Phosphorus residues accumulate in the soil when phosphorus fertilizer application exceeds its utilization by crops. Continuous use of phosphatic fertilizers in excess of crop needs thus results in a gradual increase in available phosphorus status of the soil. Buildup of phosphorus depends upon many factors which include the rate and period of phosphorus application, the kind and number of crops grown, the nature of the soil, and soil management factors.
Changes in available phosphorus status (Olsen P) as a result of phos-horus fertilization and cropping were monitored in a long-term study under three annual cropping systems (Table 10.4). After seven rounds of phosphorus fertilization and cropping, available phosphorus content reached the levels of 13.8, 10.3, and 14.5 mg/kg under wheat-maize, wheat-fallow, and wheat-soybean systems, respectively. This corresponds to a three to four-fold increase over initial available hosphorus level (2.8 mg/kg) at the phosphorus application rate of 32.8 mg/kg (150 kg P20s/ha). At the moderate phosphorus application rate of 19.7 rug/kg (90 kg P20s/ha), which is the recommended dose for most crops in Pakistan, available phosphorus levels did not exceed 7 rug/kg. At this rate, despite some increase . available phosphorus, the soil remained deficient in phosphorus.
. Table 10.4 Available phosphorus (Olsen) content and phosphorus fertility buildup factor (PFBF) in relation to P fertilization and cropping for seven consecutive cropping seasons under three systems.
P rate/season” |
|
|
| Increase in |
|
(mglkg) (kg | Total Padded | Available P” available p •• | PFBF# | ||
P2Os/ha) | I | (mg/kg) | (mg/kg) | (mg/kg) | (mg/kg) |
/ | WHEAT-MAIZE SYSTEM |
|
| ||
6.6~. | 45.9 | 4.7 | 1.9 | 24.3 | |
13.1 ~ ) |
| 91.7 | 4.9 | 2.1 | 44.3 |
19.7 (90) |
| 137.6 | 5.6 | 2.8 | 49.3 |
26.2 (120) |
| 183.4 | 6.6 | 3.8 | 48.4 |
32.8 (150) |
| 229.3 | 13.8 | 11.0 | 20.9 |
|
| WHEAT-FALLOW SYSTEM |
|
| |
6.6 (30) |
| 26.2 | 5.0 | 2.2 | 11.8 |
13.1 (60) |
| 52.4 | 5.0 | 2.2 | 23.5 |
19.7 (90) |
| 78.6 | 6.9 | 4.1 | 19.1 |
26.2 (120) |
| 104.8′ | 7.3 | 4.5 | 23.5 |
32.8 (150) |
| 131.0 | 10.3 | 7.5 | 17.4 |
WHEAT-SOYBEAN SYSTEM
6.6 (30) 45.9 5.0 2.2 20.6
13.1 (60) 91.7 6.2 3.5 26.6
19.7 (90) 137.6 7.0 4.2 32.4
26.2 (120) 183.4 8.9 6.1 29.9
32.8 (150) 229.3 14.5 lL7 19.6
“Original soil P = 2.8 mglkg.
.- Increase in available (Olsen) P as a result of fertilization and cropping for seven seasons.
# PFBF = Total P added/increase in available P.
Source: Puno (1991)
It is of practical importance to know the rate of change in available phosphorus status in relation to the rate of phosphorus application under different soil and cropping systems. Tandon (1987) therefore defined the Phosphorus Fertility Buildup Factor (PFBF) as the units of fertilizer phosphorus required to raise the available soil phosphorus level by one unit after accounting for phosphorus uptake by the crops grown. The higher the PFBF, the more difficult, or slower it is to build up available phosphorus status in the soil. It usually takes about 2~0 units of fertilizer phosphorus to raise the Olsen phosphorus level by one unit, after accounting for phosphorus uptake under field conditions. The data obtained by Puno (1991) for a Sultanpur silt loam soil (Table 10.5) showed that PFBF values ranged from
Mean PFBF values were lower under the wheat-fallow system(19.1) than those for wheat-maize (37.4) and wheat-soybean (25.8) systems, indicating that phosphorus fertility could be built up more easily under the wheat-fallow system than under the wheat-maize system.
