Campbell Biology · Chapter 37

Soil and Plant Nutrition

pp. 785–800 · 3 sections

Plants build almost their whole bodies from air and water, but they also need a short list of mineral elements that only soil can supply. This chapter treats soil as a living system, explains which elements plants need and how a shortage shows, and introduces the bacteria and fungi that help roots get nitrogen and phosphorus. Soil science and plant nutrition aren't tested in the current AP course, but the nitrogen cycle, mutualism and water potential all are, and this chapter is packed with examples of them.

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Chapter quiz (35 questions)Flashcards (39 terms)

37.1 Soil: texture, chemistry and care

pp. 785–789

On the AP exam? Background

Soil science isn't in the current course. What carries over is water potential (Topic 2.7), nutrient cycling by decomposers (Topic 8.2) and human impacts such as eutrophication from fertilizer runoff (Topic 8.7).

In the course: Topic 2.7 Tonicity and Osmoregulation, Topic 8.2 Energy Flow Through Ecosystems, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Soil starts as rock broken down by freezing water, weak acids and living things such as roots. Mixed with organisms and humus (decaying organic matter), it forms layers called horizons. The top layer, the topsoil, is where most roots feed.
Texture depends on particle size: sand grains are largest, silt is in between and clay particles are microscopic. Wide gaps between sand grains drain fast and let air in; tiny clay particles hold water and ions, sometimes too much. Loam, a mix of all three, usually grows plants best, with its pores roughly half water and half air.
Most soil particles carry negative charges. Positive ions (cations) such as K⁺, Ca²⁺ and Mg²⁺ cling to them and resist washing away, but negative ions such as nitrate (NO₃⁻) and sulfate (SO₄²⁻) don't, so rain leaches them out. Phosphate is an exception: it binds tightly to certain soil minerals, so it rarely leaches but is often hard for roots to get.
Roots free stuck cations by cation exchange. They release CO₂, which forms carbonic acid in soil water, and pump out H⁺. The H⁺ ions take the cations' places on the particles, and the freed cations enter the soil solution, where roots can absorb them.
Humus keeps soil crumbly, holds water and cations, and releases nutrients slowly as decomposers break it down. Topsoil teems with bacteria, fungi, protists, worms, insects and roots, whose activity mixes, aerates and enriches it.
Farming can wear soil out. Irrigation can drain aquifers and leave salt behind, which drags down the soil's water potential so roots struggle to take up water. Harvests carry minerals away, so farmers add fertilizer, but excess fertilizer washing into lakes can cause eutrophication.
Good soil care includes matching soil pH to the crop (below about pH 5, toxic aluminum ions dissolve; lime raises pH), slowing erosion with windbreaks, terraces, contour rows and no-till planting, and phytoremediation, which uses plants to pull pollutants such as heavy metals into shoots that can be harvested.

Key terms (14)

topsoil: The uppermost soil layer, a mix of rock bits, living things and decaying matter. It's where most roots feed.
soil horizon: One of the distinct layers you see if you dig a deep pit through soil, from rich topsoil down to barely changed rock.
humus: Dark, partly decayed organic matter in soil. It holds water and cations, keeps soil crumbly and feeds nutrients back slowly as it breaks down.
soil texture: How coarse or fine a soil is, set by its mix of sand (largest grains), silt (middle) and clay (tiniest particles).
loam: A soil with a fairly even mix of sand, silt and clay. It holds enough water and minerals but still lets air reach the roots.
soil solution: The water in the gaps between soil particles plus everything dissolved in it. Roots take up minerals from this water, not straight off the particles.
cation exchange: Swapping of positive ions on soil particles: H⁺ from roots takes the place of ions like K⁺ or Mg²⁺, which then float free for roots to absorb.
leaching: Loss of dissolved nutrients as rain or irrigation water soaks down through the soil and carries them out of reach of roots.
aquifer: A layer of rock or sediment underground that holds water. Most irrigation water is pumped from aquifers.
salinization: Buildup of salt in soil, often because irrigation water evaporates and leaves its salts behind. It lowers the soil's water potential.
fertilization: Adding mineral nutrients to soil to replace what harvests remove. Fertilizer labels list their nitrogen, phosphorus and potassium content as three numbers.
no-till agriculture: Planting seeds in narrow slots without plowing the whole field, so roots and crop leftovers keep holding the soil in place.
phytoremediation: Using plants that pull pollutants such as heavy metals out of soil and store them in parts you can harvest and dispose of safely.
sustainable agriculture: Farming that keeps land productive for the long run while protecting soil and water and still paying the farmer.

Check yourself: 37.1 Soil: texture, chemistry and care

4 questions on 37.1 Soil: texture, chemistry and care. Pick an answer to see if you got it, and why.

Question 1 of 4

Most soils have negatively charged particles, but in some deeply weathered tropical soils at low pH, many particle surfaces carry positive charges instead. A researcher adds equal amounts of potassium (K⁺) and nitrate (NO₃⁻) to a column of this tropical soil and to a column of a typical soil with negative particles, then rinses both with the same volume of water. Which result is most likely?

Question 2 of 4

Root hair cells use ATP-powered proton pumps to move H⁺ out into the surrounding soil. A researcher treats the roots of a plant growing in clay soil with a drug that blocks these pumps. What is the most likely effect on the plant's uptake of magnesium ions (Mg²⁺) held on the clay particles?

Question 3 of 4

Three garden soils were tested in the lab (invented data). Soil | Water draining through in 10 min (%) | Water held after draining (g per 100 g soil) | Total particle surface area 1 | 82 | 9 | low 2 | 45 | 24 | medium 3 | 6 | 41 | very high After a week of steady rain, seedlings in one of these soils wilt and their roots start to rot from lack of oxygen. Which soil is it most likely to be, and why?

Question 4 of 4

In a hot, dry valley, a field has been irrigated for decades with water containing a little dissolved salt. Over that time, the soil solution's water potential has dropped from −0.2 MPa to −0.9 MPa. The crop's root cells have a water potential of about −0.6 MPa. What will happen now?

0 of 4 answered

37.2 Essential elements and deficiency signs

pp. 789–792

On the AP exam? Background

Topic 1.2 tests which elements build which molecules, and Topic 3.4 that a plant's carbon comes from CO₂. You won't need the list of 17 essential elements, the macro- and micronutrient split or deficiency symptoms.

In the course: Topic 1.2 Elements of Life, Topic 3.4 Photosynthesis, Topic 6.8 Biotechnology (notes, videos and more questions)

Key points

A fresh plant is mostly water, often well over three quarters of its mass. Nearly all of the dry mass left after drying is organic matter built by photosynthesis. Its carbon, and most of its oxygen, came from CO₂ in the air, and most of the hydrogen came from water. Soil minerals are only a few percent of dry mass.
An element is essential only if a plant can't complete its life cycle, seed to seed, without it. Scientists find essential elements with hydroponic culture, growing plants in nutrient solutions and leaving out one element at a time.
All plants need 17 essential elements. The nine macronutrients, needed in larger amounts, are carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus (the main ingredients of organic molecules) plus potassium, calcium and magnesium. The eight micronutrients are iron, zinc, copper, manganese, boron, molybdenum, nickel and chlorine.
Micronutrients are needed only in traces mainly because they work as cofactors, helping enzymes run the same reaction over and over. Iron, for example, is part of electron carriers in chloroplasts and mitochondria. A shortage of even one micronutrient can still cripple or kill a plant.
Nitrogen limits plant growth more than any other mineral, because proteins, nucleic acids and chlorophyll all contain it. Shortages of nitrogen, phosphorus and potassium are the most common.
Where symptoms show depends on mobility. A mobile nutrient, such as nitrogen or magnesium, is moved in the phloem from old leaves to growing ones, so old leaves show the shortage first. An immobile one, such as iron, stays put, so the youngest leaves suffer first. Yellowing from too little chlorophyll is called chlorosis.
Besides changing the soil, scientists can change the crop: bred or engineered varieties can tolerate problem conditions such as acidic, aluminum-rich soil or long flooding.

Key terms (10)

essential element: An element a plant can't do without: if it's missing, the plant can't complete its life cycle and make healthy offspring.
hydroponic culture: Growing plants with their roots in a nutrient solution instead of soil, so you control exactly which elements they get.
macronutrient: An essential element plants need in fairly large amounts: carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, potassium, calcium and magnesium.
micronutrient: An essential element plants need only in tiny amounts, such as iron, zinc or molybdenum. Most act as enzyme helpers.
cofactor: A non-protein helper, often a metal ion, that an enzyme needs to work. Because it's reused, only a little is needed.
chlorosis: Yellowing of leaves because they've lost or can't make enough chlorophyll. Shortages of nitrogen, magnesium or iron can all cause it.
mobile nutrient: A nutrient the plant can move through the phloem from old leaves to growing ones. When it runs short, old leaves show symptoms first.
immobile nutrient: A nutrient that stays where it was first used, like iron. When it runs short, the youngest leaves show symptoms first.
dry mass: What's left of a plant after all its water is driven off. Almost all of it is organic matter made by photosynthesis.
deficiency symptom: A visible sign, like yellow leaves, purple edges or stunted growth, that a plant is short of a particular nutrient.

Check yourself: 37.2 Essential elements and deficiency signs

4 questions on 37.2 Essential elements and deficiency signs. Pick an answer to see if you got it, and why.

Question 1 of 4

A growing oak adds new cellulose to its cell walls every day. Cellulose is a polymer of glucose (C₆H₁₂O₆) made from sugars produced in photosynthesis. Which atoms in this new cellulose came mostly from water the tree absorbed through its roots?

Question 2 of 4

Researchers grow bean plants in nutrient solutions, each missing one element. Plants lacking element Q grow as tall and green as the controls and flower on time, but their seeds are shriveled and none sprout. Element R builds up to high levels in the leaves of the control plants, yet plants grown without it look just like the controls and make plenty of seeds that sprout and grow normally. Which conclusion is best supported?

Question 3 of 4

Tomato plants were grown in a complete nutrient solution, then moved to a solution lacking either element E or element F. The table shows how much of the missing element was in old and young leaves 3 weeks after the move, as a percentage of the level at the move (invented data). Element removed | Old leaves (%) | Young leaves (%) E | 35 | 90 F | 98 | 30 Which conclusion is best supported?

Question 4 of 4

A shortage of which element would most directly slow a root cell's production of new DNA and new membrane phospholipids while having little direct effect on the amino acids it can build?

0 of 4 answered

37.3 Partners in plant nutrition

pp. 792–798

On the AP exam? Yes

Topic 8.2 tests the nitrogen cycle (fixation, ammonification, nitrification, denitrification and assimilation), and Topic 8.5 tests mutualism, commensalism and parasitism. Details like leghemoglobin or the two kinds of mycorrhizae are extra.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 8.5 Community Ecology, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

The rhizosphere, the thin zone of soil hugging each root, is crowded with microbes because roots leak sugars, organic acids and amino acids. Some of these rhizobacteria help the plant by making growth signals, fighting pathogens or freeing nutrients.
Air is about 78% N₂, but plants can't use it, because its triple bond is very hard to break. Plants take up nitrogen as ammonium (NH₄⁺) or, more often, nitrate (NO₃⁻). In natural soils almost all of it comes from bacteria (a little from lightning), not from weathered rock.
The soil nitrogen cycle: nitrogen-fixing bacteria turn N₂ into ammonia (NH₃); decomposers release ammonia from dead matter (ammonification); ammonia picks up H⁺ to become NH₄⁺; nitrifying bacteria oxidize ammonium to nitrite and then nitrate; and denitrifying bacteria in oxygen-poor soil turn nitrate back into N₂. Plants assimilate the nitrogen they absorb into amino acids and nucleotides.
Fixation is run by the enzyme nitrogenase and is costly, about 16 ATP per N₂, or 8 per NH₃. In legume root nodules, Rhizobium bacteria fix nitrogen for the plant and get sugar in return, a mutualism. Nitrogenase is ruined by oxygen, so nodules use leghemoglobin to keep free O₂ low while still supplying the bacteria's respiration.
A legume and its Rhizobium strain recognize each other through an exchange of chemical signals that switches on nodule-building genes. Farmers rotate legumes with other crops, or plow legumes back into the soil, to add nitrogen.
Most plants form mycorrhizae, mutualisms with fungi. Fungal threads add huge surface area for taking up water and minerals, especially phosphate, and the plant pays in sugar. Ectomycorrhizal fungi wrap the root in a sheath; the far more common arbuscular fungi push branched structures through root cell walls without breaking the membranes. The partnership is ancient and probably helped plants colonize land.
A few plants feed in unusual ways. Epiphytes perch on other plants but gather their own water and minerals; parasitic plants tap a living host's tissues; and carnivorous plants still photosynthesize but trap animals for nitrogen and other minerals missing from poor, boggy soils.

Key terms (15)

rhizosphere: The thin layer of soil right around a root. Root secretions make it far busier with microbes than the soil a few millimeters away.
nitrogen fixation: Turning N₂ gas from the air into ammonia (NH₃). Only some bacteria and archaea can do it, using an oxygen-sensitive, ATP-hungry enzyme called nitrogenase.
ammonification: Release of ammonia by decomposers as they break down proteins and other nitrogen-containing matter in dead things and wastes.
nitrification: A two-step process in which soil bacteria oxidize ammonium to nitrite and then to nitrate, the form most plants take up.
denitrification: The conversion of nitrate back to N₂ gas by bacteria in oxygen-poor soil. It returns nitrogen to the air.
assimilation (of nitrogen): Building absorbed nitrogen into the plant's own molecules, such as amino acids and nucleotides.
root nodule: A swelling on a legume's root where plant cells house nitrogen-fixing Rhizobium bacteria.
leghemoglobin: An iron-containing protein in legume nodules that grabs oxygen, keeping free O₂ low enough for nitrogenase while still supplying the bacteria's respiration.
crop rotation: Growing different crops on a field in different years, such as a legume one year and a grain the next, to restore soil nitrogen and break pest cycles.
mycorrhiza: A mutualism between a fungus and a plant's roots. The fungus brings in water and minerals, especially phosphate, and the plant gives it sugar.
ectomycorrhiza: A type of mycorrhiza in which the fungus forms a thick sheath around the root and grows between, not into, the root's cells. Common in trees like pines and oaks.
arbuscular mycorrhiza: The most common mycorrhiza. The fungus pushes branched structures through root cell walls without breaking the cell membrane, giving a big area for swapping nutrients.
epiphyte: A plant that grows perched on another plant for support but collects its own water and minerals, mostly from rain and dust.
parasitic plant: A plant that taps into a living host's tissues to steal water, minerals and sometimes sugars.
carnivorous plant: A photosynthetic plant that traps and digests small animals to get nitrogen and other minerals missing from poor soils.

Check yourself: 37.3 Partners in plant nutrition

4 questions on 37.3 Partners in plant nutrition. Pick an answer to see if you got it, and why.

Question 1 of 4

Some nitrogen fertilizers are sold with an additive that slows the soil bacteria that turn ammonium (NH₄⁺) into nitrite. Compared with the same fertilizer without the additive, what is the most likely effect?

Question 2 of 4

Question 3 of 4

A mutant pea plant can't make leghemoglobin. Its roots still form nodules full of Rhizobium bacteria, but the nodules fix very little nitrogen. What is the most likely reason?

Question 4 of 4Calculator allowed

The overall reaction run by nitrogenase is: N₂ + 8 e⁻ + 8 H⁺ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pᵢ A clover root nodule makes 1.2 µmol of NH₃ per minute. About how much ATP does it use each minute for fixation?

0 of 4 answered