Abstract: Effect of applied and residual P from two sources,di calcium phosphate (DCP) and single super phosphate (SSP), were evaluated by growing wheat, brassica and berseem crops in a pot experiment. The criteria used were plant yield and P uptake. Both of the P sources for grain and straw yield of brassica, residual as well as applied P did not improve the yield. However, grain yield of wheat, fodder yield of berseem and total P uptake by these crops were found to improve significantly due to residual and applied P as DCP compared to SSP. A heavy P application from DCP showed positive residual effect and resulted in increased yield and P uptake by wheat and berseem. These results, therefore, strongly advocate the suitability of DCP as a P fertilizer source.
Introduction
In intensive agriculture, crops are grown one after the other and fertilizers are applied at higher doses to maximize crop production. Repeated application of P fertilizer may result in “P built up” due to increase in phosphorus concentration in the soil solution, particularly when the amount of P added to soil as fertilizer exceeds removal by the crop (Tisdale et al., 1985; Memon, 1996). Positive effects of residual phosphorus on succeeding crops have been reported by (Hussain et al., 1992; Khan and Makhdum, 1990; Saeed et al., 1992). In a pot study two phosphatic fertilizers, DCP and SSP were repeatedly applied at 0, 22, 44, 88 and 176 mg kg–1 to three successive crops grown to various periods of time (Alam et al., 2001; Latif et al., 1998). Analyses of soil showed increase in available P content after each crop harvest, depending on source and rate of P applied. However, the cumulative mean available P, after three crop harvests, were found to be higher in DCP. Repeated applications of P upto 44 mg kg–1 rate resulted in equivalent available P (about 20 mg kg–1) from both the sources, but higher application rates showed 2-3 times more P in DCP as compare to SSP supplied treatments. It was postulated that heavy P application as DCP may maintain relatively more available P in soil for a longer period of time and thus may have a greater residual effect than SSP. Fresh additions of P fertilizer from either source may also result in variable response by crops depending on their P requirements. By this experiment effect of applied and residual P from two sources, DCP and SSP, on yield and P uptake by three crop species, brassica, wheat and berseem was assessed.
Materials and Methods
To determine the effect of applied and residual P on crop growth, the treatments having 22 and 88mg P kg–1 from previous applications (Alam et al., 2001) were given 44 mg P kg–1 from the DCP and SSP, while the other treatments were kept as residual. Ten seeds of brassica (CV. Inmol), wheat (CV. Punjab-96) and berseem (CV. Local) were sown in each pot and after germination thinned to maintain 5 plants per pot. A basal dose of 25 mg N kg–1 as urea solution was applied to brassica after 4 weeks growth while 100 mg N kg–1 was applied to wheat, 3 weeks after sowing and again 100 mg N kg–1 was added after 8 weeks. Brassica and wheat were grown upto maturity for grain and straw yield (growth period 129 and 154 days respectively) while 4 cuttings of berseem were obtained for the estimation of fodder yield (growth period 147 days). Plant samples were dried in oven at 70°C for 3 days to record dry weight. The samples for wheat and berseem were then ground to fine powder in a wiley mill and 1g portions were digested in triacid mixture and analyzed for P concentration using Barton's reagent (Jackson, 1962). Data was analyzed statistically using MSTAT software. Duncan's multiple range test was used to compare the means.
Results and Discussion
Brassica: The effect of residual and applied P on straw and grain yield of brassica were not significantly different from control (Table 1). The two P sources, DCP and SSP were also not found to differ for the production of dry matter yield. Thus the native soil P was fairly adequate for growth and yield of brassica, indicating its low P requirement. The results agree with earlier report by Alam et al. (2001).
Wheat: Residual as well as applied P increased (p<0.05) wheat straw and grain yield over control (Table 2). Application of 44 mg P kg–1 to residual P from a lower P rate (T2) produced grain and straw yield equivalent to residual P from higher P rates (T3 and T5). Application of the same 44 mg P kg–1 at a relatively higher residual P (T4) had beneficial effect for both straw and grain yield compared to control but resulted in similar yield as obtained by higher residual P (T5). Thus heavy P application had more residual effect and persisted for a longer period of time than a normal or low P application rate.
Grain yield due to applied and residual P from DCP was higher (p<0.05) than that from SSP, the difference in grain yield was only significant at the highest residual P rate (T5), otherwise, the behavior of the two sources were almost similar for both straw and grain yield at each P rate. Similar results were earlier reported for applied DCP and SSP on wheat growth (Latif et al., 1998).
Application of 44 mg P kg–1 to lower (T2) or higher (T4) residual P treatments increased (p<0.05) P concentration over control in grain while the increase in straw was significant only for the higher rate (T4). The residual P from lower (T3) and higher (T5) rates also had a positive effect on P concentration in grain and resulted in significantly (p<0.05) improved P concentration where the source was DCP. Total P uptake increased over control due to applied and residual P.
| Table 1: | Effect of applied and resiudal P on grain and straw yield of brassica |
| * Additional 44mgPkg–1 was applied as DCP or SSP to respective pots, Source rate and SxR interaction were all nonsignificant | |
| Table 2: | Effect of applied and residual P on grain and straw yield of wheat |
| Source x rate interaction was not significant for grain or straw yield, *R-residual; A-Applied P @ 44 mg kg–1. | |
| Table 3: | Effect of applied and residual P on P concentration in grain and straw and total P uptake by wheat |
| Sources x rate interaction for P uptake was highly significant (p<0.01) as determined by DMR test. * R-residual, A-applied P @ 44 mg kg–1 | |
For P uptake DCP appeared significantly better than SSP. Source x rate interaction effect for total P uptake was also highly significant (p<0.01). Thus application of 44 mg P kg–1 as DCP at a lower (T2) or higher (T4) residual P resulted in equivalent or higher P concentration as well as P uptake compared to applied and residual higher P rate from SSP. This may be attributed to increased availability of P in soil from DCP compared to SSP at equivalent rates of applied and residual P. Akhtar and Alam (2001), reported increased availability of P in DCP compared to SSP from a clay loam soil incubated for 120 days (Table 3).
Berseem: The effect of applied and residual P on fresh and dry fodder yield of berseem is given in Table 4 and Fig. 1. Applied P increased fresh and dry fodder yield over control. Application of 44 mg P kg–1 to lower (T2) or higher (T4) residual P produced dry fodder yield equivalent to dry fodder yield from relatively high residual P (T5). Residual P from a lower rate (T3), however, could not increase the dry fodder yield over control. The DMY obtained from applied and residual P from DCP was significantly higher than that from SSP and this was particularly true for treatments T4 and T5 where the DMY was almost 2 times higher. In addition to increase in DMY, applied and residual P increased (p<0.05) P concentration in plants. Residual P had more pronounced effect compared to applied P (Table 5) and in this case again DCP proved better than SSP.
| Table 4: | Effect of applied and residual P on dry fodder yield and total P uptake by berseem |
| *R-residual, A-applied P @ 44 mg kg–1; source x rate interaction for DMY and P uptake was significantly different at P<0.01 as determined by DMR test respectively. | |
| Table 5: | Effect of applied and residual P on P concentration (mg kg–1) in berseem |
| * R-residual, A-applied P at 44 mg kg–1 | |
| Fig. 1: | Effect of applied and residual P on fresh fodder yield of berseem |
The concentration of P decreased in plants with subsequent cuttings, that may be attributed to increased dry matter yield, because P uptake increased at each cuttings and total accumulative P uptake also increased over control. Application of 44 mg P kg–1 to residual P (T2 and T4) resulted in increased P uptake and was equivalent to P uptake of the next higher residual P (T3 and T5), respectively. Thus heavy P application showed a positive residual effect and resulted in increased P uptake and the dry matter yield of berseem.
Source x rate interaction was also significant for both DMY and P uptake. The dry matter yield and P uptake by berseem was significantly higher (p<0.05) in treatments receiving P as DCP compared to SSP. It may be observed that application of 44 mg P kg–1 as DCP on low residual P (T2) produced DMY and P uptake equivalent to that recorded from applied and residual higher P from SSP. This indicates that where DCP was applied, the P content in soil was maintained consistently available for plant uptake, as compared to SSP, where it was largely fixed in the soil and was not available for crop.
Brassica, a low P responsive crop did not respond to higher residual P from previous application nor to the fresh P addition from the two P sources. However, wheat, a moderately responsive crop, showed significantly higher response of grain yield from residual DCP at the highest rate only while berseem, a high responsive crop to P, produced 2 times higher dry matter yield and P uptake in both applied and residual higher P compared to SSP. Thus we may conclude that DCP proved to compete favourably with SSP as a P fertilizer source. The increased availability of P in soil from DCP compared to SSP at equivalent rates of applied and residual P could make it a favorable choice for long term benefits.