Boron is mobile in the soil and is subject to leaching, like nitrate and sulphate. Boron is not transferred easily from older to younger (upper) leaves, so younger leaves show symptoms first. Symptoms include rosetting, yellow top, poor flowering, death of the terminal buds and poor seed set. Boron is required for sugar transport cell division, pollen tube germination and amino acid production. Boron toxicity is indicated by yellowing, followed by browning of the leaf margins and tips with sharp boundaries between the yellow and/or brown and unaffected green area. Additions of high rates of B on soils where B is not required can result in toxicity and a reduction in yield. There is a narrow range between deficiency and toxicity; extreme care must be taken to apply B uniformly and to avoid overlap.