April is the time for spring ploughing. It is the time when farmers plow purple flowers into the rice fields using a tractor. By May, these flowers will become green manure which can boost land yield and the quality of rice planted later.

The purple flower is called Astragalus sinicus, commonly known as Chinese milkvetch. It often grows in wet places, such as fields and river beaches.

Astragalus sinicus is used in livestock feed and green manure since it contains various nutrients, such as nitrogen, phosphorus, potassium, zinc, manganese, etc. In ancient China, farmers used it as green manure. The earliest record of its use can be traced back to the Ming Dynasty (1368-1644).

Native to China, the little purple flower grows in all provinces along the Yangtze River. It blooms from February to June with purple or orange corollas. Its leaves are generally green, but researchers of Hunan Agricultural University have cultivated a new variety with red leaves.

Its seeds are difficult to germinate because they have a hard coating – it takes three to four months under natural conditions.

Astragalus sinicus needs a lot of nitrogen throughout its life, but it has limited nitrogen absorption from the outside. According to related researches, a type of bacteria called "rhizobium" helps fix nitrogen within its root nodules.

Read more from the series:

Season of flower

(All images via VCG)

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