Effects of struvite application on soil and plants: a short‐term field study

Summary

The soil P concentration commonly declines over time by organic management, if the farm does not acquire enough fertilisers or other inputs containing P from outside. An interesting source to supply farms with P is struvite (magnesium‐ammonium phosphate). This mineral easily precipitates when the concentrations of these nutrients is high enough, such as in a toilet separating urine. Struvite
may be produced in a wastewater treatment plant, as a method to reduce the P concentration in the water reaching the recipient. If struvite is allowed as a mineral fertiliser in the general fertiliser regulation of the European Union,which is currently under revision, this mineral may be allowed also in certified organic agriculture. A wastewater treatment plant in Hamar (south‐eastern Norway), Hedmarken inter‐municipal
wastewater corporation (Hias IKS) has developed a patented method to precipitate struvite efficiently. A sample of struvite produced by this process at Hias was sent to Tingvoll (north‐western Norway) to be tested in an experimental field with perennial ley, called “SoilEffects”. This experiment, established in 2011, is used to study the effect on yields and soil characteristics when the slurry from the farm’s herd of organic dairy cows is anaerobically digested to produce biogas before being applied as fertiliser. Non‐digested and digested slurry are compared in two levels of manure application, low and high (30 or 60 tons of slurry per ha and year), and with a control with no manure application. Eight replicates are available of each treatment, and to half of these plots, struvite was applied shortly before the slurry was applied, in the end of April 2018. We applied an amount of struvite corresponding to 40 kg P per ha. Before the application, soil samples had been
collected from two depths, to study the effect of struvite on soil characteristics. Yields of ley were recorded in the two cuts, which occurred in June and August. The yield level at the 1st cut was generally low due to drought, but the increase in yield was significant with application of
struvite in the plots receiving no manure, and with low manure application. At the 2nd cut, yield levels
were generally higher, and again a positive effect of struvite was found in these treatments. On average, the total yield (sum of yields at the 1st and 2nd cut) was 2.4 tons of dry matter (DM) per ha with no application of manure or struvite, and 4.1 with application of struvite. In treatments receiving a low amount of manure, the mean total yield was 5 tons per ha with no struvite, and 6 tons with struvite application. In treatments receiving high amounts of manure, the mean total yield was 6.4
tons per ha without struvite, and 6.2 with struvite.
The concentrations of minerals in aboveground plant material from both cuts were analysed, and
new soil samples collected after the 2nd cut of ley. Application of struvite increased the P concentrations in plant material and the concentration of AL‐extractable P in soil. Struvite further increased the concentration of magnesium in plant material and soil. Soil pH increased from spring to autumn by application of manure, but the application of struvite reduced this effect. The amount
of P being adsorbed to oxides of iron and aluminium in the experimental soil from a solution containing dissolved phosphate was also recorded. The soil has a certain amount of such oxides, and long‐term application of P in manure, as well as application of P in struvite, could possibly reduce the amount of P being adsorbed from the solution. However, no significant differences were found between soils which had received low or high amounts of manure over several years, and we did not find any effect of struvite application on this characteristic in this soil.