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Soil temperature (°C)

figure 3.8 Soil test phosphorus level using phosphorus sorption (PS-P) required for maximum yield of lettuce as affected by soil temperature. (Adapted from Gardner and Sanchez, unpublished data.)

3.3.4 Sources of Phosphorus

Most phosphorus-containing fertilizers are derived from mined phosphate rock. In some unique production situations on acidic soils, phosphate rock can be used directly as a phosphorus source. Most cropping systems show the best response to water-soluble phosphorus fertilizers. Water-soluble phosphorus fertilizers are produced by reacting phosphate rock with sulfuric or phosphoric acid (90). Ammonium phosphates are made by passing anhydrous ammonia through phosphoric acid. This production includes diammonium phosphate and monoammonium phosphate.

The agronomic effectiveness of phosphorus fertilizers was reviewed by Engelstad and Terman (91). Most crops require readily available phosphorus, and most soluble sources perform similarly. However, in some situations the ammonium phosphates produce phytotoxicity (92), and their use is often discouraged when high amounts of phosphorus are required. For example, for economic reasons, diammonium phosphate typically is broadcast applied for lettuce production in the southwestern desert, but its use is discouraged when broadcast rates are high or when phosphorus fertilizer is banded near the plants.

Soluble, dry fertilizers and solution fertilizers perform similarly under many production systems. However, there are some unique production situations where solution sources may present logistical advantages. Often solution sources are easier to use in band placement or point-injection technologies. Generally, solution sources would be utilized in application with irrigation water.

In conclusion, under most conditions, cost considerations, available application technologies, and the potential for phytotoxicity are the major determining factors influencing the selection of sources of phosphorus fertilizers.

3.3.5 Timing of Application of Phosphorus Fertilizers

Overwhelming evidence indicates that for annual crops, phosphorus fertilizers should largely be applied preplant. Phosphorus moves to plant roots primarily by diffusion, and young seedlings of most annual crops are very sensitive to phosphorus deficits. Furthermore, yields of some crops often fail to recover fully from transitory phosphorus deficits (93).

Grunes et al. (94) showed that the proportion of fertilizer phosphorus absorbed by sugar beets (Beta vulgaris L.) decreased as the time of application was delayed. Lingle and Wright (95) reported that muskmelons (Cucumis melo L.), which showed large responses to phosphorus at seeding, showed no response to sidedressed phosphorus fertilization. Sanchez et al. (96) reported that a preplant phosphorus deficit in lettuce could not be corrected by sidedressed fertilization. Preplant broadcast or band applications are usually recommended for annual crops.

3.3.6 Placement of Phosphorus Fertilizers

The literature contains many accounts recording the positive effects of applying phosphorus fertilizer to a localized area, usually near the plant roots, as opposed to a general soil broadcast application. Reviews on the subject of fertilizer placement should be consulted for detailed information (97,98). Localized placement of phosphorus fertilizers might include row, band, or strip placement.

It is generally presumed that a localized or band application reduces fertilizer contact with the soil thereby resulting in less phosphorus sorption and precipitation reactions and, thus, enhanced availability to crops. However, for soils with a high phosphorus-fixing capacity, where phosphorus is relatively immobile, placement of the fertilizer where root contact is enhanced may be an equally or more important mechanism than restricting fixation (99-101).

The relative benefits of localized placement of phosphorus fertilizers are neither constant nor universal across crop production situations. This fact is illustrated by a series of experiments that the author conducted to improve phosphorus fertilizer use for vegetable crops produced on Histosols (102,103). The amount of phosphorus required for lettuce production could be reduced by at least 50% if phosphorus was banded instead of broadcast (Figure 3.9). However, band placement was not a viable strategy for improving phosphorus-use efficiency for celery under the

figure 3.9 Marketable yield of lettuce as affected by phosphorus rate and placement. (Adapted from C.A. Sanchez et al. J. Am. Soc. Hortic. Sci. 115:581-584, 1990.)

figure 3.9 Marketable yield of lettuce as affected by phosphorus rate and placement. (Adapted from C.A. Sanchez et al. J. Am. Soc. Hortic. Sci. 115:581-584, 1990.)

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