Because of their root systems some plants affect the ecology of an entire community

As we saw in chapter 2, ecological relationships between organisms in a community are often complex—as in the food web of a community. An example of a complex ecological relationship involves the tamarisk, or saltcedar (Tamarix species), which thrives along the stream banks of many western states. Originally introduced to America from its native habitats in Asia and the Mediterranean, the tamarisk was planted as an ornamental tree and as a way to stabilize

  • t "r- ■■ - ..^mta
  • t "r- ■■ - ..^mta

FIGURE 7.25

Anabaena and rice production. (a) Cultivation of rice is aided by nitrogen fixation by the cyanobacterium Anabaena azollae. Anabaena lives and fixes nitrogen in cavities of this water fern, Azolla. (b) Azolla is grown in rice paddies of the warmer parts of Asia. Later, the paddies are drained, and the Azolla is plowed into the soil, thereby enriching the soil with nitrogen for the rice crop. Nitrogen fertilizers do not have to be used in such paddies.

FIGURE 7.25

Anabaena and rice production. (a) Cultivation of rice is aided by nitrogen fixation by the cyanobacterium Anabaena azollae. Anabaena lives and fixes nitrogen in cavities of this water fern, Azolla. (b) Azolla is grown in rice paddies of the warmer parts of Asia. Later, the paddies are drained, and the Azolla is plowed into the soil, thereby enriching the soil with nitrogen for the rice crop. Nitrogen fertilizers do not have to be used in such paddies.

Putting Things Back

Plants extract large amounts of nutrients from soil. For example, during one growing season, a wheat crop on 1 hectare of land removes 85 kilograms of nitrogen, 47 kilograms of potassium, and 17 kilograms of phosphorus from the soil. Some of these nutrients are replenished by decaying humus and by plowing under the remaining parts of the crop. However, such replenishment does not match what is lost when the crop is harvested if crops are grown repeatedly on the site. Consequently, these lost nutrients must be replenished with fertilizers. For example, the yield of an unfertilized soil that initially produced 100 bushels of corn per acre diminished to only 23 bushels per acre in 70 years. When this soil was fertilized, the yield increased to more than 130 bushels per acre. Although fertilization increases plant growth and crop yield, it rapidly reaches a point of diminishing returns: doubling the yield of already fertile soil often requires adding as much as five times more fertilizer.

Chemical Fertilizers

Most chemical fertilizers have a rating that consists of three numbers, such as 12-6-6. These numbers refer to the amounts of nitrogen, phosphorus, and potassium, which are the three elements most likely to be deficient in soil. Thus, a 12-6-6 fertilizer contains 12% nitrogen (usually as ammonium salts), 6% phosphorus (as phosphoric acid), and 6% potassium (as potash).

Nitrogen, which is the most expensive of these elements to produce, is incorporated into fertilizer via the HaberBosch process:

300 atmospheres pressure

> J .u ^

The application of fertilizer increases plant growth by increasing the availability of nutrients.

2 NH3

The application of fertilizer increases plant growth by increasing the availability of nutrients.

Nitrogen can either be added directly to the fertilizer as an ammonium salt or be converted to nitrate and then added as a nitrate salt (e.g., NaNO3).

More than 40 million metric tons of nitrogen produced by the Haber-Bosch process are added to soil each year. However, this represents only about one-fifth the amount of nitrogen added to the world's soil by nitrogen-fixing bacteria (nitrogen is also added by thunderstorms and atmospheric deposition). Furthermore, the Haber-Bosch process is expensive in terms of energy; producing 2.5 kilograms of ammonia via the Haber-Bosch process requires the energy equivalent of 1,000 kilograms of coal. The costs of producing nitrogen account for about half of our $16-billion fertilizer bill. Consequently, manufacturing nitrogen-containing fertilizer requires more energy than any other aspect of crop production in the United States. To compound this problem, applications of nitrogen-containing fertilizers are inefficient, because crops absorb only about half of the nitrogen that is applied. The rest is absorbed by other organisms, leached from the soil in rainfall, or reconverted to gaseous nitrogen (N2) by denitrifying bacteria such as Micrococcus denitrificans.

Chemical fertilizers are concentrated, easy to apply, and allow a grower to apply specific amounts of various nutrients. However, these fertilizers do not replenish humus in the soil. To maintain humus, growers usually plow under either the unharvested plants or a subsequent cover crop of barley or rye. The latter process is called green manuring and provides an excellent example of another kind of fertilizer: organic fertilizer.

Organic Fertilizers

Organic fertilizers are essentially the same thing as humus. Although hardly new (planting a fish with corn seed is proverbial), the increased costs of chemical fertilizers have prompted a growing number of gardeners and farmers to rediscover organic fertilizers, which increase both the water retention and fertility of soil. Organic fertilizers include manure, dead animals and plants, fish scraps, and cottonseed meal. On a smaller scale, backyard gardeners often use compost, fish meal, lawn clippings, garbage, and a concoction called manure tea as organic fertilizers. We do not recommend fertilizing your houseplants with manure tea if guests are coming.

Foliar Fertilization

Despite the presence of a thick cuticle, many plants can absorb nutrients through their leaves and stems. For example, iron is sprayed on azaleas and pineapples, and copper and zinc are sprayed on citrus to prevent mineral deficiencies. This type of fertilization is called foliar fertilization and is restricted primarily to micronutrients (i.e., nutrients of which plants need small amounts).

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment