Fertigation for citrus trees By Mongi Zekri, Brian Boman and Tom Obreza icroirrigation is an important with overhead sprinkler irrigation. component of citrus produc- Research has also shown the impor- Mtion systems in Florida. For tant advantage of microsprinklers for citrus trees, microirrigation (Figure 1) freeze protection of citrus. is more desirable than other irrigation Microirrigation combined with methods for several reasons: water properly managed, water savings with fertigation (Figure 2) — applying conservation, fertilizer manage- microirrigation systems can amount to small amounts of soluble fertilizer ment efficiency and freeze protec- as much as 80 percent compared with through irrigation systems directly tion. Research has shown that when subirrigation and 50 percent compared to the root zone — provides precise Figure 1. Microsprinkler irrigation of citrus trees. Figure 2. Fertigation system for citrus trees. Naturally Chelated ORDER Foliar Blends ONLINE Clear natural chelates for higher analysis, better results and affordable prices. www.moreoranges.com (866) 375-2487 14 CITRUS INDUSTRY • March 2014 Figure 3. Fertigation system including backflow prevention devices. timing and application of water and ces are in place and working properly. solution should be mixed with irriga- fertilizer nutrients in citrus production. The time required for water to tion water in a jar (at the same dilu- Fertilizer can be prescription-applied travel from the injection point to the tion rate that is used in the irrigation in small doses and at particular times farthest emitter is generally 20 to 30 system) to determine if any precipitate when those nutrients are needed. This minutes for most microirrigation sys- or milkiness occurs within one to two capability helps growers increase tems. Therefore, a minimum injection hours. If cloudiness does occur, there fertilizer efficiency, reduce nutrient time of 30 to 45 minutes is recom- is a chance that injection of the chemi- leaching by excess rainfall or over- mended. This time should be sufficient cal will cause line or emitter plugging. irrigation, and should result in reduced to achieve uniform distribution of When urea, ammonium nitrate, cal- fertilizer rates for citrus production. nutrients throughout the irrigation cium nitrate and potassium nitrate are The two most common nutrients distribution system. After fertigation, dissolved, heat is absorbed from the applied to citrus through fertigation are continue to run water for at least 30 water and a very cold solution results. nitrogen and potassium. minutes to completely flush the fertil- Consequently, it may not be possible Florida state law requires that back- izer from irrigation system lines and to dissolve as much fertilizer as needed flow prevention equipment (Figure 3) emitters to minimize clogging poten- to achieve the desired concentration. be installed and maintained on irriga- tial. Keep in mind that excessive flush- It is often necessary to let the mixture tion systems that have fertilizer injec- ing time beyond 30 minutes can leach stand for several hours and warm to tion capability. The function of the plant nutrients below the root zone. a temperature that will allow all the backflow prevention device is to pre- mixture to dissolve. vent contamination of ground or sur- FERTILIZER SOLUBILITY Nitrogen. Urea, ammonium nitrate, face water by the applied chemicals. Before injecting fertilizer solutions, calcium nitrate, potassium nitrate, Therefore, before injecting fertilizer a “jar test” should be conducted to ammonium sulfate and ammonium into any irrigation system, make sure determine clogging potential of the thiosulfate are very soluble in water. all required backflow prevention devi- solution. A sample of the fertilizer Phosphorus. Most dry phosphorus fertilizers (including ammonium phos- phate and superphosphates) cannot be injected into irrigation water because they have low solubility. Monoammo- nium phosphate (MAP), diammonium phosphate (DAP), monobasic potas- sium phosphate, phosphoric acid, urea phosphate, liquid ammonium polyphosphate and long chain linear polyphosphates are water soluble. However, they can still have precipita- tion problems when injected into water with high calcium concentration. Phosphoric acid is sometimes injected into microirrigation systems. It not only provides phosphorus, but also lowers the pH of the water, and this can prevent the precipitation 16 CITRUS INDUSTRY • March 2014 problems previously mentioned. This tion products. When urea-ammonium 0-26) (NH4)2S2O3 is used as both a practice will be effective as long as nitrate solutions are combined with fertilizer and as an acidulating agent. the pH of the fertilizer-irrigation water calcium nitrate, a thick, milky-white When applied to the soil, Thiobacillus mixture remains low. As the pH rises insoluble precipitate forms, presenting bacteria oxidize the free sulfur to due to dilution, phosphates precipitate. a potential plugging problem. sulfuric acid. The acid then dissolves One approach that is sometimes suc- Calcium Nitrate (15.5-0-0-19 Ca) lime in the soil and forms gypsum. cessful is to supplement the phos- 5Ca(NO3)2-NH4NO3-10H2O: This fer- The gypsum helps to maintain a good, phoric acid injections with sulfuric tilizer is high in nitrate-nitrogen (14.5 well-granulated, aerated and porous or urea sulfuric acid to assure that the percent) with 1 percent ammonium- soil structure. Ammonium thiosulfate irrigation water pH will remain low nitrogen, and it supplies calcium. The is ideal for treatment of calcareous (between a pH of 4 and 5). Phosphoric product can be combined with ammo- (high lime) soils. It is compatible with acid should only be injected when the nium nitrate, magnesium nitrate, potas- neutral or alkaline phosphate liquid combined Ca and Mg concentration sium nitrate and muriate of potash fertilizers and nitrogen fertilizers. of the water is below 50 ppm and the (KCl). It should not be combined with Ammonium thiosulfate can be applied bicarbonate level is less than 150 ppm. any products containing phosphates, in liquid mixes or by itself. Ammo- Potassium. Most potassium fertil- sulfates or thiosulfates. nium thiosulfate should not be mixed izers are water soluble, and injection of Ammonium Thiosulfate (12-0- with acidic compounds because it will K through microirrigation systems has been very successful. The problem most often associated with potassium injec- tion is solid precipitants that form in the mixing tank when potassium is mixed with other fertilizers. The potassium sources most often used in microirriga- tion systems are potassium chloride (KCl) and potassium nitrate (KNO3). Potassium phosphates should not be injected into microirrigation systems. Calcium. Fertilizers containing calcium should be flushed from all tanks, pumps, filters and tubing prior to injecting any phosphorus, urea-ammo- nium nitrate or urea sulfuric fertilizer. Calcium should not be injected with any sulfate form of fertilizer because it combines to create insoluble calcium sulfate, or gypsum. Micronutrients. Several metal micronutrient forms are relatively insoluble and therefore are not used in fertigation. These include the carbon- ate, oxide or hydroxide forms of zinc, manganese, copper and iron. The sulfate form of copper, manganese and zinc is the most common and usually the least expensive source of micronu- trients. These metal sulfates are water soluble, but through fertigation they are not very successful in alleviating a micronutrient deficiency. SOME COMMON FERTIGATION MATERIALS Ammonium Nitrate Solution (20-0-0). NH4NO3-H2O is ammonium nitrate fertilizer dissolved in water with a density of 10.5 pounds per gal- lon. It is the most widely used nitrogen source for Florida citrus. Urea-ammonium Nitrate Solution (32-0-0). (NH2)22CO-NH4NO3: Urea- ammonium nitrate solution is manufac- tured by combining urea (46 percent N) and ammonium nitrate (33 percent N) on an equal nitrogen content basis. The combination of urea and ammonium nitrate contains the highest concentra- tion of nitrogen of all the nitrogen solu- CITRUS INDUSTRY • March 2014 17 decompose into elemental sulfur and tion. This product can be blended with CRYSTALLIZATION ammonium sulfate at pH less than 6. other fertilizers, but KTS blends should Solution fertilizer salt-out, crystalli- Application to neutral and acidic soils not be acidified below pH 6.0. The zation, or precipitation in storage tanks (without free lime) may result in a pro- proper mixing sequence for KTS is first can be a problem during the winter. As nounced drop in soil pH over several water, then pesticide (if any), and then a rule of thumb, the more complex the weeks. The extent of the pH drop in KTS and/or other fertilizer. Potassium formulation, the greater is the tendency these types of soils depends upon the thiosulfate provides not only potas- for salt-out. The most important factor total amount of this fertilizer applied, sium, but the thiosulfate is oxidized by affecting salt-out temperature of a the cation exchange capacity of the Thiobacillus bacteria to produce sul- fertilizer solution is its concentration soil and the buffering capacity of the furic acid. This acid reacts with calcium of N and K. The higher the analysis soil. (Most Florida citrus soils are very carbonate in the soil, which releases of a solution, the higher is the crystal- weakly buffered.) additional calcium for the plant. Thus, lization temperature. For example, a Phosphoric Acid (0-54-0) H3PO4 potassium