Methods and Rates of Application

The boron requirement of crops varies considerably, so recommendations must take these differences into account. Although plant species having high boron requirements are more likely to become boron deficient under boron-limiting conditions in the soil, their recommended boron rates may vary according to other conditions such as differences in root systems, effects of other soil parameters, and available soil calcium. Therefore, generalized boron recommendations must take all such factors into account.

Application of boron fertilizers at the recommended rate for a high-boron-requiring crop may provide excessive available boron for another crop. Tolerance to higher levels of available boron varies considerably, and species with high boron requirements do not necessarily have high tolerance and vice versa. For example, alfalfa and cabbage (Brassica oleracea var. capitata L.) have high boron requirements but are only semitolerant to high boron levels (249).

Recommended rates of boron application generally range from 0.25 to 3 kg ha^-1, depending on crop requirements and methods of application. Higher rates of boron generally are required for broadcast soil applications than for banded soil application or foliar sprays.

Rates are usually similar for all boron sources, except for higher rates with slowly soluble sources such as colemanite or fritted products. Recommended boron rates and methods of application for some commonly fertilized crops are summarized by Mortvedt and Woodruff (116).

A primary consideration for soil application of boron is the soil surface texture and depth. In coarse-textured soils, under high rainfall, boron may move rapidly downward and from the root zone (250). In a loamy sand with the argillic horizon more than 40 cm deep, boron side-dressed is more effective than broadcast applications for corn (251). Fine-textured soils have the capacity to restrict boron leaching from the upper layers. Tap-rooted crops such as soybeans, may absorb nutrients from deeper layers, especially in dry weather, and benefit from boron in subsurface layers.

The two chief methods of boron fertilization are by adding it directly to the soil or by foliar spraying. Generally, soil and foliar applications of B are effective for crops. Soil applications are generally used for applying boron to field crops, but foliar sprays are more common on perennial crops such as fruit trees. Foliar application rates are usually about 50% lower than soil application rates. However, Murphy and Lancaster (252) obtained maximum yields of cotton with either 0.5 kg B ha^-1 applied as a foliar spray (five times at 0.1 kg ha^-1 each) or with <0.3 kg B ha^-1 applied to the soil. For soybeans in a silt loam, foliar boron sprays were effective in increasing the number of pods per branch, but soil-applied boron had no effect (253).

Either broadcast or banded applications to soil are recommended, depending on the crop and soil conditions. Broadcast applications are used to establish and maintain alfalfa and other nonrow crops. Banded applications may result in greater efficiency of applied boron. Root growth may be depressed in soil near banded boron fertilizers. Mortvedt and Osborn (233) reported soluble boron concentrations as high as 75 mg kg^-1 in soil near banded NP fertilizers with 1% B as Na₂B₄O₇.5H₂O. Root growth of alfalfa and oats was depressed in soil containing soluble boron concentrations <10 mg kg^-1. Soluble boron concentrations in soil would be much lower if the same boron rate was broadcast rather than banded to soil.

Applications of boron to the soil alone or with mixed fertilizers are common, and most data reported on plant boron accumulation have been obtained with boron-containing fertilizers applied broadcast or in bands. In field studies on rutabaga, band applications of boron resulted in greater boron concentrations in leaf tissue than did broadcast applications (254). In fact, boron applications of 1.12 kg ha^-1 applied in bands resulted in greater boron concentrations in leaf tissue than did 2.24 kg ha^-1 applied broadcast. Studies on rutabagas (254) and on corn (108,255) indicated that band- or foliar-applied boron resulted in greater boron accumulation by plants than did boron applied broadcast. Greater boron accumulation when it is applied in bands is likely due to the fact that a large quantity of the available nutrient is concentrated in the immediate root zone. Thus, boron applied in bands would be concentrated over a small area and would be taken up by the plants rapidly.

Applications of nutrients by foliar spray are effective in areas of California and Arizona where soil applications of micronutrients are ineffective because elements such as zinc, manganese, and copper are fixed in forms that are not readily available to certain crops (256). Foliar applications, besides resulting in higher boron accumulation in plants, could be used to advantage if a farmer omitted the addition of boron in the NPK bulk fertilizer or if boron deficiency was suspected. Foliar spray applications in the early growth stages resulted in greater absorption of boron than did those applied at later stages of growth (254). Mortvedt (257) stated that early-morning applications of foliar-applied nutrients may result in increased absorption, as the relative humidity is high and the stomata are open. It should however be pointed out that more than 98% of the boron applied as a foliar spray on white clover (Trifolium repens L.) remained at the point of application, and less than 2% was useful to the growth of the plant (258). This small but efficient portion of boron was quite mobile and was distributed to the different parts of the plants and then transferred from the oldest parts to the newly formed leaves. In a study on barley, soil applications of boron produced higher boron concentrations in the boot-stage tissue and grain, than similar amounts of boron applied as foliar spray (259). This result indicates that boron uptake, at least by barley, is more efficient through soil–root systems than through the leaves.

For some elements such as molybdenum, which plants require in extremely small amounts, seed treatment with a preparation containing molybdenum will prevent a deficiency. However, because of the comparatively higher requirement of boron than molybdenum, and because of the toxic effect of boron on seeds or seedlings, seed treatment for boron fertilization has not received attention.