Humic Substances Provide No Consistent Agronomic Benefit in Representative Field Soils

Soil application of humic acid (HA), generally derived from leonardite shale, is a common practice in California vegetable production.  Five commercial HA formulations were evaluated for their effects on soil microbial activity, seedling emergence, crop productivity and nutrient uptake when applied to representative agricultural soils.  Two soils differing in organic matter content (8 and 25 g kg^-1) were wetted to field capacity moisture content with solutions of water, N and P fertilizer, HA, or fertilizer + HA, and incubated aerobically at 25 ^oC.  In the lower organic matter soil a synergistic effect of fertilizer and HA was observed after 7 d of incubation on both microbial respiration and the amount of phospholipid fatty acids detected; these stimulatory effects were not observed in the higher organic matter soil.  In a greenhouse pot study the effects of HA on seedling emergence, dry mass accumulation and P uptake of romaine lettuce (Lactuca sativae L.) were evaluated in four soils of low P availability; HA was applied to the soil at a rate simulating a field application of 2.2 kg ha^-1 a.i.  HA had no significant effect on emergence rate or percentage, or P uptake, in any soil; plant dry mass was increased with HA in one soil.  Field trials were conducted in 2008 and 2009 evaluating the effects of pre-transplant soil application of HA at 1.1 or 3.4 kg ha^-1 a.i. on growth, nutrient uptake and fruit yield of processing tomato (Lycopersicon esculentum Mill.).  In neither year was macro- or micronutrient uptake increased with HA.  Similarly, there was no significant HA effect on plant dry mass accumulation or fruit yield.