Cal Poly SS 2221 Midterm 2

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

Higher crop yields, intensive agriculture, and recognition that micronutrient deficiencies can limit crop productivity and quality.

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

Micronutrients are required in very small quantities and their availability depends more on chemical form and soil conditions than on total abundance.

Based on your understanding of micronutrient cycling

They are commonly fixed to soil particles, precipitated into insoluble compounds, or exist in forms that roots cannot readily absorb.

How does pH and oxidation status affect micronutrient availability?

High pH generally decreases Fe, Zn, Mn, and Cu availability; reducing conditions often increase Fe and Mn solubility, while oxidizing conditions tend to reduce their availability.

Explain how mycorrhizae and chelation affect micronutrient availability?

Mycorrhizae expand root access to soil nutrients, while chelating compounds keep micronutrients soluble and available for uptake.

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

Plant strategies: use efficient cultivars, inoculate with mycorrhizae, apply foliar micronutrients. Soil strategies: adjust pH, add organic matter, apply micronutrient fertilizers.

How common are boron deficiencies in grape production in California?

Boron deficiencies are relatively common in California vineyards, especially on low-boron soils.

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

Foliar application near flowering is most effective because boron is delivered directly to reproductive tissues when demand is highest.

Why would one desire to increase zinc concentrations in wheat?

To improve the nutritional value of wheat for human consumption and help reduce dietary zinc deficiency.

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

Organic matter produces chelating compounds that keep zinc soluble and increase plant uptake.

What is the role of S in the plant?

Sulfur is essential for amino acids, proteins, enzymes, vitamins, and chlorophyll formation.

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

They contain sulfur-rich compounds responsible for their characteristic flavors, odors, and many health benefits.

How common are S deficiencies in crops?

Sulfur deficiencies are becoming increasingly common due to reduced atmospheric sulfur deposition and greater crop demand.

What are some visual symptoms of S deficiencies in plants?

Uniform yellowing of young leaves, stunted growth, delayed maturity, and reduced vigor.

Draw the S cycle?

Atmosphere → Deposition → Soil Sulfate ↔ Organic Sulfur → Mineralization → Plant Uptake → Plant Residues → Soil Organic Matter; losses occur through leaching and gaseous emissions.

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

Mollisols contain high organic sulfur near the surface due to organic matter accumulation; Spodosols accumulate sulfur in spodic horizons through leaching; Oxisols contain sulfate adsorbed to Fe and Al oxides due to intense weathering.

How common is calcium in the Earth's crust?

Calcium is the fifth most abundant element in the Earth's crust.

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

Liming supplied calcium, reduced acidity and aluminum toxicity, and greatly increased agricultural productivity.

What is the role of calcium in the plant?

Calcium strengthens cell walls, stabilizes cell membranes, supports cellular signaling, and is essential for growing tissues.

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

Brassicas generally have greater calcium uptake and accumulation due to differences in physiology and tissue composition.

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

Poor calcium transport to developing fruit caused by water stress, irregular irrigation, or rapid growth.

Draw the calcium cycle?

Parent Material Weathering → Soil Solution Ca²⁺ ↔ Exchangeable Ca → Plant Uptake → Plant Residues → Soil; losses occur through leaching and erosion, and additions come from fertilizers and lime.

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

Higher pH, greater CEC, higher calcium saturation, calcium-rich parent material, and adequate moisture generally increase calcium availability.

Explain the relationship between calcium and soil structure.

Calcium promotes clay flocculation and aggregate formation, improving soil porosity, infiltration, aeration, and root growth.

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

Liming materials: calcitic lime, dolomitic lime, burnt lime. Non-liming calcium sources: gypsum, calcium nitrate, calcium chloride.

List 2 potential challenges related to magnesium management.

Magnesium deficiency and nutrient imbalances caused by competition with calcium and potassium.

What is the role of Magnesium in the plant?

Magnesium is the central atom in chlorophyll, activates many enzymes, and is important for photosynthesis, energy transfer, and carbohydrate transport.

Name all the pools and processes in the P cycle and how they connect.,

Mineral P ↔ soil solution P ↔ adsorbed P ↔ organic P ↔ plant P. Processes include weathering, adsorption/desorption, precipitation/dissolution, mineralization/immobilization, plant uptake, residue return, erosion, runoff, and leaching.

What is the plant available form of P in soil?

Orthophosphate ions: H₂PO₄⁻ and HPO₄²⁻.

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

Usually less than 1% of total soil phosphorus.

What is the role of P in the plant?

Phosphorus is important for ATP and energy transfer, DNA and RNA synthesis, root development, flowering, seed formation, and overall growth.

Name at least two P-rich minerals.

Apatite and vivianite.

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

Adsorbed P is attached to mineral surfaces and can be relatively reversible, while precipitated P forms new solid minerals and is generally less available.

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

Commonly about 20-80% depending on soil type and organic matter content.

How can we conceptualize soil organic P?

As phosphorus contained within living organisms, residues, microbial biomass, and soil organic matter that must be mineralized before plant uptake.

What is the typical C:P ratio of microorganisms?

Approximately 60:1.

What is the typical uptake efficiency of applied fertilizer P?

Often only 10-30% in the first growing season.

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

Use soil test results (such as Olsen P), crop requirements, yield goals, and field history.

Which factors impact adsorption/desorption and precipitation/dissolution?

Soil pH, clay content, mineralogy, Fe and Al oxides, calcium concentrations, moisture, and temperature.

Which conditions favor P mineralization?

Warm temperatures, adequate moisture, active microbial communities, and low microbial demand for phosphorus.

Which conditions favor P immobilization?

High carbon inputs with low phosphorus content and strong microbial growth.

Explain the difference between enzymatic hydrolysis and biological solubilization.

Enzymatic hydrolysis releases phosphate from organic compounds using enzymes such as phosphatases; biological solubilization dissolves mineral phosphorus through microbial secretion of acids and other compounds.

Explain how and why P availability is affected by total soil P content.

Higher total P generally increases the potential supply of P, although availability still depends on chemical forms and soil properties.

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

High Ca can precipitate P in alkaline soils, while Fe and Al oxides strongly adsorb or precipitate P in acidic soils.

Explain how and why P availability is affected by pH.

Maximum availability usually occurs around pH 6-7; acidic soils increase Fe and Al fixation, while alkaline soils increase Ca fixation.

Explain how and why P availability is affected by contact of soil solution to soil minerals and roots.

Phosphorus moves mainly by diffusion, so greater root-soil contact improves uptake.

Explain how and why P availability is affected by temperature.

Higher temperatures generally increase microbial activity, mineralization, and root growth, increasing P availability.

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

Clays and oxide-rich colloids provide adsorption sites that can retain phosphorus and reduce availability.

Challenge yourself! Based on your understanding of the P cycle propose management practices that could improve P availability.,

Maintain pH near neutral, add organic matter, place fertilizer near roots, encourage mycorrhizae, use cover crops, and reduce fixation.

List 4 ideas to increase P availability for your crop.

Band P fertilizers near roots, maintain optimal pH, increase organic matter inputs, and promote mycorrhizal associations.

Explain how chickpea increases P availability and what the implications are for developing other management strategies to improve P availability.,

Chickpea releases organic acids that mobilize soil phosphorus, suggesting that crops, microbes, or practices that enhance acid exudation may improve P acquisition.

Which P can you find in an Olsen P extract?

Primarily plant-available inorganic phosphorus in the soil solution and readily desorbable phosphorus.

List advantages and limitations of the Olsen P test.

Advantages: widely used, simple, good predictor in neutral to alkaline soils. Limitations: less reliable in some acidic soils and does not account for all P pools.

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

Phosphorus binds strongly to soil particles, so it is commonly transported with eroded sediment or surface runoff.

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

Sandy soils, highly weathered soils, organic soils, or soils with low P adsorption capacity.

What are the consequences of P loss for the environment?

Eutrophication, algal blooms, hypoxia, reduced water quality, and ecosystem degradation.

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

The P Index is a risk assessment tool developed by Lemunyon and Gilbert to estimate the risk of phosphorus loss from fields and guide management decisions.

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

Soil test P, fertilizer and manure application, erosion, runoff potential, slope, drainage, and management practices.

How can the risk for P loss be reduced?

Reduce erosion, improve nutrient management, establish buffers, avoid overapplication, and time applications appropriately.

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

Root foraging improves root exploration, P mining mobilizes previously unavailable phosphorus, and P use efficiency increases yield per unit phosphorus absorbed.

How can the plant make "cheap" roots?

By producing thinner roots with lower tissue density and lower construction costs.

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

Shallow root angles improve access to surface phosphorus, while longer root hairs increase soil exploration and phosphorus uptake.

What are cluster roots?

Dense clusters of short roots that release organic acids to mobilize nutrients, especially phosphorus.

What is phosphatase?

An enzyme that releases phosphate from organic phosphorus compounds.

List the 4 pools of K in the soil.

Soil solution K, exchangeable K, nonexchangeable (fixed) K, and mineral (structural) K.

What is the role of K in the plant?

Potassium regulates enzymes, osmotic balance, stomatal function, water relations, and stress tolerance.

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

Exchangeable K is readily available and serves as a major short-term reserve.

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

Primarily the exchangeable K pool and some soil solution K.

When would you expect high rates of K fixation?

In soils rich in 2:1 clays such as illite and vermiculite, particularly when soils dry.

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

Apply K in smaller, more frequent applications, band fertilizers, and maintain adequate soil moisture.

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

Texture, rainfall, drainage, CEC, slope, erosion risk, and fertilizer management.

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

Reduced fertilizer efficiency, lower crop productivity, and economic losses.

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

Higher CEC and SOM improve K retention, sandy soils lose K more readily, and clay mineralogy influences fixation and release.

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

N has major gaseous forms and biological fixation; P has strong adsorption, precipitation, and minimal gaseous losses; K exists only in mineral and ionic forms with no biological fixation or gaseous phase. All three include soil solution pools, plant uptake, residue return, and fertilizer inputs.