Source: Fluid Fertilizer Foundation
Like zinc, discussed in an earlier issue, boron is one of seven plant food elements required in micro amounts but producing a macro impact on plant development. Seven decades have passed since scientists first demonstrated that boron is essential for plant growth. Although many functions of boron in plant growth are not fully understood, a great deal is known. Some of the functions in plant nutrition this unique micronutrient is involved in are:
- cell wall structure
- sugar transport
- cell division
- plant hormone regulation
- flowering and fruiting.
Crops need boron throughout their life cycles. Soils, for one reason or another, may not be able to supply boron in quantities to meet those needs. Boron availability is dependent on several factors and conditions existing in the soil-plant system, and is strongly influenced by rainfall — or lack of it — during the growing season. Some of the factors that influence boron availability are:
Soil organic matter. This is the primary source of "reserve" boron. Organic matter complexes with boron to remove it from the soil solution when levels are high after fertilization. It then resupplies the soil solution to maintain adequate levels when boron is removed by crops or leaching. Soils with low organic matter will usually need more frequent boron fertilization at lower amounts per acre.
Soil texture. Sandy soils that are well drained are most likely to be boron deficient because of leaching. If subsoils are fine-textured, less frequent additions of boron fertilization may be needed. Total boron is usually highest in clay soils with high organic matter. However, plant-available boron may be quite low because of the strength by which boron is held on clay surfaces.
Cultivation. Boron is made more uniformly available to plant roots when mixed throughout the upper soil profile by plowing. Plowing also speeds up the rate of organic matter breakdown, releasing boron into the soil. As crop production systems shift to reduced tillage and no-till management, organic matter will accumulate on and near the soil surface. As this happens, boron availability will become more dependent on surface moisture and rainfall patterns. Fertilizer management will become more critical.
Drought. During periods of drought, topsoil dries out. Crops are unable to feed in the uppermost part of the soil, thus are subject to temporary boron deficiency. Since boron moves by mass flow of soil water, dry weather limits availability by restricting flow of water.
Microbial activity. Microorganisms break down soil organic matter, which allows the release of boron from organic complexes. Microbial activity is lowest under drought conditions, or in cold, wet soils. It is highest when soils are moist and warm. Where microbial activity is highest, boron release is highest.
Soil pH and liming. Boron availabili-ty decreases with increasing pH. A drop in plant uptake is often dramatic at soil pH levels above the 6.3 to 6.5 range. On the other hand, crops such as alfalfa, which have a high boron demand, also require a soil pH above 6.5 for optimum growth. Liming acid soils sometimes induces a temporary boron deficiency.
Soil Fertility. Availability and use of soil boron depend on fertility levels. Balance among the various soil nutrients, as well as actual boron level, influence its plant-use efficiency. There are particularly strong boron interrelationships with nitrogen, potassium, and calcium. For example, work by Woodruff in South Carolina on corn has shown that boron fertilization prevented yield reduction where high K fertilization was used.
Yield determines need
Crop production will fall short of genetic potential when any of the essential elements is in short supply. This is as true for boron as it is an element like nitrogen. The difference is a shortfall of nitrogen is often more visible.
Total requirement by plants for a specific nutrient can be highly variable. Reasons are sometimes climate-driven, but more often they are management related. Total boron need can be traced to differences among crops and production practices such as yield goal or variety selection needed for improving productivity and profitability. For example, total boron removed is six times higher for alfalfa yielding 10.8 tons/A than that yielding 6 tons/A. By comparison, boron content of high-yielding corn is about the same as that for alfalfa yielding 6 tons/A. The need for boron fertilization is increasing because of higher crop yields and reduced levels of organic matter (the primary soil source), and because of years of intensive boron removal by crops.