Boron can be toxic and can give you as many problem as if it was deficient. Toxic levels of boron are associated with high-boron waters. Check your boron level in your area before applying any. Deficiencies are generally related to high rainfall areas as well as areas irrigated for a considerable time with low boron surface waters in light sandy soils.
Plants deficient in boron show a wide variety of symptoms, depending upon the species and plant age, but the earliest symptom is failure of root tips to elongate normally, accompanied by inhibited synthesis of DNA and RNA. Cell division in the shoot apex is also inhibited, as well as in young leaves. Boron plays an undetermined but essential role in elongation of pollen tubes.
Symptoms of boron deficiency in plants include: death of terminal growth, causing lateral buds to develop and producing a "witches-broom" effect. Thickened, curled, wilted and chlorotic leaves. Soft or necrotic spots in fruit or tubers. Reduced flowering or improper pollination.
There have been no acute visible symptoms in animals demonstrated by boron.