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SALLY’S LAST OBSERVATIONS OF OVERALL GROWTH
Sally was excited to record the last growth data for her plants, not because she wanted to finish the experiment, but rather because she wanted to have real numbers to show how every- thing worked out. She already knew from watching the plants for weeks that the soil treatments watered with nutrient solution had grown the best, but now she would be able to quantify it. She took pictures, made her observations, and recorded them in her laboratory notebook. Sally concluded that the plants grown in the washed sand treatment were small and unhealthy-looking compared to those in the soil treatments. The plants watered with nutrient solution were greener and healthier-looking, but also stunted and small, and made only a few flowers. The plants grown in the two soil treatments and watered with nutrient solution were healthy, green, and with flowers.
Sally was partly relieved to have finally made her last observations, but also she was a little disappointed. She had truly begun to enjoy the experimentation, observation, and informed reasoning involved in the project. Rather than kill her plants, she kept growing them, watering them with the same treatments she had used all along, and thought about what else she might learn from them.
WHAT IS THE RHIZOSPHERE?
Plant stems and leaves have fairly predictable lives: they live in air, which will vary in terms of temperature and relative humidity, but otherwise is essentially the same year round. Roots, on the other hand, live in an ever-changing world of soil, water, air space, and other organisms. This dynamic and complex environment is called the rhizosphere. It encouraged the evolution of a highly adaptable, efficient root structure that permitted plants to maintain their photosynthetic organs outside an aqueous medium.
Soil Water
Soil water is a critical component of the rhizosphere, as it is both the material in which mineral nutrients are dissolved and the source water consumed in photosynthesis and transpiration. Soil water occurs in two main forms, bound waterand free water. Bound water is the sum of all the water molecules that are adsorbed by hydrogen bondingto the surfaces of the various soil components. This water, while important in governing some of the physical and chemical properties of the various components of the rhizosphere, is not biologically available to the plant or other organisms. Free water is the liquid water that fills the interstices between the various components of the rhizosphere. Depending on the overall moisture content of the soil, a con- tinuum from no free water to completely saturated soils may exist. The amount of free water in a soil is related to the size of the soil particles; smaller particles can hold more free water than larger particles.
Air Spaces
All the spaces occupied by free water in a fully saturated soil could be occupied by air. Air spaces are critical because they allow oxygen for respiration to travel through the soil. Most aerobic organisms can tolerate only a limited time with no oxygen, and so fully saturated or flooded soils tend to encourage the growth of either facultative or obligate anaerobic organisms (organisms that can grow without oxygen, either as needed or requiring the absence of oxygen). Most plant roots have only a limited ability to survive without oxygen, so the air spaces of the rhizosphere, governed in part by soil particle size, is a critical aspect in plant growth and nutrition.
Soil Particles
The physical matter of soil is much more than dirt—it is a complex medium of many kinds of soil materials, each with
different properties. It is the subject of entire fields of ongoing research. Soil is composed of two main classes of material: inorganic soil materialsand organic soil materials.
Inorganic Soil Materials
The inorganic materials of soil are the minerals and compounds derived from the Earth’s crust. These particles are largely com- posed of various types of oxides and silicates in conjunction with other elements and minerals. Most mineral nutrients of plants, with the main exception of nitrogen, are released from soil as part of natural physical processes. Soil particles range in size from roughly 2 mm (millimeters) in diameter all the way down to 2 µm (micrometers) or smaller. (A millimeter equals 0.039 inch; a micrometer equals 0.000039 inch.) A convenient scheme for organizing this range into size classes is as follows:
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Coarse sand particles have diameters from roughly 2 mm to 200 µm•
Fine sand particles range from 200 µm to 20 µm in diameter•
Silt particles range from 20 µm to 2 µm•
Clay particles range from roughly 5 µm to 0.01 µm in diameter Note that this system has some overlap between categories. It is a necessarily artificial way of compartmentalizing the variation that exists (Figure 6.1). Each class of particles has associated physical and chemical properties that greatly affect their role in the rhizosphere.The water swelling capacityand the cation exchange capacity are related to the surface area to volume ratio of the particle, which is inversely related to particle diameter. That is, large particles have less surface area for the volume that they occupy, whereas smaller particles have a greater surface area per unit volume. Clay soils, which have the smallest particles, have a high