Plant growth whether in agricultural, grassland, forest, or natural ecosystems is quantitatively limited by the amount of water and nitrogen that can be recovered from the environment. Water is needed to allow plants to open stomata in the leaf surfaces so that carbon dioxide can diffuse into leaves for photosynthesis. Since the pathway for carbon dioxide to diffuse into leaves is the same as water vapor diffusion from the leaves, every increment of plant growth requires a well defined amount of water uptake by plants from the soil. There is no genetic solution to the physical laws of gas diffusion. In addition, a major factor in the amount of soil water available to plants is the rate of water evaporation from the soil surface. Removal of plant residues for biofuel necessarily results in an increase in the total water requirement for subsequent plant production. Equally important is the nitrogen resource. Nitrogen is required by every living cell as an essential component in nucleic acids and proteins. Virtually all growth processes, including photosynthesis, are dependent on the amount of nitrogen accumulated by the plants. Due to the large amounts of nitrogen needed by plants, productivity is commonly limited by the amount of nitrogen available to and accumulated by plants. Consequently, both water and nitrogen place unavoidable quantitative limits on plant productivity. Without renewed water and nitrogen resources, plant productivity will decrease, and eventually fail, no matter the plant species or their genetic make up.