Boron Soil Test and Leaf Analysis Correlate with Fruit Yield of Sweet Oranges.
Dirceu Mattos Jr.1, José A. Quaggio1, Heitor Cantarella2, and Eduardo S. Stuchi3. (1) Instituto Agronômico (IAC), Rod. Anhanguera, km 158, Cordeiropolis (SP), 13490-970, Brazil, (2) Instituto Agronômico, Av. Barão de Itapura, 1481, Campinas (SP), 13001-970, Brazil, (3) Embrapa Mandioca e Fruticultura e Estação Experimental de Citricultura de Bebedouro, Rod. Brigadeiro Faria Lima, km 384, Bebedouro (SP), 14700-971, Brazil
Foliar spray of boron has been the most common method of supplying citrus trees with this essential element. Since there is little evidence of boron phloem redistribution in citrus, applications have to be directed towards new vegetative and reproductive flushes during spring and summer. However, soil application of boron fertilizers appears to be more efficient than leaf sprays because it is possible to build up boron levels in the soil and raise the nutrient availability during periods of nutrient plant demand. In order to adopt this strategy, it is necessary to establish correlations between soil boron levels, plant nutritional status and fruit yield. Recent studies have suggested that current interpretations for adequate levels of soil hot-water extracted boron (0.20-0.6 mg/dm3) and total leaf nutrient concentration (36-100 mg/kg) have to be reviewed in the citriculture. Experiments using 3-yr-old Natal sweet orange trees [Citrus sinensis (L.) Osbeck] on either Rangpur lime (C. limonia Osbeck), Sunki mandarin [C. sunki (Hayata) hort. ex. Tanaka] or Swingle citrumelo [Poncirus trifoliata (L.) Raf. x C. paradisi Macfad.] were set up in the field. The soil at the planting site is a medium textured (clay content 380 g/kg) Oxisol with pH (CaCl2) 5.7, CEC 55 mmolc/dm and base saturation 70%. Trees received boron at rates 0, 2, 4, and 6 kg/ha/yr as boric acid (BA), colemanite (Col) and ulexite (Ul) split in three equal applications during the rainy season. Soil samples were collected from the 0-20, 20-40, and 40-60 cm depth layers as well leaf samples for chemical analyses. Yield and quality of fruits were evaluated after harvesting in November 2004 (two years after beginning experiments). Soil B content in the control plots remained constant, around 0.25 mg/kg. Significant effects of fertilizer sources and rates (Prob. < 0.05) on soil-B availability are probably related to varying fertilizer solubility. Boron levels at 0-20 cm soil depth in the plots that received 6 kg/ha were 1.8 mg/kg (BA), 2.7 mg/ kg (Col) and 3.7 mg/kg (Ul). Our results also demonstrated significant boron leaching to deeper soil layers (B-levels up to 3.3 mg/kg). This was expected since non-ionic boric acid is the predominant form of this element in the soil solution with pH between 5 and 9. Overall means for boron leaf concentration were well correlated (Prob. < 0.01) with fertilizer rates (in kg ha-1) as follow: rate 0 (228 g/kg), rate 2 (400 g/kg), rate 4 (439 g/kg) and rate 6 (455 g/kg). Symptoms of leaf toxicity were observed for B rates greater than 4 kg/ha. Fruit yield in the second year after fertilizer applications (3-4-yr-old trees) was dependent on rootstock: Sunki mandarin (7.6 t/ha) > Rangpur lime (6.7 t/ha) > Swingle citrumelo (6.3 t/ha). Fruit yield was also affected by boron application and, for trees on Swingle citrumelo was maximum (8.9 t/ha) at B = 2 kg/ha. Linear regression pooled across rootstock varieties pointed out that maximum fruit yield (7.6 t/ha) was obtained with soil-B = 1.5 mg/dm3 (at 0-20 cm soil depth) and B-leaf concentration = 405 mg/kg. Such B leaf content is greater than those taken as adequate for citrus.