Importance of nutrients for citrus trees By Mongi Zekri and Tom Obreza Let’s talk about potassium (K). o maintain a viable citrus industry, Florida growers must sprays of potassium nitrate (KNO3) Tconsistently produce large, high- or monopotassium phosphate (MKP), Effects of K on citrus fruit quality quality, economic fruit crops from (KH2PO4). Precautions should be Variable K taken to avoid foliar burn from high year to year. Efficiently producing Juice Quality maximum yields of high-quality fruit spray concentrations. Fruit burn was Juice Content – is difficult without an understanding not observed when 25 lbs. KNO3/acre of soils and nutrient requirements were applied in 125 gal. of water/ Soluble Solids (SS) – of bearing citrus trees. Most Florida acre, 15 lbs. MKP/acre were applied Acid (A) + citrus is grown on soils inherently low in 32.5 gal. of water/acre, or when 15 SS/A Ratio – in fertility with low cation exchange lbs. MKP/acre were applied in 10 gal. Juice Color – capacity (CEC) and low water-holding of water/acre by airplane. Solids/Box – capacity; thus soils are unable to retain POTASSIUM FUNCTIONS Solids/Acre + sufficient quantities of available plant External Fruit Quality nutrients against leaching by rainfall Citrus fruits remove large amounts or excessive irrigation. of K compared with other nutrients. Size + Potassium (K) is one of the most Potassium moves from leaves to fruit Weight + important nutrients applied as fertil- and seeds as they develop. Potassium Green Fruit + izer in Florida citrus groves. Potassium is necessary for several basic physi- Peel Thickness + (also called potash) is listed on the ological functions like the formation Increase (+), Decrease (-) fertilizer label as K2O. Potassium plays of sugars and starch, synthesis of a key nutritional role in determining proteins, normal cell division and Increasing potassium in the tree yield, fruit size and quality. Florida growth, and neutralization of organic from deficient to optimum concentra- sandy soils used to produce citrus are acids. Potassium is important in fruit tion increases yield, fruit size, fruit naturally low in K. Furthermore, K is formation and enhances fruit size, fla- weight, green fruit, peel thickness and not fixed and does not accumulate in vor and color. This nutrient also helps acid content. Concurrently, juice con- those sandy soils, even with repeated to reduce the influence of adverse tent, total soluble solids (TSS), TSS/ fertilizer applications. Thus, K fertil- weather conditions like drought, cold acid ratio and juice color decrease. izer application is required every year and flooding. Sufficient potassium reduces inci- in Florida citrus groves. Potassium de- Potassium is known to influence dence of fruit splitting, creasing, fruit ficiency is not common when a grove is many enzymatic reactions and is as- plugging and stem-end rot of fruit fertilized normally, but deficiency can sociated with almost every major plant in storage. develop on high pH soils or when high function. Potassium helps regulate Timing is important for K applica- N rates stimulate high fruit production. the carbon dioxide supply to plants by tions to enhance fruit size and yield. Apply K controlling Potassium is a primary component of fertilizer at Guidelines for interpretation of orange tree leaf K (%) analysis based stomata cell walls, accounting for more than on 4- to 6-month-old spring flush leaves from non-fruiting twigs a K2O rate opening 40 percent of fruit mineral content. equal to the Deficient Low Optimum High Excess and closing. About 70 percent of final fruit size is N rate. Potassium related to the number of cells in the If leaf K <0.7 0.7 – 1.1 1.2 – 1.7 1.8 – 2.4 >2.4 improves fruit, so more cells usually means is consis- the efficien- larger fruit. Cell division typically tently less Oranges Grapefruit Other cultivars cy of plant stops by late April; thus fruit size than opti- Tree age water and change throughout the rest of the year mum from Pounds of K 0/acre/year (range) sugar use comes from cell enlargement. There- year to year, 2 for main- fore, the maximum effect of foliar K 4-7 120 – 200 120 – 160 120 – 200 increase the tenance (8 lbs./acre K2O) is achieved from K2O rate by 8 and up 140 – 250 120 – 160 120 – 300 and normal applications that make this important 25 percent. Yield based Yield based growth nutrient available during bloom and Low leaf K functions. post-bloom when it can be used dur- concentra- Pounds of K20/tree/ Additionally, K moves ing both cell division and rapid cell tions are Tree age year (range) sugars from the site of enlargement phases. common photosynthesis to other in groves 1 0.15 – 0.30 storage sites. Potassium POTASSIUM DEFICIENCY planted on 2 0.30 – 0.60 works with phosphorus (P) The rate of photosynthesis drops calcare- 3 0.45 – 0.90 to stimulate and maintain sharply when plants are K deficient. ous soils. rapid root growth of plants, Too much N with too little K can If low yield, small fruit, fruit split- and stimulates the synthesis of protein result in a backup of the protein build- ting and/or creasing are observed, from amino acids. Potassium improves ing blocks, set the stage for disease application of additional K fertilizer plant health and resistance to disease problems, reduce production of carbo- is justified. If trees do not respond and tolerance to nematodes and in- hydrates, reduce fruiting and increase to K soil application, an alternative sects. The rate of photosynthesis drops fruit splitting, fruit creasing, plugging approach to increasing leaf K is foliar sharply when plants are K deficient. and drop. A shortage of K can result 8 CITRUS INDUSTRY • May 2012 in decreased yield and low fruit qual- ity. Negative effects of low K on fruit yield and quality generally precede FDOC is on a Mission appearance of leaf deficiency symp- toms. Decreased yield and small fruit Strategic Plan have been observed on trees with leaf K in the range of 0.5 to 0.8 percent, to Identify Future Goals while K concentrations of 1.2 percent By Douglas Ackerman or more have been associated with maximum yield of high quality fruit. “A good plan is like a road map: Moderately low concentrations of K in the tree will cause a general It shows the final destination and usually the best way to get there.” reduction in growth without visual — H. Stanley Judd deficiency symptoms. The onset of visual deficiency symptoms means t’s been several years since the current Florida Department of that production has already been seri- Citrus (FDOC) mission statement was created and approved by ously impaired. the Florida Citrus Commission (FCC). With a new FCC and In Florida, low K fertilization can I lead to deficiency symptoms that FDOC executive director, the time is right to take a fresh look at the develop in late summer and fall on the FDOC’s role in charting the course for a sustainable future for the recently matured spring flush leaves. Florida citrus industry. When K is low, the general leaf pat- tern begins as a yellowing of the tips To initiate this process, the FDOC leadership team is conducting and margins, which then gets broader. a series of strategic planning sessions to discuss very basic questions Necrotic areas and spotting can de- such as, “Who is the FDOC? What is the role of the FDOC? How velop on the leaves. Symptoms appear can the FDOC best serve the needs of the Florida grower?” Our goal first on older leaves because K tends to concentrate in the rapidly grow- is to develop a mission and an action plan that will guide FDOC ing tissues. Potassium deficiency will efforts over the next few years. cause slow vegetative growth, small Our first step was to review the current mission statement adopted leaves, fine branches, thin canopy, an increase in susceptibility to drought in 2005. We believe there is an opportunity to further refine the and cold, reduction in fruit size, very language so that our purpose is more clearly defined and understood. thin peel of smooth texture, prema- Next, we identified strengths, weaknesses, opportunities and ture shedding of fruit, and lower acid concentration in the fruit. threats for the FDOC. We also began to outline a list of core values Potassium deficiency symptoms that describe how we conduct ourselves and our business. usually result from an insufficient K As the strategic planning process continues, the leadership team supply in the soil. Potassium defi- ciency may occur on acid sandy soils will draft key strategic initiatives (KSIs) which will serve as guiding where leaching may be consider- principles for the future. We will create an action plan that maximizes able. The supply of K to plants may FDOC strengths and opportunities while minimizing weaknesses and be decreased by soils that have very high concentrations of calcium and external threats. Each KSI will include a specific timeline and mea- magnesium or by heavy application surements so that we can determine progress toward goals. of N. Decreased K uptake is typical When completed, the staff will present the proposed mission state- on some calcareous soils. Lack of soil moisture also reduces K uptake and ment and FDOC Strategic Plan to the FCC and industry for input and may lead to K deficiency. If the supply consideration. Having a well-thought-out strategic plan will allow us of N and P is high relative to that of K, to align all department functions and allocate resources effectively growth may be rapid at first, but the K concentration in the plant may ul- and efficiently. Our goal is to provide the maximum return on invest- timately decrease to cause deficiency. ment for growers and help ensure the sustainability and economic Addition of K would be necessary to well-being of the Florida citrus industry. maintain the nutrient balance required for uniform and continued growth. In The mission of the Florida Department of Citrus is to grow the market for the Florida situations where available K is high citrus industry to enhance the economic well-being of the Florida citrus grower, compared with the N or P supply, citrus industry and the state of Florida. Douglas Ackerman, executive director, can be luxury consumption of K can occur. reached at (863) 537-3999. For more information, visit www.FDOCGrower.com (Luxury consumption means the plant takes up more of the nutrient than it actually needs for maximum growth.) Under most soil conditions, K deficiency can be corrected by ap- plying potassium chloride (muriate of potash) or potassium sulfate to the Column sponsored by the Florida Department of Citrus soil. However, in fine textured, saline P.O. Box 9010, Bartow, FL 33831-9010 or calcareous soils, K applications to CITRUS INDUSTRY • May 2012 9 From top left, clockwise, potassium deficiency symptoms (small fruit, chlorosis and necrotic spotting of leaves) spray application of • Potassium nitrate K has been demon- K nutrient sources used to manu- strated to increase fruit facture solutions include: size. Foliar-applied K • Potassium chloride has also corrected K • Potassium nitrate deficiency of citrus on • Potassium sulfate calcareous soil. Foliar • Potassium thiosulfate application of potas- K nutrient sources applied in foliar sium nitrate (KNO3) sprays include: increased leaf K more • Potassium phosphite rapidly compared with • Potassium nitrate soil-applied fertilizers • Potassium thiosulfate because plant uptake • Monopotassium phosphate the soil are sometimes ineffective or was much faster, but the positive effect • Dipotassium phosphate slow to correct K deficiency. Foliar was shorter-lasting. application of potassium nitrate or Common solid sources of K applied CONCLUSION mono-potassium phosphate can be to citrus groves include: Supplying sufficient nutrition should very effective and rapid to correct K • Potassium chloride (muriate of be a high-priority management prac- deficiency. The recommended foliar potash) tice for every grower. Furthermore, to K spray for mature citrus trees is 8 • Potassium sulfate achieve optimum plant nutrition, an lbs. K2O/acre per application. Foliar • Potassium-magnesium sulfate appropriate nutrient balance is neces- sary. Correct ratios of nutrients are critical to fertilizer management and sustainability. If an element is below the critical level, yield production will fall, even though the other elements are kept in sufficient supply. Too much N with too little K can reduce fruiting Blue Goose has the people, and result in lost crop yield and qual- experience and resources ity. High K with low N and P supply to improve your return on your investment will induce luxury consumption of K, delay fruit development and reduce Growers Cutting juice content. Balanced use of plant Heavy equipment nutrients corrects nutrient deficiencies • Installation services • Services for Agricultural and and toxicities, improves soil fertil- • Grove care • Hedging Commercial construction ity, increases nutrient and water use • Crop manage- • Excavation • Grading • Land clearing efficiency, enhances crop yields and ment • Topping • Skirting • Waterway maintenance – Water fruit quality, develops tree tolerance to • Property pests, diseases and other stresses, and management • Cross Hedging control structure installation improves environmental quality.

FORT PIERCE Mongi Zekri is a multi-county citrus 772-461-3020 Main Office Real estate Extension agent and Tom Obreza is a 9901 Okeechobee Rd. 34945 • Leasing Services professor and interim associate dean for ARCADIA CLEWISTON • Agricultural sales and purchases 863-993-9661 863-228-7126 Extension; both with the University of 6057 Nichols St. 34266 2550 Witt Rd.. 33440 Florida-IFAS. 10 CITRUS INDUSTRY • May 2012