Boron in vegetables: Not too little, not too much
Boron is important in protein synthesis; seed and cell wall formation; pollen grain germination and pollen tube growth and, therefore, fruit set and yield; regulation of carbohydrate metabolism; and sugar translocation. Boron is quite soluble and readily leached in some soil types, especially sandy soils typically used for vegetable production.
Vegetables vary considerably in their B requirements with those requiring high levels having quite dramatic deficiency symptoms. The growing point in deficient plants will break down, resulting in a stunted or rosette appearance; leaves may have a yellowish or reddish cast and members of the cabbage family will often have a hollow stem.
Boron requirements range from 0.5 to three pounds of available B per acre with two pounds per acre a good average. Since B is readily leached from sandy soils and it is not included in a standard soil test, growers with sandy sites should consider adding a B evaluation to their soil test. Boron should not be applied without a test since over-application can lead to toxicity, and there is not much difference between levels that help and levels that hurt. Boron applications should be made to bring mature leaf tissue to between 20 and 100 ppm. Tissue levels over 200 ppm are excessive and can be toxic.
Boron deficiencies can be corrected with foliar or ground applications. By the time foliar symptoms are observed and the cause determined, significant economic damage may have already occurred. So, the small expense of a B test on questionable sites prior to planting is well worth the cost. Once an application level has been established, future tests may be unnecessary. Growers can yearly apply the needed amount as a pre-plant broadcast application. Such small amounts of B are generally needed that granular B products should be well blended with a pre-plant broadcast of N–P–K.