It’s a familiar story. Despite all our care and effort, when plants wilt, young leaves turn yellow, and healthy roots blacken and die, we’d like to know why and how to prevent them.
Soil enhancement and the right fertilizers can prevent plant malnutrition symptoms such as 1) chlorosis (abnormal coloring), 2) interveinal chlorosis (chlorosis between veins), 3) stunting, as well as 4) necrosis (death of plant tissue).
The most common causes are too much or too little light, water, or nutrition. Clearly, the more confusing part is nutrition, simply because we can’t see nutrients and what it is exactly that they do.
This guide summarizes everything you should know about plant nutrition and what you can do to fix common issues.
We often take for granted the expected signs of balanced plant nutrition: plants germinate, flowers bloom, and fruits ripen perfectly. We don’t need to know the 21 nutrients that make these happen.
However, the most common issues involve six macronutrients that plants absorb a lot of 3 primary nutrients) and moderate quantities of 3 secondary nutrients). Why do we need to know this?
- Nutrient competition: Plant nutrients can induce plant malnutrition when they compete against each other.
- Symptom confusion: The symptoms of plant malnutrition are similar even if they’re caused by different nutrients or triggered by non-nutrient causes.
- Multiple options: Treatments and interventions are available from the air, the soil, and fertilizers as natural or synthetic as well as organic or inorganic nutrients.
FREE PDF: Plant Nutrients (6 pages)
USEFUL LINK: What Makes Plants Grow?
RELATED VIDEO: Keep your garden blooming forever (3:34 minutes)
This guide will answer three key questions:
- Sufficiency: When do you know if your plant has enough of a nutrient?
- Toxicity: When do you know when there’s too much of that nutrient?
- Deficiency: How can you know that there’s not enough of that nutrient?
How Do Nutrients Cause Malnutrition?
Of course, fertilizers provide nutrients to plants. The thing is, one nutrient can also cause malnutrition when it prevents your plants from absorbing other important nutrients. For example:
USEFUL LINK: Growing Indoor Plants with Success
What’s Confusing About Plant Malnutrition?
Whether plants absorb a little or a lot, each nutrient is equally important. You should know when there is nutrition toxicity (too much) or nutrition deficiency (too little).
- Nutrition deficiency: When one nutrient is lacking, a plant may not germinate. It may fail to develop roots, stems, and leaves. Or the plant may not flower or create seeds. In many cases, plants simply die due to nutrition starvation.
- Nutrition toxicity: On the other hand, too much nutrition can cause problems. For instance, too much nitrogen creates too many leaves and no fruits; overfeeding with manganese turns leaves yellow/ and too much boron kills plants.
When the symptoms of nutritional lack or excess are similar, diagnosing plant malnutrition can be confusing. For example:
RELATED LINK: The Definition of Necrosis in Plants
FREE PDF: Micronutrient Chlorosis
The 3 Macronutrients
Just like the vitamin supplements we take, macronutrients are food supplements for plants except that plants can get their nutrients from the air, from the soil, or from fertilizers.
Nitrogen (N), Phosphorus (P), and Potassium (K) is the plant’s macronutrients. This because they are consumed in the largest quantities. That’s why these are considered the three primary nutrients.
PRO TIP: Plants tend to absorb less nutrients in the cold. Warm up the soil before planting.
FREE PDF: Early Detection of Nutrient Deficiencies & Toxicities (36 pages)
Nitrogen is probably the most famous nutrients for plants, and for a reason. It is responsible for the number of leaves among other things. However, having more nitrogen than needed, is not good neither. Why? Let’s check below.
To measure nitrogen sufficiency, many are using chlorophyll meters for precision. However, most gardeners simply look for healthy green leaves and fruits that ripen with enough sweetness. In addition, they also check that:
- plants are strong and do not fall over
- plants produce sufficient amounts of sugars and starches
- plants are healthy and not overly succulent
- plants contain sufficient trace elements such as copper and boron
- tubers do not rot due to accumulated water
- plants flower and fruit evenly and on time
USEFUL LINK: More Problems with Synthetic Nitrogen Fertilizer
FREE PDF: The Nitrogen Cycle (5 pages)
In general, nitrogen toxicity shows as succulent plants with dark green leaves that flower or fruit poorly and can’t resist diseases or pests. Plants need longer growing periods while fruit maturity may be delayed.
- Usual Causes: Excess nitrogen (NH4+) can happen when farmers have access to cheap fertilizer but are confused or misinformed about proper and correct usage. Other causes are:
- Groundwater pollution.
- Raw animal manure in soil
- Ammonia (NH3) that prevents plants from absorbing nitrogen
PRO TIP: Always compost animal manure before using it as fertilizer.
- Diagnosis: When healthy leaves are falling, wilting, and yellowed, and when edges of leaves turn brown, you’ve got nitrogen toxicity.
- Effects: There effects of nitrogen toxicity includes problems in plant health, garden production, and crop management.
- Plants that absorb too much nitrogen may be stunted, spindly and tall.
- Too many leaves grow.
- Stems are weak, with little or no flowers or fruits.
- Roots are weaker and shorter.
- Plants show less resistance to frost damage
- Plants attract more insects and pathogens.
- Eventually, nitrogen toxicity kills the plants.
- Suggested solutions: To draw out some of the excess nitrogen, lay mulch over the soil. For instance, when soft bark or wood break down, they use up nitrogen. For rare or isolated symptoms, use a dry fertilizer with low N and high K ratios. For more serious or widespread symptoms, use dry fertilizer with a zero N ratio.
FREE PDF: Know Your Nitrogen (4 pages)
Nitrogen Deficiency Sympthoms
General indicators of nitrogen starvation include small plants with chlorotic (yellowing) lower leaves while younger leaves seem normal. Plants with normally red or purple tissue may look darker.
Some flowers may appear late but bloom in profusion. Eventually, the leaves may wither, drop early, or die. Most fruits are smaller, more vividly colored, and mature early.
- Usual Causes: In most cases, plants absorb less nitrogen when soil has high phosphorus or carbon such as from sawdust;
- Diagnosis: To diagnose nitrogen deficiency, many farmers eyeball indicator crops such as cereals, pulses, sorghum, and maize. However, most gardeners use other physical indicators.
Nitrogen deficiency cause stunted plants, younger leaves to look pale or bleached, while older leaves turn yellow-green then dry out and crumble easily.
Initial Indicators of Nitrogen Deficiency: Plants that are starved of nitrogen generally look pale and stunted. Onset nitrogen deficiency is signaled by smaller plants, flowers, or fruits as well as lower or older leaves and stems turning an unusual yellow-green, yellow, or even pink.
Advanced Indicators of Nitrogen Deficiency: Serious signs of nitrogen deficiency include yellowing of the entire plant or entire rows of crops as well as necrosis, when lower leaves rot or die. Some of the more common signs of nitrogen deficiency are:
- plants that are overly succulent
- tubers that accumulate water and rot
- plants that are weak and fall over
- delayed flowering and fruiting
- fruits that ripen unevenly
- leaves in unusual colors
PRO TIP: Do not be confused. Although chlorosis or discolored leaves do indicate nitrogen deficiency in the soil, these may also indicate:
- protein deficiency in the plant,
- a disease in the plant
- insects attacking the plant,
- insufficient sunlight, or
- soil that’s too dry, too wet, or too cold.
- protein deficiency in the plant,
Soltions to Nitrogen Deficiencies
Isolated or rare symptoms of nitrogen deficiency can be corrected by using slow-release, dry fertilizer at the dripline, which is then activated with water. To enrich the soil with organic nitrogen:
- Use a mulch of nettles, a rich source of natural nitrogen
- Add composted manure to the soil
- Plant a green manure crop (e.g., borage, grass, legumes, clover, vetch)
- Plant nitrogen-fixing plants (e.g., legumes such as peas or beans), or
- Add coffee grounds to the soil
- Tested Treatments: In more serious or widespread symptoms of nitrogen starvation, use fast-release liquid fertilizer.
PRO TIP: The correct nitrogen-potassium ratio is critical for instance, high nitrogen with low potassium is best for vegetables while low nitrogen with high potassium is best for flowers and fruits.
PRO TIP: When using granular nitrogen fertilizer, sprinkle and rake on plant beds or mix in topsoil. When using liquid nitrogen fertilizer, dilute in water and spray or pour over soil around plants, but never as a foliar treatment because nitrogen can burn the leaves.
Extra Nitrogen Recommended Plants that require more nitrogen include kitchen crops such as greens, cucumbers, muskmelons, okra, peppers, pole beans, squash, sweet corn, and tomatoes.
Phosphorus is our second macronutrients. Despite being less know that nitrogen is role is quite important and you will definitely notice any problem related to its absence or excess.
Plants with healthy roots and shoots are indicative of sufficient phosphorus. As well, stalks, stems, and branches are firm and strong. In addition:
- Plants mature earlier and more uniformly.
- Plants produce better seeds and flowers.
- Plants are more resistant to plant diseases.
- Legumes have more capacity for nitrogen-fixing.
Causes: Since phosphorus doesn’t leach from soil, phosphorus-rich manure, compost, or inorganic fertilizer causes phosphorus toxicity build-up in soil as well as in rivers, lakes, streams, and oceans.
Diagnosis: Since symptoms are similar to manganese and iron deficiencies, phosphorus toxicity is often difficult to diagnose. General symptoms include leaf tissues with watery edges that become necrotic.
Effects: High-phosphorus content in soil prevents plants from absorbing iron, manganese and zinc micronutrients in the soil, thus inducing malnutrition, which can be followed by fatal necrosis. Excess phosphorus absorption induces deficiencies in nitrogen (N) zinc (Zn), iron (Fe), or cobalt (Co). Severe phosphorus toxicity is fatal to plants.
Suggested interventions: For isolated or rare indicators of phosphorus toxicity, eliminate all use of organic compost, mulch, or any type of manure. You can also avoid soil additives such as compost containing crab or shrimp shells, hair ash, mushrooms, or cucumber skin.
Suggested treatments: To treat more serious or widespread symptoms, switch to low K, slow-release dry fertilizers from mineral sources. If necessary, use very low-phosphorus, slow-release, organic nitrogen sources or mulches such as pine bark mulch or blood meal
PRO TIP: Never spray high-phosphorus liquid fertilizer on plants. It burns leaves.
Usual Causes: Most cases of phosphorus loss in soil is attributed to runoff leaching or soil erosion as well as soil temperature that is less than 55 degrees F.
Diagnosis: Many find it difficult to diagnose phosphorus deficiency. Symptoms are similar to other nutrition imbalances (e.g., shorter roots, purple chlorosis). Since the only sign is stunting at first stages of growth, it’s too late to correct when other symptoms show.
To diagnose phosphorus deficiency, some farmers use indicator crops such as maize, leucerne grass (Medicago sativa L.), tomatoes, and cereals.
Initial Indicators: The onset of phosphorus deficiency shows on older leaves. Symptoms include abnormal colors such as purplish stems and bluish-green or darker lower leaves. Plants that are starved of phosphorus produce less seeds, flowers, and fruits.
In large, leafy vegetables, leaves look darker or leathery with a purplish tinge. Chlorosis may also show as purplish, reddish, or dull-bronze petioles, veins, and lower stems. Other visual signs of phosphorus deficiency include:
- smaller plants
- leaves are few, distorted, or smaller
- shortened or stunted growth
- older leaves look reddish or bluish-green, later becoming bronze or red
- anthocyanin (red or purple pigments) on the underside of leaves, stems, seedlings, transplants, and on some lower stems
- stems are normal in length but smaller in diameter
- premature dropping of lower leaves
- reduced flowering
- fruit ripening is delayed
Advanced Indicators: Plants that are starved of phosphorus mature early with low- quality fruit or leaves. In more serious cases, phosphorus deficiency shows as lower leaf necrosis. In addition, plants suffering from serious phosphorus starvation show:
- older leaves that shrivel, die and drop
- weak, thin, spindly stems
- darker older leaves with shortened stems
- less or defective seeds, flowers, and fruits
Suggested Interventions Deficiency: To rise the phosphorus level mix some bone meal, guano, manure, or powdered rock phosphate in the soil around the plant. While the affected leaves may not recover, new leaves will grow and appear healthy.
For some isolated or rare instances, the following interventions are recommended:
- Use a phosphate fertilizer as the one below
PRO TIP: Cow manure has high ammonia content but it also contains pathogens.
Tested Treatments: For more serious or widespread symptoms, apply fast-acting colloidal phosphate, rock dust, or liquid high-phosphorus fertilizer.
- To unlock phosphorus in soil for increased plant absorption, add lime (calcium hydroxide) to reduce soil acidity.
- To offset removal by crops, the best phosphorus content in soil is from 30 to 50 ppm.
Extra Phosphorus Recommended: Plants that require a lot of phosphorus include all legumes and plants with fast-growing leaves. Cold-weather plants with short roots and quick top growth (e.g., lettuce) also consume lots of phosphorus.
In addition, the start of the growing period of annuals – such as corn, beans, rice, peas, watermelon, wheat, marigolds and zinnia – requires more phosphorus.
FREE PDF: Phosphorus Dynamics – From Soil to Plant (9 pages)
Potassium is the last of the three plants macronutrients. Very rarely my plans have been affected by potassium deficiency. Nonetheless, being able to spot what potential problem might cause it is important especially when you use new soil or plant in new gardens.
Plants that get enough potassium have sturdy stems, resist cold temperatures, are hardy and flowering well. In addition:
- Plants are highly resistant to drought; they don’t droop or wilt between watering.
- Flowers, tubers, cucurbits and fruits are well-developed.
- Plants have deep and healthy roots.
- Plants can resist damage and frost.
- Perennial plants survive winter temperatures.
USEFUL LINK: How to Make Homemade Potassium for Plants
Potassium toxicity is caused in case of weather degrades mica in clay or feldspar in sand and silt, organic reactions result in organic acids that increase the potassium content of soil.
- Potassium is released into the soil by tree roots as well as by the early stages of decomposition of leaves, seeds, bark, and other plant matter.
- Some potassium is released into soil from aerosols, dust, and rainfall.
- Draining or running water from forested to lowland areas also inject potassium into soil.
PRO TIP: Lower potassium soil content to minimize phosphorus that increases algae growth in waterways and kills aquatic organisms.
Diagnosis: There are no indications that excess potassium affect plants at toxic levels. However, deficiencies in other nutrients such as magnesium, calcium, and nitrogen can be indicative of potassium toxicity.
Effects: So far, there have been no cases where plants that naturally absorb excessive amounts of potassium from soil show effects of imbalanced nutrition.
Research indicates that excess potassium can induce nutrient deficiencies as plants are unable to absorb nitrogen, iron, zinc, manganese, and magnesium.
Suggested Solutions: Although excess potassium is apparently nontoxic to plants, it induces nitrogen, magnesium and calcium deficiency. Thus, soil testing and assessment is recommended. Other options include:
Interventions: For isolated or rare indicators of potassium toxicity, apply slow-release fertilizer with a low potassium ratio. Remove mineral-containing rocks from the soil.
Treatments: To treat more serious or widespread symptoms, drain, dry and filter the soil. Subsequently, apply only liquid, low K fertilizer.
There are two causes of potassium deficiency: one happens in the plants, and the other happens in the soil.
In plants: As needed, plants can move potassium in the cell sap from its older leaves to the newest.
When potassium moves from the lower leaves to upper leaves and is not replenished, water replaces the lack of potassium, causing turgid plant cells to become flabby instead. The plant cells weaken and susceptible to pathogen attacks.The plants cannot withstand frost, drought, or high salt content.
In soil: Extreme liming or true soil deficiency can cause potassium deficiency. Moreover, aside from lack of moisture or water content in the soil, the most common triggers of potassium deficiency in plants include soil that is:
- with low pH
- in low temperature
- highly compacted
Diagnosis: To diagnose potassium deficiency, many large-scale farmers use indicator crops such as banana, cotton, and potato.
PRO TIP: A little potassium in the fall helps plants to be stronger in winter.
Symptoms of potassium deficiency show differently at the onset compared to during the more advanced stage. Here’s how you can tell.
Initial indicators of potassium deficiency: The onset of potassium deficiency shows on leaf edges going inwards, and on older leaves first then spreading to younger leaves, as follows:
- new leaves are smaller than old leaves; stunted roots, shorter internodes; slow, stunted growth
- brown or purple spots on older leaves while veins look normal
- leaf edges curl downwards or crinkle and roll upward
- leaf tips and edges look bronzed, dry out and become brittle; shoot tips may show short, bushy zigzag growth, or die out later
- stems, branches weak, break too easily; more pests and pathogens
- plants wilt readily, such as between watering
- flower buds are few
- fruits are small and show poor color
- may need more water yet produce less output
Advanced Indicators: In more serious or widespread cases, potassium deficiency shows as lack of flowering, more and increasingly bigger dead tissue (necrotic lesions) on other leaves, followed by death of plant tissues (necrosis). Other symptoms include:
- stems and stalks are shorter than usual
- upper leaves appear healthy while lower leaves show necrotic spots
- lower leaves show chlorosis: reddish veins and brown or yellow margins
- rare or zero flowering or fruit
- deformed roots
- low resistance to insects or diseases
Suggested Interventions: For mild or isolated symptoms, options to increase potassium in the soil include:
- Increase moisture in soil to help potassium move towards plant roots.
- Bury some crushed banana peel an inch below the topsoil.
- Mix fruit, coffee grounds, urine, manure or wood ash to compost and mix with soil.
- Use kelp, greensand, or fertilizer made from mined rock powder.
PRO TIP: Use wood ash with caution; it can burn the leaves of plants.
Tested Treatments: For more serious or widespread symptoms, use comfrey liquid to add natural potash or liquid fertilizer with high potassium content as a foliar shower. Here are some product options:
- Muriate of potash (KCl) in 50, 41, or 33% concentrations as the one below
- Potassium sulfate (K2SO4) in 43% concentration
- Potassium nitrate (KNO3) in 37% concentration
- Sulfate potash magnesia (K2SO4 or MgSO4) in 18% concentration
- Kainite (KCl + NaCl + MgSO4) in 10% concentration
PRO TIP: Deep comfrey roots supply potash to fruiting plants grown in pots.
Extra Potassium Recommended: Soil with high potassium and pH levels are best for fruiting or flowering plants such as currants, strawberries, quince, apples, cucumber, squash and tomatoes. In particular, perennials, biennials and carbohydrate-rich plants such as potatoes, beets, cassava, and yam need a lot of potassium to produce bigger tubers.
FREE PDF: Plant Nutrients – Potassium (2 pages)
RELATED LINK: Why the potassium deficiency in my crop?
Congratulations! You’ve just completed your review of the three primary nutrients. Up next: the three secondary nutrients.
The 3 Micronutrients
Plants consume secondary macronutrients much less than primary nutrients. However, make no mistake: any calcium, magnesium, or sulfur deficiency causes serious problems.
Let’s begin with calcium, a hard mineral that’s still soft enough for microbes.
As strange as it sounds calcium is not only a human nutrients, but also a plant one. however, differently from teh three we discussed boefore (N-P-K) plants they have only very small intake of it.
Sufficient calcium nutrition is expected from soil with appropriate pH levels. Plants remain healthy in cold weather or even in highly humid air. In addition:
- Fruits do not show rot or deterioration.
- Young leaves do not show curling ends or discoloration.
- Older leaves do not show yellowish edges or necrotic spots.
Usual causes: When calcium toxicity occurs, it is often attributed to over-liming or excessive use of gypsum. Another cause is when the soil pH level doesn’t agree with plant needs.
Diagnosis: One indicator of calcium toxicity is pH imbalance in the soil, which causes blossom end rot in plants.
Effects: While calcium toxicity is rare, it indicates high soil pH where soil is so alkaline that plants can’t absorb nutrients, thus inducing deficiencies in potassium and magnesium.
PRO TIP: Never use calcium in soil with high magnesium content.
- apply elemental sulfur that converts to urea-sulfuric acid fertilizer
- apply an acid that converts to sulphuric acid
- use organic mulches to gradually increase soil acidity
- increase soil acidity with ammonium sulfate as the one below
The lack of water and not lack of calcium that causes calcium deficiency. In other cases, it’s too acidic soil that prevents plants from absorbing calcium in the soil.
Calcium deficiency happens rarely. However, when it does happen, symptoms begin to show on growing ends or tips and on the youngest leaves, and more frequently on fleshy fruits that are just developing.
PRO TIP: The symptoms of calcium deficiency can be confused with those of magnesium toxicity.
RELATED VIDEO: Complete Guide to Fertilizing Houseplants (37:59 minutes)
Diagnosis: Generally, when younger or newer parts of plants are smaller than older plant parts, there is potassium deficiency.
Initial Indicators: The most common indicators of calcium deficiency symptoms first show on the plant’s fast-growing tissues and in new growths such as buds, tips, and young leaves. Other onset indicators are:
- New leaves hooked back and yellowish while old leaves are dark green.
- Mature leaves form curl into cups.
- spots of necrosis on margins of young leaves (brown lesions, end rot)
- stunted growth
- burn tips in leafy vegetables (lettuce, spinach, cabbage, etc.)
- chlorosis of new leaves with reddish veins, brown or yellow edges
- weak stems and stalks
- little or no flowering or fruiting
- Less buds grow, end buds die.
- reduced weak, shorter or deformed roots
- low resistance to insects or diseases
Advanced indicators of calcium deficiency include plants that stop growing, or growths are twisted, disorganized, and die off. Roots are most severely affected. Other symptoms include:
- necrotic spots on leaves grow larger
- newest tips and leaves are deformed, crinkled, hooked or curled, similar to herbicide poisoning.
- outer edges of leaves wither and die
- bitter pit in fruits (apples, pears, and so on)
- rust spots in tubers (potatoes, yam, taro, etc.)
- small nuts or empty shells in ground nuts
- roots are leaky, thin, or stunted with root disease (e.g., Pythium) or show malformed brown or black tips.
- shoots are distorted and eventually show terminal dieback
- stems break, or the entire plant collapses
- necrosis (death) of buds, root tips.
PRO TIP: Root diseases can cause random nutrient deficiencies.
Suggested Interventions: For isolated or mild symptoms, increase calcium intake in growing plants. Some options are:
- Cut away or pick off the affected buds, leaves or fruits.
- Use mulch or compost rich in calcium or high in nitrates such as oat hay, sorghum, corn, or weeds.
- Spray a mild calcium solution thrice a week.
- In a pinch, dilute spoiled milk and water your plants with it.
PRO TIP: For acute calcium deficiency, microwave eggshells (or sun-dry for two days), grind to powder, and mix with two spoons of lime and vinegar. After a few hours, dilute with water and spray on plants.
Tested Treatments: In more serious or widespread symptoms, apply organic matter and agricultural lime to reduce soil acidity to pH 6.5.
- Use low-nitrogen, high-phosphorus fertilizer like the one below.
- Dissolve 4 tbsp calcium nitrate per gallon of water and apply to root areas.
- Apply calcium carbonate, phosphate salts, or magnesium carbonate.
- Apply gypsum or lime sulfate (calcium sulfate dehydrate), silica gel or hydrated aluminium oxide to clay soil.
PRO TIP: Use gypsum on soil with high clay content.
- Add expired calcium carbonate (e.g., chalk, Alka-Seltzer, Tums, Dicarbosil, Rolaids) to your compost.
- Cool the water used for boiling eggs and use it to water your plants. The is the fastest way to provide calcium to plants. Eggs shell do not work, if not in the very long term once broken down.
PRO TIP: If soil is slightly acidic (pH 6.5 or more), plants cannot absorb calcium from bone meal, only phosphorus.
To prevent calcium deficiency: (1) Completely dissolve water-soluble calcium nitrate and water the plants with it. (2) Make sure that soil and water have low sodium, and soil pH is 5.5 or less to prevent calcium deficiency. (3) For indoor plants, keep humidity between 40 and 60%. Use fans to keep leaves moving gently and remove excess moisture.
Extra Calcium Recommended: All plants need calcium, but extra calcium generally benefits plants such as tomato, potato, pepper, peanut, legume, cauliflower, celery, carrot, melon, and broccoli, to mention a few. Plants that produce high calcium include cucurbits, green leafy vegetables (collards, mustard, kale, etc.) and beans (soybeans, mung beans, kidney beans, etc.)
Magnesium is something often forgotten but can affect significantly the way leaf looks like. Here’s why and how to spot potential deficiencies or excess.
Plants with sufficient magnesium have bright green leaves that are free of spots or discoloration. In addition, they have potent seed or pollen, starchy tubers, as well as tasty fruits.
Magnesium deficiency is often caused by drought or excess potash. Magnesium deficiency happens in soil with pH 5.2 or less, it is more often caused. Also, magnesium accumulates in soil with high organic matter or clay content.
Confusingly, high magnesium content has also been found in soil that is tilled, used for travel when wet, and includes waterlogged and compact subsoil.
Diagnosis: First of all, it’s rare to find instances of magnesium toxicity. In most cases, calcium or potassium deficiency precedes that issue. Moreover, symptoms of magnesium toxicity are too similar to those of calcium deficiency.
Second, it’s not easy to spot when plants absorb too much magnesium. For instance, it’s rarely seen at the growing stage, in nursery crops or in greenhouse plants.
Effects: Excess magnesium induces potassium or calcium deficiency, so you might suspect too much salt (e.g., dark leaves, stunted growth). To complicate matters, excessive magnesium also increases salt levels in soil.
Suggested solutions: There are two suggested ways to address magnesium toxicity. First, help plants absorb more magnesium: increase soil pH or spray diluted Epsom salts every few weeks on cool days. Second, feed the soil and shift to low-magnesium fertilizers or soil additives.
Usual Causes: You might be delighted by leaves showing unfamiliar reds or yellows between green veins. However, when no fruits or flowers appear, you may have magnesium deficiency which is either a soil problem, or a plant problem.
In soil: Magnesium levels are very low in acidic soils (pH 6.0 or less). Although adding magnesium and raising garden soil pH are common practices, magnesium deficiency continues due to practices such as:
- excess irrigation
- more intensive harvesting
- more crop rotations per site
- more use of fertilizers lacking in secondary nutrients
- excessive use of fertilizers
In plants: Most cases of magnesium deficiency in plants are attributed to temperature and acidity in growing media. For instance:
Cold: Plants growing in low-temperature soil tend to absorb less magnesium
Low-pH: Plants can’t absorb magnesium in soil with pH 5.5 or less.
PRO TIP: When soil is rich in phosphorus, adding Epsom salts don’t work.
Diagnosis: Keep in mind that the following symptoms differ in each plant, so much so that a generalized description is difficult. Having said that, most magnesium deficiency happens in sandy soil with high levels of nitrogen and potassium.
Initial Indicators: The onset of mild magnesium deficiency often shows during the 4th to 6th week as interveinal chlorosis in older leaves first. Plants will show clearer and more symptoms under bright light such as:
- newer leaves that are smaller than older leaves
- bleaching in new leaves that are paler or lighter-colored leaves
- leaves show curling edges and spots of dead tissue (necrosis)
- leaf veins that are dark green while leaf tissue are chlorotic: yellow-green, purple, white, or reddish
- in older leaves, edges and between veins turn yellow, brown, purple, or red
- yellow spots show between the leaf veins but not on young leaves
chlorosis increases on younger leaves; more leaves turn pale, brown, and die.
- older leaves begin to turn yellow, reddish, or purple.
- necrotic spots show on the older leaves at the lower levels
- growth is stunted, less seeds are produced
- twigs are weak
- leaves are smaller, thinner, turn brittle, and drop prematurely
- more susceptible to plant diseases
- fruits ripen with less sweetness and may drop prematurely
Suggested Interventions: For quick remedy of isolated or limited symptoms, use natural (unfiltered) water, or magnesium hydroxide (milk of magnesia). If these don’t work and soil testing confirms magnesium deficiency:
- Add rock dust.
- Apply Epsom salts whatever the soil pH.
- In cold weather, use cloches to raise soil temperature. For indoor plants, maintain a soil or root temperature of 20 to 25 degrees Celsius.
- Plants can uptake in soil with sufficient magnesium at pH 6.5 to 9.5. To stabilize soil acidity at pH 5.5, add lime to increase magnesium and soil pH.
- Add lime to highly acidic soil as the one below
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PRO TIP: There is more magnesium in dolomite than in standard lime.
- To add magnesium without changing soil pH:
- Dissolve 2 tablespoons of Epsom salts (hydrated magnesium sulfate) per gallon of water.
- Spray until moisture reaches the drainage holes at the bottom of each pot.
- Use as foliar spray 3x a week when new leaves sprout.
- Spray again when flowers start blooming.
- Sprinkle Epsom salt in the soil, about one tsp per foot of the plant’s height.
- Dissolve 2 tablespoons of Epsom salts (hydrated magnesium sulfate) per gallon of water.
PRO TIP: Epsom salt is not a fertilizer. It has no essential plant nutrients.
Tested Treatments: To increase magnesium content in soil:
- add organic matter to sandy soil
- stop using potassium additives
- improve soil drainage in anaerobic soil or in soil with high clay content
In more serious or widespread symptoms in soil with high calcium and potassium, try the following:
- use irrigation water (it contains sufficient Mg2+)
- broadcast inorganic fertilizer such as dolomitic limestone (8-10% Mg) before planting in acidic soil
- use sulfate of potash magnesia (11% Mg, 22% S, 22% K2O) to maintain current soil pH of low-potassium soil
- use soybean meal, cottonseed meal, or seed cake as magnesium sources (Note: these contains up to 4x potassium more than magnesium)
- Poultry manure contains magnesium and less than 50% potassium.
- Till the soil to at least 6 inches and amend the soil with lime granules or Epsom salts dissolved in water. Then, thoroughly mix the soil.
Extra Magnesium Recommended: Plants need more magnesium during dry periods, and during their growing stage. More magnesium may be required by plants such as azaleas and blueberries that grow well in soil with a pH of 9.5 or more.
At the same time, note that potassium in garden soil competes with magnesium and induces magnesium deficiency in plants.
PRO TIP: Balancing calcium and potassium while adding magnesium in magnesium-deficient soils with a low organic content is difficult and needs planning.
When mentioning sulfur you might be tempted to think that sulfur is something that plants should not take as sulfur is a pollutant. Almost. Sulfur acid is bad, but sulfur, on its own, and in small quantity, is necessary for plants.
In addition, plants have high tolerance against pests, diseases, drought, waterlogging, or extreme temperatures. The thing is, different triggers cause sulfur toxicity.
Sulfur toxicity, is rare and is often caused by plants with sulphide (H2S) injury. Most plants can absorb high levels of SO2 from air pollutants. At the same time, sulfur toxicity also occurs in
Diagnosis: The ideal sulfur concentration in soil is between 0.1 and 0.2 mg (SO2/m3). Thus, when it exceeds 0.6 mg, sulfur toxicity symptoms show as necrosis, first as spots that later spread over the entire leaf.
Effects: Plants can burn and die from oversupply of sulfur, which can result in adding too much salt in soil.
Suggested Solutions: While sulfur can decrease soil pH slightly, there are more detrimental effects that outweigh that small benefit. For isolated or rare symptoms:
- switch to fertilizers with low or zero sulfur content
- increase irrigation
For more serious or widespread symptoms:
- Replace the potting mix.
- Wash leaves regularly or spray with clean water
- As an alternative to slower elemental sulfur, apply iron sulfate in moderation (it burns plants) to quickly infuse sulfur nutrients as well as lower soil pH.
- Do not spray sulfur on sensitive plants such as cucurbit, raspberry, peach, and apricot, particularly during the growing period when leaves begin to sprout.
Other causes include:
- Excess application of manure can induce sulfur deficiency in plants.
- Less use of traditional organic manures
- Less use of fertilizers with high sulfur content
- More use of sulfur-free fertilizers such as urea, potassium chloride, and DAP (diammonium phosphate)
- Insufficient organic matter in soil
- More intensive planting that removes more sulfur from the soil
- Soil high in acids or alkalinity
- Excessive watering or high rainfall in winter that causes leaching of sulfur
- Dry soil that causes low mobility of sulfate nutrients
- Low soil temperature that causes low rates of sulfate mineralization
Diagnosis: Some Indicator crops commonly used to diagnose sulfur deficiency in soil include tea, cereals, and leucerne (Medicago sativa).
Similar to nitrogen deficiency, lack of sulfur first shows as leaves that are either light-green or pale yellow. However, unlike nitrogen starvation, sulfur deficiency first shows on younger leaves and remains even after adding nitrogen.
Initial Indicators: Initial sulfur deficiency is signalled by smaller plant size and young leaves that show interveinal chlorosis, stripes of light yellow or light green. These can later spread to the entire plant. However, older leaves remain green. Shoots are stunted though soil contains enough sulfur and persists after adding nitrogen.
Advanced Indicators: Advanced sulfur deficiency shows as yellowing of the entire plant, red discoloration, and lower leaf necrosis. Also:
- Chlorosis shows all over each leaf, not just on veins or edges.
- Plants are stunted and spindly with short and slender stalks.
- Plant growth is retarded, and maturity is delayed.
- Some plants show reddish stems and petioles.
- Legumes produce small or distorted nodules while fruits stay green and do not mature.
- Later, older leaves turn yellow green with long and woody stems.
Suggested Interventions: The recommended soil sulfur content is at least 5.5 ppm. For light or isolated symptoms of onset sulfur deficiency:
- Use slow-release fertilizer with a high sulfur content
- Increase clay content and balance soil pH (acidity)
- Regularly apply sulfur to sandy, well-irrigated soil.
PRO TIP: Powdered sulfur on hot days can burn leaves. Reapply after heavy rain.
On the other hand, if the soil contains adequate sulfur but plants are not absorbing enough, you can increase microbial activity with the following interventions:
- Till to reduce soil compaction.
- Improve drainage of water-saturated soil.
- Increase soil temperature by aerating cold soil.
- Apply sulfur to soil about 12 months before planting.
- Mix some ammonium sulfate to a side-dress fertilizer.
Tested Treatments: For more serious or widespread symptoms of sulfur starvation:
- Increase sulfur, calcium or phosphorus content of soil.
- Only use sulfur that soil bacteria has transformed into sulfate ion (SO₄²⁻).
- Increase the organic matter in soil.
- Apply pest management measures to stop root pruning by insects (e.g., rootworms).
Extra Sulfur Recommended: Plants require little sulfur but a deficiency can cause serious problems. Forages, oil crops, and a number of vegetables from the allium family (leeks, garlic, onions), legumes, alfalfa, and canola consume high amounts of sulfur nutrients.
Here are some of the most common questions about plant macronutrients.
Aside from macronutrients, do plants need other nutrients? Some plants need minute quantities of sodium, nickel, vanadium, cobalt, and silicon. Plants also need micronutrients such as iron, copper, manganese, boron, chlorine, zinc, and molybdenum.
What are the different types of fertilizers? Fertilizers are either organic (from plant or animal matter) or inorganic (artificial, synthesized). These can be grouped as compound versus straight fertilizers, and as controlled-release versus slow-release fertilizers.
Are there different ways of providing nutrition through fertilizers? Different types of fertilizer can be used as 1) top dressing, 2) base dressing, by foliar feeding, 3) base dressing by watering. Other methods fertilizer application are broadcast, banded, incorporated, seed-placed, banded near the seed, and dribble banded.
When is a foliar spray necessary? Foliar sprays are best for quick feeding via leaf tissue, particularly when the plant only needs a small quantity of the nutrient.
Is it important to use a chelated micronutrient? Chelation helps leaves to absorb some nutrients. It also protects nutrients from microbes and bio-chemical elements in soil.
Wow! You’ve just completed a thorough review of plant nutrition deficiency and toxicity.
Before you go, here are 4 takeaways to remember.
- Some nutrients induce nutrition toxicity or deficiency.
- Some symptoms of different nutritional imbalances are similar
- A nutrient deficiency may show symptoms associated with other nutrients.
- Any combination of pollutants, pathogens, pests, temperature, herbicides, drought, or soil acidity can create symptoms that may be confused as nutrient disorders.
Now that you’re informed about what nutrients do, you won’t be easily confused. Even better, you can now provide better plant nutrition practices.
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