Citrus orchards occupy more than 280,000 ha in Florida. Soils planted to citrus require intensive nutrient management to maximize fruit yield and quality. Traditionally, orchards receive ≈40 kg phosphorus (P) per ha at planting followed by applications of up to 100 kg·ha−1 P per year until they enter the fruit-bearing years after age 4. At this point, fertilization diminishes to an average of 20 kg·ha−1 P per year, but some growers apply up to 50 kg·ha−1 P per year (Tucker et al., 1990).
Excessive P fertilization has lowered orange juice soluble solids (Anderson, 1966) and delayed external color development (Koo, 1988). In Florida, the surface layers of some acidic, poorly drained soils planted to citrus are composed of stripped quartz sands that have little capacity to retain soluble P, so the potential for P leaching exists when the orchard is drained (He et al., 1999, 2006; Li et al., 1999; Zhang et al., 2002). Thus, overapplying P should also be avoided to prevent potential water quality degradation.
Bearing citrus trees rarely respond to P fertilizer (Smith, 1954) except when planted on soils with extreme P fixation capacity or soils nearly devoid of P. For example, Cantarella et al. (1992) and Quaggio et al. (1998) observed positive yield responses of ‘Valencia’ oranges and lemons [C. limon (L.) Burm.] to annual P fertilizer rates up to 62 kg·ha−1 P on a high P-fixing Brazilian soil. In Florida, Spencer (1963) found that P improved growth of orange trees planted on previously nonfertilized soil but observed a positive response in only 1 of 4 years when 60 kg·ha−1 P per year was applied to bearing trees. Anderson (1966) observed an orange yield response to P fertilizer in 3 of 5 years in central Florida where the Bray P1 soil test was near the accepted minimum adequate value of 40 mg·kg−1 (Koo et al., 1984). This result emphasized the need for a calibrated P soil test for citrus using a common extractant like Mehlich 1.
Soil test data from bearing Florida citrus orchards reflect the ability of many surface soils to retain applied P to a limited extent. For example, the Bray P1 soil test was 44 mg·kg−1 at the beginning of the Anderson (1966) P fertilizer experiment, but it increased to 100 mg·kg−1 after 8 years. Obreza (1993) observed that Mehlich 1 soil test P increased from 15 to 70 mg·kg−1 in a newly planted orchard where a total of 160 kg·ha−1 P was applied during the first 3 years after planting. Anderson (1983), Tucker et al. (1990), and Alva (1993) found that soil test P in a wide range of mature Florida citrus orchards was five to 10 times higher than what would be expected in similar, nonfertilized soils.
No calibrated P soil test exists specifically for Florida citrus orchards. The critical value of Mehlich 1-extractable P currently used (30 mg·kg−1) was determined for agronomic crop production (Obreza and Morgan, 2008). Environmental concerns about P loss to surface water and grower consideration of nutritional effects on citrus yield and fruit quality emphasize the importance of judiciously using P fertilizer. The objective of this experiment was to establish guidelines for P management in developing Florida grapefruit and orange orchards by determining the effect of P fertilizer rate on soil test P and subsequently calibrating a P soil test for citrus yield and fresh fruit quality.
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