| Popis: |
Two rates of crop residues (-CR and +CR) and six rates of P fertilizer (0, 100, 200, 300, 400, and 500 kg superphosphate fed-1) were applied each growing season (corn and clover) in a long-term field experiment started at the Experimental Farm of Assiut University, Assiut, Egypt. The crop residues were added to the soil at the end of each growth season at rates of 5.45 and 3.71-ton fed-1 corn stover and Egyptian clover straw. The response of soil P fractions to both CR and superphosphatr fertilizer was small at the very beginning of the experiment, however it became more pronounced starting from the year 1999 up to the year 2003. Resin-P accounted for 0.81- 0.93%; and 1.30- 1.52% of total P in soil after 5 and 16 years of continuous application of CR and SP, respectively.At zero level of SP and no addition of CR the resin-P fraction was markedly decreased from 6.5 to 5.65. The size of inorganic (Pi) and organic (Po) phosphorus forms of the NaHCO3 fractions were increased with the continuation of applying the P fertilization management for 16 years and ranged from 2.73 to 3.13 and from 1.84 to 2.19 for Pi and Po forms, with and without crop residues, respectively. Five years after continuous application of SP and without application of CR, the relative concentrations of NaOH-Pi and NaOH-Po increased from 3.69 to 4.44% and from 1.60 to 2.00% of total P, respectively. Also, increasing the application level of SP from zero (P0) to 500 kg SP fed-1 (P5) each growth season for five years increased the HCl-P and residual-P fractions linearly and significantly. The path analysis showed that both resin-Pi and NaHCO3-Pi were mainly and directly affected by P fertilization. In soil that received P fertilization management with CR, resin-P was largely controlled by added-Pi, NaHCO3-Pi and HCl-P fractions. |
