Flashcards Soil Fertility: Phosphorus, Potassium, Sulfur, and Calcium

What are the two main plant-available forms of phosphorus?

H₂PO₄⁻ and HPO₄²⁻.

What percentage of plant dry weight is phosphorus?

About 0.2%.

What major plant molecules contain phosphorus?

DNA, nucleotides, ATP, and NADP.

What soil pH maximizes phosphorus availability?

pH 6-7.

Why is phosphorus unavailable in acidic soils?

It becomes fixed by Fe and Al compounds.

Why is phosphorus unavailable in alkaline soils?

It precipitates with calcium.

What is mineralization?

Microbial conversion of organic P into plant-available inorganic P.

What is immobilization?

Microbes take up available phosphorus and temporarily store it in biomass.

What is adsorption?

Phosphorus binding to soil colloid surfaces.

What is desorption?

Release of phosphorus from soil particle surfaces.

What is precipitation?

Formation of insoluble phosphorus minerals.

What is dissolution?

Release of phosphorus from minerals into solution.

What percentage of applied fertilizer P is typically taken up during the first season?

10-30%.

Why is fertilizer phosphorus often inefficient?

Much becomes adsorbed to colloids or precipitated into secondary minerals.

How do roots increase phosphorus availability?

Through organic acid exudates and phosphatase enzymes.

How do mycorrhizae help phosphorus uptake?

They expand soil exploration and increase P acquisition.

What is legacy phosphorus?

Phosphorus accumulated in soil over time from past applications.

Why can legacy phosphorus create environmental problems?

It contributes to runoff and eutrophication.

Why can soil tests underestimate phosphorus risk?

They may not capture large legacy phosphorus pools.

What enzyme is commonly involved in phosphorus mineralization?

Phosphatase.

What is biological solubilization of phosphorus?

Organic acids released by plants or microbes dissolve phosphorus compounds.

What do chelating agents do?

Bind Ca, Fe, or Al and free phosphorus for plant uptake.

Why are soil aggregates beneficial for phosphorus cycling?

They help retain and cycle phosphorus within the soil.

Which two nutrients are the main causes of eutrophication?

Nitrogen and phosphorus.

What is the purpose of the P Index?

To estimate phosphorus loss risk from a site.

What P Index score indicates very high risk?

Greater than 32.

What management practices reduce P loss?

Cover crops, grass strips, proper fertilizer placement, and lower P applications.

What is flocculation?

Soil particles clumping together into larger aggregates.

What promotes flocculation?

Calcium and gypsum.

What is dispersion?

Soil particles separating into individual suspended particles.

What promotes dispersion?

Sodium.

In what form do plants absorb potassium?

K⁺.

What is potassium's role in stomata?

Controls opening and closing.

How does potassium affect crops?

Improves quality, root growth, disease resistance, and water regulation.

Which K pool can plants directly access?

Soil solution K.

Typical concentration of K in soil solution?

1-10 ppm.

What is readily available K?

Exchangeable K held on soil particles.

What is slowly available K?

K trapped between clay layers.

Which clay fixes K most strongly?

Vermiculite.

Which minerals contain most soil potassium?

Feldspars and micas.

In which soils is K leaching most important?

Sandy soils.

What form of sulfur is plant available?

Sulfate (SO₄²⁻).

What are sulfur's major functions?

Amino acids, proteins, and enzyme activation.

Where does most sulfur come from in non-calcareous soils?

Organic matter.

Sulfur deficiency appears first on which leaves?

New leaves.

What ion form do plants absorb calcium as?

Ca²⁺.

What are major calcium functions?

Cell wall strength, cell division, and stress tolerance.

What disorder is associated with calcium deficiency in tomatoes?

Blossom-end rot.

What lettuce problem can calcium deficiency cause?

Tip burn.

How does liming affect calcium?

Increases soil pH and calcium availability.

What ion form do plants absorb magnesium as?

Mg²⁺.

What essential molecule contains magnesium?

Chlorophyll.

What role does Mg play in ATP reactions?

Regulates phosphate transfer reactions.

What is unusual about serpentine soils?

They often have low Ca:Mg ratios.

State Liebig's Law of the Minimum.

Growth is limited by the most scarce resource.

What are chelates?

Organic molecules that keep micronutrients soluble and available.

What conditions commonly cause micronutrient deficiencies?

Sandy soils, high-pH soils, intensive cropping, and macronutrient-only fertilization.

What is the most important micronutrient in these notes?

Zinc (Zn).

Which fungi commonly improve zinc uptake?

Mycorrhizae.

Why is zinc important beyond plants?

Zinc deficiency contributes to human malnutrition.

Which micronutrient is easily leached and frequently toxic at high levels?

Boron.

What is unusual about boron compared with many micronutrients?

It is neutral/uncharged.

What are the 4Rs?

Right Source, Right Rate, Right Time, Right Place.

What information should be gathered before determining fertilizer rates?

Soil tests, plant tests, and field history.

Fertilizer grades express phosphorus as what?

P₂O₅.

Fertilizer grades express potassium as what?

K₂O.

Conversion factor from P₂O₅ to elemental P?

0.44.

Conversion factor from K₂O to elemental K?

0.82.

Optimum P availability?

pH 6-7.

Fertilizer P recovery in the first season?

10-30%.

Major nutrients responsible for eutrophication?

Nitrogen and phosphorus.

Flocculation is promoted by?

Calcium and gypsum.

Dispersion is promoted by?

Sodium.

Plants absorb potassium as?

K⁺.

Sulfur deficiency appears first on?

New leaves.

Magnesium is the central atom in what molecule?

Chlorophyll.

Most important micronutrient discussed in the notes?

Zinc.

What are the 4Rs of nutrient stewardship?

Right Source, Right Rate, Right Time, Right Place.

Q: What gave rise to increased attention to micronutrient management in agriculture?

A: Increased crop yields, intensive agriculture, widespread use of high-analysis fertilizers lacking micronutrients, and recognition that micronutrient deficiencies can limit production even when macronutrients are adequate.

Q: What are two important differences between micronutrient cycles and many other nutrient cycles?

A: Micronutrients are required in very small quantities, and their availability is controlled more by chemical reactions (adsorption, precipitation, oxidation-reduction) than by total soil concentration.

Q: Why are micronutrients often unavailable to plants even when soils contain them?

A: Micronutrients may be adsorbed to soil minerals, precipitated into insoluble forms, trapped in organic matter, or present in an oxidation state that plants cannot easily absorb.

Q: How does soil pH affect micronutrient availability?

A: Most micronutrients (Fe, Mn, Zn, Cu) become less available as pH increases because they precipitate into insoluble compounds. Availability generally increases in acidic soils.

Q: How does oxidation status affect micronutrient availability?

A: Oxidation-reduction reactions change nutrient solubility. Reduced conditions often increase Fe and Mn availability, while oxidized conditions can decrease availability through precipitation.

Q: How do mycorrhizae increase micronutrient availability?

A: Mycorrhizal fungi extend the root system, explore more soil volume, and improve uptake of relatively immobile nutrients such as Zn and Cu.

Q: How does chelation increase micronutrient availability?

A: Chelating compounds bind micronutrients and keep them soluble, preventing precipitation and making them easier for plants to absorb.

Q: Give three plant management strategies that increase micronutrient availability.

A: Select efficient crop varieties, encourage mycorrhizal associations, and use foliar micronutrient applications.

Q: Give three soil management strategies that increase micronutrient availability.

A: Adjust soil pH, apply chelated micronutrient fertilizers, and increase soil organic matter.

Q: How common are boron deficiencies in California grape production?

A: Boron deficiency is one of the most common micronutrient deficiencies affecting California vineyards.

Q: What is the best method of boron application to grapes based on the study discussed in class?

A: Foliar boron application was found to be the most effective method.

Q: Why was foliar boron application more effective than soil application?

A: It bypasses soil fixation processes, rapidly corrects deficiencies, and delivers boron directly to actively growing tissues and reproductive structures.

Q: Why would producers want to increase zinc concentrations in wheat?

A: To improve the nutritional value of wheat for human consumption and help address zinc deficiencies in human diets.

Q: What is zinc biofortification?

A: The process of increasing zinc concentrations in edible crop tissues to improve human nutrition.

Q: Why is zinc considered an important human nutrient?

A: Zinc is essential for immune function, growth, enzyme activity, and overall human health.

Q: How can organic management increase zinc concentrations in wheat compared to conventional management?

A: Organic systems often increase soil organic matter, which promotes chelation of zinc and keeps it in more plant-available forms.

Q: What role does chelation play in zinc availability?

A: Chelation binds zinc in soluble complexes, reducing precipitation and improving plant uptake.

Q: Why can increased soil organic matter improve zinc nutrition?

A: Organic matter produces natural chelating compounds that help keep zinc available for plant absorption.

Q: Which micronutrients generally become less available as soil pH increases?

A: Fe, Mn, Zn, and Cu.

Q: What two biological mechanisms commonly increase micronutrient uptake?

A: Mycorrhizae and chelation.

Q: What is the primary reason micronutrient deficiencies occur?

A: Low availability, not necessarily low total soil content.

Q: Which management practice directly avoids soil fixation of micronutrients?

A: Foliar application.