Improving phosphorus availability and reducing emissions through soil health – experiences from 24 test fields

A healthy ecosystem can retain phosphorus in a plant available form. Soil health is commonly described though physical, chemical and biological components.
This poster was presented at 3rd European Sustainable Phosphorus Conference 2018 (ESPC3).

Phosphorus availability to plants and its potential to pollute waters is commonly evaluated through measuring soil phosphorus with extractants which emulate plant root exudates. Several different extractants have been proposed, and there is no clear consensus on what method to use. In addition some work has highlighted that it is the phosphorus saturation (i.e. ratio of phosphorus to iron and aluminium), which is more relevant for both plants and emissions. However the plant availability and emissions depend also on other factors: soil quality and soil health. We studied 24 Finnish test fields intensively from 2015-2018 to identify factors which could influence phosphorus emission risks and plant availability. The fields had varying soil types (clay, peat and sandy) and cultivation backgrounds. The fields were tested for phosphorus concentrations using ammonium acetate, Mehlich 3, H3A, hydrochloric acid and water to give a range of phosphorus solubilities. In addition the phosphorous uptake was measured and several soil quality indicators were assessed (e.g. aggregate stability, water infiltration, microbial respiration, soil structure, rooting depth). The fields were found to differ more in their soil health parameters than in their phosphorous concentrations, which opens possibilities for increasing phosphorus uptake and decreasing emissions by improving soil health.

Soil phosphorous has traditionally been managed from a chemical concentration viewpoint. For plant nutrition, the levels should be high enough to ensure adequate plant uptake. In contrast for the environment, the levels should be low enough to prevent excessive emissions. Phosphorus saturation thresholds have been proposed as a way to manage the issue (Nair et al. 2016). However this approach does not take into account other factors which influence soil emissions such as soil structure, aggregate stability or overall health of the soil ecosystem.

Figure 1. Phosphorus saturation (ratio of P to aluminium and iron) has been considered to be a good measure of phosphorus availability and risks for leaching. However it does not consider soil physical or biological components. Data based on measurements from 24 test fields. P saturation from H3A extraction.