were able to stabilize night soil and make it safe to use.

The answer is the night soil reservoir. Night soil reservoirs are facilities to ferment night soil and convert it into fertilizer. These facilities could be as simple as a hole dug beside a field and covered with a lid.

Farmers mixed rice straw into the night soil inside the reservoir. When night soil, which is a mixture of carbohydrate, fat, nitrogen compounds and many other organic substances, is stored in an airtight chamber with the lid closed (under anaerobic conditions) and with rice straw added, it then decomposes into substances with a relatively simple structure (such as organic acids, fatty acids and amino acids) as a result of the metabolic activities of various anaerobic bacteria. Subsequently, methane bacteria then generate gases such as carbon dioxide, methane, hydrogen, nitrogen, ammonia and hydrogen sulfide. Eventually, night soil is stabilized and rendered free of almost all roundworms, which are killed by the heat generated during the fermentation process.

In the Edo era, the shogunate government ordered all citizens to install a large cesspool with each toilet, in order to efficiently store all night soil. This led to the installation of cesspools in rural and urban houses alike. The night soil stored in these tanks underwent partial anaerobic decomposition before being collected and transported to suburban farms by boat or by horse or ox-drawn wagons. This night soil was then stored in night soil reservoirs for stabilization and sanitation before being spread on the fields as fertilizer.

During the Edo era, Japan therefore had a true SMC Society in which night soil was sanitarily treated for effective use without affecting the living environment, thereby making the best possible

use of the limited resources available. People today, who no longer make use of night soil, can learn a great deal from the sanitary recycling of night soil in the Edo era. It is important that Japan should share the benefits of its experience with developing countries, especially from the perspective of hygiene education and village development.

[Sanitary use of the night soil reservoir]

Source: Ministry of the Environment

## Column

## Night soil prices

In the Edo era, a price was placed on night soil for trading purposes, and these prices varied depending on the person's class in society. Since the nitrogen and phosphorus contents were an essential determinant of fertilizer quality, it is believed that the value placed on night soil as a fertilizer was determined by the person's diet.

An analysis of night soil in the early Showa period (performed by Dr. Kerner, professor of the School of Agriculture and Forestry (now the Faculty of Agriculture, the University of Tokyo)) shows that the nitrogen and phosphoric acid contents of night soil differed between people with different jobs, such as farmers and soldiers.

Composition of night soil (%)

Contents	Class	Farmer	Tokyo citizen	Middle class gov. official	Military person
Water	95.4	95.4	94.5	94.6	94.6
Organic matter	3.03	3.18	3.89	4.07	4.07
Nitrogen	0.55	0.59	0.57	0.80	0.80
Phosphoric acid	0.12	0.13	0.15	0.30	0.30
Potassium	0.30	0.29	0.24	0.21	0.21
Soda	0.51	0.41	0.45	0.26	0.26
Lime	0.01	0.02	0.02	0.03	0.03
Magnesium	0.03	0.05	0.06	0.05	0.05
Sulfuric acid	0.07	0.04	0.05	0.07	0.07
Chlorine	0.70	0.55	0.61	0.51	0.51
Silicic acid & sand	0.04	0.10	0.11	0.04	0.04
Iron oxide & aluminite	0.03	0.02	0.06	0.06	0.06

Source : Kazue Kurokawa, "A Study of Soil Fertilizers since the Meiji Era in Japan"