Above ground and below ground biomass are strong indicators of crop health and yield potential.
With all things being equal, a crop with poorly developed roots that brandish small, spindly slightly coloured leaves will more often than not yield much less than the same crop that has a long, highly developed root system and large, dark green foliage.
In the plot shot, we're quantifying the biomass by weight. We will correlate that with any vigor observations, canopy fullness data and yield effects for the treatment. We cut the crop at the above ground, below ground point and get the weights of each.
No matter how good a crop looks or how healthy the plant might be, external environmental factors such as drought, pest pressure, or acts of God can limit the crop's potential. The limiting factors of crop potential are often referred to in the context of Liebig's Law of the Minimum (a.k.a Liebig's Law, or the Law of the Minimum). The scarcest resource available dictates growth, not the total resources available.
In ecosystems, this resource is often times water, sunlight or a mineral nutrient in the soil.