Pingdi 平糴 (also called hedi 和糴, tuizhi 推置, biaodi 俵糴, jundi 均糴, bodi 博糴, duidi 兌糴 or kuidi 括糴) and pingtiao 平糶 were two elements of an ancient system of grain price adjustment. In times of good harvest the local authorities purchased grain (pingdi) at favourable prices and stored it in public granaries, as a reserve for times of bad harvest, when the grain prices soared. Throwing the grain reserves on the market (pingtiao) during times of famine, the volume of grain on the markets was increased and its market price remained on an acceptable level.
This kind of economic policy was seen as a cornerstone of good government. It was promoted by Fan Li 范蠡, a counsellor to the king of Yue 越 during the Spring and Autumn period 春秋 (770-5th cent. BCE), and was also part of the legalist policy of Li Kui 李悝, the counsellor of Wei 魏 during the Warring States period 戰國 (5th cent.-221 BCE). Li Kui explained that high grain prices would harm the consumers, and low ones the producers. The government would therefore have to see to it that the grain price was balanced into two directions. Fan Li defined the ideal price range as between 30 and 80 qian 錢 (per shi 石, see weights and measures), so that both, producers and consumers would benefit (nong mo ju li 農末俱利).
Li Kui defined "good" and "bad" years were defined by certain criteria and ranked into three grades each (shangshu 上熟, zhongshu 中熟, xiashu 下熟, xiaoji 小饑, zhongji 中饑, daji 大饑). In times of excellent harvest (dashu 大熟), the government purchased three quarters of the surplus grain, in years of relatively good harvest (zhongshu) two thirds, and in times of good harvest half, and threw the respective amounts on the market in years of crop failure, namely half of the average individual yield in suboptimal years, three times in years of bad crop failure, and ten times when crop failure was disastrous. Li Kui's calculation was based on the assumption that the average yield of 100 mu 畝 of farmland was 150 shi 石 of grain. His data for the six levels of harvest were 600, 450, and 300 for good harvest, and 100, 70, and 30 for bad ones. Li Kui did not speak of a price range, but apparently aspired to keep the price stable on a normal level. Marxist historians therefore call Li Kui's policy a kind of control over the emerging class of grainbrokers, while Fan Li was had a more positive attitude towards these "profiteers". A similar procedure is mentioned by the master of interventionist economic policy, Guanzi 管子 (ch. Qingzhong 輕重篇).
During the Han period 漢 (206 BCE-220 CE), Emperor Xuan 漢宣帝 (r. 74-49 BCE) adopted the suggestion of Geng Shouchang 耿壽昌 to build state granaries, the so-called "ever normal granaries" (changpingcang 常平倉) in the border regions, in order to supply the garrison troops there. The system remained a core part of economic policy until the end of the empire in 1912. It was also a component of the reforms in economic policy carried out during the Song period 宋 (960-1279) by Wang Anshi 王安石.
During the Qing period 清 (1644-1911) a handbook for magistrates called Huangzheng zeli 荒政則例 regulated the purchase of grain by the government and its storage for bad times. Yet in practice, there was much abuse with the system. Peasants or grain brokers were forced to sell their grain at cutthroat prices, and grain purchases were measured by a large bushel (dadou 大斗), while the grain thrown on the markets was sold by the small bushel (xiaodou 小斗). During the Tang period 唐 (618-907) there were cases where the government did not pay at all, and during the Song period the government purchase of grain was by the peasantry perceived to be just a kind of additional tax.
Information on the early system can be found in the chapter on "profiteers" (129 Huozhi liezhuan 貨殖列傳) in the history book Shiji 史記 and the chapter on food and commodities (24 Shihuo zhi 食貨志) in the official dynastic history Hanshu 漢書·