Nitrogen Availability From Liquid Organic Fertilizers

Supplying adequate N for maximum crop productivity is often problematic in organic vegetable production.  A number of liquid organic fertilizers are available for in-season use, but information on their N mineralization characteristics is limited.  N availability of 5 commercial liquid organic fertilizers was quantified in an aerobic incubation experiment and a greenhouse bioassay.  The fertilizers, containing either animal byproducts (fishery wastes and seabird guano) or plant extracts, ranged in N concentration from 26-60 g kg^-1.  Soils from two organically-managed fields were brought to field capacity moisture content, fertilized at a rate of 100 mg kg^-1 N, and incubated at 15 and 25 C.  Fertilizer N availability was estimated as the increase in soil mineral N concentration of fertilized soil compared to unfertilized soil.  Across soils and temperatures fertilizer N availability was quite rapid, with an average of > 70% of initial N content in mineral form after 1 week of incubation.  Such quick N availability was attributed to substantial fertilizer NH₄-N content (a mean of 38% of initial N across fertilizers) and rapid hydrolysis of simple N compounds.  Soil and incubation temperature had modest but statistically significant effects on fertilizer N availability.  High N availability was confirmed in a greenhouse experiment in which N uptake by fescue (Festuca arundinacea Schreb.) was evaluated.  The N content of fescue clippings from pots amended with the organic fertilizers was compared with that from unfertilized pots, and from pots receiving an equivalent N rate from ammonium sulfate.  Fescue N removal in 4 weeks of growth attributable to fertilization reached as high as 69% of the organic N applied, with the N uptake from all organic fertilizers at least equaling that from ammonium sulfate.