, 50 kg grain is produced per kg N uptake). At yield levels beyond 60% of Yw, the relationship between uptake and yield becomes non-linear, i.e. the concentrations of nitrogen in grain and stover start to increase, even at balanced nutrition. In both cases (target yield below or above 60% of Yw), we assume the minimum nitrogen input required for a given target yield equal to the nitrogen uptake in the aboveground biomass (in kg N/ha).
Note that the above coefficient (19 kg N per ton grain) is presumed valid on the condition of ‘balanced nutrition', i.e. assuming correct stoichiometric ratios between N, P and K. Also, other management factors are presumed optimal, so as to avoid yield limitations other than due to water (in rainfed conditions), while yield reducing factors (pests, weeds etc.) are presumed absent. Based on data from Janssen et al. (1990) on physiological efficiencies of N, P and K (kg grain yield per kg uptake of a nutrient), balanced crop nutrition of maize implies that 0.125 kg ha-1 of P and 0.667 kg ha-1 of K be taken up to balance each kg ha-1 of N taken up. Hence, the minimum nitrogen input requirements mapped in the Atlas can also be used to derive required uptakes of P and K to realize the specified target yields. These are, 0.125 (for P) and 0.667 (for K) times the minimum N input requirements. This approach of so-called ‘crop nutrient equivalents' was further documented by Janssen (1998; 2011).
One may argue that these minimum nitrogen input requirements are a very optimistic (i.e., ‘low') estimate of N input requirements. This is certainly true for a system where all residues are harvested, as our approach then implies that all added fertiliser N is either taken up by the crop or substitutes N taken up from the soil pool, at any ratio between these two. That implies zero losses via volatilisation (NH3), denitrification (N2 and N2O), or leaching (nitrate). In contrast, for a system where all residues are returned to the soil, the proposed minimum N input requirement theoretically implies that annual N losses are equivalent to the amount of N in the stover. (The implications for both these systems refer to steady state conditions.) Note, however, that decomposition of maize straw (with a high C/N ratio) requires N in the short term; only in the long run will N from residues become available for crop uptake, through increased N supply from the soil pool. For this reason and because it is uncertain how much of the residue-nitrogen is retained in the field, we consider it correct to coin our N input requirement as ‘minimum input requirement'. It indicates a well-defined target, which is highly efficient and with low environmental losses, and shows the minimum amount of nitrogen that is likely needed to achieve a defined target yield.
For a more comprehensive description of quantifying nutrient uptake requirements, minimum nitrogen input requirements and balanced nutrition, see:
De Vries, S.C., H.F.M. ten Berge, M.K. van Ittersum, 2017. Estimating Nutrient Uptake & Input Requirements and Gaps - Concept Note. Crop Nutrient Gap project, Wageningen University & Research and CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS): http://www.cropnutrientgap.org/ and www.yieldgap.org
*These parameters are consistent with those of http://www.ipni.net/article/IPNI-3346. For other examples of analyses of yield-uptake relationships or the use of balanced nutrition, see e.g. Witt et al. (1999) for rice and Setiyono et al. (2011) for maize.
In general, the translation from nitrogen uptake requirement to nitrogen input requirement is complex, as it involves estimating various loss terms on the nitrogen balance. These losses together constitute – in the long run – the nitrogen surplus: the difference between input and offtake in harvested product. To equate – as we do here - the minimum nitrogen input requirement to the uptake in the aboveground biomass is obviously a gross simplification. Nevertheless, it represents a realistic approximation as illustrated by the following example. Our calculations presume a steady state (‘equilibrium') condition: the soil nitrogen pool has attained a constant size and no longer changes over the years due to mining or accumulation. For a grain yield of 8 t/ha, the required crop N uptake is 160 kg/ha (20 kg N per t of grain), and hence the minimum N input requirement would be 160 kg/ha. With a nitrogen harvest index of 60%, 96 kg N/ha is harvested in grain. The straw contains 64 kg N/ha, and this amount represents the sum of nitrogen lost by all biophysical processes plus nitrogen deliberately removed in straw (if any). As a consequence, if all straw is harvested, the assumption is that no losses occur. In such case, the input of 160 kg/ha is clearly insufficient to sustain the system, because the condition of zero N loss is unrealistic. Alternatively, if the straw is retained in the soil, or returned as manure after feeding the straw to cattle, 64 kg N (or 40% of the annual input) can be lost from the cycle. It can be shown that - for a case with an apparent nitrogen recovery of 60% (only achievable with best crop and nutrient management practices) – this means that about 62% of fertiliser N not taken up in the first year and of N in crop residues must be retained in the soil pool for release in later years. While this seems possible, such high retention fraction requires careful N management and even then results in sufficient N supply only after equilibrium has been reached. Until then, losses must be kept even smaller than presumed above.
Janssen, B.H., Guiking, F.C.T., van der Eijk, D., Smaling, E.M.A., Wolf, J., van Reuler, H., 1990. A system for quantitative evaluation of the fertility of tropical soils (QUEFTS). Geoderma 46, 299–318.
Janssen, B.H., 1998. Efficient use of nutrients: an art of balancing. Field Crops Res.56, 197–201.
Janssen, B.H., 2011. Simple models and concepts as tools for the study of sustained soil productivity in long-term experiments. II. Crop nutrient equivalents, balanced supplies of available nutrients, and NPK triangles. Plant Soil 339, 17–33.
Setiyono, T.D., Walters, D.T., Cassman, K.G., Witt, C., Dobermann, A, 2010. Estimating maize nutrient uptake requirements. Field Crops Research 118, 158–168.
Witt, C., Dobermann, A., Abdulrachman, S., Gines, H.C., Wang, Guanghuo, Nagarajan, R., Satawatananont, S., Tran, Thuc Son, Pham, Sy Tan, Le, Van Tiem, Simbahan, G.C., Olk, D.C., 1999. Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia. Field Crops Research 63, 113–138.