This research article published by Elsevier Ltd., 2021
Phosphorus (P) is a vital plant macronutrient required for plant growth which usually available in
limited amount. P availability for plant uptake in highly weathered soil is controlled by soil erosion
and high fixation. The availability of P applied from fertilizers depend on the soil pH, soil sorption
capacity (PSC) and P saturation status (PSD), which determines P storage, losses, fixation, and
additional P to be added with minimal loss to the environment. PSC and PSD are agroenvironmental
indicators
used
to
estimate
P
availability
and
P
loss
to
the
environment.
However,
PSC
and PSD of agricultural soils had been never studied in Tanzanian soils. This study was
conducted to assess and estimate P availability, PSC and PSD and the risks of P losses in tropical
soils from Usangu basin popular for paddy farming. In total, 198 soil samples from 10 paddy
irrigation schemes were collected (November–December 2019) and analyzed for inherent P
(PM3), metal oxides of Aluminium (Al M3), iron (Fe M3), and calcium (Ca M3) as main PSC and
PSD determinant. The determined concentrations were in range of; P M3 014.9–974.69 mg/kg, Al
M3 234.56–3789.36 mg/kg, Fe M3 456.78–2980.23 mg/kg, and Ca M3 234.67–973.34 mg/kg.
Estimated PSCM3 ranged 5.62–34.85 mmol/kg with a mean value of 14.14 mmol/kg
corresponding to high status, ensuring high P holding capacity for plant uptake. However, some
soils had very low PSCM3 creating a risk of P loss to environment. Among soils, the estimated
PSD M3 ranged from 0.01 to 17.57% and was below (<24%), indicating low P loss risks to surface
and groundwater, however, some soils were observed to have PSDM3 above 15% which
correspond to a critical degree of phosphate saturation of 25% in a watershed using oxalate
extraction method. Therefore some sites were associated with high P loss to the environment,
immediate and precautionary actions for sustainable P management to increase productivity,
environmental safety and sustainability are needed to be in place.