ONION RESPONSE TO AMMONIATED PEAT AND AMMONIUM-SULFATE IN RELATION TO AMMONIUM TOXICITY

The objective of this study was to evaluate the effects of intermediate to high rates of granulated mineral or organo-mineral fertilizers on onion N uptake, nutrient balance, root growth and water influx. Ammonium sources were ammoniated peat (AP), ammonium sulfate (AS) and peat treated with ammonium sulfate (PAS). Onion was grown for 84 d in four soils at five N levels under growth-chamber conditions. Root length, N uptake, and root growth rate were influenced significantly by soil type, N source and N rate and were highest with 133 and 266 mg N kg(-1) on loamy soils. On a sandy soil, maximum N uptake was obtained with 133 mg N kg(-1). Water influx was influenced significantly by soil type and N application rate and reached its maximum value with 133 mg N kg(-1). AP produced the greatest N uptake, root length and root growth rate. Except in the sandy soil, the highest water uptake was also obtained with AP. Compositional nutrient diagnosis (CND) indicated that plants fertilized with AP achieved a more adequate nutrient balance (or the lowest d value) over a larger range of N rates (133-266 mg N kg(-1)) than AS and PAS. AP, thus, attenuated ammonium toxicity to the crop more than AS and PAS. At AP rates exceeding 266 mg N kg(-1), CND indicated a nutrient imbalance involving P or K deficiency.