The researchers wanted to find out how soil density affects soybean growth in Brazilian Amazon soils. They tested different soil densities and measured root growth. They found that when soil density is too high, soybean roots can't grow well. The critical soil density for good root growth was around 1.5 to 1.8 Mg m -3, depending on the soil type. This means that if the soil is too dense, soybean plants won't grow as well.