| Humic acid possesses many chemical properties beneficial to soil conditions and plant growth. Its association with soil should not be viewed only from the standpoint as a supplier of plant nutrients, but also as a protective or neutralizing agent against phytotoxic conditions in soils: Humic acid is thought to be capable of reducing toxic levels of Al and Mn in the soil solution by chelation and complex formation, and effects a controlled release of these ions to plants when they are needed. To test these hypotheses above, the present greenhouse experiments were conducted in these replications using sand and Bladen soil. The humic acid was obtained from soils by extraction and purification.; The Bladen soil was selected for its native Al stress. Corn (Zea mays L.) and soybean (Glycine max L.) were grown in the sands for 32 days receiving 0, 100 and 350 ppm humic acid and a modified Hoagland-Arnon nutrient solution. The Al and Mn stresses were created by adding 0, 20 and 50 ppm Al, or 5, 25 and 75 ppm Mn per pot, respectively. The Bladen soil received no additional Al or Mn was added, but a decreasing Al-stress was induced by changing the soil pH through the addition of 0, 0.8, 1.5 and 2.3 g Ca(OH)(,2), respectively. The plants received 0, 150 and 475 ppm humic acid. The experimental design was completely randomized.; The results from plants grown in the sand medium indicated that humic acid and Bladen soil was capable in offsetting phototoxic levels of Al or Mn. Grown in the absence of humic acid in sand, the plants showed severe toxicity symptoms at 20 and 50 ppm Al, but in the presence of humic acid, the plants were green and healthy. Dry weight content of plants decreased with increased Al stress at 0 ppm HA. In the presence of humic acid, the plants exhibited dry weight contents that were considerably higher than plants grown at 0 ppm HA. Plant tissue Al concentration, though drastically reduced, in plants grown in the presence of humic acid, remained at about 100 ppm for treatments with 50 ppm Al in the growth medium. No ill-effects were observed on these plants, however. The interaction of humic acid with Al stress brought changes in the other element composition of the plants. Calcium and P concentration and uptake decreased with increasing Al-stress at 0 ppm HA. K concentration in plant tissue increased, but uptake was decreased as a result of Al stress in the absence of humic acids. The addition of humic acid to the growth medium had effected in general a higher Ca and P concentration in plant tissue, the uptake of Ca, P and K was increased when humic acid was present in the growth medium. |
