In 1840, Justus von Liebig observed and reported that the rate of growth of a plant, the size to which it grows, and its overall health is dependent on the availability of the scarcest of its essential nutrients. Scientists now know it is also true for humans and animals. This observation is known as Liebig’s “Law of the Minimum”. Liebig’s law is often depicted by a barrel with staves of various lengths representing nutrient deficiency in the plant. Hence, you can fill the barrel with water (representing plant growth, health and yield) to the lowest stave in the barrel. Supplemental addition of non-deficient nutrients will not improve plant performance and can possibly be detrimental to tree health. Thus, the Minimum Relative Yield is limited to the lowest stave in the barrel.
But, we need to realize that is not all Liebig said. When we fertilize to address our limiting nutrient, calcium in the graphic above, the next lowest stave which is zinc (Zn) becomes the limiting nutrient. Therefore, we must determine what our limiting nutrients are, apply them in a balanced nutrition program and “fill the barrel to the top”. Also, we must consider how we should adjust our balanced nutritional program to make sure we have adequate nutrients to address all plant processes with HLB infected trees.
During the past five years, we have produced the Ready, Set, Grow Citrus Newsletter addressing nutrient deficiencies that we have observed in many citrus groves we have visited. We started with nitrogen, potassium and calcium fertility based on our observations. Why? Because the three most important nutrients for tree growth, fruit production and disease mitigation via the citrus tree’s “immune system” in descending order are N>K>Ca. By doing this, we were trying to increase Liebig’s “Minimum Relative Yield” (graph above) of fruit production and tree health for the Florida citrus grower based on substantiated scientific studies during this critical time of HLB infection. But we have also seen multiple groves that are deficient in magnesium, boron, zinc and manganese. We have also seen healthy trees with a good fruit-set, but small fruit indicating inadequate potassium application. These deficiencies must also be addressed to fill the barrel.
We are very pleased that Florida citrus growers have concluded that proper plant nutrition is paramount and the key to tree health and productivity. A healthy person or plant receiving proper nutritional management has the ability to fight disease through their immune system. Occasionally, antibiotics are needed when the disease is severe. However, if a person or plant, does not receive proper nutritional management and becomes very weak, then even antibiotics cannot save them. Tree or human health, when resisting a pathogen attack, must be sustained with a strong nutritional program. Then, additional antibiotic application may be advantageous. But, please do not reduce balanced citrus nutrition that leads to weakened tress, to provide funds for antibiotic application.
So now, we must drill down further with a plant nutritional program unknown to Liebig. It is a balanced nutrition program based on the 4 R’s: Right source, Right rate, Right timing and Right placement that maximizes plant response without creating extra tree stress in HLB infected trees. Let’s look at the nutrients we have identified as limiting (N, K, Ca, Mg, Zn, Mn, B).
Citrus is a C 3 crop (mechanism for carbon dioxide uptake) like most vegetable crops. The biochemistry within these crops make them susceptible to ammonium toxicity. Hence, like vegetable crops, nitrate nitrogen is the preferred source for citrus trees. This important information was published by UF scientists in 1999. Further published work sponsored by Yara supports this finding. We can see the influence of using the right source (YaraLiva Calcium Nitrate) in comparison to ammonium nitrate in the yield graph below:
Using the right source, calcium nitrate, increased fruit yield at all nitrogen rates of application when compared to ammonium nitrate. So knowledge of the right source is extremely important to address the needs of citrus and reduce ammonium stress that limits growth and production. With calcium nitrate, a linear increase in yield was observed as the N rate increased compared to the curvilinear response to AN declining yield as the N rate increased. Thus, from the last 3 years of our Brazilian study, we see that 142 lbs. N/a (160 kg. N/ha) applied to mature trees from calcium nitrate produced higher yield than any rate of ammonium nitrate on healthy trees. During the study period, trees were not infected with HLB. Using only nitrate nitrogen from calcium nitrate as their N source for application, many Florida growers with 100% HLB infection have found their fruit production has increased to pre-HLB levels.
Plants require potassium (K) in similar quantities to N. In the plant, K activates enzymes, is involved in protein synthesis, photosynthesis, water regulation, stomatal movement, and phloem transport. Potassium plays a major role in the plant protection response to pathogen attack. Finally, pound solid production is dependent on adequate K. In general, when crops are grown with insufficient K they produce less than optimum yields, and they do not use water or N efficiently. Therefore, we must apply sufficient K to maximize our fruit size and pound solid production. It is essential that we use the right source. HLB infected trees have a weak rooting system and we should not create additional stress. Potassium chloride has the highest salt index of all potassium sources and could negatively influence citrus roots. Potassium sulfate/KMag should be very beneficial K sources and both supply sulfur which is the sixth most important nutrient for citrus nutrition.
Calcium (Ca) is one of the most important nutrients in a balanced nutritional program for HLB-infected citrus. Cell division or mitosis will not occur without it. Plant cell walls and plasma membranes are weak and limited without Ca resulting in easier access for pathogens and less efficient water and nutrient uptake. Root hair, which are the primary nutrient uptake vessels for plants, require Ca to grow. Below, you will see that Ca is the main driver for root hair development. Also, when water soluble Ca is removed from growing trees, shoots, leaves and more importantly roots suffer. Healthy roots must be continually grown to replace ineffective HLB infected roots and Ca is a key nutrient for this to occur.
But most important of all, Ca is the messenger for initiation of HLB defense by the plant protection mechanism along with K. Therefore, we must apply adequate soluble Ca and K to obtain the immune effect. Interestingly, essentially all HLB-infected trees are Ca deficient.
Optimal photosynthesis is required for healthy trees and fruit production. But before trees can have photosynthesis, they need chlorophyll (light interceptor and energy fixation). The magnesium (Mg) ion is incorporated into the very center of the chlorophyll molecule and trees will not have photosynthesis without it. Plants obtain their carbon from CO2. But C 3 plants need the RuBP carboxylase enzyme to obtain CO2 and the enzyme will not function unless activated by Mg. Magnesium nutrition is also required for ATP, the energy compound for plants. Photosynthesis, CO2 and biochemical energy require Mg. When Mg is deficient, plant growth and fruit production is greatly decreased. Magnesium is considered the fourth most important nutrient in citrus production.
Micronutrients play a vital role in citrus nutrition. Extremely small quantities of boron (B) are necessary for cell division, hormone synthesis, protein synthesis, flowering and fruit set and very importantly, balance of starch and sugars. Boron and Ca work together in many of these functions. Zinc (Zn) is needed for protein and hormone synthesis, chloroplast formation, cell elongation, and enzyme activation. Manganese (Mn) is required as a coenzyme for oxidation/reduction reactions, N and CO2 utilization and assimilation, and chlorophyll synthesis. We can see the importance of micronutrients listed above and are not surprised at the plant response growers are currently obtaining based on our micronutrient deficiency observations in many citrus groves we have visited.
But all the above information means nothing unless we can provide a balanced plant nutritional program to address the deficiencies that exist in Florida citrus using the 4 R concept.
A blend with a composition of 9-1-14 11Ca 7.7S 2Mg 0.025%B 0.04%Mn 0.08%Zn. This blend can be made by combining 1161 lbs. of YaraLiva Tropicote, 44 lbs. of 0-46-0 (triple superphosphate) , 400 lbs. of SOP, 370 lbs. of K-Mag, and one gallon of Procote BMZ applied to all blend components for even distribution of all micronutrients. Growers using this blend have obtained excellent results. We are just now introducing our ground applied Procote micronutrients for optimal distribution and tree utilization. Use the 9-1-14 for all applications listed below.
For mature producing trees, apply 140 to 200 lbs. N/a based on tree age and fruit production potential.
During Mid-September/October apply 30% of total N for nutrient storage in trees prior to Spring Flush; during late January/early February apply 20% of total N for tree growth/health after winter; during mid-March/early April apply 30% of total N for fruit set and tree nutrition; during late May/early June apply 20% of total N for tree health/fruit development prior to the rainy season.
Surface apply 9-1-14 to soil area around the tree that contains citrus roots.
Contact us if you would like more information about citrus crop nutrition.
Director, Agronomic Services - firstname.lastname@example.org