Sewage sludge has been used as a fertilizer, particularly in poor soils. Great amount of sewage sludge are produced in Cairo-Egypt. In greater Cairo alone about 150,000 ton/year of dry sludge is produced. Most of the domestic sludge are recycled in agriculture as a fertilizer. Sludge is applied to sandy soil in order to provide organic matter and nitrogen & phosphorus. 

The isotopic exchange kinetic technique was applied to quantify the availability of phosphorus from the following Egyptian sludges to sandy soil with low P content (9 ppm) 1) raw, domestic 2) anearobic digested, domestic 3) secondary treated, domestic + industrial. One half of the sludge received 6 KGy gamma radiation dose. The soil + sludge mixture was incubated for 7 weeks. Soil and sludge were sequentially extracted for P forms (rapidly available form, plant available form, P associated with oxide and organic form and carbonate precipitated form) and determined for isotopic exchange kinetic parameters (r1/R, n, Cp). The calculation of isotopically exchangeable phosphorus (E1min, E1min-3 month, E >3 month) was conducted.

The sludge total P content ranged from 6231 to 8330 mg Kg^-1 whereas the sandy soil contains as low as 9.3 mgKg^-1. The percentage of organic P ranged from 31.3 to 52.1 of the total P. Gamma radiation treatment resulted in a reduction in the organic P. The reduction reached 30% in the secondary treated domestic + industrial sludge, whereas it reached 8% in the raw domestic sludge. The sequential extraction of P showed that, in all kinds of tested sludges P content occur in the following order: oxide & organic form> carbonate precipitated form> plant available form> rapidly plant available form.

In general, the concentration of water extractable inorganic phosphate (Cp) of the three kinds of sludge increased as the incubation time increased. However, there was variation in the degree of the increase between the different sludges. Although Cp recorded the highest value for the control treatment (KH₂PO₄), a sudden reduction in its content was observed after one week of incubation, then the Cp values become almost constant throughout the rest of the incubation period. This phenomenon could be explained by immobilization of inorganic phosphate. The radioactivity remaining in solution (r1/R) after 1 minute of exchange showed an increase with the incubation time as the fixing capacity of the soil decreased. The E1min values were highest in digested sludge but lowest in raw sludge and intermediate in secondary treated sludge. The E1min-3month, give an indication of the amount of available P that is ready to be consumed by the root system through active uptake of annual crops. The E1min-3month value was highest in the secondary treated, domestic + industrial sludge (4186 mg P Kg^-1) but lowest in the raw domestic sludge (304 mg P Kg^-1).

There is an increase in P content of (soil + sludge mixture) during the 7 weeks of incubation, in the meantime there is a decrease in P content of (soil + KH₂PO₄ mixture) during the same period. Thus it is concluded that the addition of sludge to sandy soil increases the availability of P from sludges more than the mineral fertilizer applied in soluble form. Results of P sequential extraction (H₂O-extractable P) were correlated with P isotopically exchangeable within 1min. A highly significant linear relationship was observed (r² =0.94) between those two parameters. Thus, water extractable P could be used to evaluate the availability of P in most sludge.

Keywords: 33P, Cp, n, r1/R, E1min, E1min-3month, E>3 month, KH₂PO₄ , incubation time, gamma radiation, sandy soil