This is more or less completely wrong.
Firstly, many crop growers, indeed, do not rest the soil every seven years - for organic farmers, for example, it's more like every three or four years, by using a crop rotation system, where fields are planted with different crops, and one is left to lie fallow, and which field is which rotated around every year. Other methods used are cover crops, which are crops grown specifically to improve various aspects of the soil (such as preventing soil erosion and improving soil fertility), which are then either allowed to die and mulch down naturally or plowed under into the soil, and, of course, for non-organic farmers, the use of artificial fertilizers to replace the nutrients taken out of the soil by the crops. In short, farmers don't simply shut down their farms for a year every seven years because farming knowledge, methods and technology have advanced in the past couple of millenia.
Secondly, there is no such thing as 'vitamin B17'. Amygdalin is a substance found in the seeds of several fruits, such as apples, peaches and plums. It was a supposed treatment for cancer in Russia in the 1840s, and in the US in the 1920s, but was found to be incredibly toxic and banned. In the 1950s, a supposedly non-toxic, synthetic form of it, called laetrile, was developed as a meat preservative, but then used as a cancer treatment. However, the FDA banned the interstate shipment of it, despite it being rebranded as a vitamin. This is where the supposed 'vitamin B17' comes from. There is no solid evidence amygdalin or laetrile actually works as a cancer treatment, and the possible side-effects are, basically, cyanide poisoning.
