key concept 35.4 Soil Organisms Increase Nutrient Uptake by Plant Roots

One gram of soil can contain 6,000–50,000 bacterial species and up to 200 meters of fungal hyphae (the long branching cells of fungi), although both are largely invisible to the naked eye. In Chapter 38 you will learn how plants prevent infection by harmful soil microbes. But plants actually encourage a few species of fungi and bacteria to infect their roots and even invade root cells. In this section we will describe the mutually beneficial relationships in which products are exchanged between the plants and these special soil microbes.

focus your learning

  • Mutualistic relationships between soil organisms and plant roots are facilitated by chemical signal exchange.

  • Although abundant in Earth’s atmosphere, nitrogen is not a readily available nutrient for plants.

  • Humans have used various methods, including crop rotation and use of natural and human-made fertilizers, to replenish soil nitrogen depleted by crop harvesting.