SS 2221 Midterm 2

All pools and processes in the P cycle and how they connect

Pools: SOM, secondary minerals, colloids, primary minerals, and orthophosphate. Mineralization, immobilization, leaching, dissolution, precipitation, desorption, adsorption, wetathering, and runoff

What is the plant avalible form of P in soil?

H2PO4 and HPO4

How much of the total soil P is typically foind in plant avalible form?

less than 1%

What is the role of P in the plant?

Nucleotides- DNA, ATP, and NADP molecules for energy storage and transfer; phospholipids-

Membrane synthesis and stability; cofactors of some enzymes(activates/inactivates enzymes)

Photosynthesis, Glycolysis (conversion of glucose into pyruvate to produce energy), Respiration, Carbohydrate metabolism, N fixation

Name P-rich minerals

apatite, strengite, and variscite

What is the difference between P adsorbed on colloids versus P precipitatied in secondary minerals?

Adsorbed P: phosphate sticks to colloid surfaces.

Precipitated P: phosphate reacts with ions and forms a new solid mineral.

How much of total P is typically found in the soil organic matter pool?

30-65%

How can we conceptualize soil organic P?

phosphorus held in soil organic matter (living organisms, residues, and humus). It becomes plant-available only after microbes mineralize it into inorganic phosphate.

What is the typical C:P ratio of microorganisms?

between 6 and 13

What is the typical uptake efficiency of applied fertilizer P?

after first season 10-30%

How do you determine whether P fertilizer application is required or not?

Olses P test. Look at levels for crop being grown. dont want to over fertilize

What factors impact adsorption/desorption and precipitation/dissolution?

pH, and concentrations of Al, fe, and or Ca

Which conditions favor P mineralization?

Oxide soils, Hot and wet well aerated

Which condtitions favor P Immobilization?

High C soils

What is the difference between enzymatic hydrolysis and biological solubilization?

Enzymatic hydrolysis: enzymes break organic P compounds into inorganic phosphate that plants can use.

Biological solubilization: microbes release acids or other compounds that dissolve inorganic P minerals or release P from soil surfaces.

How is P avalibility affected by total soil P content?

A soil can have a high total P content but still have low plant-available P because much of it is adsorbed to minerals, precipitated, or held in organic matter.

How is P avalibility affected by Ca, Fe, and Al concentrations?

• In high-pH/calcareous soils, P reacts with Ca and can form calcium-phosphate minerals.

• In acidic soils, P is strongly adsorbed to or precipitated with Fe and Al oxides.

How is P avalibility affected by pH?

• Low pH: P is tied up by iron (Fe) and aluminum (Al).

• High pH: P is tied up by calcium (Ca).

How is P avalibility affected by contact of soil solution to soil minerals and roots?

More contact = less P stays available. As phosphate in soil solution contacts soil minerals, it can be adsorbed or precipitated; as it contacts roots, plants can take it up.

How is P avalibility affected by temperature?

Cold soil lowers P availability. Microbial activity, organic-P mineralization, root growth, and diffusion all slow down, so less P reaches roots.

How is P avalibility affected by clay type and types of colloids?

More reactive clay and oxide colloids usually reduce P availability because they adsorb phosphate strongly.

- Fe and Al oxides hold P very strongly, especially in acidic, highly weathered soils.

- 1:1 clays (such as kaolinite) can adsorb more P than less-weathered 2:1 clays.

4 ways to increase P avalibility to crops based on our understanding of the physiochemical processes governing P cycling

Adjust pH to ~6–7 to reduce P fixation by Fe/Al in acidic soils and Ca in alkaline soils.

Band/place P fertilizer near roots to reduce contact with P-fixing soil minerals.

Add organic matter or use cover crops to promote mineralization and organic acids that can release P.

Encourage mycorrhizae to extend the root’s reach and improve P uptake.

How does chickpea increase P avalibility and what are the iplicaitons for developing other P avalibility strategies?

Chickpeas release organic acids from their roots that help dissolve or release phosphorus held by calcium and soil minerals. This makes more P available near the roots.

What does an Olsen P test mean?

Olsen P is a soil test that estimates the amount of plant-available phosphorus, especially in neutral to alkaline/calcareous soils.

It uses a sodium bicarbonate solution to pull out some of the phosphate that could become available to plants. A higher Olsen P value generally means more P is available and less P fertilizer may be needed.

Why is runoff the dominant loss pathway in most ecosystems?

Runoff is usually the main P-loss pathway because phosphorus is held tightly by soil and does not leach easily. Instead, rain or irrigation can carry away dissolved P and soil particles with attached P across the surface into waterways.

Under what circumstances does leaching become and important P loss pathway?

Low natural P binding, or P saturated soils

What are the consequences of P loss for the environment?

contamination of ground and surface water, harmful to animals

What is the P index? Who developed it and how do you use it?

developed by the USDA and you use it to determine what the risk you site is at for leaching P

What are the factors in the P index used to determine the risk of P loss?

Soil erosion, irrigation, fertilizer rate and methods, distance from watersource

Explain the difference between root foraging, P mining, and increasing P use efficiency

Root foraging: Plants grow more roots/root hairs into soil areas where P is available, helping them find more phosphorus.

P mining: Plants and microbes release acids, enzymes, or other compounds that release P that is chemically or organically tied up in the soil.

Increasing P use efficiency: The plant grows more or produces the same yield using less P—for example, by recycling P internally or needing less P in its tissues.

How can the plant make 'cheap' roots?

make root hairs

How does the root angle and root hair length affect P uptake?

Root angle: Shallower, more horizontal roots explore the P-rich topsoil, where most phosphorus is located, so they can increase P uptake.

Root hair length: Longer and denser root hairs greatly increase root surface area and reach into tiny soil pores, helping the plant absorb more slowly moving phosphate.

What are cluster roots?

specialist root structures and exudation of organic anions (carboxylates) by roots

What is phosphatase?

Phosphatases are enzymes made by plant roots and microbes that break phosphorus out of organic matter, turning organic P into inorganic phosphate that plants can absorb.

What is the role of S in the plant?

it is involved in protein synthesis and enzyme activation

What do garlic, cabbage, sprouts, and onion have in common in relation to sulfur?

it forms the flavor componds in these plants

How common are S deficiencies in crops?

More common since the reduction of greenhouse gas emmisions containing sulfur and higher demand on soils due to higher crop yields. less common in western soils, some coarse texture soils that are irrigated with low S water are deficient.

What are some visual symptoms of S deficiencies in plants?

appears on new growth, looks like pale yellow or light green on leaves.

What are the dominant forms and distribution of S in the soil profile of a Mollisol, Spodosol, and Oxisol? Explain the observable patterns.

Mollisol: Mostly organic S at the surface; it decreases sharply with depth because organic matter is concentrated in topsoil.

Spodosol: Mostly organic S, with a peak in the spodic/B horizon because organic matter and Fe/Al compounds move downward and accumulate there.

Oxisol: Organic S is near the surface, but inorganic S dominates deeper down and increases with depth because sulfate is strongly held by Fe and Al oxides in the highly weathered subsoil.

How common is calcium in the Earth’s crust?

5th most abundant element in the crust. 3.63% by weight

How did clacium affect the productivity of the Cerrado region in Brazil?

it reduced the pH which reduced Al toxicity and allowed roots to grow better.

What is the role of calcium in the plant?

for cell wall stiffness, required for cell division, and plays a role in plan’t response to stress.

Why do you think calcium tissue concentrations are greater in Brassicas compared to grasses?

Ca moves more easliy in water and barssicas have higher transporation .

In soils with high calcium concentrations, what could cause blossom end rot in tomatoes?

low soil moisture. the Ca cant reach the plant

How do soil pH, CEC, and cation saturation affect calcium availability?

Ca is lower in low pH and is higher in high pH but less avalible. CEC low = less Ca. %Ca saturation Abnormally high levels, or application rates of other cations, in the presence of low to moderate soil Ca levels tends to reduce the uptake of Ca.

Explain the relationship between calcium and soil structure

calcium can cause soil to flocculate

Name 3 calcium containing agricultural inputs that are liming materials and 3 that don't have an immediate effect on soil pH

Lime: Different kinds of limestone, blast furnace slag. Ca ferlilizer: gypsum, CalCl2, Ca-chelates

List 2 potential challenges related to magnesium management.

Mg can be leached out easily, causes dispersion. When fertilized with K+ it can cause the K to get taken up rather than the Mg causing deficiency

What is the role of Magnesium in the plant?

consituent of the chlorphyll molecule, involved in protein formation(ribosomes), phosphate tranfer in ATP Reactions, activates enzymes.

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

deficiencies in human diets and accidental toxixity in plants due to high levels of micronutrients.

Point out two important differences between micronutrient cycles and many other nutrient cycles?

Macronutrients are more prone to leaching, erosion, gaseous loss, and loss thorugh plant uptake and harvest. Macronutrient cycles are more connected to OM breakdown with microbial transformations being involved. Micronutrient avalibility is more dependant on soil pH than macronutrients.

Based on your understanding of micronutrient cycling, explain why micronutrients are often unavailable to the plant?

Micronutrient avalibility is more dependant on soil pH than macronutrients. micronutrients tend to move less through the soil and need direct contact with roots for uptake to occur.

How does pH and oxidation status affect micronutrient availability?

Mo and Cu increase in avalibility as pH increases. Zn and Mn decrease as pH increases. Zn tends to be more available in flooded soils. In wet, waterlogged soils, Fe and Mn become more soluble, and the opposite for dry soils.

Explain how mycorrhizae and chelation affect micronutrient availability

Mychorrhizae help the plant access mictronutrients as they extend farther than the root zone and release compounds that help dissolve nutrients from soil particles. Chelation holds micronutrients in solution so it can be uptaken by the plant rather than becoming insoluble.

Give 3 examples of plant management strategies and 3 examples of soil management strategies to increase micronutrient availability.

Reduce tilage, maintain soil mositure, and rotate types of crops grown in a field.

How common are boron deficiencies in grape production in California?

Very common on old floodplain soil and alluvial fans in Stanislaus, Merced, San Joaquin,

Kings and Kaweah rivers, the Sierra Nevada foothills, and

the North coast. Less common on soils from marine sedimentary soils of the central coast.

Based on the study discussed in class, what is the best method of boron application to grapes. Explain why this method was most effective.

Precise application through drip or foliar application as it can be directly absorbed through the leaves through the stomata which is an easier method of absorption than through the roots.

Why would one desire to increase zinc concentrations in wheat?

Nutritional value for human consumption

Explain how organic management can increase zinc concentrations in wheat compared to conventional management

Chelation