Assessing the horizontal and vertical distribution of phosphorus in agricultural soil around maize root using a fertigation method in surface drip irrigation

The availability and efficient utilization of phosphorus, an essential nutrient in agricultural production, through fertigation systems requires precise determination of application rates and an understanding of phosphorus movement and distribution in the soil layers surrounding crops. This study aimed to evaluate the distribution of soluble phosphorus (SSP) and available phosphorus (SAP) in agricultural soil under maize cultivation and in bare soil during two fertigation cycles over various time periods at the Soil and Water Research Institute (Karaj). After tillage, maize was planted using a strip and single-row method, with row and plant spacings of 60 cm and 15 cm, respectively. Following fertigation in both cycles, using micro auger soil samples were collected from both bare soil and soil under maize at horizontal distances of 5 and 10 cm from the dripper and at depths of 0-5, 5-10, 10-15, 15-20, 20-30, and 30-40 cm at time intervals of 4, 24, 98, and 960 hours, with two plant replications (two sides of each plants). The samples were subsequently transferred to the laboratory for chemical analysis using conventional measurement methods. The results indicated that fertigation, depth, and time significantly affected the distribution of SSP and SAP in both systematic and non-systematic manners. The analysis of variance revealed that the effect of the fertigation cycle treatment on SSP and SAP under maize cultivation at 5 and 10 cm from the fertilizer injection site was significant (at the 1% statistical level), demonstrating a phosphorus movement of approximately 10 cm within this system. As the depth increased, the average SSP and SAP values exhibited a decreasing trend. The maximum average concentration across all treatments was observed 98 hours after fertilizer injection, indicating an optimal time range for easily accessible phosphorus availability. Overall, due to the unique properties of phosphorus in soil—such as low solubility, low mobility, and high stabilization/fixation by the soil matrix—along with its availability for plants, it is preferable to apply phosphorus fertilizers in a soluble form, approximately 5 centimeters away from the crop, and in more frequent applications.