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What resources do plants need for growth?
- Light
- Soil
- Air (carbon dioxide and oxygen)
- Water (transport, structure and cooling)
- Nutrients (essential elements)
- Space
- Temperature
What are essential elements for plants?
They are chemical elements that a plant must be supplied with and cannot survive without.
Which nutrients are considered macro or major elements with removal measured in tens of kg/ha/year?
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Magnesium (Mg)
- Sulphur (S)
- Calcium (Ca)
Which macro nutrient is essential for sugar beet and some vegetable crops?
Sodium (Na)
Which nutrients are considered micro or trace elements with removal under 1 kg/ha/year?
- Molybdenum (Mo)
- Boron (B)
- Iron (Fe)
- Manganese (Mn)
- Zinc (Zn)
- Copper (Cu)
- Chlorine (Cl)
Which micronutrient is never deficient due to its extremely small requirement?
Chlorine (Cl).
What is pH in the context of the soil?
A measure of the soil solution's acidity or alkalinity, ranging from 0 to 14, with 7 as neutral.
What is the general pH range of UK soils and what is the optimum pH on mineral soils?
UK soils typically range from pH 4.0 to 8.0, with 6.5 considered optimum for most mineral soils.
Why is the optimum soil pH for UK mineral soils specified as 6.5?
It ensures trace elements are available whilst still avoiding heavy metal poisoning
Which trace element has limited availability at pH 6.5?
Manganese (more readily available at lower pH's)
How does a one-point shift on the pH scale affect acidity or alkalinity?
A single-point shift on the pH scale results in a tenfold change in acidity or alkalinity.
Which trace element becomes more available at high pH levels in contrast to others that become less available?
Molybdenum remains more available at high pH, while most other trace elements decrease in availability.
Why might foliar feeding be preferred when soils have a high pH?
It corrects any deficiencies in crops grown on alkaline soils in the most cost effective way, since adjusting soil pH down is often impractical or too costly.
How can a low soil pH lead to heavy metal poisoning in plants?
Acidic conditions increase the solubility of metal ions (like manganese), potentially causing toxicity if the plant takes up too many.
What can be done to prevent soils becoming too acidic which may lead to heavy metal poisining?
Liming to increase pH
Which major nutrient is notably affected by soil pH levels?
Phosphorus is significantly influenced by pH, as it can become locked up at very high or very low pH values.
Which micronutrient has significantly reduced availability below pH 6.5?
Molybdenum
Which macronutrient has significantly reduced availability below pH 7?
Magnesium
Which micronutrient has reduced availability above 6.5 but is not effected by lower pH levels?
Iron
What can be said about the relationship between micro and macro nutrients and soil pH?
In general, the following will be more available at:
Macronutrients = Higher pH (except P and N)
Micronutrients = Lower pH (except Mo)
Which crops are most sensitive to acidic pH values (between 5.9 and 6.0)?
- Field beans
- Peas
- Sugar beet
- Barley
Which crops are most sensitive to acidic pH values (around 4.9 to 5.0)?
- Ryegrass
- Rye
- Potatoes
What are the critical pH values for wheat and maize?
5.5
What are the critical pH values for OSR?
5.6
What are the critical pH values for linseed?
5.4
What are the critical pH values for oats?
5.3
How does soil become more acidic over time when organic manures containing ammonium are applied?
Ammonium ions are nitrified by soil microbes into nitrates, leaving excess hydrogen ions that lower the soil pH.
Why might grassland pH be maintained below the standard optimum?
A slightly lower pH helps avoid trace element deficiencies in the grass, which in turn benefits livestock health.
Why should organic matter (OM) soils have a lower than optimum pH?
OM molecules bind to nutrients, so keeping the pH slightly lower helps prevent additional trace element deficiencies.
What factors should be considered when making liming recommendations?
- Measure soil pH
- Consider soil type (especially clay vs sandy)
- Crop rotation needs
- Reference tables (RB209)
Why do clay soils require liming less frequently but often need a higher amount of lime when they are limed?
Clay particles hold onto calcium ions more effectively but also have a high buffering capacity, meaning more lime is needed to neutralise excess acidity.
What is meant in relation to liming if clay soils are said to have a high buffering capacity?
The clay particles can bind and neutralise many hydrogen ions due to their large surface area, requiring lots of calcium ions to displace them and therefore more lime
What does the fineness/particle size of a liming material determine?
How quick the lime will act and increase the soil pH
What does the neutralising value of a liming material determine?
How concentrated it is
What used to be the most effective liming material and why is this no longer used?
Burnt Lime (calcium oxide) at 80% NV was banned due to exothermic reactions with water
What is the most effective liming material available today?
Hydrated / Slaked Lime (calcium hydroxide) at 70% NV - burnt lime reacted with water (very expensive)
Which liming materials are most commonly used?
- Ground Chalk
- Limestone
- Magnesium Limestone
What is the NV of Ground Chalk, Limestone and Magnesium Limestone?
54%
What two factors of liming materials determine their effectiveness?
Particle size and neutralising value
What is the primary function of nitrogen (N) in plants?
Nitrogen promotes leaf growth, chlorophyll and enzyme production, and overall vegetative development.
How does nitrogen deficiency typically appear in plants (SCFE)?
- Stunted root and shoot
- Chlorosis of the older leaves
- Fewer tillers and branch stems
- Early ripening
Which leaves does a nitrogen deficiency effect?
Older leaves
Is nitrogen mobile in the plant?
Yes - moves from older leaves to new leaves if deficient
Why is nitrogen usually applied in multiple spring applications?
Splitting applications helps match plant demand, reducing the risk of leaching as more will be up-taken
What happens to the remaining nitrates which are not up-taken by the plant roots?
- Leaching
- Denitrification
What is the legal drinking water limit for nitrate levels and why was it set?
50 mg/L - originally established over concerns about "blue baby" syndrome and possible cancer risks, though evidence is inconclusive.
What is denitrification and under what conditions does it occur?
The conversion of nitrate to nitrogen gas by anaerobic bacteria, commonly happening in waterlogged or compacted soils.
What is immobilisation in the nitrogen cycle?
Where nitrates in the soil are converted back into organic proteins
How does most nitrogen exist in the soil and how does it become plant-available?
Most nitrogen is in organic matter as proteins; microbial activity mineralises these proteins to ammonium, which is then nitrified to nitrate for plant uptake.
What is volatilisation in relation to nitrogen?
When ammonium is transformed into gaseous ammonia and lost to the atmosphere.
What is mineralisation in relation to nitrogen?
Where nitrogen stored in organic proteins is broken down into ammonium ions
What is nitrofocation in relation to nitrogen?
Where ammonium ions in the soil are converted to nitrates
In what order do the following five materials containing nitrogen contribute to soil N content?
- Organic manures
- Crop residues
- Pollutants
- N fixation
- Fertilisers
Why is phosphorus (P) so vital in plants?
Phosphorus is crucial for energy transfer (ATP), photosynthesis, nucleic acid formation, and oil transport within the plant.
What are typical phosphorus deficiency symptoms (SCL)?
- Stunted root and shoot
- Chlorosis of the older leaves
- Leaf discolouration (purple)
Which leaves does a phosphorus deficiency effect?
Older leaves
Is phosphorus mobile in the plant?
Yes
How is phosphorus taken up by plants and why is it harder to absorb than nitrate or potassium?
Plants absorb phosphorus as phosphate ions, but these ions exist in very low concentrations in soil solution, requiring significant energy to acquire.
Why is phosphate generally not prone to leaching?
Phosphate levels in soil solution are very low, so there is little excess to wash away (< 1kg/ha/year) unless the soil phosphate index is extremely high (4 or above)
What is phosphite and how does it relate to phosphate in plants?
Phosphite ions enter a plant quicker when foliar applied, but the plant must convert phosphite into phosphate, and the timing of this conversion is unknown.
Which property unique to phosphate allows it to "piggyback" on cation exchange sites and is also classed as readily available P in the soil?
Phosphate, despite being an anion, can attach alongside magnesium or calcium ions bound to the soil's exchange sites due to opposing charges. Weak attraction so P ions readily move back in to the soil solution for crop uptake.
What is precipitated calcium/magnesium phosphate?
It forms when calcium or magnesium ions react with phosphate, creating a less soluble compound that the plant cannot uptake which precipitate out of solution as tiny solid particles
Why does calcium/magnesium phosphate dissolve at optimum pH (> 7.0)?
The available phosphate in solution is depleted and so the calcium/magnesium phosphate must dissolve back in to solution to maintain equilibrium
When will the solubility of calcium/magnesium phosphate decline?
if the soil pH increases above 7 (more available phosphorus)
When will calcium/magnesium phosphate become less available?
The higher the pH (more phosphorus available)
Why does phosphate lock-up occur in clay soils below pH 6?
More metallic (iron and aluminium) ions become soluble in acidic conditions, reacting with phosphate to form complex molecules that are then permanently unavailable to the plant
Why should you prevent the pH from becoming to acidic when dealing with phosphorus?
Phosphate lock-up will occur which is irreversible
How does index 2 describe the available phosphate level in combinable crops and grassland?
It is at maintenance level - crops will not suffer deficiencies and fertiliser applications will replace crop P off-take
What is advised if the soil index for phosphorus is less than 2?
Seed bed applications are necessary before drilling the following crop
Why is it vital to apply phosphorus on potatoes even when it has a soil index of 2?
Potatoes have a high yield response to phosphorus - ensuring no deficiencies is key so it is better to be safe than sorry
What role does potassium (K) play in plants?
Potassium regulates water movement and acts as a catalyst for biochemical reactions
Is potassium mobile within the plant?
Yes
What are visible symptoms of potassium deficiency in cereals (LSF)?
- Leaf tip chlorosis
- Stunting
- Fewer tillers
What are visible symptoms of potassium deficiency in broadleaved crops (OSF)?
- Older leaf margins will show chlorosis
- Stunting
- Fewer tillers and branch stems
What are visible symptoms of potassium deficiency in potatoes?
Bruising occurs more easily
Which leaves does a potassium deficiency effect?
Older
Where is most of the potassium found within plant cells?
It doesn't form any major structures and so floats around as an ion in the cytoplasm
What form is potassium found in the soil?
Potash, taken up by the plant as potassium ions (K+)
Where do clay soils have naturally abundant potassium levels (Native-K)?
Potassium is naturally released from broken-down minerals within the clay structure
What is luxury uptake of potassium?
Plants can absorb more potassium than they actually require, which can lead to nutrient imbalances (unique to K)
How can high potassium levels affect magnesium (Mg) availability especially for livestock?
Excess potassium uptake can lead to magnesium deficiencies and for lactating livestock fed those crops it can cause hypomagnesemia.
Why is magnesium (Mg) important for plant growth?
Magnesium is at the core of the chlorophyll molecule and functions as a catalyst in various enzymes.
What are the main functions of magnesium within the plant?
Constituent of chlorophyll and enzymes
What are magnesium deficiency symptoms in crops as well as the specific symptom seen in potatoes?
- Older leaves show interveinal yellowing (veins remain green while the tissue between them turns yellow)
- Necrotic spots also seen in potatoes
Which leaves does a magnesium deficiency effect?
Older leaves
What does the saying "old maggie young man" refer to in terms of defficiences?
Older leaves - Mg
Younger leaves - Mn
What form is magnesium taken up by the crop?
As Mg²⁺ ions
Which soils are most prone to magnesium deficiency and why?
Sandy soils have fewer cation exchange sites to hold Mg²⁺ ions
Which crops have higher magnesium demands?
Sugar beet and potatoes often need more magnesium than most combinable crops.
How is magnesium usually supplied to crops and how can it also be applied if needed quickly?
Soil uptake usually but foliar application (epsom salts) can address deficiencies quickly.
Which other nutrient does magnesium work together with?
Calcium - they are the most important nutrients for growth
Why is calcium (Ca) so crucial in plant cell walls?
Calcium strengthens cell walls and helps neutralise acidity within plant cells, maintaining structural integrity.
Is calcium mobile within the plant?
No - it is very immobile
Which crops often show calcium deficiency and why?
Fast-growing crops (e.g., salad crops) may show deficiencies because calcium is immobile and struggles to reach rapidly developing younger tissues.
What are signs of calcium deficiency in cereals (DSNDW)?
- Discolouration/misshapen leaf tips
- Stunted growth
- Necrosis of leaf margins
- Death of growing point
- Weak stems
What are signs of calcium deficiency in potatoes?
Internal rust spot (economic impact)
What are signs of calcium deficiency in beans?
Twisted plants
Which leaves does a calcium deficiency effect?
Younger leaves
Which calcium-containing fertiliser is the most soluble but also very expensive?
Calcium Nitrate
What are typical sulphur deficiency symptoms and what specific symptom is seen in OSR?
- Yellowing and stunted growth of younger leaves (similar to N)
- Causes pale petals in OSR.
