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  Section: Plant Nutrition » Macronutrients » Phosphorus
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Fertilization in Irrigation Water

  Historical Information
  Phosphorus Functions in Plants
  Nature and Transformations of Soil Phosphorus
Diagnosing Phosphorus Deficiency
  Visual Symptoms of Deficiency and Excess
  Tissue Testing for Phosphorus
  Soil Testing for Phosphorus
Factors Affecting Management of Phosphorus Fertilization
  Crop Response to Phosphorus
  Soil Water
  Soil Temperature
  Sources of Phosphorus
  Timing of Application of Phosphorus Fertilizers
  Placement of Phosphorus Fertilizers
  Foliar-Applied Phosphorus Fertilization
  Fertilization in Irrigation Water

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).

Phosphorus source seems to be another important factor affecting phosphorus movement in soils and thus the efficacy of fertigation. Stanberry et al. (124), using radioautographs to trace P32 movement in Superstition sand, noted that phosphorus from phosphoric acid and monocalcium phosphate moved vertically across the length of the photographic film (20 cm) compared to dicalcium phosphate and tricalcium phosphate, which showed negligible movement. Lauer (122) reported that sprinkler-applied monoammonium phosphate, urea phosphate, and phosphoric acid showed similar movement in soils. However, ammonium polyphosphate penetrated only to 60 to 70% of the depth of the other sources. Rauschkolb (125) reported that glycerophosphate moved slightly farther than orthophosphate when injected through a trickle-irrigation system but phosphorus from both sources moved a sufficient distance into the root zone such that phosphorus availability was adequate for tomatoes. O’Neill (126) reported that orthophosphoric acid applied in the irrigation water for trickle-irrigated citrus (Citrus spp. L.) was delivered to a greater soil volume than triple superphosphate applied directly below the emitter. The phosphoric acid also lowered the pH of the irrigation water sufficiently to eliminate clogging problems associated with the precipitation of phosphorus in the irrigation lines.

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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