Fertiliser from Urine (Struvite)

Nitrogen and phosphorus are two major valuable nutrients found in urine, which are both essential macronutrients for the plants (see also nutrient requirements). Huge quantities of urine are most often difficult to transport and to keep due to its unpleasant odour. To overcome these issues, Struvite can be produced. Struvite (MgNH₄PO₄•6H₂O), sometimes also called Magnesium Ammonium Phosphate Hexahydrate (M-A-P), has many of the fertilising properties of urine with several advantages: the volume and weight are reduced, it can be stored in a compact form and it is easy to handle, transport and apply- especially in a granulated form.

Struvite is a naturally occurring precipitate in aged urine. It is formed out of some of its compounds when the pH-level of the urine exceeds a distinctive level due to chemical changes happening during storage. The Struvite molecules are formed through a combination of dissolved magnesium, ammonium and phosphate ions. Since the concentration level of magnesium is considerably lower than the concentration of the other compounds this natural process stops when all the magnesium ions in solution are fixed. However, this process can be stimulated again by artificially adding magnesium to the stored urine. By this, the precipitation of Struvite continues until one of the other in the process involved components (ammonium and phosphate) are depleted. In normal urine, the concentration of phosphate is subsequently the limiting factor. The artificial addition of magnesium to urine is therefore a very convenient way of fixing the phosphorus and a part of the nitrogen that is contained in urine (TILLEY et al. 2008; UDERT 2002; UDERT et al. 2006) (see also producing struvite from urine).

According to it’s chemical composition, Struvite has an NPK-value of approximately 6:29:0:(Mg)10. It shows some advantageous properties like a slow nutrient release and good P availability.As regards the plant availability of phosphorus (P) from struvite, it was found that there is no significant difference between struvite and mono-calcium phosphate, a source of water soluble P that is generally considered to be fully plant available. However, the plant available nitrogen is not enough for optimal growth, but higher struvite application would significantly increase the soil pH level to a degree where it might have negatively affected nutrient availability and uptake. Moreover, the magnesium content is very high. In general, the average amount of nutrients that should be applied to the soil should follow the order N >= K > P > Mg. It is therefore suggested that for an optimal use, a combination of other fertilisers together with struvite should be used. Especially an increase of the nitrogen content would be advisable since nitrogen is the nutrient that most of the farmers look for. Most suitably a ready-made mixture with an optimal nutrient ratio could be supplied.

One of the big advantages of Struvite that might be recognised by the local farmers is for its purity and product reliability. Whereas the industrially supplied chemical fertilisers are likely to be polluted with unwanted side products and heavy metals, even when coming from official channels. For fertilisers coming from informal sources (>60%) even the actual nutrient concentration itself is of big question. As locally produced fertiliser like struvite would surely get more credit from farmers because its source is known and a reliable availability can be expected. This might even be a factor supporting a higher price compared to other fertilisers.

For further information, see also producing struvite from urine or nutrient requirements.