Comparing the phosphorus use efficiency of pre-treated (organically) rock phosphate with soluble P fertilizers in maize under calcareous soils

Experimental site

A field experiment was conducted to study the effect of different P sources (RP, ARP, SSP and DAP) alone and pretreated with organic amendments (HA, Enriched compost and compost) along with control on yield and P uptake of maize at agronomic research farm (34.02, 71.48), The University of Agriculture, Peshawar. The soil of the experimental site was alkaline (pH = 7.4), calcareous (16.9%), non-saline (EC = 0.17 d Sm⁻¹) in nature and silt loam in texture. It was low in organic matter (0.76%) and AB-DTPA extractable P (3.33 mg kg⁻¹).

Preparation of P fertilizers

Experimental description

The experiment was laid out in randomized complete block design (RCBD) with factorial arrangement. Sixteen (16) treatments along with control were replicated three times. Azam variety of maize was sown at the rate of 30 kg ha⁻¹ on 15^th June, 2019 in 3 × 5 m³ plots and then plants were thinned after 3 weeks of emergence. All treatments were treated with basal dose of 150 kg N ha⁻¹ as a urea inclusive of N obtained from organic amendments. Hoeing and weeding were done manually. However, weedicides were also sprayed for controlling weeds. Standard agronomic practices were applied to all experimental units uniformly throughout the whole growing season.

Agronomic attributes

Soil and plant analysis

Surface soil samples (0–20 cm) were collected at three stages (before sowing, at boot stage and postharvest stage) for evaluating the treatments role in P status of the soil. Soil organic matter was determined by the procedure according to Nelson & Sommers (1983). Extractable P in soil was quantified on spectrophotometer according to the procedure of Soltanpour & Schwab (1977). The pH and EC of the soil was determined in 1:5 soil water suspensions by the methods of MeClean (1982), and Richards (1954), respectively. Plant P content was determined by using the protocol described by Jones, Wolf & Mills (1991) While, P use efficiency was determined by the method described by Johnston & Syers (2009) via the following formula (Eq. (4)):

$$\mathrm{P}\mathrm{u}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{k}\mathrm{g}\mathrm{k}{\mathrm{g}}^{-1}\right)={\displaystyle \frac{\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\left(\mathrm{m}\mathrm{g}\mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}{\mathrm{t}}^{-1}\right)}{\mathrm{P}\mathrm{a}\mathrm{v}\mathrm{a}\mathrm{i}\mathrm{l}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\left(\mathrm{m}\mathrm{g}\mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}{\mathrm{t}}^{-1}\right)}}$$

Statistical analysis

Statistical analysis of data was performed through analysis of variance (ANOVA) technique as suggested by Steel & Torrie (1981) according to factorial randomized complete block design (RCBD). The means were compared with least significant difference (LSD) at p < 0.05 via computer software Statistix 8.1.

Biological yield (kg ha⁻¹)

Application of different phosphate fertilizers significantly increased biological yield of maize as shown in Table 1. Maximum biological yield of 9,150 kg ha⁻¹ was produced by DAP which was statistically different from the rest of inorganic fertilizers. The SSP and ARP produced statistically similar biological yield of 8,703 kg ha⁻¹ and 8,620 kg ha⁻¹ respectively. Among the chemical fertilizers RP produced minimum biological yield but it was higher than control plot (6,099 kg ha⁻¹).

When averaged across amendments, the maximum biological yield of 9,442 kg ha⁻¹ was observed for simple compost treated plots which slightly declined with P enriched compost. It should be noted that for P sources with compost the amount of P applied was 10 t ha⁻¹ whereas, in case of enriched compost its amount was 2 t ha⁻¹, Thus, it is certified that the bulk of compost could be reduced by five times through its enrichment without any remarkable reduction in plant growth and yield. HA combined with chemicals fertilizers improved biological yield but their effectiveness was not enough like compost and enriched compost.

In case of integrated use of inorganic and organic fertilizers the remarkable biological yield of 10,047 kg ha⁻¹ was obtained at DAP + compost treated plots which was significantly higher than the rest of treatment combination. The SSP + compost and ARP + compost receiving plots have also produced the biological yield of 9,606 and 9,316 kg ha⁻¹ respectively (Fig. 1). The RP applied alone was not effective but its effectiveness was pronounced when applied in combination with compost or P enriched composts.

Grain yield (kg ha⁻¹)

Grain yield of maize significantly increased with both sole application of chemical fertilizers and their combination with organic amendments (Table 2). The plots treated with sole DAP produced grain yield of 3,256 kg ha⁻¹ that was higher than control (1,839 kg ha⁻¹) but produced superior grain yield of 3,877 kg ha⁻¹ when DAP was applied with compost. This treatment (DAP + compost) showed the highest grain yield among all the applied treatments indicating that the growth and yield of the present crops is associated with solubility of P from the applied fertilizers. The ARP and SSP produced statistically similar grains yield of 2,986 kg ha⁻¹ and 2,935 kg ha⁻¹ respectively, those were 62.36% higher than control. The grain yield further increased when ARP and SSP were applied with different types of compost. The sole application of SSP increased grain yield by 40% but rose up to 60% when applied in combination with compost (Fig. 2). Similarly ARP applied alone showed 30% increase in grain yield over control but it was further enhanced when applied with compost suggesting that local acidification is as effective as SSP fertilizer.

Harvesting index (%)

Analysis of variance regarding harvest index (HI) showed significant differences for phosphorus sources applied alone or in combination with organic amendments (Table 3). When data were averaged across inorganic p sources, maximum HI (35.4%) was noted for DAP followed by ARP (34.9%) and SSP (33.4%) but minimum was calculated for RP applied alone (32.6%) which was better than control (30.2%). Similarly, data revealed that in case of organic P amendments higher HI (35.5%) was observed for compost followed by enriched compost (34.8%). Plots treated with HA gave a HI of 33.1% which was comparatively lower than compost and enriched compost.

Cost benefit ratio

The benefit cost ratio (CBR) analysis of maize crop and fertilizers are presented in (Table 4). Maximum benefit of Rs. 98,762 and 98,762 ha⁻¹ were obtained from DAP + enriched compost and DAP + compost, respectively. However, their cost benefit ratio were too low (3.1 and 5) compared to ARP enriched compost having net return of 95,540 and CBR of 10.3. The sole RP though had a higher BCR (46.4) but its net return was as low as Rs. 74,637 ha⁻¹. Any treatment combination who’s CBR is greater than 3.0 has considered economical but its benefit should be also kept in mind. The two fertilizers having the same benefit net return then the fertilizer with high CBR would be selected. Similarly, the difference in net return between DAP and ARP enriched compost was too low that’s why, ARP enriched compost is recommended under existing soil and climatic conditions.

Soil organic matter (%)

Data regarding soil organic matter (SOM) as affected by sole application of chemical fertilizers or in combination with different amendments are shown in Table 5. Higher SOM was observed at DAP (0.94%) followed by ARP (0.83%) and SSP (0.82%) while minimum was observed in control plot (0.43%). When data were averaged across the P amendments, significantly higher SOM was observed in plots treated with simple compost. The SOM observed for enriched compost was 0.93% while for HA treated plots was 0.66%. The ARP + compost produced at par SOM with other conventional fertilizers.

Soil phosphorus concentration at boot stage and after harvest (mg kg⁻¹)

The ABDTPA extractable phosphorus in soil at boot stage and after harvesting the crop as influenced by the application of organic and inorganic fertilizers are shown in Tables 6 and 7, respectively. There were no significant differences among the sources for soil phosphorus at boot stage but they were higher than control. When the data were averaged across P amendment maximum soil P concentration (6.28 mg kg⁻¹) at boot stage was observed for plot receiving compost followed by P enriched composts (6.22 mg kg⁻¹). These treatments were statistically different from HA treated plot having P contents of 5.37 mg kg⁻¹.

Post-harvest soil AB-DTPA extractable P was significantly altered by applied treatments. The soil P significantly increased with the sole application of chemical fertilizers or applied with different organic amendments. When data were averaged across fertilizer types, maximum P (5.29 mg kg⁻¹) was noted for DAP followed by ARP (5.04 mg kg⁻¹). Minimum P concentration (2.17 mg kg⁻¹) was observed for control. Similarly, highest P contents of 5.59 mg kg⁻¹ was observed with the addition of compost followed by different P enriched composts (5.57 mg kg⁻¹). Addition of HA did not improve soil P compared to composts. Interactive effect demonstrated that soil P was highly improved (6.02 mg kg⁻¹) with DAP + compost followed by DAP enriched compost. As DAP is considered highly soluble then other P sources (mineral) hence, it added more P to soil with the combination of compost.

Phosphorus concentrations in grains and stalk (%)

Data regarding phosphorus concentration in grains and stalk as affected by organic and inorganic P fertilizers are presented in Tables 8 and 9 correspondingly. Chemical fertilizers didn’t show any significant effect for grain P concentration in maize. However, when data were averaged across different P amendments significant differences were observed. Higher P content (0.20%) was observed for the compost treated plots which was statistically higher than that observed for P enriched composts (0.18%).

Significant differences were observed among the P sources for stalk P content in maize. Averaging the data across fertilizers, maximum P concentration of 0.26% was observed at DAP followed by SSP (0.24%) which were statistically different from treatments fertilized with ARP (0.21%) and RP (0.18%), however, they were higher than control plots (0.13%). When data were averaged across P amendments, compost treated maize plants absorbed optimum P (0.27%) than other P amendments and it was statistically higher than the rest of treatment combination. In Enriched compost fertilized plots, plants absorbed slightly higher P (0.23%) than sole chemical fertilizers.

Phosphorus use efficiency (%)

Phosphorus use efficiency (PUE) was significantly affected by various P sources either applied alone or in combination with different amendments (Table 10). Maximum PUE was observed for DAP (36.17 kg kg⁻¹) followed by SSP (33.7 kg kg⁻¹) and ARP (32.02 kg kg⁻¹) while the minimum PUE (23.12 kg kg⁻¹) was observed for control. When data were averaged across organic source, maximum PUE was observed for sample compost (37.29 kg kg⁻¹) which was statistically different from all other amendments. Although P enriched compost had closely similar PUE value 34.95 kg kg⁻¹ suggesting that chemical fertilizers enriched with compost has responsive effect because they may release much phosphorus to the soil which is the key factor in organic agriculture for improving yield and yield related components. The HA treated plots resulted a PUE of 30.66 kg kg⁻¹ indicating that it had also additional beneficial P value when applied with chemical fertilizers, it may be meaningful in releasing more P from in-organic sources.