The use of phosphates to nourish agricultural soils aims to replenish the removal of phosphorus from soil by harvests and erosion losses. Adopting the average phosphorus contents in crops and the harvest index, the global crop production harvested 12.7 MMT P from soils in 2005 as shown in Table 1. A study of Chinese phosphorus flows suggested that the national harvest in 2000 removed 3.4 MMT P from croplands, based on a set of 'domestic' data for the phosphorus contents and the harvest index. These two estimates agree that (1) cereals accounted for a major part of the harvested phosphorus, that is, 70% at the global level, and/or 68% in China; and (2) about two-thirds of the annually harvested phosphorus is contained in grains, and rest is contained in straw and other agricultural waste.
Since natural weathering and atmospheric deposition, as discussed elsewhere, cannot compensate the amount of phosphorus uptake from soils, application of phosphates, in both inorganic and organic forms, becomes essential to sustain today's harvests. There are several means of organic phosphorus reuse. The most direct means is to recycle crop residues in situ. Assuming roughly half of the annual output of crop residues (mostly cereal straw) is not removed from fields, the amount of the direct reuse of crop residues is about 2.2 MMT Pyr-1.
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