AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 1997, No. 5
Phosphorus that is bound with iron and aluminum oxide compounds becomes more soluble under flooded conditions. When the soil dries, however, iron re-oxidizes and begins to form complex compounds with phosphate, resulting in less plant-available phosphorus.
Phosphorus is also present in soils in organic forms which are not utilized directly by crops. As organic matter decomposes with sufficient oxygen, moisture, and microbial activity, phosphate is released. Under flooded or water-logged conditions, organic matter mineralization and release of phosphates is limited. On soils that have been land leveled, subsurface layers that are high in iron and aluminum and also low in organic matter are often exposed, resulting in chemical reactions that reduce phosphorus availability.
On soils that are acidic and have high iron and aluminum contents, and which also have low organic matter contents, phosphate…that which is native in the soil or that added as fertilizer…is subject to 'fixation'. In simple terms, this means that phosphorus is bound in forms unavailable or only very slowly available to plants. Mixing phosphorus fertilizer with the soil results in more fixation than when phosphorus is banded.
In soils that are alternately wetted and dried (flooded and drained) the phosphorus fixation is much more extensive and less reversible than under continuously flooded (wet) conditions. This helps explain why crops rotated with rice have an increased need for phosphorus and why banded phosphorus applications are often superior to broadcast applications.
In recent years, even on soils that were thought to provide phosphorus to rice in adequate amounts under flooded culture, responses to phosphorus additions have been measured. If rice responds to phosphorus additions under flooded culture, then the phosphorus needs of other crops in rotation should also receive increased attention.
Many plants have a beneficial association with specialized soil fungi called mycorrhizae. These fungi are present in most field soils and help plants utilize phosphorus, zinc, and other nutrients. Mycorrhizal populations are reduced by flooded soil conditions to the point that corn plants are unable to take up the necessary phosphorus for normal development. This reduction in the mycorrhizal association increases the need for phosphorus fertilization for corn, particularly on soils testing medium or lower in phosphorus.
On soils with high phosphorus fixation, phosphorus should be applied as close to planting as possible. For row crops, a starter fertilizer that includes phosphorus may need to be considered as a supplement to normal broadcast applications. Starter fertilizers tend to perform best when crops are planted early into cool, moist soils and those with high residue covers. If the equipment is available and the opportunity to band phosphorus exists, growers may want to experiment with phosphorus placement methods to determine the most desirable application method for their farming system. The key is to get phosphorus applied when and where it is needed.