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Phosphorus is considered a major pollutant of lakes in central Florida, and growers producing crops in the Lake Okeechobee watershed are being challenged to reduce use of P fertilizer. Caladium (Caladium ×hortulanum Birdsey) tubers are produced on organic soils within this area. This study was done to determine if current commercial P fertilization rates could be reduced or eliminated, since these organic soils have high levels of water extractable P (Pw). Two farms were selected with low (Farm A 19 lb/acre; 21 kg·ha–1) or high (Farm B 59 lb/acre; 66 kg·ha–1) preplant Pw levels. Production of caladium tubers with the standard grower P fertilization practice (Farm A = P at 39.2 lb/ acre; 43.9 kg·ha–1, or Farm B = P at 15.9 lb/acre; 17.8 kg·ha–1) was compared to production with either one-half the standard grower rate of P or no P. The percentage of harvested tubers in each of five grades and the estimated harvested tuber value index were similar irrespective of the amount of P fertilizer used on either farm. These results indicate that P could be eliminated from the fertilization program for caladium tuber production on organic soils. Caladium tubers are produced on organic soils (Histosols) in central Florida and are irrigated with surface waters from canal systems connecting a series of lakes. Phosphorus is considered a major pollutant in these surface waters, and agricultural enterprises are always challenged to reduce use of P fertilizers to minimize the impact from P in the Lake Okeechobee watershed (Izuno et al., 1991). The potential for P pollution from organic soils is common in other areas of the United States and Canada as well since organic soils are often linked to environmentally sensitive wetlands; drainage and farming practices often lead to mineralization of P and other nutrients (Cogger and Duxbury, 1984; Lucus, 1982). The effect of rates of complete (N–P–K) fertilizers on caladium tuber production have been reported (Harbaugh and Overman, 1983), but reports dealing with the ratio of N to P to K to optimize caladium tuber production under field conditions are unknown. Fertilization practices have often developed for ornamental crop production without detailed studies on the effects of the ratio of N to P to K. This may be due to several factors including 1) the total fertilization cost is generally <3% of the variable production cost (Taylor et al., 1990), 2) the ratio of P in the macro nutrient mixture has a relatively insignificant effect on total fertilization costs, 3) the crop value is high compared to many crops, and 4) the crop response (yield and/or crop quality) can be greatly influenced by fertilization. Consequently, there has been a tendency to overfertilize with one or all three of the primary macro nutrients on high-value ornamental crops. Recognition of the potential pollution of surface or ground water sources, however, has led to the desire to develop more precise and conservative fertilization practices on many ornamental crops, including caladiums. Phosphorus mineralization rates from central Florida organic soils ranged from 34 to 164 lb/acre (38 to 184 kg·ha–1) per year (Reddy, 1983), and from 5 to 64 lb/acre (5.6 to 72 kg·ha–1) for drained soils or 32 to 78 lb/acre (35.8 to 87.4 kg·ha–1) for flooded South Florida organic soils (Diaz et al., 1993). Before this study, we randomly sampled soil from several caladium fields that had been farmed for at least 20 years; Pw soil levels ranged from 19 to 55 lb/acre (21 to 62 kg·ha–1) (B.K. Harbaugh, unpublished data). These Pw soil levels were generally well above levels shown to be adequate for root or tuberous vegetable crops on organic soils (Sanchez, 1990). For example, the critical Pw value for radishes and potatoes was 14 lb/acre (16 kg·ha–1) and 16 lb/acre (18 kg·ha–1) for carrots. We hypothesized, therefore, that caladiums could be produced with lower rates of P fertilizer than were currently being used. This study was conducted to determine if P applications could be reduced or eliminated compared to current fertilization practices for caladium tuber production.

Phosphorus Fertilization for Caladium Tuber Production on Organic Soil