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Promoting Phosphorus in Soils

Plants require nutrients for healthy growth, and of these nutrients, phosphorus is very important. However, in agricultural systems, the amount of phosphorus that is available to the plant rapidly decreases after the farmer applies it to the field. This occurs because phosphorus interacts with soil material at the soil surface. Phosphorus can then be washed away into streams and local water-bodies, which can have harmful effects on the environment.

Teagasc researchers at Johnstown Castle have been investigating the ability of earthworms to reduce this accumulation at the soil surface, since earthworms have a substantial effect on the soil by redistributing and mixing material. “Earthworms are classified into three groups, those that live in the litter layer at the surface, those that live below ground, and those that produce semi-permanent vertical burrows. By feeding on material on the soil surface, earthworms can mix phosphorus present at the surface with soil below the surface,” explains Dr Rachel Creamer.

An experiment was conducted to investigate how earthworms redistribute phosphorus and how plant communities assimilate redistributed phosphorus. To replicate this accumulation at the surface, two soils with contrasting phosphorus concentrations were used. The upper soil (1 cm) was approximately seven times greater in phosphorus concentration than the lower soil (30 cm). Plant communities (grass-only or grass, forb and legume communities) were planted and established for six months before earthworms were introduced. Following a further nine months, the experiment was deconstructed and divided into different layers for laboratory analysis.

“This experiment showed that earthworms promoted a more even distribution of phosphorus by removing this nutrient from the soil surface (0 – 1 cm depth range). Furthermore, it was shown that more phosphorus was assimilated by plants in the presence of a grass, forb and legume community compared to a grass-only community,” explains Dr Creamer.

Experimental mesocosms designed to investigate the effects of the soil biota on the vertical distribution of phosphorus