Development of An Environemntal Soil P Test for Muck (organic) Soils.

Soil phosphorus (P) testing method developed for mineral soils may not be suitable to organic soils, due to the differentiated nature in chemistry and behaviours of P. A study was conducted to develop (a) soil P test(s) to predict availability of P to crops and to assess environmental risk of P in organic soils. Soil samples were collected from eight soil series in Ontario, with 8 or 9 sites each to cover a range of soil P levels. Three studies were conducted, including a surface runoff box, an intact soil core leaching, and a greenhouse pot trial. The results from both runoff and leaching studies showed that dissolved reactive P in both surface runoff and leachate were strongly and linearly related to WEP or CaCl₂-P, and split-linearly related to Bray1-P or FeO-P concentration and their P sorption index (PSI)-based DPS derivations (i.e. Bray1-P/PSI, Bray1-P/(Bray1-P + PSI),  FeO-P/PSI and FeO-P/(FeO-P + PSI)). The greenhouse study revealed that FeO-P and Bray1-P were closely related to ryegrass biomass production and P uptake. As it is desirable from practical point of view that assessment of environmental risk is conducted using the same soil test as for agronomic calibration, whenever possible, the FeO-impregnated filter paper extraction may be recommended for both environmental and agronomic prediction of soil P in muck soils. However, the method must be validated under field conditions, with critical values respectively determined for agronomic calibration and environmental assessment, before practical implementation.