Phosphorus (P) is considered one of the macronutrients that plays an important role in the growth and development of crop plants (Barman et al., 2023). The total content of this element in the soil surface is low and its average is 0.6% P. Phosphorous is available in two forms of organic and inorganic forms needed by crop plants (Griffith, 2011). Most soils all over the world have low available phosphorus due to the nature of their composition, despite the fact that the total amount of it is still high (Nussaume et al., 2011). In addition, most of the phosphorus is chelated by cations like calcium and magnesium (Von Vexhull and Mutert,1998), which makes it less available to plants (Raghothama, 1999). The behavior of adsorption, desorption and absorption for phosphorus is an important part of its migration and transformation in soils (Perazzolli et al., 2016). Large quantities of phosphate fertilizer added can stimulate a rapid reaction between phosphorus and iron, calcium and aluminum, leading to the formation of complexes and making the nutrients difficult to absorb by the plant root. In addition, many factors affect the amount of fixed phosphorus, including soil pH, humidity, clay minerals in soil and temperature (Toro, 2007). Therefore, calcareous soil has a sufficient content of phosphate, but it is in the form of low-soluble compounds as a result of the reactions to which it is exposed, such as fixation with carbonates in the soil solution (Al-Husseini, 2013).
       
Organic fertilization is an important technique used to help manage land. Organic fertilizers are added to the soil and will help to facilitate the reaction between P and Ca⁺², Al⁺² and Fe⁺³ as they chelate to these cations, which enhances the solubility of phosphate compounds and, more importantly, the concentration of nutrients such as phosphorus in the soil. The addition of organic fertilizers will also lower pH, cover the adsorption zones and prevent P from precipitating as hydroxyapatite (HA) (Shariatmadari, 2006). Organic fertilizers can be made from either plant or animal materials. Charcoal has been shown to improve both chemical and physical properties of soil (Ahmed et al., 2025). The addition of organic matter to the soil will readily increase the amount of available phosphorus and the soil fertility (Spencer, 2008; Bader et al., 2021; Raju, 2025). An example of a viable alternative to chemical fertilizers is poultry manure (pm). The primary purpose of using (pm) in agriculture trials is to act as an organic soil amendment to introduce nutrients to the soil that will help sustain crops. Due to the presence of NPK and other high nutrient levels within (pm), it is considered a unique organic fertilizer (Garg and Bahla, 2008). Sheep manure is considered an effective source of organic fertilizer that helps improve soil properties. Sheep droppings improve the availability and soluble phosphorus by extending the incubation time, as per AL-Magimia (2019).
       
This study was focused on the interaction between organic fertilizer and phosphorus to increase the availability of nutrients, especially phosphorus. As mentioned by Matura et al., (2023), the organic fertilizer makes the soil environment rich in nutrients by increasing fixation (Nitrogen), mineralization (phosphate) and solubilization (potassium). It is believed that the decomposition of organic matter over time and the release of essential nutrients like phosphorus may be the reason. Therefore, this research is aimed at finding out the impact of various doses and kinds of organic waste on phosphorus availability during different incubation periods.

Soil sampling and analyses
 
The laboratory experiment was conducted at the plant production department at Northern Technical University Technical Institute, Hawija, Iraq, during the summer season 2024-2025. The sampling was taken from the surface layer at a depth of 0-30 cm from the agricultural field at Kirkuk state, Iraq, which is located at longitude 43.78106 east and north latitude 35.33603. The soil sample was air-dried and then sieved through a 2 mm sieve. Soil pH and electrical conductivity (EC)1:3 soil to distilled water ratio, 35 min shaking, according to Black (1965), organic matter (OM) (Walkley-Black method) by using wet oxidation and described in Jackson (1958), for CaCO₃ was used back titration method (Jackson,1958) and soil texture (hydrometer method) contents were determined according to the methods of Black (1965).While the available nutrients for each of the nitrogen estimated by potassium chloride (2M KCl) according to the method of Mulvaney and Bremner (1982), phosphorus was estimated by soil extraction with sodium bicarbonate (NaHCO₃ 0.5M) at pH 8.5 (Olsen et al., 1954) using a spectrophotometer at 840 nm. The potassium was estimated by ammonium acetate(1N) (Pratt, 1965). The results of soil properties are shown in Table 1.


 
Poultry and sheep manure analysis
 
Organic extract (1:5 organic fertilizer: water) was used to estimate pH according to Jackson (1958). Organic fertilizer was digested by the wet method, taking 0.2 g of manure and adding concentrated sulfuric acid for 24 hours and then heating up for 30 min then adding the acid concentration 70% until a clear solution was obtained. Organic carbon calculated by wet oxidation, Walkley-Black method, according to Jackson (1958). Total nitrogen in manures was estimated using concentrated acids (H₂SO₄ and HClO₄) microkeldhal (Page et al., 1982). Estimated total phosphorus using a spectrophotometer (882 nm) after digesting the samples, following the method mentioned by Matt (1970). For determining total potassium using a flame photometer, the following method by Richards (1954). The content of poultry and sheep manure is shown in Table 2.


 
Incubation experiments and design
 
The experiment was conducted in the laboratory of the plant production department at Northern Technical University Technical Institute, Hawija, Iraq. To carry out the factorial experiment with three factors, a completely randomized design (CRD) with three replications was employed. Since the first factor was a phosphate fertilizer and the mono potassium phosphate fertilizer (KH₂PO₄) was the source of P, the four levels of phosphate fertilizer (0, 10, 20 and 30) mg P kg^-1 soil were denoted as (P0, P1, P2, P3), respectively. The content of KH₂PO₄ fertilizer is P₂O₅ = 52% and K₂O = 34%. The second factor is the type of organic waste (sheep and poultry manure), these being indicated by S1 and S2, respectively. The third factor, organic waste level (0, 1, 2) % for both kinds of manure, is indicated by (M0, M1, M2). Soils after air drying and sieving through a 2 mm mesh were weighed at 1 kg and put in plastic containers. Phosphorus and organic waste were added at the levels specified above. After mixing fertilizer and organic waste with soil, distilled water was sprinkled to bring the soil moisture back to field capacity and then it was placed in a conditioner at 25°C. Data collection was done at three dates (15, 33 and 60 days) during the two-month (60-day) incubation period. At the end of these times, the Olsen extractable-P was extracted with NaHCO₃ at pH 8.5 and its concentration was determined by a spectrophotometer at 840 nm.
 
Statistical analysis
 
All data in the research were collected and analyses using a SAS program. The Duncan test was set up to determine the significant differences among means at the confidence level (0.05) (Al-Rawi and Khalaf Allah, 2000).

The results in Table 3 show a significant difference among phosphorus levels at the incubation period 15 days, which increased with higher addition levels. Notably, P3 achieved the highest value (28.07) mg kg^-1, compared to the control treatment, which provided the lowest value (8.75) mg kg^-1.


       
The results obtained from poultry manure excelled those on sheep manure in increasing the availability of phosphorus on average (21.62, 15.92) mg kg^-1, respectively. This corresponds because poultry manure has a higher content of available nitrogen and phosphorus (Table 2), which leads to an enhancement of the availability of phosphorus in the soil, which is consistent with Al-Obaidi (2022). While the level (M2) significantly excelled at the rest of the levels and provided a value (25.40) mg kg^-1 compared to the levels (M0, M1), which were given average (11.50, 19.41) mg kg^-1, respectively. These results are in line with the outcomes by Joseph et al., (2025).
       
The highest significant interaction between the phosphorus and the type of OM was at treatment P3S2 (32.33) mg kg^-1, while the lowest rate was with treatment P0S1 (7.72) mg kg^-1. As a binary interaction between the type of OM and its level, the highest value was at treatment M2S2 (31.33) mg kg^-1 and the lowest value was at treatment M0S1 (11.50) mg kg^-1. The interaction between M x P resulted in the treatment P0M0 having the lowest value (3.66 mg kg^-1), while the highest value was observed in the treatment P3M2 (38.88 mg kg^-1).
       
While the interaction of triple factors had a significant effect, as the P3S1M2 provided the highest mean (51.33) mg kg^-1, which surpassed the rest of the treatments, meanwhile, the lowest value was for the P0S1M0, which was at a rate of (3.66) mg kg^-1.
       
The results of Table 4 indicate that the concentration of available phosphorus in the soil increased with increasing phosphorus levels, as the (P3) offered the highest concentration average of phosphorus (29.02) mg kg^-1, whereas the level without the addition (P0) reached (9.66) mg kg^-1 at the incubation period of 33 days. The results obtained showed that the available phosphorus increased with the increase in the levels of added phosphate fertilizer, which may be attributed to the fact that these soils are considered poor in their content of this element and respond to fertilizer additions. In addition to this increase is attributed to the role played by these wastes in increasing the availability of phosphorus, due to their production of carbon dioxide gas, which, through its release from the process of decomposing organic wastes and as a result of the dissolution of CO₂ in water, produces carbonic acid, which works to dissolve the precipitated phosphate compounds (Jenkinson, 1991). These results are consistent with AL-Migimia (2019). As for the type or source of animal manure, poultry manure was significantly superior to sheep manure and each of them reached (22.56 and 16.59) mg kg^-1, respectively. These results are consistent with what was reached by Donatus (2017). In addition to the level of animal fertilizer, which exceeded the third level (M2), its average reached (27.19) mg kg^-1 compared to the control treatment (10.03) mg kg^-1.


       
The results of the Table 4 show that the interaction between phosphorus levels and the type of organic fertilizer was positive, P3S2 treatment recorded the highest average (33.69) mg kg^-1, while the control treatment P0S1 was (8.58) mg kg^-1 recorded the lowest value. While the interaction of the type and level of OM and its levels, the treatment S2M2 had an average (33.22) mg kg^-1 that exceeded all averages, as well as the combination S0M0 provided the lowest value (10.03) mg kg^-1. The interaction between phosphorus and organic matter level had a positive effect as the treatment P3M2 achieved (40.92) mg kg^-1, which was the highest value compared to the control treatment was (3.00) mg kg^-1 was provided the lowest value. The treatment resulting from the triple interaction of phosphorus, OM type and levels P3S1M2 provided an average of (53.81) mg kg^-1, which was significantly superior to the rest of the treatments.
       
Table 5 displays the effect of added phosphate, the type of OM and its level on phosphorus availability at the incubation period of 60 days. The adding phosphate fertilizer controlled to a significant enhancement in phosphorus availability at all levels and its averages reached (10.54, 14.79, 26.77 and 29.67) mg kg^-1 for levels P0, P1, P2 and P3, respectively and with an increase related to the control treatment (40.32, 153.98, 181.49) % for levels P1, P2, P3, respectively. This increase may be attributed to the fact that increased phosphorus levels led to an increase in the forms of exchangeable and dissolved phosphorus, which leads to increased availability (AL-Migimia, 2013). While poultry manure fertilizer outperformed sheep manure, both of them reached (23.54 and 17.35) mg kg^-1, respectively. In addition to the superiority of the last level over the rest of the levels, M2) of OM provided the highest rate (28.96) mg kg^-1 and the lowest rate was in the control treatment (M0), which was (8.79) mg kg^-1.
       
Table 5 shows the results of the interaction of phosphorus levels and the type of OM, which was highly significant and the treatment P3S2 achieved the highest interaction rate (34.56 mg kg^-1). While the lowest interaction for that combination, P0S1, was (9.21 mg kg^-1). The result of the level and type interaction of OM was positive and the treatment S1M2 achieved significant differences, as it provided the highest average (34.75 mg kg^-1). However, S1M0 treatment provided the lowest average (8.80 mg kg^-1). The interaction of phosphate fertilizer and the level of OM, as the treatment P3M2 presented the highest average (43.34 mg kg^-1), while the lowest interaction was with the P0M0 treatment, which had a rate of (2.31 mg kg^-1). The resulting combination of the triple interaction between phosphorus, OM types and its level achieved P3S1M2 (56.66 mg kg^-1), the highest value and the treatment P0S1M0 provided the lowest value (2.30 mg kg^-1). It was observed that with increased incubation periods, an increase in phosphorus availability occurred, as the longer the incubation period, the higher the phosphorus availability, because the organic waste released phosphorus during the decomposition stage and the reactions to which it is exposed, including adsorption and precipitation. These results were in line with the findings of Mohsen (2025), who found that the organic acid present in organic fertilizers is the main agent that breaks down phosphate rock, which subsequently increases the level of released phosphorus in soil and enhances plants’ absorption of it.

The higher level of phosphate fertilization and organic matter addition provided a higher phosphorus availability and the poultry manure surpassed the sheep manure. For the incubation periods, the longer the incubation period provided the higher the phosphorus availability. Moreover, the combination of phosphate fertilizer addition with organic matter resulted in an enhancement in phosphorus availability over time. Therefore, for sustainable agricultural management for using fertilizer with higher efficiency, it’s recommended to mix it with organic waste to ensure a continuous supply of phosphorus for plants during the growth seasons.

The authors would like to thank the Northern Technical University, Technical Institute/Hawija, for providing the laboratory equipment for the completion of this research.

The authors state that there is no conflict of interest.