The productivity of a citrus grove with variation in tree growth was mapped to delineate zones of productivity based on several indicator properties. These properties were fruit yield, ultrasonically measured tree canopy volume, normalized difference vegetation index (NDVI), elevation and apparent electrical conductivity (EC a ). The spatial patterns of soil series, soil color and EC a , and their correspondence with the variation in yield emphasized the importance of variation in the soil in differentiating the productivity of the grove. Citrus fruit yield was positively correlated with canopy volume, NDVI and EC a , and yield was negatively correlated with elevation. Although all the properties were strongly correlated with yield and were able to explain the productivity of the grove, citrus tree canopy volume was most strongly correlated (r = 0.85) with yield, explaining 73% of its variation. Tree canopy volume was used to classify the citrus grove into five productivity zones termed as 'very poor', 'poor', 'medium', 'good' and 'very good' zones. The study showed that productivity of citrus groves can be mapped using various attributes that directly or indirectly affect citrus production. The productivity zones identified could be used successfully to plan soil sampling and characterize soil variation in new fields.
Spatial variability of citrus yield is very common in Florida, but very little has been done to identify the responsible soil factors. Our objective was to characterize variations in soil physical properties in sandy soils where citrus is produced in Florida. A citrus grove was divided into five productivity zones defined as Bvery poor, [ Bpoor,[ Bmedium,[ Bgood,[ and Bvery good[ based on tree canopy volume. Six random soil samples were collected from each zone at depths of 0 to 15, 15 to 30, 30 to 45, and 45 to 60 cm. Samples were analyzed for soil color, particle size and sand size distribution, and water retention parameters. Although the coefficients of variation (1%Y60%) were large for color, texture, and water retention, the values were generally higher in areas with large trees and vice versa. The trend was opposite for soil lightness and sand content, which were lower in areas of greater growth. Most of the soil properties had a large spatial variation with semivariogram range of 100 to 250 m and varied greatly with increased productivity. Differences between the productivity zones were even more prominent at greater soil depth. Characterization and quantification of spatial soil variability in citrus groves would assist in planning future soil sampling in new fields showing soil and/or tree growth variability. The selection of soil sampling sites should be based on the sampling interval suggested by the variogram range (G75 m) and include the depth (45Y60 cm) influencing productivity.
Variability in soil properties is a complication for fertilization, irrigation, and amendment application. However, only limited progress has been made in managing soil variability for uniform productivity and increased water-use efficiency. This study was designed to ameliorate the poorproductivity areas of the variable sandy soils in Florida citrus groves by using frequent small irrigations and applying organic and inorganic soil amendments. Two greenhouse experiments were set up with sorghum and radish as bioassay crops in a randomized complete block design (RCBD). The factors studied were two soil-productivity classes (very poor and very good), two water contents (50% and 100% of field capacity), two amendments (phosphatic clay and Fe humate), and two amendment rates (10 and 25 g kg -1 for sorghum and 50 and 100 g kg -1 for radish). Amendments applied at 50 and 100 g kg -1 increased the water-holding capacity (WHC) of poor soil by 2-to 6-fold, respectively. The lower rates (10 and 25 g kg -1 ) of amendments were not effective in enhancing sorghum growth. The higher rates (50 and 100 g kg -1 ) doubled the radish growth as compared to the control. The results indicate that rates greater than 50 g kg -1 of both amendments were effective in improving water retention and increasing productivity. Irrigation treatment of 100% of field capacity (FC) increased the sorghum and radish growth by about 2-fold as compared with the 50%-water content treatment. The results suggest that the root-zone water content should be maintained near FC by frequent small irrigations to enhance water availability in excessively drained sandy soils. In addition, application of soil amendments in the root zone can enhance the water retention of these soils. Furthermore, managing variable sandy soils with WHC-based irrigation can increase water uptake and crop production in the poor areas of the grove.
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