The 48% drop in Soya stocks are a bit of an anomaly caused by Trump's trade war with China. When Trump started the trade war with China they retaliated with tariffs on US farm produce. China was the main export destination for US Soya beans and Soya bean oil, so China's tariffs drastically reduced China's purchases of US soya products and reduced the price paid to US farmers. This in turn caused China to aggressively buy from other sources, mainly Brazil, and used up some?????????????????? of their stocks, it also made Soya less attractive to grow for US farmers. Medium term China can not avoid buying US Soya without causing a massive spike in prices in China and or use up all their stocks, or both. So China had to jump back into the US Soya market, which it seems they did in style and bought up a lot of cheap Soya.
So in short, stocks built in '18 and '19 because of China not buying as much meaning stocks were at record levels in '19 and then when China jumped back in to the US Soya market, to rebuild stocks and stop price spikes in China, it caused stocks to drop back to more normal levels.
The trade wars have acted as an anchor on grain and proteins market prices, because the US was dumping to try keep stocks under some sort of control.