This chapter, is a pretty comprehensive look at soils. The level is just about exactly where I would want it for this course. The chapter is well-organized and there are VERY FEW errors of any kind here.
Many authors would include a horizon D which is the bedrock itself. In the case of Connecticut, this bedrock is usually granite. This causes, in part, our soil to run on the acid side. Some locations along the Connecticut River valley have a limestone bedrock and that causes their soil to be more nearly neutral in pH and gives the soil a much better buffering capacity against acid rain. In those locations are the remnants of farming in Connecticut.
You'll want to read carefully the section on cation exchange. I consider this to the the "function" in soils, so in a physiology course this is an important aspect.
The various kinds of water in soil are also physiologically significant. If you start with mud, allow it to drain. The water that drips out due to gravity is called gravitational water. This water is only marginally useful to plants as it is around for only a short time. In fact this water is dangerous as it leaches nutrients and erodes soil particles. After everything has drained, the soil is left at field capacity; it has all the water it can hold without dripping any more. Some water remains held in the small spaces between soil particles; it is called capillary water. This water is useful to plants; it is in this capillary water that cation exchange occurs and released nutrients are taken from the capillary water and brought into roots for transport in xylem. As long as there is capillary water in soil, the soil is called friable. If all the capillary water is removed, then the soil has reached the wilting point. Cells in plants having no access to capillary water will lose their turgor pressure and wilt (droop). There is still water in the soil, it just isn't available to plants. The remaining water is called hygroscopic water; it is held tightly to particle surfaces and cannot be taken in by plant roots.
The experiment by van Helmont (page 469) is somewhat misleading. It is doubtful that van Helmont was able to accurately separate soil from roots and so the 60 grams of soil he could not recover could be minerals removed by the plant. They could also be the accumulation of very small particles coating the roots of his willow. Your book author carefully says "less than 60 grams" and this is important. Also, it is doubtful that his balance for 90 kg of soil was accurate to 60 grams missing. Randy Moore is properly cautious here!
The words macronutrient and micronutrient are used here. Please note that nutrient is a difficult term. Usually we use nutrient to mean a foodstuff...an energy producing fuel for an organism. The nutrients described here are NOT NOT NOT foodstuffs, but are minerals used in cells as cofactors for enzymes and as elements needed in biochemistry (N and S for amino acids, etc.). Please be aware that mass media calls fertilizer plant food and this is WRONG WRONG WRONG!
The chart on pages 472-3 and the figure on page 474 are critical for your mineral nutrition lab exercise. You will probably want to bring your textbook to lab on the days that we are in Shafer 215 so you can analyze the results against what is written and shown here.
Absorption of minerals by selective uptake through the endodermis especially as shown in Figure 20-14 is likely the most important function of a root. Most of this is by active transport allowing movement of minerals against the concentration gradient. However, it should be (but is seldom) pointed out that the main method of concentrating minerals in a plant is carried out in the leaves. By evaporating water from the transpiration stream supply, the minerals arriving at the leaf are concentrated greatly. Your Eosin Y project in lab should have given you strong evidence in support of that.
Chelators are described on page 475. In your mineral nutrition project, we used Sequestrene. It is an analog to Versene mentioned here in your text. This compound prevents iron from precipitating out of the solutions; thus it is available to roots.
At the top of page 476 your book talks about minerals used primarily in leaves. Omitted from the list is probably the most obvious one: Magnesium! Now why would I say that? Hint: Figure 7.5, Page 138.
I'm sure you are wondering why Molybdenum is a critical element. Your book mentions the importance of nitrate reductase in Nitrogen metabolism (page 476). If you check out table 20.2 on page 273, you will see that molybdenum is a cofactor for nitrate reductase. Is it any surprise then that one of the symptoms for molybdenum deficiency is chlorosis?
On page 479, your book talks about nitrogen fixation by bacteria in root nodules. On page 480, the process is shown in Figure 20.18. This process is greatly inhibited by the presence of oxygen. The nodules (Figure 20.19A) house the bacteria in the roots and provide a microaerobic compartment for them. The oxygen is reduced, in part, by the presence of leg-hemoglobin that scavenges the oxygen. Note the pink color of the nodules.
On page 482 the plant Genlisia is mentioned...oh how I wish there were more on this one. I've never seen it (no photo here either), and I don't know how the traps work (basic description is vague). Maybe in the next edition there will be something more.
The description of mistletoe and witchweed are intriguing. I need to get some witchweed going in our greenhouse. I find the non-functional RuBisCo particularly interesting...why be green if you can't do photosynthesis? I wish the book had put a reference here or in the literature cited at the end of the chapter. I would have liked to have you read the experimental design here! The boxed essay referred to 15.1 is excellent but also lacks a literature reference.
The story of Eperua purpurea is well done...you get a flavor for Sanford's experiment here. This and other examples were why I chose this textbook over others on the market. I wish more of the stories in this book had a similar level of science exposed. I also wish the reference were here for us to check the primary source.
Go back to the Course Schedule.