The process has an active source and sink and is passive in between. The companion cells provide ATP to load and unload sucrose. They are important because they are components of organic molecules, enzyme activity, the maintenance of cell walls and membranes, and energy production. Their most common functions are as enzyme activators and in the formation of chlorophyll. Fertilizers are required because cultivation increases erosion and the loss of nutrients, large amounts of nutrients are taken away with the crops, and soils may be naturally poor.
Natural communities recycle all available nutrients so that they are used over and over.
The three mineral nutrients most commonly found in commercial fertilizers are nitrogen, phosphorus, and potassium. Organic fertilizers may be superior because they help build up the humus content of the soil, enhancing water and nutrient-retaining properties.