Soil studies using sweet vernal to assess element availability IV. Phosphorus

Summary

The phosphorus status of New Zealand topsoils is surveyed in terms of total, organic, and inorganic phosphorus fractions, solubility of the inorganic phosphorus in N H₂SO₄ and phosphorus content of the grass, sweet vernal (Anthoxanthum odoratum). Use is made of the genetic soil classification to show how the plant and soil phosphorus values are influenced by the nature and degree of the soil-forming processes.

The total phosphorus contents of mineral topsoils at early stages of weathering reflect the nature of the parent rock in that red-brown loams from basalt and yellow-brown loams from andesite contain larger amounts of phosphorus than soils from more acidic rocks such as yellow-brown pumice soils and the zonal soils. Soil development sequences, representing increasing degrees of weathering and leaching, from groups of parent rocks, show a decrease in the phosphorus content of the topsoils. The greater the iron and aluminium content of the parent rock, the greater is the proportion of the original soil phosphorus retained in the strongly weathered, strongly leached, soils. Soil development is accompanied by a decrease in the solubility in n H₂SO₄ of the inorganic phosphorus which reflects the progressive fixation of the soil phosphorus.

The capacity of topsoils to retain added phosphate has been related to the degree of soil weathering, leaching and podzolisation.

The proportion of phosphorus in organic form is high but does not form a systematic pattern within soil sequences. It is generally low in recent soils and high in organic soils.

The amount of phosphorus taken up by the grass, sweet vernal (Anthoxanthum odoratum) from non-topdressed sites is related to the stage of soil development rather than to the amount of phosphorus present. High levels of phosphorus are found in the grass on recent soils and on weakly weathered, weakly leached soils, irrespective of their parent rock, while strongly weathered, strongly leached, soils have low levels of phosphorus in the grass.