Phosphorus is one of the three primary nutrients commonly found in fertilizers along with nitrogen and potassium. Phosphorus occurs naturally in soil, but it is usually in forms unavailable for plant uptake. Plants require phosphorus to grow, which is why farmers have been feeding it to their crops for centuries. Proper management and application of phosphorus is critical because overloaded phosphorus in the soil can runoff into water supplies and become a major pollutant, and there has been a phosphorus fertilizer shortage due to depleted sources of phosphate rocks. Many of the 550,000 acres of corn grown in Louisiana are cultivated on the alluvial plain of the Mississippi River. Phosphorus deficiency symptoms on corn seedlings are common and most pronounced on these sandy loam and silt loam alluvial soils.

As part of a study how phosphorus nutrition affected corn crops, soil samples were collected from a greenhouse pot experiment conducted in 2011 at LSU. In this study, corn seeds were sown on potted Commerce silt loam and Perry clay soils and grown until harvest. A phosphorus fertilizer, triple superphosphate (TSP, 46% P₂O₅), was applied at rates of 0, 26, 53, 80 and 120 pounds per acre (0, 30, 60, 90 and 120 kilograms P₂O₅ per hectare). Soil samples from individual pots were obtained 30 days after phosphorus application or prior to sowing and after harvest and tested.

This study revealed that the two alluvial soils, Commerce silt loam and Perry clay, tested in low to medium levels for extractable phosphorus but responded differently with phosphorus fertilizer application. Grain phosphorus uptake and yield of corn grown on Perry clay increased with the phosphorus rate but not in Commerce silt loam soil. The application of 120 kilograms P₂O₅ per hectare or 107 pounds P₂O₅ per acre obtained a grain yield of 90 grams per pot, while the check plot obtained 28 grams per pot; This represents a 62 gram per pot increase in yield. However, the increase in grain yield was notably not proportionate with the phosphorus rate. For example, pots that received 30 kilograms P₂O₅ per hectare (27 pounds P₂O₅ per acre) obtained a higher grain yield of 71 grams per pot more than pots that received 60 and 90 kilograms P₂O₅ per hectare or 54 and 80 pounds P₂O₅ per acre (51 and 56 grams per pot), respectively. Furthermore, the phosphorus application also affects the quality of the corn grains. The check pots, which have no phosphorus fertilizer applied, have empty corn cobs, compared with the phosphorus-treated pots. This means that regardless of the phosphorus rate, phosphorus fertilizer application improves the yield of the corn. The grain phosphorus uptake for both soils showed similar patterns and responses to phosphorus application as grain yield. It is important to note that even if the soil test phosphorus levels are below the critical phosphorus level in a specific region, the crop response to phosphorus applications is not always certain. The reason why these two soils respond differently might be explained by different soil properties. The results showed that the unutilized phosphorus fertilizer was transformed into calcium-bound phosphorus (Ca-P) in Commerce silt loam soil, while in Perry clay soil it was transformed into iron and reductant-phosphorus. The results also showed that the inorganic phosphorus pools for both soils decreased at harvest. This means that the corn plant was able to take up the phosphorus in the soil.

Root growth is another factor that will affect phosphorus availability and uptake. Both the phosphorus fertilizer rate and placement method influence the pattern of corn root growth. The phosphorus band is applied by placing the fertilizer 2 inches to the side and 2 inches below the seed.. Applying phosphorus in a band is advantageous when the soil test level is low and there is a tendency to “fix” phosphorus in unavailable forms. Fixation happens when phosphorus reacts with other minerals and forms insoluble compounds, which become unavailable to crops. The broadcast method involves placing the fertilizer in the top 6 inches of the soil surface with subsequent mixing. This method produces the most uniform phosphorus distribution within the root zone and provides more root contact with phosphorus. However, it also maximizes contact between the soil and fertilizer so the opportunity for fixation is higher. The deep subsurface placement is the application of fertilizer 6 inches below the soil surface.

For the phosphorus band application, corn roots mostly concentrated at the top 6 inches of the soil. For the broadcast phosphorus application, the roots grow until 10 to 12 inches deep. With the deep subsurface-applied phosphorus, roots grew beyond 15 inches deep. Furthermore, broadcast-applied phosphorus, regardless of the rate, showed different patterns of corn root growth in Perry clay soil that were not observed in the Commerce silt loam soil. The corn roots grown in the check pot were more plentiful and denser compared to the corn roots grown on the phosphorus-treated pots 60 and 120 kilograms P₂O₅ per hectare or 54 and 107 pounds P₂O₅ per acre. This shows that the corn roots explore the entire soil of the check pot to take up phosphorus available in the soil.

The outcome of this study shows that corn, a high-yielding row crop, requires an adequate supply of primary nutrients like phosphorus throughout the rooting zone for good growth and high yield. With the increasing demand for the same amount of production area, farmers should maintain soil productivity, such as fertilizer application, incorporation of plant residues or manures and minimum tillage, to maximize yield potential through an effective nutrient management approach. Therefore, it is very important for producers to know the right source of fertilizer, right amount to apply, the right application method, and the right time to apply the fertilizer.

Marilyn Dalen is a postdoctoral researcher in the LSU School of Plant, Environmental and Soil Sciences. Brenda Tubana holds the Jack E. and Henrietta Jones Professorship in the LSU School of Plant, Environmental, and Soil Sciences.

(This article appeared in the spring 2022 issue of Louisiana Agriculture.)

Corn grain grown on Commerce silt loam and Perry clay soils in response to different phosphorus fertilizer rates. Photo by Marilyn Dalen

Corn roots grown on Perry clay soil (A) and Commerce silt loam (B) in response to different phosphorus fertilizer placement methods. Photo by Marilyn Dalen

Corn roots grown on Perry clay soil (C) and Commerce silt loam (D) in response to different phosphorus fertilizer rates applied through broadcast method. Photo by Marilyn Dalen