The results and discussion of the study focused on the total and available phosphorus content in the surface horizons of agricultural soils (Jakubus, 2015). We used the sequential extraction procedure described by Hedley et al. to fractionate inorganic phosphorus and determine its speciation (Saleem et al. 2017). We also investigated the influence of soil factors on phosphorus forms to understand the dynamics of P availability. The study showed that agricultural soils had varying levels of total and available phosphorus, indicating differences in soil fertility. These differences could be attributed to various soil factors, such as pH, organic matter content, and nutrient management practices.
Overall, the study provides insights into the dynamics of phosphorus availability in the soil and highlights the importance of considering soil factors in phosphorus fertilization practices to optimize nutrient availability and improve soil health.
The findings of this study suggest that proper management of phosphorus fertilizers, taking into account soil factors and nutrient management practices, can significantly improve soil health and ensure efficient phosphorus fertilization in grassland soils. By understanding phosphorus dynamics in the soil and optimizing the effectiveness of applied phosphorus fertilizers, agricultural practices in the Prayagraj district of Uttar Pradesh can be optimized to achieve sustainable agricultural development and enhance crop yield. By adopting integrated nutrient management approaches and preserving the natural soil microbiome, farmers in Prayagraj can promote sustainable agriculture while effectively managing phosphorus fertilizers.
3.1 Characterization of the physicochemical properties of the soil
The physicochemical properties of the soils collected from Inceptisols are presented in Table 1. The soil profiles contained a high clay content compared with the sand and silt contents; the mean sand content ranged from 71.81%, the silt content ranged from 11%, and the clay content ranged from 16.75% for all the samples. The sandy loam texture was used for all the soil profiles, and the clay and sand contents varied from the topsoil to the subsoil. The bulk density of the profile ranges from 1.00 to 1.25 Mg m3, the particle density varies from 2-2.8 Mg m3, the percent pore space varies from 10–17%, and the solid space ranges from 83–90%. The water-holding capacity of the soil profiles ranges from 47–71%, and the specific gravity varies from 11–28 gcm3. The soil samples were neutral in nature, with a medium exchangeable Ca content ranging from 15 to 40 Cmol/kg− 1 soil. The reaction was neutral to alkaline, as reflected by the pH values ranging from 6.8–7.9. With an average annual rainfall range of approximately 1042 mm in the Prayagraj district, all soluble salts may have leached out from the profile. This was reflected in the very low-salinity soils in this region. The EC values ranged from 0.12 to 0.69 dSm-1. These soils are non-saline in nature, which might be due to the availability of sufficient rainfall to leach out soluble salts from the root zone or down to the profile (Arbind et al.2022). The organic carbon content of the soil ranges from 0.15 to 1.59%, with a mean of 0.69%. The organic carbon content of the topsoil was slightly greater than that of the subsoil. The medium to high OC content in these soils can probably be explained by the fact that most inceptisols are rich in organic matter. The available nitrogen content in the soil ranged from 31 to 333 kg ha− 1 from the topsoil to the subsoil. The available phosphorus content in the soil was low and decreased significantly with depth, ranging from 11–21 to kg ha− 1. Available potassium varies from 51–112 kg ha− 1.
Table 1
Physiochemical properties of the soil samples.
| %Sand | %Silt | %Clay | BD | PD | PS | SS | WHC | SG | pH | EC | OC | Ca2+ | Mg2+ | N | P | K | Fe2O3 | Al2O3 | R2O3 |
| % | Mgcm− 3 | % | % | gcm− 3 | 1:2 | dSm− 1 | % | C mol/kg− 1 | kg ha− 1 | % |
Min | 64.10 | 4.00 | 12.28 | 1.00 | 2.00 | 10.00 | 83.00 | 47.00 | 11.00 | 6.80 | 0.12 | 0.15 | 15.00 | 4.20 | 31.00 | 11.00 | 51.00 | 0.08 | 1.00 | 1.11 |
Max | 80.16 | 17.85 | 26.23 | 1.25 | 2.80 | 17.00 | 90.00 | 71.00 | 28.00 | 7.90 | 0.69 | 1.59 | 40.00 | 51.40 | 333.00 | 21.00 | 112.00 | 0.13 | 2.40 | 2.48 |
Mean | 71.81 | 11.08 | 16.75 | 1.11 | 2.34 | 13.48 | 86.46 | 57.63 | 19.04 | 7.40 | 0.34 | 0.69 | 26.24 | 13.55 | 144.71 | 13.54 | 82.71 | 0.10 | 1.53 | 1.63 |
Std. error | 0.82 | 0.99 | 0.74 | 0.02 | 0.05 | 0.41 | 0.40 | 1.44 | 0.98 | 0.07 | 0.04 | 0.07 | 1.18 | 2.07 | 14.79 | 0.40 | 3.29 | 0.00 | 0.09 | 0.09 |
Stand. dev | 4.01 | 4.87 | 3.64 | 0.09 | 0.22 | 2.00 | 1.98 | 7.07 | 4.81 | 0.34 | 0.19 | 0.35 | 5.80 | 10.16 | 72.44 | 1.98 | 16.10 | 0.02 | 0.45 | 0.45 |
Variance | 16.08 | 23.68 | 13.27 | 0.01 | 0.05 | 3.99 | 3.91 | 49.98 | 23.17 | 0.12 | 0.04 | 0.12 | 33.67 | 103.17 | 5247.35 | 3.91 | 259.17 | 0.00 | 0.21 | 0.20 |
The abbreviations in the table are BD, bulk density; PD, particle density; PS, pore space; SS, solid space; WHC, water-holding capacity; SG, specific gravity; pH, hydrogen power; EC, electrical conductivity; OC, organic carbon; N, nitrogen; P, phosphorus; K, potassium; Fe2O3, iron oxide; Al2O3, aluminum oxide; and R2O3, total sesquioxide. |
3.2 Soil phosphorus fractions
The results for the soil phosphorus fractions are presented in Table 2, and the correlation coefficients among the P fractions and soil properties are presented in the figure.
Table 2
Distribution of phosphorus fractions in soils.
| S-P | Al-P | Fe-P | Occl-P | Red-P | Ca-P | Mineral P | Total-P | Organic-P |
Min | 0.86 | 0.86 | 4.82 | 1.26 | 1.12 | 10.65 | 20.25 | 52.15 | 7.52 |
Max | 7.06 | 2.24 | 8.24 | 5.70 | 2.81 | 35.09 | 54.50 | 97.19 | 53.25 |
% of total | 4.14 | 2.15 | 10.45 | 3.98 | 3.01 | 31.57 | 55.30 | - | 44.70 |
Mean | 2.53 | 1.36 | 6.54 | 2.56 | 1.87 | 19.73 | 34.58 | 62.82 | 28.24 |
Std. error | 0.42 | 0.09 | 0.22 | 0.23 | 0.10 | 1.12 | 1.61 | 1.83 | 1.90 |
Stand. dev | 2.08 | 0.44 | 1.09 | 1.13 | 0.47 | 5.48 | 7.90 | 8.99 | 9.30 |
Variance | 4.31 | 0.19 | 1.20 | 1.27 | 0.22 | 29.98 | 62.38 | 80.75 | 86.40 |
Table 3
Depthwise distribution of phosphorus fractions in soils.
| S- P | Al-P | Fe-P | Occl-P | Red-P | Ca-P | Mineral P | Total-P | Organic-P |
0–15 | 3.1 | 6.5 | 2.8 | 3.9 | 2.2 | 25.9 | 44.1 | 72.4 | 28.3 |
15–30 | 2.7 | 1.4 | 7.0 | 2.6 | 1.8 | 19.9 | 35.4 | 61.4 | 26.0 |
30–45 | 2.3 | 1.2 | 6.2 | 2.2 | 1.4 | 17.3 | 31.0 | 60.3 | 29.3 |
45–60 | 2.0 | 1.0 | 5.7 | 1.5 | 1.8 | 15.9 | 27.8 | 57.2 | 29.4 |
Min | 2.0 | 1.0 | 5.7 | 1.6 | 1.4 | 15.9 | 27.8 | 57.2 | 26.0 |
Max | 3.1 | 1.8 | 7.3 | 3.9 | 2.2 | 25.9 | 44.1 | 72.4 | 29.4 |
Mean | 2.5 | 1.4 | 6.5 | 2.6 | 1.8 | 19.7 | 34.6 | 62.8 | 28.2 |
Std. error | 0.2 | 0.2 | 0.4 | 0.5 | 0.2 | 2.2 | 3.5 | 3.3 | 0.8 |
Variance | 0.2 | 0.1 | 0.5 | 1.0 | 0.1 | 19.5 | 50.1 | 44.2 | 2.5 |
3.3 Saloid-P (S-P)
Saloid-P and S-P are influenced by various soil characteristics, such as the silt content, pore space, sand content, clay content, bulk density, solid space, water holding capacity, specific gravity, pH, exchangeable magnesium, and free iron oxide. Overall, saloid-P is a critical phosphorus fraction in the soil that is influenced by various soil characteristics and plays a significant role in nutrient availability and optimal plant growth. Saloid-P, or S-P, is a key phosphorus fraction in the soil that is influenced by various soil characteristics, such as the silt content and pore space. Overall, saloid-P is an important factor for nutrient availability and optimal plant growth. Saloid-P, also known as S-P, is a crucial fraction of P in the soil.
The soil P content ranged from 2.0 to 3.0 mg/L and was 4.13%. Sal-P was significantly positively correlated with the silt content (r = 0.61*) and pore space (r = 0.55*). It was significantly negatively correlated with sand (r = -0.41* ), clay (r = -0.32*), bulk density (r = -0.33* ), solid space (r = -0.53*), water holding capacity (r = -0.39* ), specific gravity (r = -0.41* ), and pH (r = -0.47* ) and could be carried out by the transformation of loosely bound surface-adsorbed P into a less soluble form of P(Sacheti and Saxena 1973). Similar results were observed for clayey soils. exchangeable magnesium (r = -0.37*) and free iron oxide (r = -0.528*). There was a significant positive correlation between Ca-P (r = 0.56*) and inorganic P (r = 0.60*) due to strong weathering and a negative correlation between Fe-P (r = -0.42*) and organic P (r = -0.63*). This difference might be due to the neutral pH. Saloid P is loosely or weakly correlated with Fe-P, Red-P and Organic P, demonstrating that a profound correlation with the distribution of Saloid-P was detected, similar to the findings of Vishwanath and Doddamani 1991.
To further understand the dynamics of phosphorus distribution in soil, it is important to consider the various phosphorus fractions and their relationships with soil characteristics. The negative correlation between Fe-P and organic P may be influenced by the neutral pH of the soil. In addition, the relationships between saloid-P and other phosphorus fractions, such as Fe-P and Red-P, indicate complex interactions and distributions of phosphorus in the soil. Interestingly, similar findings were reported by Vishwanath and Doddamani, highlighting the significance of saloid-P for soil fertility.
3.4 Aluminium-P (Al-P)
The Al-P content in these soil profiles ranged from 1.61 to 1.83 mg/L, and the mean value of Al-P was 1.35 mg/L. Al-P comprised 2.15% of the total P, and a very low content of total P in the soils is an indication of strong weathering and well-drained humid tropics (Sarkar et al.2014). Among the fractions, Al-P was significantly positively correlated with Occl-P (r = 0.43*), Ca-P (r = 0.60*), mineral or inorganic P (r = 0.67*), and total P (r = 0.47*) and negatively correlated with organic P (r = -0.11*), while Al-P was positively correlated with pore space (r = 0.33*), aluminium oxide (r = 0.89*), and total sesquioxide (r = 0.89*). It was significantly negatively correlated with the clay content (r = -0.16*), solid space (r = -0.30*), pH (r = -0.40*), and exchangeable Mg (r = -0.21*).
3.5 Iron-P (Fe-P)
The Fe-P content of the soil is one of the most important P fractions influencing its availability in the soil. among inorganic fractions. Fe-bound P was the second largest pool. This was expected because the soils were dominated by Fe sesquioxides (Arbind et al., 2020). The Fe-P in the soils ranged from 5.61 to 7.29 mg/L, with a mean value of 6.53 mg/L. Fe-P comprised 10.44% of the total P. A significant positive correlation was observed between Fe-P and the sand percentage (r = 0.64*), water-holding capacity (r = 0.86*), and free iron oxide (r = 0.56*). It was significantly negatively correlated with silt (r = -0.65*) and particle density (r = -0.42*), and Fe-P was positively correlated with Occl-P (r = 0.62*), Red-P (r = 0.64*), and total P (r = 0.54*); similar findings were found with Prahalad et al. 2014 and Organic P (r = 0.29*) due to mineralization and an increase in biological activity in soil. Similar findings were found by Sacheti and Saxsena (1973), Watham et al. (2018), Arbind et al. (2020) and Bhavsar et al. (2018).
3.6 Occluded-P (Occl-P)
The Occl-P in the soil P fractions ranged a medium level and it varies from 1.54 to 3.89 mg/L, with a mean value of 2.55 mg/Land it representing 3.98% of the Total-P. Significant positive correlations between Occl-P and sand (r = 0.35*), water holding capacity (r = 0.54*), available phosphorus (r = 0.76*), available potassium (r = 0.42*), free iron oxide (r = 0.49*), aluminum oxide (r = 0.51*), and Total Sesquioxide (r = 0.53*) were observed and were negatively correlated with silt (r = -0. 35*), Occl-P was positively correlated with total P (r = 0.80*) and inorganic P (r = 0.55*). Ca-P (r = 0.42*) and Organic P (r = 0.31*). Similar findings have been reported by Bhavsar et al. (2018) and Arbind et al. (2020).
3.7 Red-P
The soluble P reductant in the soil is adsorbed by the oxides or hydroxides of Fe and Al. The Red-P in the studied soils ranged from 1.6 to 2.1 mg/L, with a mean value of 1.86 mg/L occupying 3.0% of the total P. Red-P was significantly positively correlated with the water holding capacity (r = 0.56) and EC (r = 0.45). It was significantly negatively correlated with silt content (r = -0.21*), particle density (r = -0.33*), and pore space (r = -0.31*). Red-P was significantly positively correlated with inorganic P (r = 0.27*); similar findings were found by Bhavsar et al.2018, Prasad et al. 1986, and Arbind et al.2020. This fraction was the second least abundant fraction. A low Red-P value was observed in soils with a relatively high pH and sand content. This difference might be due to the increase in the iron- and aluminium-bound P contents and in the content of calcium-bound P, similar to the findings of (Viswanatha and Doddamani 1991, Sharma and Tripathi 1992 and Trivedi et al. 2010).
3.8 Calcium-P (Ca-P)
The amount of Ca-P in the soils ranged from 15 to 25 mg/L, with a mean value of 19.72, accounting for 31.57% of the total P and resulting in the highest fraction among all the fractions. This may be because of the average range of Ca2 + ions in the soil. Ca-P was significantly positively correlated with pore space (r = 0.39*), organic carbon (r = 0.55*), available nitrogen (r = 0.55*), available potassium (r = 0.35*), aluminum oxide (r = 0.60*), and total sesquioxide (r = 59*). Ca-P was significantly negatively correlated with clay (r = -0.32*), solid space (r = -0.403*), pH (r = -0.226*), EC (r = -0.232*), exchangeable magnesium (r = -0.388*), and free iron oxide (r = -0.22*); significantly positively correlated with mineral P (r = 0.96*); and negatively correlated with organic P (r = -0.53*), similar to the findings of Bhavsar et al.2018 and Arbind et al. (2020). Calcium-bound P is typically the dominant inorganic soil P fraction. The close association between Ca-P and inorganic P was due to the dominance of Ca-P in soils (Bhavsar et al.2018), while the pH varied from 7.2 to 7.4. Within this range, calcium-bound P is often the dominant inorganic soil P fraction, and the calcareous character of most of these soils may be attributed to the large proportion of calcium-bound P.
3.9 Inorganic-P
Inorganic P exists as an orthophosphoric acid salt. In general, inorganic P is the predominant form of soil P, constituting 20–80% of the total P in the surface layer (Tomar 2003). Inorganic P included all the fractions of P, that is, S-P, Al-P, Fe-P, Occl-P, Red-P, and Ca-P; the values ranged from 27 to 44 mg/L; and there was a significant decrease in the levels of P from the surface layer to the subsurface layers in all the profiles. Inorganic P was significantly positively correlated with EC (r = 0.491*), available nitrogen (r = 0.48*), available phosphorus (r = 0.33*), available potassium (r = 0.35*), aluminum oxide (r = 0.67*), and total sesquioxides (r = 0.66*). It was significantly negatively correlated with the clay content (r = -0.30*), solid space (r = -0.35*), and pH (r = -0.32*). Among the fractions, inorganic or mineral P was significantly and positively correlated with saloid P (r = 0.60*), Al-P (r = 0.67*), Occl-P (r = 0.55*), and Ca-P (r = 0.96*), as they are the major contributors to the inorganic form of P.
3.10 Organic-P
The total organic P concentrations ranged from 25 to 28 mg/L, and it has been shown that there was a significant decrease in and irregular arrangement of organic P in a few of the profiles. This arrangement, compared to that of inorganic P, is due to the availability of Fe and Al oxides, which are available in inorganic P (Arbind et al.2020). Organic P was significantly and positively correlated with clay content (r = 0.42*), bulk density (r = 0.52*), solid space (r = 0.41*), available phosphorus (r = 0.66*), and free iron oxide (r = 0.49*). It was significantly negatively correlated with silt (r = -0.45*) and pore space (r = -0.410*), showing that more soils containing more silt had less organic P; similar results were found by Arbind et al. (2020) and available nitrogen (r = -0.34*). Organic P was positively correlated with Occl-P (r = 0.31*) and Total-P (r = 0.62*) The association of organic-P and Total-P highly correlated was reported by (Dongle 1993). Organic P was negatively correlated with saloid P (r = -0.63*), Ca-P (r = -0.53*), and inorganic P (r = -0.46*).
3.11 Total-P
The average total P values ranged from 57 to 72 mg/L. A significant decrease in the P concentration from the surface layer to the subsurface layers was observed in all the profiles, possibly due to the continuous application of manures and P fertilizers to the top layers; similar results were found by Arbind et al.2020 and Dongale 1993. Total P was significantly positively correlated with bulk density (r = 0.47*), water-holding capacity (r = 0.57*), available phosphorus (r = 0.97*), available potassium (r = 0.39*), and free iron oxide (r = 0.38*). Aluminum oxide (r = 0.47*) and total oxides (r = 0.48*) were observed. It was significantly negatively correlated with silt (r = -0.29*) and exchangeable Mg (r = -0.23*). Total P was positively correlated with Al-P (r = 0.478*), Fe-P (r = 0.546*), Occl-P (r = 0.806*), Ca-P (r = 0.300*), and the mineral P (r = 0.399*).
According to the results for soils in Prayagraj, which are neutral to alkaline, all the soil samples were under permissible limits, with the nonsaline condition of electrical conductivity being suitable for crops. Of the soil samples, 58.3% had medium-high organic carbon contents due to low and high temperatures and less decomposition of organic matter in the soil; more than 50% of the samples had low and medium ranges of nitrogen and phosphorus and a high range of potassium; more than 52% of the samples had a high range of exchangeable calcium ions; and 70% of the soil samples had low levels of magnesium. The total sesquioxide hydroxides of iron and aluminium ranged from low to medium for free iron oxide, and the overall saloid-P values ranged from an average of 2.53 mg/L (4.13%), whereas the average Al-P of the samples ranged from 1.35 mg/L (2.15%), the lowest among all the fractions. The Fe-P values ranged from second highest at 6.53 mg/L (10.44%). The Occl-P values ranged from 2.55 mg/L (3.9%), which was within a medium range compared to all the other fractions. Red-P ranged from 1.86 mg/L (3%). The Ca-P values ranged from 19.72 mg/L (31.27%), with the highest values among all the fractions. The total P concentration ranged from 34.58 mg/L, while the inorganic P concentration ranged from 62.84 mg/L, while the total organic P concentration ranged from 28.24 mg/L (44.70%).