Agronomy

describe the soil classifications in the Northern region

Vertosols - clay >35%, cracks, slickensides

Sodosols - strong texture-contrast (2 completely different soil textures on top of one another), sodic (high sodium) B horizon. When it rains, these soils erode fast. Can cause water logging. 

Chromosols - strong texture contrast, pH > 5.5 in B horizon. Not sodic. 

Kandosols - Lacking strong texture contrast, massive B horizon. Low nutrient content. 

Describe the soil classifications in the Southern region

Sodosols - strong texture-contrast (2 completely different soil textures on top of one another), sodic (high sodium) B horizon. When it rains, these soils erode fast. Can cause water logging.

Chromosols - strong texture contrast, pH > 5.5 in B horizon. Not sodic. 

Calcarosols - lacking strong texture-contrast, high in calcium

Describe the soil classifications in the Western region

Tenosols - does not retain moisture well. 

Sodosols - strong texture-contrast (2 completely different soil textures on top of one another), sodic (high sodium) B horizon. When it rains, these soils erode fast. Can cause water logging.

Calcarosols - lacking strong texture-contrast, high in calcium

Kandosols - Lacking strong texture contrast, massive B horizon. Low nutrient content. 

What is PAW? What can it be affected by?

Plant Available Water. Can be affected by rainfall, infiltration, drainage, runoff, evaporation, transpiration, and soil-available water capacity

What is PAWC? What is needed to calculate it?

Plant available water capacity. You need

1. DUL (= Drained Upper Limit - field capacity; amount of water a soil can hold against gravity)

2. CLL (= Crop Lower Limit - permanent wilting point; amount of water remaining in a soil after a specific crop has extracted all available water) OR LL15 (= Lower Limit 15 - Water remaining in soil at 15 bar pressure.)

3. Bulk density (g/cm^3) (the mass of dry soil per unit volume, including both solids and pore space.)

What is fallow efficiency and how can it be affected?

Fallow efficiency is how much water is able to be retained in the soil profile within a fallow. It can be affected by infiltration, runoff, erosion, evaporation. 

Some crops like wheat leave residues that are good for fallow efficiency. Weeds can make a big difference in fallow management. 

How do you calculate fallow efficiency?

2 soil samples, one at start of fallow and one at end. Know water inputs into the system during the fallow (rainfall). 

Fallow efficiency = change in plant-available water during fallow / rainfall during fallow

What is Soil Organic Matter, how is it measured?

Everything in or on the soil that is of biological origin, whether it is alive or dead.

SOM all contain carbon. SOM is often measured by determining soil C content. It’s assumed that SOM (on average) contains 58-60% C. So, a soil containing 1% C will have approx. 1.7% SOM. Not all carbon in soil is organic.

What is Soil Organic Carbon?

the carbon component of soil organic matter (SOM), representing the accumulation of decomposed plants, microorganisms, and animal materials

Why should we maintain a high level of SOM?

Reduced bulk density, Increased aggregate stability, Resistance to soil compaction, Enhanced fertility, Reduced nutrient leaching, Resistance to soil erosion, Increased soil biological activity, Soil C sequestration

Describe the biological, chemical, and physical functions of SOM.

Biological functions. Energy for biological processes, reservoir of nutrients/nutrient cycling (N, S, P), disease suppression/resilience.

Chemical functions. Cation exchange capacity, buffers changes in pH, complexes cations) SOM compounds have a net negative charge that holds cations (calcium, magnesium, potassium) - contributes to cation exchange capacity. Very important in lighter/sandier soils. Soils with low CEC tend to lose nutrients via leaching. SOM absorption of H inhibits Al and Fe hydrolysis. It effectively ‘locks up’ toxic ions like Al. Helps prevent precipitation of in Fe and Al oxides.

Physical functions. Improves structural stability, influences water retention, infiltration/drainage and aeration.

Why can SOM decline?

Tillage (reduces SOM by mixing soil and bringing soil organisms into contact with plants with plant residues which speeds up SOM decomposition. It breaks soils aggregates and exposes SOM to air and soil organisms)

Soil erosion

Nutrient export in crop yield

Burning/harvesting of crop residues

Nutrient mining (long term fertiliser inputs)

Frequency and duration of fallow periods

Low crop diversity

Describe soil organisms and their importance

Bacteria - Important for building soil structure and fertility. Can form mutualistic or antagonistic relationships with plants. E.g nitrogen fixers

Fungi - Dominant decomposers. Drives decomposition of cellulose and lignin. Hyphae important for soil aggregation. Slow turnover rate. Promotes nutrient retention. E.g arbuscular mycorrhizal fungi obtains C from plant hosts and provides P, Zn, Cu. in return

Nematodes - useful soil health indicators. They mineralise nutrients during the feeding process, regulate populations of bacteria and fungi, and a food source for microarthropods.

Microarthropods - mineralising nutrients, regulates populations of soil fauna, large numbers and diversity found in healthy soils.

Macrofauna - Earthworms: soil formation, litter decomposition, and redistribution of organic matter. Isopods and millipedes: major consumer of organic debris. Insect larvae such as cane grubs: consume root material. Centipede, spiders, scorpion, and beetles: dominant predators. Ants: generalist predators and scavengers. Termites: important decomposers

What is the energy source that sustains the soil

Carbon

What is the rhizophere

C-rich zone that is an area of intense biological activity. Huge numbers and a diverse range of bacteria and fungi occupy the rhizosphere. Plant roots excrete monosaccharides, organic acids, and dead cells as root exudates.

5 functions of the soil biota

1. Provide a physical and chemical environment suitable for plant growth. This means it can decompose organic materials into particulate and mineral-associated organic matter, stabilise, and results in improved aeration, water holding capacity, water infiltration, and mineralisation.

2. Influence water availability. Improves soil structure so that rainfall infiltrates rather than runs off. Building SOC increases soil water holding capacity. Plants allow mycorrhizal fungi to colonise their roots because they act as an extension of the root system.

3. Control nutrient availability. Nitrogen is an essential macronutrient (proteins, chlorophyll production). Soil microbes play a key role in the soil nitrogen cycle. Nitrogen is fixed into the system. Transformation of organic and mineral forms of N. Also can impact the availability of other essential macronutrients, e.g., phosphorus and sulphur.

4. Regulation of pests and pathogens. E.g Bacteria produce antibiotics that reduce the growth of Phytophthora and nematode-trapping fungi.

5. Cause plant disease

   
   

Describe the types of nitrogen fixing bacteria

Symbiotic - rhizobia bacteria enter root tissue and modify host cells so that nodules are formed. Bacteria convert N2 from the atmosphere to ammonia. Quantity of N fixed depends on legume species.

Free living - When high amounts of carbon are available as an energy source, a range of free-living bacteria will fix nitrogen. Quantity fixed is generally low.

What happens chemically during denitrification

Nitrate (NO3) is converted to gaseous N (NO2) by denitrifying bacteria. This is an anaerobic process. If only partial denitrification, produced nitrous oxide (N2O) (a very potent greenhouse gas).

How does tillage impact soil biota

Destroys the topsoil layer and its structure. Fungal hyphae networks are lost, leading to greater bacterial dominance. Soil particles lost = less rainfall infiltration. More surface runoff and erosion occurs. Loss of micro, macro, and faunal diversity.

In a no-till/reduced till system, it is stable and undisturbed. Crop residues are retained on the soil surface. Greater fungal dominance with development of extensive hyphae networks. Diverse communities.

How does soil compaction impact soil biota

Uncontrolled traffic and heavy machinery lead to soil compaction, especially on wet soil. Reduces bacterial and fungal abundance. Shifts community in favor of anaerobes: methanogens and denitrifiers. Negatively impacts crop root growth and earthworm activity. Reduces nodulation and rhizobium colonisation. Reduces root colonisation of symbiotic soil fungi. .Controlling traffic is an essential practice. Compactions destroy the pore-spaces inhabited by the soil fauna.

How do monocultures impact soil biota

Crop-specific pathogens become dominant in monocultures and low diversity systems.

Rotating crops allows break in the pathogen life cycle and improves weed control.

How do bare fallows and the removal of crop residues impact soil biota

Bare fallows deplete SOM, with flow-on effects to soil health.

Wide-row and skip-row planting of grain crops to maximize plant available water.

Arbuscular mycorrhizal fallow do not do well with long-fallows (areas where nothing has grown for a while since the last crop) in the northern cereal cropping zone.

12-14 month fallows accumulate soil water, but root colonization is reduced because mycorrhizal inoculum declines in absence of host plants.

Use of cover crops during fallows can help.

Surface residues should be retained rather than burnt or sold. They provide important carbon inputs, protect the soil from wind and water erosion, improve soil moisture conditions, and provide a home for organisms.

How do fertilisers and pesticides affect soil biota?

Inputs of N usually decrease fungal biomass and diversity

High N and P levels are detrimental to AMF (arbuscular mycorrhizal fungi) abundance and function

Fertilisers also have positive effects; when plant growth increases, roots exude more labile C compounds and more organic matter is returned to the soil. SOM inputs critical for maintaining aggregate structure and soil function. Maintaining soil fertility is essential for sustainable agriculture.

Imidacloprid - systemic insecticide (within all parts of the pants). Residues from these plants go into the soils however and can kill other organisms like earthworms and honey bees. Global collapse in bee populations cause

What is cover cropping, why is it useful?

A non-cash crop that is planted between cash crops (can replace fallow phase, but sometimes only during a period of a fallow). It’s one of the simplest ways of improving soil health. Protects soil from erosion and nutrient losses. Provides carbon inputs and increases soil N.

Cover crops may have detrimental effects. Populations of pests and pathogens may increase if you plant cover crops that are hosts.

What are the 3 principles of conservation agriculture?

1. Minimum soil disturbance

2. Permanent or semi-permanent organic soil cover (ex. Cover crop)

3. Crop rotations

What is stubble retention? Benefits and drawbacks?

Crop residues remain primarily on the soil surface. It’s easier than planting cover crops. It consists of standing plant stems and straw and chaff cut out by the harvester and separated from the grain. Stubble is retained to protect against erosion.

Disadvantages of stubble retention: it can make cultivation difficult especially if there are high stubble loads without the gear to handle it. Requires special seeders: knife point or disc seeders. High C:N residue can cause nutrient immobilization. Also is a fire risk. Can cause more weeds (because there's no tillage) and pathogen/pest accumulation (because of herbicide resistance).

What are problems with conservation agriculture

Can get water repellent sandy topsoils. When crop residues decompose, they deposit a waxy layer residue which coats the sand particles, making them water repellent. Can be resolved by tilling

Herbicide resistant weeds. No mechanical control can result in herbicide resistance. Weed populations can adapt to an environment especially when only one crop is being grown.

Stratified organic matter. Nutrients get concentrated into the topsoil.

Compact, acid subsurface

Describe strategic tillage

Most no-till adopters continue some strategic tillage (~30% cropped area). Not on an annual basis, usually 4-5 years between. Respond to economic and seasonal drivers. Can help alleviate problems associated with no till systems. It is implemented to combat: • Build-up of herbicide resistant weeds• Increased incidence of soil- and stubble-borne diseases • Enrichment of nutrients and carbon near soil surface(address nutrient stratification).

What are the downsides to strategic tillage

Strategic till can cause more nitrogen runoff in dermosol soils. For sodosol soil, there is a huge increase in erosion and nitrogen runoff with strategic tillage compared to no till. There is also a decrease in infiltration for strategic tilling in sodosol and dermosol soils. Sodic soils have a high concentration of sodium which can break up clay soils (high dispersal rate), meaning there's a higher risk of erosion. Without infiltration this will lead to more erosion.

What are soil constraints? Give some examples

Soil constraints are physical, chemical, or biological features in the soil that limit the ability of crops to access nutrients and water.

Salinity = osmotic effects, toxic effects

Acidity = toxic affects, deficiencies

Alkalinity = toxic effects, deficiencies

Nutrient toxicities = toxic effects

Nutrient deficiencies = deficiencies

Sodicity = dispersion (a significant effect on Australia. ($1.3 billion/year))

Cation imbalance = dispersion

Compaction = high soil strength

What is soil pH? What does it affect? What is an optimal pH?

pH controls availability of nutrients (several different mechanisms), availability of toxins, rate of microbiological processes.

H+ limits root growth and pH 3.9 to 4.5 depending on species.

OH- limits root elongation. Exact pH not known.

Soil pH effects on nutrient availability. The vast majority of nutrients are available between pH 6 and 7.5.

What is salinity in soil? Where does it come from and what does it affect?

Saline soils contain an excess of soluble salts at a level that limits sustainable use of that resource in plant growth, land use, or infrastructure.

Primary sources = natural salinity. It comes from the weathering of rocks (releases ions that form salts as they break down), rainfall and ocean-derived aerosols, aeolian (wind-blown) deposits, and connate salts form ancient seas.

Secondary sources = human-induced salinity. It comes from land clearing (dryland salinity, brings stored salts to the surface as groundwater table is raised), irrigation (excess irrigation water moves past the root zone, recharging groundwater and mobilising salts), industrial and agricultural wastewater, and urban water leaks and coastal groundwater extraction.

High salinity reduces plant available water.

What is sodicity in soil? What does it impact? How to fix it?

Conditions where exchangeable Na occupies a high proportion of soil’s cation exchange sites. Causes clay particles to disperse. Leads to poor structure, restricted infiltration, waterlogging, surface sealing, and reduced root growth.

Physical impacts: clay dispersion (breakdown of aggregates, loss of soil pores), surface sealing and crusting (poor seedling emergences and low infiltration), low infiltration nd permeability (increased runoff, ponding, and erosion), hardsetting and poor tilth (soil difficult to work, roots struggle to penetrate), slow internal drainage (perched water, waterlogging, and poor aeration), reduced root growth and exploration (limits access to nutrients and water.)

Chemical impacts: high exchangeable Na+ (displaces Ca and Mg, weakening clay bonds = Risk of cation leaching), increased soil pH (affects nutrient availability), reduced availability of key nutrients due to high pH and poor root access, toxicity risks from high Na, poor flocculation stability (soils disperse quickly in rain or irrigation water flow).

Amelioration (fixing) = applying calcium-based amendments like gypsum, which displace harmful sodium ions on soil particles. This stops clay from dispersing, improves soil structure, and restores water and air movement.

What are benefits of crop rotations?

For a disease to occur there must be a susceptible host, a virulent pathogen, and an environment favorable for the pathogen. Crop rotation plays a major role in managing the disease triangle. Beyond pests and disease management, crop rotations can also provide opportunities to diversify risk, help address fertility issues, enable more effective weed control, improve management of water use efficiency (some crops can extract deeper and some have shallow roots), and provide biodiversity and ecosystem services. Pathogens are a major system driver regarding crop rotations. E.g. root lesion nematodes are present in sorghum, so planting something like maize which is not susceptible would be smart.

Strategic sequencing of deep vs shallow-rooted crops improves water capture and reduces deep drainage.

How does rhizobium symbiosis work?

• Microbe supplied with a habitat and carbohydrates by the plant

• Plant receives N2 fixed from the air by the microbe

Why is soil nitrate not great?

High levels of mineral N in the 0-10 cm of soil at sowing may reduce nodulation and N fixation. Avoid applying fertilizer N in legume crops unless essential.

What are challenges to adopting legumes?

Reduced margins driving economics

Less profitable sometimes

Mainly winter crop

High disease risk

Easier to use N fertiliser

Soil constraints (don’t do well in sodic soils, saline soils, waterlogging, toxicities, drought and nutrient deficiencies.) Acute deficiency of P can prevent nodulation by legumes. P is required for module metabolism.

What are some types of soil amendments?

• Animal Manures

• Slurries / pond sludges / digestate

• Commercial / industrial residues

(food & fibre processing)

• Composts (can be made from different feedstocks)

• Biosolids (processed sewage sludge, cheap, high contamination levels)

• Biochar (from various feedstocks, carbon sequestration objective, expensive, usually mixed with compost)

What are some characteristics of compost? What are some drawbacks to it?

• Macro nutrients (N, P, K, Mg, Ca) – you need total & available content

- often limited N supply but considerable P and K

• Micro nutrients (S, Zn, Mn, B, Mo….)

• Neutral pH (7 - >8) / Liming Effect

• Both labile and stable carbon compounds (the energy for soil)

• High cation exchange capacity

• High water holding capacity

• Low bulk density

• Diverse microbial population

Drawback: Hard to balance its components. Not often in the ratio that plants need nutrients. Therefore, farmers may often just end up buying fertilisers.

Conclusions: compost can replace P, K, and Mg fertilizer and substitute liming

What is the “Law of the Minimum”?

Nutrient in least supply limits growth. Growth (biomass) is determined by whichever nutrient is present in the shortest supply.

In other words, you can’t “make up” a lack of other nutrients to allow growth. Also, amounts of nutrients are all relative.

Describe the types of multiple nutrient deficiencies (single limitation, simultaneous co-limitation, independent co-limitation, and serial limitation)

• Single limitation: Occurs when plant growth is constrained by one nutrient only. Adding that nutrient alone increases growth.

• Simultaneous co-limitiation: Occurs when two or more nutrients limit growth at the same time, such that adding only one nutrient does not improve growth but adding them together does.

• Independent co-limitation: Occurs when two nutrients each limit growth independently, so that adding either nutrient alone increases growth and adding both gives an additive response.

• Serial limitation: Occurs when one nutrient limits growth first, and once it is corrected, a second nutrient becomes limiting

Plants need 17 nutrients. Try to name most of them (3 basic, 3 primary, 3 secondary, and micro)

Basic nutrients: C, H, O. Primary macronutrients: N, P, K. Secondary macronutrients: Ca, Mg, S. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.

Whats the difference between essential nutrients and beneficial elements

Essential nutrients: 

1. Needed for all plants to complete a full life cycle 

2. Can not be replaced by another naturally occurring element

3. Are directly involved in metabolism (e.g. as structural component), OR, it must be required for a distinct metabolic step (e.g. an enzyme reaction)

Beneficial elements - only required in some plant groups. 

• Stimulate growth, but are not essential, or

• Are essential only for certain plant species, or

• Are essential only under specific conditions.

These include: aluminium (Al), cobalt (Co), sodium (Na), selenium (Se), silicon (Si)

What are the 3 ways nutrients move into a plant?

Mass flow - movement of dissolved nutrients along with water to the plant roots. Nutrient concentration plays a huge role: the higher the nutrient concentration, the more nutrients are moved to the root system. For more mobile nutrients. Nitrates are an example of most common

Diffusion - Movement of nutrients through water from an area of high concentration to low concentration. As the roots absorb nutrient ions from soil solution, the concentration at the root surface decreases compared to the bulk soil solution. As a consequence, the ions diffuse down the concentration gradient towards the root surface. For less mobile nutrients.

Interception - the root growing and encountering soil nutrients at the root surface. Cations like ammonium most common

How does nutrient mobility affect uptake? Give some examples of mobile/immobile nutrients

Mobile nutrients - Very mobile nutrients like nitrates and sulfur can move quickly through the profile and reach plant roots more easily. Soil volume explored is larger.

Immobile nutrients - Immobile nutrients like phosphorus (P) and potassium (K). Are absorbed from just a thin cylinder of soil around each root.

How does soil pH affect nutrient availability?

Some soils are acidic, others basic. There are exceptions, but most plants grow best at a slightly acid pH where the availability of nutrients is generally high. In strongly acidic soils, growth is often limited by increased solubility of the abundant but detrimental non-nutritive element aluminium.

Aluminum is the most abundant metal in the Earth’s crust. At neutral pH it is bound into insoluble complexes. Acidic soils release Al 3+ which arrests root growth in sensitive plants.

What are ways that nutrient deficiency can occur?

1. Uniform chlorosis = most plants all have similar appearances, likely a macronutrient issue. 

2. Topography-dependent (hot-spot)

3. Slope dependent

What are methods of seeing nutrient deficiencies?

Soil fertility status (soil sampling): Advantages are that it gives a longer term picture, enables VRT, sensitive to deficiencies and long-term changes in nutrient status, enables fertiliser planning over time, and is cost effective when done properly. Disadvantages: sampling size/depth sensitive, expensive if not done properly, requires long term commitment.

Plant nutrient status: Advantages are that in season, tailored application to crop deficiencies can be useful for top-dressing N in some crops. However, this means that you are being reactive and not proactive.

What do you need to know in order to know how much fertiliser to apply?

Species/variety of plants

Soil characteristics,

Finances

Interaction between nutrients (there can be +/- reactions between nutrients)

Abiotic and biotic factors

Cultivation practices

How does a nitrogen deficiency look? Why?

N deficiency expresses itself as pale green or yellow. Remember N is a component of chlorophyll which makes plants green/allows them to photosynthesise.

Nitrogen is also mobile in the plant. Often, the leaves will be dried up because N will be moving out of the old leaves to the new leaves.

N status can be determined by chlorophyll content, measured by reflected light.

What are the different forms of N fertiliser?

Different forms of N fertilizer

1. Urea-N. Must first be mineralised into ammonium and nitrified into nitrate. 

2. Ammonium-N (quick release)

3. Nitrate (quick release)

Slow-release fertilisers may match release to requirements. Traditional fertilisers typically do not match nitrogen availability to plant needs. Slow release fertilizers may more closely match plant needs and reduce loss pathways. 

Coated urea dissolves and releases slowly, but it can be expensive. 

Why is phosphorus important? What does a phosphorus deficiency look like? How can we increase phosphorus uptake?

P has roles in cell structure, energy, and information storage and energy and information transfer. Because of its high reactivity in soil P is generally considered immobile, however, losses can still occur e.g. via leaching (dissolved or particulate) or erosion.

Phosphorus is extremely chemically reactive in soil. It also diffuses very slowly. 

P deficiency can be expressed as purple leaves. 

Better roots may improve the chance to capture soil P. 

Plant and microbial exudates may increase P availability. 

Exudates from free-living and symbiotic microbes may also contribute to P solubilisation, though it is disputed how much they could really contribute. 

Arbuscular mycorrhizal fungi facilitate P-uptake in most plants

Why is potassium important? What does a deficiency look like?

It is a critical ion within plants which helps maintain cell turgor pressure, osmotic pressure, and self-regulation. It can do a lot, like strengthen cell walls, regulate enzyme activity, promote stress tolerance, and enhance fertility. Not many functional/structural properties like N and P.

K deficiency symptoms include

• Yellowing and browning of the leaf margins and the leaf tip

• slow or stunted growth

• poor resistance to temperature changes and to drought

• early leaf senescence

Soils usually contain large amounts of K, but most of it is unavailable for plants.

Plants can only take up K as the K+ ion.

What do micronutrient deficiencies often cause? Where is this deficiency likely?

Chlorosis (often intravenal), stunting, low yields, and death. 

General conditions conducive to deficiency are:

• Leached, acid, sandy soils

• Organic soils

• Intensive cropping (high offtake)

• Extremes of pH

• Eroded soils

• Low-nutrient parent materials

What is Water Use Efficiency? Where does it come from? What is the equation?

WUE tells us how crops convert water into biomass or grain under particular environmental and management conditions

• Water-use efficiency is not a target we aim to maximise. 

WUE comes from:

• Rooting depth and access to soil water (rainfall pattern)

• Canopy development and radiation capture

• Atmospheric demand (Vapour Pressure Deficit)

• Nutrient status (nitrogen supply = Not enough nitrogen = not great roots = can’t get water/nutrients = less capture of CO2 etc. Also, increases soil evaporations because the smaller canopy provides less shade for the soil surface)

• Seed composition (Conversion efficiency of sugar into grain: cereals > pulses > oilseeds. Reflects differences in seed energy content. A plant can produce twice as much starch as fat using the same amount of raw sugar from photosynthesis. (So, some plants will require more water/photosynthesis than others.)

• Management affects WUE indirectly, by modifying these processes

WUE helps with understanding: crop outcomes under different climates and management and what is realistically achievable with the water available.

kg/ha/mm = grain yield / (water supply – evaporation)

What is the Harvest Index? What does it depend on?

Harvest index = conversion of biomass to grain yield (grain/biomass). Depends on incidence and severity of stressors during critical windows when grain number and grain size are determined. Grain number reduced when stress occurs during critical developmental stages, usually flowering. There is less of an effect outside of this stage.

What is the Vapour Pressure Deficit (VPD)

• Rate of stomatal water loss proportional to vapour pressure deficit (VPD). 

• The maximum amount of water vapour that can be held in a parcel of air increases exponentially with temperature. 

• Usually, air is not saturated. 

• High VPD = atmospheric demand for moisture is high. 

• As temp increases, the volume that air can hold increases. 

• Even if humidity is the same, temperature still causes the most VPD. 

• Stress on crops increases with VPD. 

What is the equation to find yield gap? Both in theory and in practice.

In theory, the yield gap due to water limitation is defined as: Yp - Yw

In practice, for rainfed systems, the yield gap is defined as: Yw - Ya

Achieving 80% of Yw (Y80) is considered a reasonable performance benchmark. 

Yield potential (Yp): Maximum yield achievable when growth is limited only by solar radiation, temperature, CO 2, and genotype

Water-limited yield potential (Yw): Maximum yield achievable under rainfed conditions, i.e., water supply is the only limiting factor

Observed (actual) yield (Ya): Actual yield achieved under field conditions

How do you calculate potential grain yield using WUE? Show equation

Potential grain yield = water supply - evaporation x WUE

(water supply = soil water at planting - soil water at harvest + in crop rainfall)

What are direct and indirect solutions to

• Maximising the capture and storage of rainfall and subsequent extraction of available soil water

• Increasing the efficiency with which available moisture is used

• Minimising stress during key developmental stages

1. Directly: by altering pattern of growth and water use through variety selection, time of sowing, sowing rate, canopy management. 

2. Indirectly: minimizing the effects of weeds and disease that compete for soil moisture and limit the ability of crops to access water. 

What is GxExM?

• The ability of an individual to achieve its maximum potential as encoded in its genome. 

• A function of the environment in which it completes its life cycle

• Which can be adjusted by management

How is phenotype measured using GxExM? (equation)

Phenotype (yield, quality, etc) = Genotype + Environment + GXE + Management + GXEXM