In the 1920s demand for American farm products feel, as European countries began to recover from World War I and instituted austerity (紧缩)programs to reduce their imports. The result was a sharp drop in farm prices. This period was more disastrous for farmers than earlier times had been, because farmers were no longer self-sufficient. They were paying for machinery, see, and fertilizer, and they were also buying consumer goods. The prices of. the items farmers bought remained constant, while prices they received for their products fell. These developments were made worse by the Great Depression which began in 1929 and extended throughout the 1930s. In 1929, under President Herbert Hoover, the Federal Farm Board was organized. It established the principle of direct interference with supply and demand, and it represented the first national commitment to provide greater economic stability for farmers. President Hoover' s successor attached even more importance to this problem. One of the first measures proposed by President Franklin D. Roosevelt when the took office in 1933 was the Agricultural Adjustment Act, which was subsequently passed by Congress. This law gave the Secretary of Agriculture the power to reduce production through voluntary agreements with famers who were paid to take their land out of use. A deliberate scarcity of farm products was planned in an effort to raise prices. This law was declared unconstitutional by the Supreme Court on the grounds that general taxes were being collected to pay one special group of people. However, new laws were passed immediately that achieved the same result of resting soil and providing flood-control measures, but which were based on the principle of soil conservation. The Roosevelt Administration believed Administration believed that rebuilding the nation' s soil was in the national interest and was not simply a plan to help farmers at the expense of other citizens. Later the government guaranteed loans to farmers so that they could buy farm machinery, hybrid (杂交)grain, and fertilizers. What brought about the decline in the demand for American farm products?