Plant Mineral Nutrition and Soil

Core Idea

Plants build most of their dry mass from carbon dioxide taken from the air, but they also need mineral elements absorbed from the soil. Plant biologists generally recognize 17 essential elements: 9 macronutrients needed in large amounts (carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) and 8 micronutrients needed in trace amounts. Soil particles hold many mineral cations, and roots free them by cation exchange. Partnerships with fungi (mycorrhizae) and nitrogen-fixing bacteria greatly extend what roots can obtain on their own.

How It's Best Learned

Start with the old willow experiment: a tree gains tens of kilograms while the soil loses almost nothing, so where did the mass come from? Then use hydroponic results to show how scientists decide which elements a plant cannot do without. Finish with deficiency photos and ask students to infer which nutrient is missing from where the symptoms appear.

Common Misconceptions

• Plants get most of their mass from the soil. (Most of a plant's dry mass comes from atmospheric carbon dioxide fixed in photosynthesis; soil supplies water and minerals that make up a small fraction.)
• Plants can use nitrogen gas from the air directly. (Plants absorb nitrogen as nitrate or ammonium; only certain prokaryotes can convert nitrogen gas into usable forms.)
• Fertilizer is plant food. (Fertilizer supplies mineral nutrients; plants make their own food by photosynthesis.)

Explainer

Ask where a tree's mass comes from and many people answer "the soil." In the 1600s Jan Baptist van Helmont tested this. He planted a small willow in a weighed pot of soil and gave it only water for five years. The tree gained a large amount of mass, and the soil lost only a tiny amount. He concluded the mass came from water. Part of his answer was right: water supplies hydrogen and some oxygen. But most of a plant's dry mass came from carbon dioxide in the air, fixed by photosynthesis.

Plants still need the soil. They take up water and mineral elements there. To find out which elements are essential, scientists use hydroponic culture: plants grow with roots in a solution of known composition. If leaving out one element stops the plant from completing its life cycle, that element is essential. By this test, plant biologists generally list 17 essential elements. Nine are macronutrients, needed in large amounts: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium and magnesium. Eight are micronutrients, needed in trace amounts: chlorine, iron, manganese, boron, zinc, copper, nickel and molybdenum. Nitrogen goes into proteins and nucleic acids, phosphorus into ATP and nucleic acids, and magnesium sits at the center of every chlorophyll molecule.

Symptoms reveal deficiencies. Yellowing of leaves is called chlorosis. If a nutrient is mobile, like nitrogen or magnesium, the plant moves it from old leaves to young ones, so old leaves show symptoms first. If a nutrient is immobile, like calcium or iron, the young leaves suffer first.

Soil chemistry controls availability. Clay particles and humus carry negative charges, so they hold positive ions such as potassium, calcium and magnesium. Roots release hydrogen ions that swap places with these cations and free them. This is cation exchange. Negatively charged ions such as nitrate are not held and wash away easily, which is why nitrogen is often the nutrient that limits plant growth.

Plants get help. Most land plants form mycorrhizae, partnerships with fungi whose hyphae extend far into the soil and deliver phosphate and other minerals in exchange for sugar. Legumes such as peas and beans house nitrogen-fixing bacteria (rhizobia) in root nodules. The bacteria convert nitrogen gas into ammonia, which the plant can use, and the plant supplies the bacteria with sugars. Farmers add fertilizers labeled with three numbers, the percentages of nitrogen, phosphorus (as P2O5) and potassium (as K2O), to replace what crops remove.