Boron is an essential nutrient required by all plants in varying degrees. It is only needed in small minute amounts, it is a micro-nutrient. Micro only refers to the quantity needed, so far as the overall health and productivity of the plant it is vital.
Boron effects fruit set and ultimate yields of fruits and vegetables. It influences how they process carbohydrates and distribute the natural sugars and pigments that control quality, taste, and color. A boron deficiency leaves plants stunted and deformed with misshapen buds and flowers. New foliage is frequently curled and thicker than normal, in some plants there is an out of the ordinary yellow-orange tinge. Misshapen flowers hamper pollination and fruit set. when fruit does set it is usually very low quality. In addition, roots fail to develop to their potential which inhibits the uptake of other nutrients. Severely deficient plants fail to produce any flowers, seeds, or fruit at all.
I like a shot of whiskey from time to time, if I drank it in excess I'd probably get alcohol poisoning. Plants like a shot of boron from time to time adding too much can create a toxic soil and kill off your plants, just as too much alcohol creates a toxic personality and kills off brain cells.
Some vegetables need more boron than others to be productive, so far as myself just a shot of whiskey once or twice a week, or at least a glass of wine will suffice. For the plants however adding boron demands meticulous care. The useful range where boron can help or hinder plant growth is very narrow. Too little boron causes poor vegetable production, but just a tad too much and you've created a toxic soil.
Adding Boron to Soil can be done by a number of methods. Whichever method you choose however, be sure to test the soil first - as I stated earlier too much boron can do more harm than good. Soil tests that are professionally done take time and or money which some gardeners are unwilling to expend.
The amount of boron needed to correct a normal soil deficiency is between 1/2 to 1 ounce per 1,000 square feet. Most gardens are much smaller than that so cut down the dosage exponentially. If you did not have a soil test done only add the barest minimum and water generously to allow it seep into the root zone.
The simplest method of applying boron is via a foliar spray. 1/2 tsp. of boric acid per gallon of water is sufficient. Don't add borax at same time as lime or even within the same month as one another.
Household products such as Borax can be used. It can be mixed in at tilling time. It is best IMO when diluted in water as it ensures a more equal distribution.
Brassicas such as broccoli and cauliflower, brussels sprouts, cabbage, and assorted greens are heavy boron users and will benefit from a light annual foliar spray. Turnips and beets also use more boron than most other crops.
Fruits and nut trees will suffer premature dieback of branches, fruit drop, poor fruit quality, oddly shaped and at times cracked fruits. Chlorosis sometimes also occurs. This includes all fruits which distribute larger amounts of sugars, apples and stone fruits, grapes and berries as well as citrus. - Moderate amounts is all that is needed.
Legumes, peas, beans need very little and rarely suffer from boron deficiency. Solanaceous crops tomatoes, peppers, eggplant, okra need only moderate amounts and rarely suffer from a deficiency, I didn't say never - I said rarely.