POLICY BRIEF: Improving manure recycling in agriculture

Past inefficient use has led to the accumulation of phosphorus on land

In the past 50 years, countries around the Baltic Sea have applied about 35 million tons of mineral phosphorus fertilisers to agricultural soils.

Decades of inefficient use of phosphorus in crop production has led to the accumulation of huge amounts of nutrients in soils. Earlier, farmers were advised to apply large amounts of mineral fertiliser and manure to build up soil phosphorus levels. Soils in several regions now contain so much phosphorus that only small amouts need to be applied to maintain yields.

For the Baltic Sea catchment as a whole, the amount of accumulated phosphorus in agricultural soils is the same order of magnitude as two decades of crop nutrient needs. There is large potential to decrease application rates of mineral fertilisers and manure in areas with substantial phosphorus surpluses. The surplus of phosphorus, which leads to accumulation, increases the risk of losses to lakes, streams, and the Baltic Sea. Since large amounts of feed and fertiliser are imported, phosphorus continues to accumulate.

The buildup of phosphorus on land can be slowed by reducing imports of feed and mineral fertilisers through more effective recycling of nutrients in manure and human sewage and by avoiding over-fertilisation.

Recycling could reduce mineral fertiliser imports

Agricultural systems will never be perfectly efficient because of unavoidable nutrient losses. But in the Baltic Sea region, there is room for improvement.

Each year, the 23 million pigs, 16 million cows, and 244 million chickens in the region produce manure that contains over 0,35 million tons of phosphorus. Much of the phosphorus in the manure originates from imported animal feed. Even though over 70% of crops grown in the Baltic Sea catchment are fed to livestock, this is insufficient to meet the animals’ nutrient requirements. As a result, 60% of the phosphorus needed by livestock is imported, mostly as soy from South America.

A recent study shows that the average phosphorus use efficiency in crop production in the Baltic Sea region is only about 60%, but it varies greatly between countries. If phosphorus use efficiency increased to 70 - 90%, imports of mineral phosphorus fertilisers could be reduced by 0,11 to 0,17 million tons.

Specialisation and spatial separation of crop and livestock production systems can cause excessive phosphorus application in animal-dense regions that have large amounts of manure in relation to crop area. There is potential to use manure more efficiently by redistributing “excess” manure nutrients from regions specialising in livestock production to regions specialising in crop production and, ultimately, reduce mineral fertiliser imports.

Table. In countries with low phosphorus use efficiency, there is the opportunity to reduce imports of imported mineral fertilisers by avoiding over-fertilization and by substitution with animal manure (excreta). Phosphorus in human excreta can also be further used better recycled in agriculture, further reducing the region’s dependence on fertiliser imports. Only parts of Belarus, Germany, and Russia that are within the catchment are included in the estimates.

Data from Hong et al. 2017.  *PUE is phosphorus use efficiency in crop production, the ratio of crop harvest (outputs) to the sum of fertiliser and livestock excreta (inputs)

Recycling sewage sludge could reduce annual fertilizer imports 0,036 million tons

About 75% of people in the Baltic Sea region are connected to centralised sewage systems, while the remainder are connected to non-centralised systems, such as septic- and cesspit-type systems.

Centralised sewage treatment practices have improved substantially in the past few decades; however, phosphorus removal efficiencies vary greatly, between 63% (Latvia) and 97% (Finland, Germany, and Sweden).

There is opportunity to recycle phosphorus from sewage sludge that is collected in centralised systems. Today, only about one-third of the sewage sludge in the region, which contains about 0,027 million tons of phosphorus, is used in agriculture, but practices differ among countries in the region. The remaining sludge, corresponding to 0,036 million tons of phosphorus, could be used in agriculture and reduce fertiliser imports.

Phosphorus that is not removed from sewage as sludge in centralised sewage systems is discharged to surface waters in effluent. Thus, improving sewage treatment capabilities is an effective way to prevent phosphorous from entering lakes and rivers that drain to the Baltic Sea. Currently, an average of 16% of phosphorus in human excreta (0,011 million tons) becomes effluent.