35

key concepts

35.1

Plants Require Nutrients

35.2

Plants Acquire Nutrients from the Soil

35.3

Soil Structure Affects Plant Nutrition

35.4

Soil Organisms Increase Nutrient Uptake by Plant Roots

35.5

Carnivorous and Parasitic Plants Obtain Nutrients in Unique Ways

Improving Plant Nutrition to Feed the World

Crops such as rice, wheat, and corn supply more than half of the human diet. Plants require good nutrition in order to grow and one of the nutrients often in short supply in soils is nitrogen. Organic farmers traditionally supply nitrogen by spreading animal waste (manure) over the field; as nature takes its course, the complex macromolecules in manure are broken down to ammonium (NH₄⁺).

In 1917, Fritz Haber and Carl Bosch invented a way to make ammonia in the factory, revolutionizing crop plant nutrition. The Haber–Bosch process has been called one of the most important scientific feats of the twentieth century. Chemical fertilizer in the form of ammonium salts led to spectacular increases in food production. But nitrogen fertilizer is expensive in two ways. First, the Haber–Bosch process of manufacturing ammonium from hydrogen and nitrogen gases is very energy intensive and costly.

Second, nitrogen fertilizer is also environmentally expensive. When it rains excessively, nitrogen fertilizer can be lost from farm fields and end up in lakes, rivers, or groundwater. When nitrogen-laden rivers enter the sea, excessive growth of marine algae is likely to result. Eventually the algae die, and the organisms that decompose the algae use up so much oxygen in the water that there is not enough left to support marine life. Nitrogen fertilizer runoff has resulted in vast “dead zones” in waters near the mouths of major rivers, including the Mississippi River Delta in the Gulf of Mexico. An additional environmental cost is the conversion of some nitrogen fertilizer to nitrous oxide gas (N₂O), which contributes to global warming.

Scientists are working on several strategies by which nitrogen fertilizer might be used more efficiently. One strategy is to improve farming practices, by applying optimal rates of nitrogen to crops while reducing losses to the environment. The other strategy is to alter the genetics of crop plants to improve their uptake and assimilation of nitrogen. Many processes are involved in a plant’s use of nitrogen from the soil, such as uptake into the roots, transport to other organs in the vascular system, and incorporation of nitrogen into organic molecules such as amino acids and nucleotides. Each process involves many genes, underscoring the complexity of inheritance.

How can nitrogen use efficiency be improved?