Purified humic acid (PHA) was studied for its effects on cucumber growth and nitrogen metabolism under different nitrogen levels. The researchers found that PHA helped cucumber seedlings grow better under low nitrogen stress, but not as much under high nitrogen stress. PHA increased dry matter accumulation and nitrate absorption in seedlings under low nitrogen stress. It also decreased ammonium content in roots and leaves under both low and high nitrogen stress. Enzyme activities related to nitrogen metabolism were improved by PHA, leading to higher amino acid and soluble protein content in cucumber seedlings. Overall, PHA alleviated the negative impact of nitrogen stress on cucumber growth.