Ammonia/Ammonium Toxicity Root Symptoms Induced by Inorganic and Organic Fertilizers and Placement

Ammoniacal fertilizers can cause seedling damage. The present aims were to characterize spatial and temporal, root morphological NH3/NH4+ toxicity symptoms, assess the extent of the toxicity zone, and relate species-specific responses to their root architecture. Wheat (Triticum aestivum L.) and canola (Brassica napus L.) were exposed to seed and deep placed urea. Faba (Vicia faba L) seedlings were grown above organic amendments. Time-sequential images of canola root apex and root hair die-back, discoloration, and accelerated lateral rooting were captured with soil-buried, high resolution digital scanners. Seed-placed urea stunted wheat shoot and root radicles, while slow-release urea reduced these symptoms. Primary axes of all three species were damaged by encountering the deep fertilizer zones. The multiple seminal axes and lateral root growth away from the fertilizer allowed greater wheat seedling survival, while toxicity-damage to a single tap root of the germinating canola and faba oft en resulted in seedling mortality. Urea and chicken manure developed expanded NH3/NH4+ toxicity zones 1.5 to 5 cm, eliciting similar toxicity symptoms initiated at the root apex. Within 3 d aft er planting, canola tap root elongation stopped, followed by progressive basal directed necrosis and shrinking of the root axis, root hairs. These characteristic symptoms may be used for future toxicity diagnostics of soil-grown plants. Elevated pH in the soil zone above the chicken manure suggested NH3 gas transported through soil pores followed by H+ consumption and elevated NH4 +. Ammonia gas toxicity and species-specific root system architecture should be considered in N placement and source selection.