As noted in the previous section, the amounts of phosphorus applied to crops should be based ideally on a well-calibrated soil test. However, even at a given soil-test phosphorus level, the amount of phosphorus fertilizer required for economic-optimum yield often will vary with crop. Generally, fast-growing, short-season vegetable crops have higher phosphorus requirements than field and orchard crops. Many deciduous fruit crops infrequently respond to phosphorus fertilization even if soil tests are low (47). It is presumed often that surface soil tests fail to characterize the full soil volume where trees take up nutrients or the fact that trees take up nutrients over a considerable time period.
Mechanisms of phosphorus-utilization efficiency have been classified into three broad classes including:
(a) secretion or exudation of chemical compounds into the rhizosphere,
(b) variation in the geometry or architecture of the root system, and
(c) association with microorganisms (74).
Future opportunities for improving phosphorus-utilization efficiency in crops through genetic manipulation of traits exist (75).
In conclusion, as available data permit, soil-test recommendations for phosphorus should be customized by crop. However, at present, soil-test-based recommendations are generally not sufficiently sensitive to allow recommendations to accommodate the more subtle genetic variation among cultivars within crop species.
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