Although application of fertilizer in irrigation water (fertigation) is a common practice with mobile nutrients such as nitrogen, it is less common with phosphorus because of concerns about efficiency of utilization. Owing to the soil reactions discussed in a previous section, it is often presumed that much of the phosphorus applied with water will be tied up at its point of contact with the soil. Nevertheless, there are some situations where fertigation is a viable and economical means of delivering phosphorus for crop production.
The downward movement of phosphorus in soil is influenced strongly by soil texture as shown in the laboratory (119,120) and field experiments (121,122). In one study, sprinkler-applied phosphorus moved to a depth of approximately 5 cm in a clay loam soil and to approximately 18 cm in a loamy sand (121). On a basin surface-irrigated Superstition sand that received 91 cm of water, phosphorus moved to a depth of 45 cm (123).
In established perennial crops such as citrus or deciduous fruits, fertigation is often a viable means of phosphorus delivery, regardless of the method of irrigation, because tractor application and incorporation would likely cause root damage and broadcast application would not necessarily be more efficient than fertigation. For fast-growing annual crops, where most phosphorus should be applied preplant, fertigation might not result consistently in production benefits compared to band application but might be economical or even necessary depending on the opportunities and constraints of the irrigation delivery system. Bar-Yosef et al. (127) noted no difference between broadcast and drip-injected phosphorus for sweet corn on a sandy soil. Carrijo et al. (128) reported that phosphorus applied through the irrigation system was more efficient than preplant incorporation for tomato produced on sandy soils testing low in phosphorus. Reports that phosphorus fertigation sometimes produced positive responses have been attributed to band-like effects where phosphorus is delivered in or close to the root zone and not widely mixed with the soil (128,129). Overall, the efficacy of phosphorus fertigation depends on soil texture, phosphorus source, irrigation method and amount, and cropping system utilized.
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