Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Plants, Soils, and Climate

Department name when degree awarded

Plants, Soils, and Biometeorology

Committee Chair(s)

Bruce G. Bugbee


Bruce G. Bugbee


Paul Grossl


Mike Amacher


Humic substances are reported to improve plant growth and nutrient uptake, with iron the most studied nutrient. The most common forms of iron in soils are iron oxides, which are stable under aerobic conditions and unavailable for plant uptake. Iron deficient plants become chlorotic, which reduces growth and yield. To determine if humic substances can reduce iron chlorosis, five commercially available organic acids were tested on maize grown in sand columns at high pH. The dry granular humic acid from Aldrich Chemical Company applied at 84.4 g/liter of sand by volume (5% by mass) and 1 g/liter added with irrigation water, significantly reduced iron chlorosis (p < 0.0001). It also increased fresh mass by 39% and improved root growth. The other products, applied at 50 µl/liter did not significantly affect chlorosis or plant growth.

A second objective was to determine if humic substances improve plant growth and yield. The effects of 11 commercially available dry granular products on tomato growth were studied in soil columns in a greenhouse. A product from Horizon Ag Products (Modesto, CA), DGX FeZnMn Blend at a rate of 44.8 kg/ha (40 lbs/acre), significantly improved root growth, but not shoot growth or yield.

In another study, 10 commercially available liquid products from Horizon Ag Products were tested. Treatments were applied at 50 ml/liter mixed with irrigation water. Treatment BA 6.6% increased fruit number, fruit dry mass and plant dry mass. Treatments QH6.6% and Charger increased plant dry mass. Treatment QH6.6%, Hydra-Hume6% and F-6000 Increased fruit dry mass. However, this was an extremely high application rate. When the study was done at an economical rate of 4.6 µl/liter, there were no significant t beneficial effects. Most application rates reported in the literature are considerably higher than economic rates applied in the field. The results of these studies indicate some effects on plant nutrition and growth when applied at high rates but limited effects when applied at low (economic) rates.