SS 2221 MT 2

What is the plant available form of P soil?

H2PO4 and HPO4

How much of the total soil P is typically found in plant available form?

Less than 1%

What is the role of P in the plant?

Component of DNA, photosynthesis, ATP and NADP for energy storage and transfers

Name at least two P-rich minerals

Apatite and Strangite

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

Adsorbed P is held on the surfaces of clay minerals and Fe/Al oxides. Precipitated P has formed solid Ca, Fe, or Al phosphate minerals. Both can reduce P availability, but adsorption is a surface interaction, while precipitation forms a separate mineral solid.

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

30-60%

How can we conceptualize soil organic P?/

large, varied reservoir of P in organic material at different stages of decomposition. Microorganisms and enzymes can release some of it, but not all organic P is readily available.

What is the typical C:P ratio of microorganisms?

Btwn 6 - 13

What is the typical uptake efficiency of applied fertilizer P?

10-30%

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

Olsen soil P test which measures yield and whether added P is likely to improve it.

Which factors impact adsorption/desorption and precipitation/dissolution

adsorption/desorption: pH, clay content, amt of soil Ca, Fe, and Al.
precipitation/dissolution: pH and the concentrations of soil Ca, Fe, and Al.

Which conditions favor P mineralization?

High OM and Low inorganic P = High Mineralization

Which conditions favor P immobilization?

Favored by low P high C:P OM

Explain the difference between enzymatic hydrolysis and biological solubilization

Enzymatic hydrolysis uses enzymes such as phosphatases to release P from organic compounds.
Biological solubilization uses organic acid anions to bind Ca, Fe, or Al and release P from phosphate minerals.

how and why P availability is affected by total soil P content:

It sets the size of the potential P supply, but only a small share is in solution and readily available. A soil can contain a lot of total P while having little immediately available P.

how and why P availability is affected by Ca, Fe, Al concentrations:

They can bind P in secondary minerals or on mineral surfaces. This reduces solution P; how strongly it happens depends on soil conditions such as pH.

how and why P availability is affected by pH:

In acidic (low pH) soils, P can be strongly associated with Fe and Al oxides; Ca phosphates are also affected by pH.

how and why P availability is affected by Contact of soil solution to soil minerals and roots:

P moves through soil mainly by slow diffusion. Roots need to encounter P in the soil solution, or reach nearby P that can be released from minerals or colloids.

how and why P availability is affected by temperature:

Temperature influences weathering and biological activity, which can affect how quickly P is released and cycled. The slides list temperature as a factor but do not specify a numerical effect.

how and why P availability is affected by clay type/types of colloids:

Clays and Fe/Al oxides provide surface area and sites that can adsorb P. Some Fe/Al oxides also have tiny pores that can trap P, making it less available.

List 4 ideas to increase P availability for your crop, based on our understanding of the physicochemical processes governing P cycling

Root exudates containing organic anions to bind Ca, Fe, and Al to release O

Use phosphates to release P from OM

Mycorrhizal associations

Improve root architecture

Which P can you find in an Olsen P extract?

extractable soil-P measure used to estimate the P available to crops. It doesn’t estimate all P in the soil only PAP

List advantages and limitations of the Olsen P test

Advantage: practical soil-test estimate that can be related to crop yield response and fertilizer need

Limitation: it is an estimate and P is very variable

Why is runoff the dominant P loss pathway in most ecosystems?

P is held strongly in soil particles so doesn’t really leach but it can be transferred with soil runoff and if recently applied can wash away

Under what circumstances does leaching become an important P loss pathway

In sandy soils where there is less CEC

What are the consequences of P loss for the environment?

Eutrophication and contamination of ground and surface water

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

A tool to estimate P loss potential for a specific site

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

Soil erosion, irrig, soil runoff class, distance from watercourse, soil p, fertilizer app rate and method, org p source app rate and method.

How can the risk for P loss be reduced?

apply only needed P, incorporate fertilizer when appropriate, avoid applying P before runoff-producing events, and keep nutrients away from waterways, soil erosion prevention

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

Root foraging: helps plants find P with less soil

P mining: release P from minerals or OM using educates/enzymes

P use efficiency: producing more growth or yield for each unit of P the plant takes up.

How can the plant make 'cheap' roots?

reducing the metabolic cost of exploring soil, When you need more nutrients but don’t have enough energy to put all of it into this, grow "shittier roots

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

The more vertical the roots grow, the lower the yield under low P input compared to high P input

What are cluster roots?

Specialized root structures that release organic anions, or carboxylates, to help solubilize P from minerals.

What is phosphatase?

An enzyme that helps break down organic P compounds, releasing inorganic P that plants can take up.

List the 4 pools of K in the soil

Soil-solution K, readily available (exchangeable) K, slowly available or fixed K, and K in primary minerals.

What is the role of K in the plant?

K helps regulate stomatal opening and closing and water use. It supports growth, root development, crop quality, disease resistance, and winter hardiness. Unlike many nutrients, it is not built into plant molecules; it remains as K⁺ in solution in plant cells.

How easily is K on the CEC available in the plant?

K held weakly on CEC sites is readily available and can replenish soil-solution K as plants take it up. K held strongly between clay layers is fixed and is not readily available in the short term.

Which K pools are targeted when conducting an ammonium acetate extraction?

It mainly measures soil-solution and exchangeable K, the readily available pools. It does not measure all K locked in primary minerals or strongly fixed between clay layers.

When would you expect high rates of K fixation?

In soils with K-fixing 2:1 clays, especially vermiculite and some illite, particularly when K is present in solution and can enter clay interlayers. Some smectitic clays release part of their interlayer K more readily

How would you ensure plant K uptake in soils with high K fixation potential?

Use soil and crop information to guide K applications, and account for the fact that some applied K may become fixed rather than immediately available

Which factors affect risk for K loss through runoff versus leaching?

Runoff risk rises when rainfall or irrigation exceeds the soil’s ability to absorb water, especially where erosion transports soil particles. Leaching is more important in soils with low K-binding capacity, such as sandy soils.

What are the implications of K loss through runoff or leaching?

K loss is not known to cause particular environmental hazard

How is K availability affected by CEC, SOM, texture, and clay type?

High CEC = Hold More K

Texture and Clay amt influence CEC = amt K held

More SOM = High CEC = Hold More K

2:1 clays can also fix K between layers

Compare and contrast which concepts, pools and processes are similar or different between the N, P and K cycle

What is the role of S in the plant?

S is part of the amino acids cysteine and methionine and is involved in protein synthesis and enzyme activation.

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

They are in plant groups that often have high sulfur needs and the S contributes to taste

How common are S deficiencies in crops?

have become more common across the United States with higher crop yields and cleaner air

What are some visual symptoms of S deficiencies in plants?

Young leaves light green with lighter colored veins

Draw the S cycle

What are the dominant forms and distribution of S in the soil profile of a mollisol, spodosol, oxisol. Explain the observe patterns!

Mollisol: organic S is concentrated near the surface and declines with depth, inorganic S increases with depth
Spodisols: High amounts of organic S in soil surface and subsurface, very little inorganic s
Oxisols: inorganic S dominates and increases with depth in the diagram. More organic S on the top and decreases

How common is calcium in the Earth's crust

fifth most abundant element in Earth’s crust

What is the role of calcium in the plant?

Ca helps form and stiffen cell walls, is required for cell division, and contributes to plant responses to stress.

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

differences in plant nutrient composition and requirements.

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

Blossom-end rot can still occur if Ca is not delivered to the fruit. Water stress reduces transpiration and slows Ca transport through the xylem, and high levels of other cations can also reduce Ca uptake.

Draw the calcium cycle



The calcium cycle part II

How does pH affect calcium availability:

Acid soils generally have less Ca. Above about pH 7.2, additional Ca may form nearly insoluble compounds, including with P. Lowering pH can dissolve Ca in some secondary minerals. 

Explain the relationship between calcium and soil structure

Ca²⁺ occupies CEC sites and influences whether soil particles flocculate (aggregate) or disperse (separate). This affects soil structure.

List 2 potential challenges related to magnesium management.

Ca:Mg balance ( add gypsum when too low)

Mg and K imbalances ( causes cow illnesses)

What is the role of Magnesium in the plant?

A big part of the chlorophyll molecule and is involved in protein formation, ATP rxns, and enzyme activations.

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

Increased knowledge of plant nutrition and improved methods of analysis in the laboratory

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

Micronutrients are needed in very small amounts, and the range between adequate and toxic levels can be narrow. They can also be present in manure or irrigation water and added unintentionally.

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

Their availability depends on soil conditions such as pH and oxidation status. They may become strongly bound to soil components or form less soluble compounds, leaving little in the plant-available form.

How does pH and oxidation status affect micronutrient availability?

pH and oxidation status change the chemical forms and solubility of micronutrients. In general, higher pH can reduce availability of several metals such as Fe, Mn, Zn, and Cu, while soil redox conditions can change the availability of Fe and Mn.

Explain how mycorrhizae and chelation affect micronutrient availability

Mycorrhizae extend the soil area explored by roots, which can improve nutrient uptake. Chelation binds a micronutrient in a soluble complex, helping keep it available for plant uptake.

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

• Plant strategies: rotate crops to support mycorrhizae; select crops or varieties with effective root uptake; support root–microbe partnerships.

Soil strategies: apply the needed micronutrient fertilizer; manage soil pH; use chelated nutrient sources where appropriate.

How common are boron deficiencies in grape production in California?

Boron is the second most important micronutrient deficiency problem in California vineyards after zinc

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

fall foliar application - was most effective in that trial because symptoms were lower after treatment

Why would one desire to increase zinc concentrations in wheat? Nutritional value for human consumption

Zinc deficiency in human diets can contribute to malnutrition and increasing Zn improves overall nutritional value

How does CEC affect calcium availability

High-CEC soils can hold more Ca; low-CEC soils hold less.

How does Cation saturation affect calcium availability

High levels or applications of other cations can reduce Ca uptake, especially when soil Ca is low to moderate.

How does Parent material affect calcium availability

Limestone and marl tend to produce Ca-rich soils; shale and sandstone tend to produce lower-Ca soils.

How does Moisture affect calcium availability

Water stress reduces transpiration and therefore slows Ca delivery through the plant’s xylem.