Understanding how nutrients and pH interact is key to understanding soil fertility.
The soil around the roots of plants provides the water and minerals needed to survive and grow. Within that soil, the pH and the availability of macronutrients--six essential minerals that include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur--interact and affect each other. For example, some macronutrients change the pH, and very high or low pH can affect the availability of different nutrients. The complexities of soil fertility can be applied to numerous science projects with very practical applications in agriculture and gardening.
Changing Soil pH
Different plants grow in different soil pH conditions, and by applying different types of fertilizers, gardeners often try to change the pH of their soil in order to grow a particular type of plant. However, as the University of Florida IFAS Extension points out, changing soil pH is difficult and uncertain at best. To learn more about how fertilizers affect soil pH, students should apply different types of fertilizers to different soil samples. Have students choose which fertilizers they want to test, but be sure they try fertilizers that contain sulfur and calcium, both of which can exert a noticeable effect on soil pH, according to the University of Florida IFAS Extension.
Diagnosing Plant Problems
Extremely high or low pH can cause nutrients to become deficient or become so available that they are toxic to the plant. Begin by having students grow plants under different soil pH conditions, including some plants grown at the neutral or preferred pH range. As the plants grow, have the students observe their characteristics: How quickly they grow, the development of flowers and foliage, the color of the foliage, and the plants' vigor when compared to control-group counterparts.
If plants show poor growth, the students should investigate possible nutrient deficiency symptoms and use research to diagnose the problem. Coupled with data on the pH of the growing medium, conclude the activity by having students form hypotheses about which nutrients become deficient under high and low pH.
Organic Solutions
Many mistakenly think the most drastic changes in soil chemistry and fertility come from applying synthetic fertilizers. However, as the Clemson Cooperative Extension points out, organic matter can also make soil more acidic. Other organic amendments, such as crushed limestone and wood ash, raise soil pH.
Have students apply organic fertilizers and amendments to soil samples, observing how pH and nutrient availability change. If you're running the same experiments with synthetic fertilizers, have students compare the results and apply what they've learned to understand the advantages and disadvantages of each type of fertilizer.
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