Economics of Controlled‐Release Fertilizer Use on Young Citrus Trees

Abstract

Controlled‐release N‐P 2 0 5 ‐K 2 0 fertilizers (CRFs) (Escote, Meister 9‐month, Meister 12‐ month, Nutricote 360, Prokote Plus, and Sierra) and conventional, water‐soluble fertilizer were applied at full, one‐half, and one‐fourth relative N rates to ‘Valencia’ orange [ Citrus sinensis (L.) Osbeck) trees from planting through 6 yr of age. The full rate, which fell within the recommended range, was 5.4 lb N/tree over 6 yr. CRFs were applied once/yr and conventional fertilizer applied 24 times in 6 yr. Fruit was harvested the third through the sixth years. Averaged across N rates, Prokote Plus, Nutricote, and Sierra produced 4‐yr cumulative fruit yields of 443 to 435 lb/tree compared with 390 lb/tree for conventional fertilizer; Prokote Plus and Sierra also produced higher pounds‐solids (“pounds‐solids” is the term used by the Florida citrus industry to express soluble solids yield per box, per tree, or per acre, and is the basis on which a grower is paid for his fruit) yield (27.4 lb/tree in 4 yr) and gross dollar return ($28.58/tree in 4 yr). Prokote Plus outproduced Escote in fruit and pounds‐solids yield, and outproduced Escote and Meister in dollar return. The response of pounds‐solids yield to N rate was described by a quadratic plateau model. Onset of the plateau (the critical N rate) varied from 76% of the full N rate for conventional fertilizer (at 26.9 lb‐solids/tree) to 100% of the full N rate for Prokote Plus (34.0 lb‐solids/tree), Nutricote, and Meister. The cost of fertilizing citrus with CRFs at the full N rate was four times conventional fertilization cost, but the return was only 15% greater. The high cost of CRF products currently makes their exclusive use to produce citrus economically unfeasible. Research Question Using controlled‐release fertilizer (CRF) in place of water‐soluble fertilizer can extend plant nutrient availability and decrease fertilizer leaching, particularly for N. The state of Florida passed an N best management practice (BMP) law in 1994 that authorized development of fertilizer BMPs designed to meet groundwater standards. One potential BMP is using CRF in place of water‐soluble fertilizer, but by definition a BMP must also be economically feasible. We investigated the suitability of several coated CRFs for supplying young ‘Valencia’ orange tree nutrient requirements, characterized the response to N rate, and economically analyzed CRF use compared with conventional water‐soluble fertilizer. Literature Summary CRF technology was developed primarily to improve N fertilization efficiency. The most technically advanced CRFs use a coating or encapsulation to change the rate of N entry from water‐soluble materials into the soil solution. Citrus fertilization research conducted in Florida within the past 15 yr showed that tree growth and fruit yield where part or all of the fertilization program included CRF were similar or greater than growth and yield resulting from an all water‐soluble N fertilization program. CRFs have also been shown to decrease N leaching potential. Study Description We evaluated CRFs applied to Valencia orange trees on Swingle citrumelo root‐stock from planting through 6 yr of age in a commercial citrus orchard near Immokalee, FL. The orchard tree density was 151 trees/acre. Soil: Pineda fine sand and Riviera fine sand (>98% sand, <2% organic matter) Irrigation: One micro‐sprinkler under each tree; irrigation schedule was determined by the orchard production manager. Fertilizers: One conventional, water‐soluble standard fertilizer (8N‐4P 2 O 5 ‐8K 2 O) treatment compared with six commercially available, coated, multiple‐nutrient CRF products: Escote (19‐6‐12), Meister 9‐mo (17‐6‐12), Meister 12‐mo (17‐6‐12), Nutricote (17‐6‐8), Prokote Plus (20‐3‐10), and Sierra (16‐6‐10). N rates: Fertilizer application rates were equalized based on N rate. The fertilizers were each applied at 25, 50, or 100% of a “full” N rate, which was being used by the industry and was within the recommended range. Full annual N rates were 0.4 lb/tree in yr 1, 0.8 lb/tree in yr 2, 1.2 lb/tree in yr 3, and 1.0 lb/tree in yrs 4, 5, and 6. Fertilizer application schedules: CRFs were applied once/yr in the spring. Conventional fertilizer was divided equally into six applications in 1991, five applications in 1992, and four applications in 1993, spaced 2 to 3 mo apart. From 1994 through 1996, conventional fertilizer was divided into three applications spaced 4 mo apart. Experimental design: Randomized complete block design with 10 replications of each treatment. Responses measured: Leaf tissue nutrient concentrations, tree canopy volume, fruit yield, juice quality, pounds‐solids yield, gross dollar return. For economic analysis, we estimated the costs of the fertilization programs by adding the fertilizer material and application costs for each N rate, and comparing them with the dollar return obtained due to fertilization. Applied Questions How did the CRF programs compare with the conventional fertilizer program? Prokote Plus, Nutricote, and Sierra produced higher fruit yields compared with conventional fertilizer (Table ). In addition, Prokote Plus and Sierra also produced higher pounds‐solids yield and dollar return. Among CRF products, Prokote Plus outproduced Escote in fruit and pounds‐solids yield, and outproduced Escote and Meister in dollar return. Juice quality was not affected by fertilizer source. How did the citrus trees respond to N rate? The pounds‐solids yield responded quadratically to N fertilizer up to a critical rate, then leveled off. The critical rate for Prokote Plus, Nutricote, and Meister was the full (100%) N rate, while the critical rate for Escote was 94%, for Sierra was 84%, and for conventional fertilizer was 76% of the full N rate. The lower critical rate for conventional fertilizer compared with the CRFs is a reflection of the lower nutrient‐use efficiency that commonly occurs with water‐soluble N. Was there a difference in CRF products? The 4‐yr pounds‐solids yield for the Meister and Escote treatments maximized at 29.6 lb/tree, which was 11% less than the Nutricote, Prokote Plus, and Sierra treatments (33.2 lb/tree). A probable reason for the difference is that Meister and Escote contained primarily urea, while Nutricote, Prokote Plus, and Sierra contained mostly ammonium and nitrate. Small amounts of urea passing through the coating of unincorporated fertilizer could have volatilized at the soil surface, decreasing the fertilizer efficiency. How did the CRF programs fare economically compared with conventional fertilizer? Using CRF exclusively to produce citrus is economically unfeasible because material costs are too high (Table ). The cost of fertilizing citrus with CRFs at the full N rate was four times greater than conventional fertilization cost, but the return was only 15% greater. The exclusive use of CRF products to produce citrus could not be considered a nutrient BMP at present. However, if the cost of coating technology decreases, CRF use could become more economical. Main effect of fertilizer source on young ‘Valencia’ orange tree growth and yield. (Values represent means across the full, one‐half, and one‐fourth relative N rates.) 4‐yr (1993–1996) cumulative 6‐yr (1991–1996) 4‐yr (1993–1996) weighted Fertilizer source fertilization cost Yield Gross return avg. juice quality $/tree lb fruit/tree lb‐solids/tree $/tree lb‐solids/box Prokote Plus 15.49 455 a 27.7 a 28.90 a 5.44 Nutricote 360 19.85 451 abc 26.5 ab 27.47 abc 5.27 Sierra 19.20 443 ab 27.0 ab 28.25 ab 5.50 Meister 15.81 424 abc 25.8 abc 26.41 bc 5.47 Escote 14.90 414 bc 24.9 bc 25.98 bc 5.44 <jats

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