Humic acid applications to lettuce do not improve yield but do improve phosphorus availability

The effects of humic acid (HA) and phosphorus (P) applications on plant growth and nutrient content of lettuce, together with available soil phosphorus remaining after harvest were examined. The amounts of phosphorus used were 0, 120, and 240 kg ha^?1 and those of humic acid were 0, 100, 200, and 300 kg ha^?1. Phosphorus increased the nitrogen content of lettuce significantly (P<0.01) while the application of humic acid did not have significant effect. Humic acid, phosphorus, and HA×P interaction increased the nitrogen content of lettuce significantly (P<0.05). All effects were not significant for plant K, Ca, Mg, Fe, Cu, and Mn contents. Application of phosphorus decreased Zn content significantly (P<0.05). Increased amount of phosphorus caused significant increases in residual soil phosphorus (P<0.01). It was concluded that the application of 120 kg ha^?1 of phosphorus together with 300 kg ha^?1 of humic acid was convenient for the head weight of lettuce.