The geography in China is varied, and the numbers matter in terms of land mass as well. While it is the third largest country in the world by area, not much of its land is suitable for farming. Considering it accounts for almost one fifth of the world's population, it actually contains only one tenth of the globe's agricultural land. Simply put, they cannot grow enough food to adequately feed everyone.
The problem is also about closely held historical beliefs. China has a strong connection to their origins as an agricultural society. Government leaders believe that the country should honor those roots and be a nation of self-sufficient producers. They have a strong manufacturing economy, they're entering into the fields of technology, space exploration and finance and the country has become a strong force in world politics. So, what about the food?
Well, part of the problem is not about the amount of food, but rather the efficiency of how it's produced. Farm subsidies are given to farmers for grains that could easily be imported from other countries. While some world markets are volatile and China is hesitant to be involved in unpredictable futures, the money spent on the government subsidies is actually more than what it would cost to buy those same grains from other parts of the world.
Add to this the fact that what little land China does have to grow food is not suited to grain production so harsh chemical fertilizers are needed. China's environmental record is problematic at best with air pollution and the pesticides and other substances used in farming run off into underground aquifers, contaminating the drinking water supply.
Corruption is often a problem as well, when government farmers subsidized for one kind of high quality crop, grow crops of lesser quality. The government is paying the going rate for the higher priced foods, but then the produce stored in government repositories is actually of poor quality. Government subsidies are inherently difficult to monitor in countries all over the world, but while the global trend is toward reducing subsidies, figures quoted in The Economist from the Organisation for Cooperation and Economic Development demonstrate that subsidies for Chinese farmers doubled in the five years between 2007 and 2012, topping out at about 165 billion dollars total.
It's an ever-enlarging equation and the numbers will only continue to rise. At some point the imbalance will force the agricultural system to correct itself, perhaps growing different plants or finally importing healthier and cheaper food. Inefficiency is one crop China can no longer afford to cultivate.
Sources: YouTube, The Economist