Nutrient interactions and salinity effects on plant uptake of P from waste-based fertilizers

Summary

A large number of European organically managed farms have low levels of soil P and it has been shown that arable farms that rely strongly on biological nitrogen fixation have rather low outputs, and tend to deplete soil phosphorus and potassium resources, whereas arable farms with lower reliance on BNF and higher reliance on external inputs have much higher outputs and low or no soil resource depletion. Therefore, research focusing on providing P, as well as N, K and S from alternative sources, is of interest to organically managed farms not only in Denmark but in Europe as a whole.
The aim of this study was to quantify P uptake in ryegrass grown in pots from different organic wastes applied alone or in combination with other organic wastes to improve the N:P:K:S ratio. We used the indirect labelling technique, where a non-labelled fertilizer is added into a soil that has been preincubated and equilibrated with labelled 33 P.
The apparent P recovery of the different organic wastes tested varied significantly, with the lowest recovery from manure and digested products admixed with ash from straw. The highest recovery was found in digested products, either manure alone or mixed with municipal waste or Fertigro. However, the mixture of digested manure and Fertigro gave rise to lower dry matter production, and Fertigro used alone gave rise to depressed shoot and root growth. The evidence points to a suite of effects. The Fertigro gave rise to salinity effects and a decrease in pH resulting in high leaf P concentrations but reduced shoot and root growth.
Digestion increased the availability of P, presumably due to the lower immobilization potential of the added organic matter. Mixing digested manure with ash increased resulted in an increase of the soil pH at the end of experiment, which may explain the lower P availability. This points to potential challenges when attempting to improve the N:P:K:S ratios of waste-based fertilizers, due to nutrient interactions. Such effects are presumably overexpressed in pot trials that have very limited soil volume, why field trials are needed to quantify such effects in practise.