Phosphorus Leaching?

The focus of nutrient management is minimizing the off-site movement of nutrients into the environment. Conventional wisdom has said that phosphorus binds so tightly to the soil that the only pathway for P loss to the environment is through soil erosion. Recent research is challenging this view.

Phosphorus in the soil solution exists as the negatively charged phosphate ion, but unlike nitrate it does not leach easily with the downward movement of water. Phosphate is extremely reactive and binds with aluminum, iron, calcium and other elements, which are present in all soils at relatively high levels. This causes the P to form new chemicals in the soil that bind tightly with the soil clay and organic matter.

Even though this phosphorus is tightly bound to the soil, we can still get what appears to be leaching loss, or downward movement of these materials into the soil profile and potentially to surface water through tile outlets or lower to ground water.

Studies conducted in Finland and the US by Martin Shipitalo of USDA in Ohio and Visa Nuuinen of MTT Agrifood Research of Finland demonstrate what can happen. Essentially, they found that soil particles, with the attached phosphorus, could be carried by water that flows in cracks and through earthworm borrows to lower levels of the soil profile.

Using a blue dye applied to the soil surface, they were able to show that soil particles could move through cracks in the soil down into the profile. If a compacted layer was encountered at the bottom of the plow layer, the water/dye combination could move considerable distance laterally from where it found the plow pan. Once this water/dye found an earthworm burrow, it could move rapidly downward in the soil profile. They further studied the concentration of earthworm burrows in the field and found that a much higher concentration of burrows were found above the tile drains then in the regions between the drains. They suggested that this was because the area above the tiles would have more air available for the earth worms since water would be rapidly draining downward toward the tiles in this area. A direct result of this is a large increase in the rate of water infiltration above the tiles, as shown in the following table. This further suggests that the rapid movement of any P enriched soil down through the profile will be greatest in the areas over tile drains. With the use of the blue dye, they identified 106 burrows per metre square at 10cm above the tile drain suggesting that rapid movement to tile drains could occur.

Following this study, they poured a fiberglass resin down the earthworm burrows. Once this had hardened, they excavated the soil in the test area and washed away remaining soil. The hardened fiberglass channels were left intact. They found that cracks allowed water to infiltrate into the soil, but that earthworm burrows carried the water from the plow layer positions either directly to or in close proximity to the tile drains. The sediment that was found around the tile drains was found to be from the surface soil, which was further evidence of surface soil movement down into the soil profile.

This work demonstrates that the movement of P down through the profile is not true leaching as we see with Nitrogen,, but rather a preferential flow through large cracks and earthworm channels. It does show that Phosphorus can move through the soil profile to tile drains and be susceptible to loss through tile outlets into surface waters.

This suggests that farmers should find ways to disrupt these channels that may conduct soil and nutrients from the soil surface deeper into the soil profile. This may be done by light tillage or even timing of fertilizer applications that minimize the presence of significant cracks at the soil surface.

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