Boron (B) is a required micronutrient for plant health and production. It is essential for cell wall formation and stability, maintenance of structural and functional integrity of biological membranes, movement of sugar or energy into growing parts of plants, and pollination and seed set. Adequate B is also required for effective nitrogen fixation and nodulation in legume crops. Boron deficiency symptoms are exhibited as stunted growing points, shortened thickened stems, discolored leaves (yellow to purple depending on the species), and death with severe deficiency. These symptoms can be confused with viral disease, frost, or hormone damage. Tissue testing can be used to determine the B status of a plant when diagnosing poor growth. In many species, including corn, soybean, wheat, and alfalfa, B is not remobilized to move from old growth to new growth, and deficiency symptoms will be observed in new growth first. Soil pH and texture impact the availability of B in the soil and need to be considered when thinking about the 4Rs (right source, right rate, right time, and right place) of B fertilizer application.
Source and Placement
Boron is mobile in the soil, and deficiency of B is a major concern in sandy soils. Matching the solubility and form of B fertilizer to ensure sufficiency to the plant throughout the growing season can be a challenge. The particle size and density of borate sources often do not match that of other macronutrient fertilizers they could be mixed with, resulting in a potential for uneven applications. The low rates of B applied to meet crop needs combined with the potential for uneven distribution from blending with macronutrient products can result in areas of the soil around the plant that might be deficient in one spot and toxic in another.
Recent research evaluated B incorporation with other fertilizer products, like muriate of potash (MOP), during manufacturing (da Silva et al., 2018). When MOP with B incorporated through compaction with borax (sodium tetraborate) was applied to a pot study, distribution of B in the soil was equal across the pot and maintained an adequate concentration over the nine weeks of the study (da Silva et al., 2018). However, when MOP granules were applied in a bulk mix with B granules, the soil where the B granule landed had toxic B concentrations over all nine weeks of the study. Also during this time, the B released from these granules did not disperse across the pot, leaving the rest of the soil in the deficient range (da Silva et al., 2018). Considering the source and mixing of B when making an application is critical to supplying B evenly across the field.
Applying B as a liquid to the leaves of a plant or as a soil nutrient solution is also an option for meeting B needs of the crop. A challenge of foliar B fertilization is the lack of movement of B to new growth in many species; in these species, applied B remains in the sprayed tissues and is not mobilized towards new growth. While B is generally not able to move from old growth to new growth in most crops, it has been demonstrated in some cotton cultivars and coffee plants to move as B is needed for new growth (Bogiani et al., 2014; Leite et al., 2007). Results in cotton found that when a labeled 10B isotope nutrient solution was applied to the leaves of the plant, the uptake rate of B was the same as when it was in the nutrient solution provided to the roots (Bogiani et al., 2014). In cotton, between 3 and 48 hours after application of the 10B nutrient solution, 10B concentration in all plant parts had increased (Bogiani et al., 2014), indicating movement from the third leaf of cotton where it was applied to the lower leaves, roots, and stem of the plants. Understanding how and if B is mobile in the crop where an in-season B application is being considered is critical to having an effective impact of the application on crop production.
Rate and Placement
The range of B deficiency to toxicity in the soil is much narrower than those established for macronutrients like P or K. In general, soil B concentrations are considered deficient under 0.5 ppm, adequate 0.5 to 5.0 ppm, and toxic above 5.0 ppm (da Silva et al., 2018). Response to B fertilizer applications, even when soil tests indicate a need for B fertilization, can be variable. Cotton grown in greenhouse experiments responded with increased dry matter yield, taller plants, and more reproductive structures per plant as concentration of B in the nutrient solution increased (Bogiani et al., 2014). However, when alfalfa grown in low-B soils in Montana was fertilized with liquid B fertilizer, there was only a yield response in one cutting out of eight measured over two years and two sites, and quality of the alfalfa hay was not impacted by the applications (Sapkota et al., 2018). Toxicity can happen at low concentrations of B, coffee plants grown in a nutrient solution displayed impacts of B toxicity at a concentration of 0.50 ppm, with death at of the plants at 2.0 ppm B in the solution (Vasco et al., 2018). It is important to understand how B is taken up by the applied crop and when or what is the best type of sample (tissue or soil) to take to determine rate of application.
Time
Cotton, soybeans, and alfalfa take up B throughout the growing season. Soybean B uptake increased at stage V4 and peaked at R3, but the plants did continue to take up B from the soil through R7 (Gaspar et al., 2018). At R5.5, 35% of the B in the soybeans was in the leaf tissue (Gaspar et al., 2018). When soybeans reached R5, 65.5% of the B needed for seed development and fill had been taken up by the plant. As seed development began, researchers measured that 21.5% of the B that had accumulated in the leaves prior to R5 was remobilized to the seed (Gaspar et al., 2018). Cotton grown in a greenhouse experiment with nutrient solution accumulated B in the lower leaves of the plants through the first nine weeks of growth rather than in the stem, upper leaves, or roots of the plants (Bogiani et al., 2014). When the nutrient solution was changed to one without B, cotton remobilized B to the new growth (Bogiani et al., 2014). However, in both cases, the amount that can be remobilized may not be enough to avoid deficiency in the plant or impact yield. There are multiple challenges of B application timing that are related to the source and ability to mix B fertilizers with other macronutrient sources, the low application rates needed to balance adequate supply with toxicity, and the potential of B in some crops to be remobilized to new growth.
Conclusions
Boron is a unique micronutrient that is needed for the growth, reproductive development, and yield of crops. The need and function of B in the plant is well understood. As with all nutrient application, it is critical to consider how and when B is taken up by the plants, if the crop will remobilize any of the B in old plant parts to new plant parts, and how to apply B uniformly to the soil.
Sally Flis, Ph.D. and CCA, Director of Agronomy, The Fertilizer Institute, Washington, DC