SS 2221 Final

5 soil forming factors

ClORPT - climate, organisms, parent material, relief(topography), and time

What is soil texture

percentages of sand, silt, and clay in a soil that make up a textural class.

implications of soil texture for plant health and nutrition

sandy soils hold less PAW, clay soils hold more PAW. Clays surface is more reactive so higher CEC.

What is soil structure and its types

defines hoow soil mineral particles are bound together and arranged in the three dimensional space. Made up of peds or aggregates. Granular, aggregated, blocky, columnar, prismatic, platey, massive.

How is soil structure formed

biological activity(fungal, bacterial, roots, or faunal) brings mineral particles together with organic matter over time.

Implications of soil structure classes on plant health and nutrition

reduces soil crusting and erosion, determines capacity of soil to hold water and air, facilitates root development, provides microhabitat for soil organisms, contributes to carbon sequestration,

Management practices to improve soil structure

reduce plowing and plant cover crop

Bulk Density

indicator of soil compaction, amount of soil in a volume. g/cm³. Includes volume of soil particles and pores but not water.

How is bulk density affected by soil texture

Fine-textured soils (clays and silts) have higher total porosity and lower bulk density, whereas coarse-textured sandy soils have less total pore space and higher bulk density.

Implications of bulk density on plant health and nutrition

water and solute movement, soil aeration, root penetration,

management practices to improve bulk density

plant a wider variety of crops to introduce more roots to increase structure, plowing

Gravimetric moisture content vs. volumetric moisture content vs. water tension/potential

Gravimetric soil moisture: soil weight before - oven dry soil weight

%volumetric water content = %gravimetric water content - bulk density

Waer tension/potential: pressure of water in soil

soil moisture along a water potential gradient

clay has more PAW at all tensions. Sand has the lowest PAW at all tensions

How does texture, structure, and bulk density affect soil moisture dynamics

texture: sand holds less water and will hold it for a shorter amount of time

structure: platy and massive textures make water infiltration very hard while single grain and granular make it very easy.

bulk density: higher bulk density = less pore space = less space for air and water.

What constituted soil organic matter?

All materials of biological origin in soil. Includes living microorganisms(bacteria, fungi, etc), newly incorporated crop residue, actively decomposing organic material, and stable humus.

What are the different pools of organic matter?

Active SOM: 1-2 years C/N ratio of 15-30, recently depositied, rapid decomp, 10-20% of SOM

Slow SOM: 15-100 years C/N ratio of 10-25, intermediate age OM, slow decomp., 10-20% SOM
Passive SOM: 500-5000 years, C/N Ratio of 7-10, very stable OM, very slow decomp, 60-80% of SOM

What are the benefits of soil organic matter?

Source of nutrients, increses WHC, increases water infiltraion rate, improves soil strucutre, increase CEC, provides microbial activity, contributes to carbon sequestation.

How can soil management increase or decrease soil organic matter?

increase carbon inputs by adding compost and decrese carbon losses by supporting photorespiration. Minimize tillage and plant cover crops.

How does pH affect nutrient availability?

pH affects the solubility and availibility of elements. Low pH can make toxic elements more soluble and thus availble to the plants.

How does climate affect the typical pH found in a soil?

Tropical soils that are highly weathered in hot, humid environments will have low pH. Young and unweathered soils will have higher pH

What is cation exchange capacity(CEC)?

The total quantity of cations a soil can hold adsorbed onto the coilloids.

Which cations dominate the CEC of western soils?

Ca(50-80%), Mg(10-40%), K(1-5%), Na(1-5%)

Which soil properties affect CEC

Clay and humus are the two most important factors that affect CEC. When both are high, CEC is high.

How does cation exchange capacity affect soil fertility and fertilizer management?

If CEC is high, there is less of a need for extra fertilizer if all nutrition needs are being met. Higher CEC = more fertile soils.

What is the difference between soil salinity and soil sodicity?

Salinity is the accumulation of cations and salts to a point where plant growth and soil function are damaged.

Sodicity is when sodium concentration is high enough to adverely affect soil structure, while overall salt concentration is below ‘saline’ threshold.

How do you diagnose salinity vs. sodicity?

Salinity: measure electrical conductivity in dS/m. Look for symptoms in the plants such as reduced water and nutrient uptake, stunted growth, yellowed leaves, and old leaves drop off.

Sodicity: Look for massive soil structure. Na enters the interlayer space of clays and disperses, which results in massive structure.

How do salinity and sodicity, respectively, affect the soil and plant growth?

Salinity causes osmotic drought stress (limiting plant water uptake) and toxic ion accumulation without disrupting soil physical structure. Sodicity causes clay dispersion and aggregate breakdown, leading to surface crusting, reduced water infiltration, poor aeration, and restricted root development.

How do you mitigate a salinity or sodicity problem, respectively?

Salinity: leach excess soluble salts below the root zone using high-quality, low-salt irrigation water with adequate drainage. Sodicity: apply calcium soil amendments (such as gypsum, CaSO4⋅2H2OCaSO4​⋅2H2​O) to displace exchangeable sodium (Na+Na+) from soil colloids, then leach out displaced sodium with water.

What are the three ways by which nutrients in soil reach the plant root surface?

1. Mass flow: movement of nutrients by water. Nitrogen, sulfur, calcium, chloride, and magnesium

2. Diffusion: Movement of nutrients along concentration gradients. High to low concentrations. Phosphorus, potassium, Fe, and Zn. Dependent on soil matrix and resistance to the flow of nutrients through the gradient

3. Interception: contact of roots and soil surface to absorb nutrients. Less than 1% of nutrient uptake

Categorize nutrients according to their molibity in soil

High mobility: SO4-, NO3-, Cl-, and Na+

Intermediate mobility: Ca2+, Mg2+, K+, and NH4+.

Low mobility: MoO4- and H2PO4

how are diffusion and mass flow affected by soil texture and structure

More compacted structure reduces mobility of nutrients. Macropores are necessary for water and nutrient movement. Higher clay content reduces diffusion due to its reactivity(colloids)

how can a plant increase nutrient uptake by interception

Symbiosis with mycorrhizal fungi. Endomycorrhizae and ectomycorrhizae.

three ways by which nutrients enter the root cell

Diffusion and Facilitated diffusion as a part of passive transport and active transport

which nutrient deficiencies will show first in old leaves and why?

Deficiencies of mobile nutrinets: N, P, K, Mg, Cl, and Mo. When deficient, the plant will prioritize new growth and send the nutrients to new growth.

Which criteria define plant essential nutrients

They are required for the completion of the life cycle by the plant, must not be completely replaceable by another element, must be firectly involved in plant metabolism or other specific physiological function, and the element mist be required by a substantial number of plant species.

14 plant essential nutrients. Categorized by primary macronutrients, secondary macronutrients, and micronutrients

Primary macro: N, K, P. Secondary macro: Ca, Mg, S. Micro: Cl, Fe, B, Mn, Zn, Cu, Mo, Ni.

Difference between primary macronutrients, secondary macronutrients, and micronutrients.

Primary macronutrients: Structural growth, energy transfer, and overall plant health.

Secondary macronutrients: cell wall strength, chlorphyll production, and enzyme activation.

Micronutrients: Catalysts for biochemical reactions, disease resistance, and metabolism.

Contrast and compare NH₄⁺ and NO₃^- in regard to the effect of plant uptake on soil pH

Ammonium absorbs a cation and causes roots to release H+ which results in a rhizosphere pH drop. Nitrate absorbs an anion, roots release OH- and pH rises.

Contrast and compare NH₄⁺ and NO₃^- in regard to the energy required by the plant to assimilate the N source

NH4+ is the less expensive form of N for plants and requires less energy to convert into NH3 for the synthesis of amino acids and proteins. NO3- is more 'expensive’ for plants to convert.

Contrast and compare NH₄⁺ and NO₃^- in regard to the pool size, mobility, and availability in the soil

NH4+ is less abundant in soils.

Contrast and compare NH₄⁺ and NO₃^- in regard to their role as as a substrate for N polluting processes

What is the role of soil organic nitrogen in soil fertility and plant nutrition?

limitations of interpretations of current soil testing methods

implications for organic agriculture

opportunities for future directions

What is the greatest N loss pathway from cropland, based on the California N assessment?

What are the consequences of N leaching?

Which interventions have been proposed to reduce N leaching, and how do they work?

Why do certain microorganisms carry out denitrification?

Under what conditions do N2O emissions spike?

What management considerations could reduce N2O emissions?

Which factors determine whether N2O or N2 will be the end product of denitrification?

Why is NH3 bad for the environment?

Conceptually, describe what reactions take place when urea is applied as a fertilizer

Which factors increase the risk for NH3 volatilization?

How can management minimize NH3 volatilization?

What is the difference between runoff and leaching?

How could you reduce N loss through runoff?

Improve soil structure

Optimize irrigation and nutrient management

Keep soil covered during rainy season - cover crop or mulch

Polycrylamide

Edge of field intervention

Give examples of factors related to root architecture, root morphology, plant physiology, and plant-microbe interaction that can be modified to increase N uptake

What are mycorrhizae and how do they benefit the plant?

Which conditions favor biological N fixation?

How do rates of biological N fixation vary between crop types?

Name 3 strategies by which biological N fixation can be used to increase N input into an agricultural system.

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.