How Plants Really Eat – Science Made It Easy


Do plants get hungry? Yes. When they’re hungry, thirsty, or nutrition-starved, leaves wilt and droop, stems are short and weak, roots and tissues darken and die (necrosis). But then again, how do they eat exactly?

Plants feed all the time, using light, air, and water to make their own food by absorbing nutrients in two ways: 1) foliar – using their leaves to absorb nutrients from the air, and 2) osmosis – using their roots to absorb from soil either natural nutrients or synthetic ones from fertilizer.

This article discusses where plants get the 6 nutrients that they consume most of all: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S).

How Do Plants Eat?

When we take care of plants, we provide the four things they need most: light, air, water, and nutrients. And then they do the rest of the work. For instance, plants make their own food – and food for us too.

So, how do plants get nutrients to feed themselves? In two ways.

  • Foliar intake: Some plants use their leaves to absorb carbon dioxide (CO2) and minerals from the atmosphere. This feeding process is called foliar from the Latin word folium, which means leaf.
  • Osmotic uptake: More often, however, roots absorb minerals and water from the soil and move these to stems and leaves. This feeding process is called osmosis from the Greek word osmos, which means push.

For either foliar and osmotic processes, plants use photosynthesis (from the Greek words that mean combine and light) to do two things: 1) create energy (ATP molecules) and 2) to convert CO2 and minerals into food such as starch and sugar (glucose).

But wait, there’s one more thing.

The entire process of how plants feed is called transpiration. To simplify, the three steps are:

  1. absorb nutrients and water from the soil or the air,
  2. move these throughout the plant, and
  3. leaves evaporate excess water and leave the nutrients in the plant

At the same time, the plants also produce oxygen that all animals need to breathe.

However, to make this magic happen, plants must absorb specific nutrients. What are these nutrients? Where do plants get them?

Great questions.

PRO TIP: Plants store starch and glucose mainly in fruits, seeds, or in tubers.

RELATED LINK: Snacking on Sunlight – Ask a Biologist

FREE PDF: How do plants get their food? Lesson Plan (16 pages)

Where Do Plants Get Their Food?

Plants need at least 21 nutrients to produce food and oxygen. However, most gardeners and farmers are concerned only with 3 primary and 3 secondary macronutrients – the largest amounts of nutrients that plants need.

The other are called micronutrients. Micronutrients are the nutrients that plants need, just as important as macronutrients, but required in smaller quantities. Examples are boron, iron, and zinc, to mention a few.

Plants get most of their food from the air, from soil and – particularly for hydroponic plants and indoor gardens – from three general types of fertilizers.

  • Organic or natural fertilizers: Contains only plant or animal tissue derived from sources composted using natural processes or from industrial byproducts.
    • Byproducts: from blood, bone, or feathers, such as from cattle lots, slaughter houses, food processing plants, etc.
    • Compost: from old and decomposing leaves, grass, carcass, worm castings, or manure from poultry or dairy farms,
  • Synthetic fertilizer: Contains nutrients from industrial byproducts or synthesized from different chemical and inorganic sources that immediately supply nutrients to soil.
  • Inorganic fertilizer: Contains artificial, quick-release nutrients from mined rocks, salts, etc.

PRO TIP: Some fertilizers are a blend of organic and inorganic nutrients.

FREE PDF: How do plants get their food? Lesson Plan (16 pages)

Fertilizers use NPK labels to show how much of each nutrients they contain. Let’s begin with the N for nitrogen. Now on, since plants have different names, scientific names are used for accuracy.

Nitrogen

Do you like your tomatoes, chili, or bitter gourd just a bit sweeter? How about sweeter berries, mangoes, peaches, or pears? Believe it or not, just a little nitrogen helps a lot.

The N in NPK fertilizer is for nitrogen, an inert gas with no taste, color, or odor. Although it makes up 78.09% of the air we breathe, inhaling pure nitrogen is fatal. Fortunately, to make leaves, fruits, or flowers, plants grab nitrogen out of air, soil, or fertilizer.

From the air: N2 gas is inert and must be changed to ammonium (NH4+) or nitrate (NO3–) that plant roots can absorb.

PRO TIP: Chemical formulas show you that different fertilizers are only variations of the same nutrients.

  • Plants need nitrogen fixation, a process that converts atmospheric nitrogen into nitrate or ammonium.
  • Only plants of the Fabaceae family (e.g., peas, clover, lentils, soybeans, vetches, peanuts, lupins, rooibos, cowpeas, and other legumes) can do this, and only with the help of particular bacteria.

PRO TIP: Plants need to absorb more nitrogen as their growths increase.

From the soil: Two things in the soil process nitrogen: water and microbes. Soil bacteria must convert nitrogen into an inorganic form before plants can use it.

  • Nitrogen in the forms of ammonia (NH3) and ammonium (NH4) attaches to organic matter or clay.
  • Water leaches nitrogen from the soil. At the same time, water causes nitrification in aerobic soil, turning nitrogen into either nitrate or ammonium.
  • In certain soil conditions, microorganisms convert nitrogen into plant-accessible nitrate or ammonium.
  • Aerobic soil microbes help some plants by fixing nitrogen in falling snow or rain and turning it into nitrate or ammonia forms.

From fertilizers: Urea is a stable and solid fertilizer ingredient unlike other nitrogen forms such as ammonia (NH3) or ammonium nitrate (NH4NO3) that plants can consume only before they disappear into air or leach away in water.

  • Volatilization: Nitrogen in urea on topsoil can escape into the air, particularly in high temperatures and low-moisture, pH 8+ soil.

PRO TIP: Mix urea under topsoil for longer-lasting nutrition.

Leaching: Nitrates (NO3) are water-soluble and can wash away with rain, particularly in sandy soil.

Natural / organic nitrogen: Volatilized and leached nitrogen can be easily replaced by well-composted manure, organic compost, or organic liquid fertilizers such as:

  • 12 to 15% – urea from urine, blood meal, feathers
  • 12% – hoof and horn meal
  • 11% – feather meal
  • 10% – blood meal or dried blood
  • 7% to 10% – bat guano, sewage sludge, manure, soybean meal
  • 6% – decaying matter from legumes or soybean meal
  • 6% – cottonseed meal
  • 4% – fish emulsion, crab meal
  • 2% – coffee grounds
  • 1 to 1.5% alfalfa, compost tea
  • 1.2 to 5% – fish meal, seaweed or kelp fertilizer

Synthetic / Inorganic nitrogen: Aside from the slow-release urea formaldehyde or sulfur-coated thiourea (13%) and anhydrous ammonia (NH4), the cheapest source with the highest (82%) nitrogen content, other sources of synthetic or inorganic nitrogen in fertilizers are:

  • 45% – urea
  • 34% – ammonium nitrate, ammonium nitrite
  • 21% – ammonium sulfate
  • 18% – diammonium phosphate
  • 16% – sodium nitrate
  • 15% – calcium nitrate
  • 13% – potassium nitrate
  • 10% – ammonium phosphate

PRO TIP: Inorganic, nitrogen-free sweet, potato activates nitrogen-fixing endophytes.

FREE PDF: Nutrient Deficiency Symptoms (8 pages)

Phosphorus

In 1669, a certain Hennig Brandt evaporated some urine and then heated the ashy residue till it was red-hot. Voila, the world’s first phosphorus. And, believe it or not, that’s how humans made phosphorus for the next one hundred years.

At room temperature, phosphorus is a soft, semi-transparent solid. Aside from making fertilizer, it is also used to make matches and toys that glow in the dark.

Plants need phosphorus to capture and convert energy from the sun, create leaves and stems, as well as roots, seeds, and flowers. Plants cannot absorb iron, zinc and other micronutrients without phosphorus using from the air, the soil, or from fertilizers.

From the air: Only air plants such as Spanish moss (Tillandsia usneoides), pink quill (Tillandsia cyanea), and sky plant (Tillandsia ionantha) can absorb phosphorus from the air.

From the soil: Most plants absorb phosphorus from the soil. In phosphorus-depleted soil, gardeners or farmers add manure or synthetic fertilizer to soil.

  • Phosphorus stays in soil but not where there are runoffs, erosions, or in soil near running water or large bodies of water.
  • Soil with more clay content can retain more phosphorus.
  • While phosphorus becomes available to plants within days, the nutrient stays in soil from four up to six months.
  • Soil with high phosphorus induces zinc and iron deficiencies as plants become unable to access these nutrients.
  • Organic farmers add phosphorus to soil by recycling compost as well as animal or green manure.
  • Plants are best able to absorb phosphorus in soil with pH 6 to 7.5.
  • Plants cannot absorb phosphorus in soil that is cold, highly alkaline or acidic.

From fertilizers: Through their roots, plants absorb in its orthophosphate forms (e.g., HPO4 2-, H2PO4– or PO4 3). Ground bone meal or pulverized rock phosphate is used to make fertilizer that releases phosphorus slowly, over several planting seasons.

Organic phosphorus: If you prefer natural fertilizers, the highest concentrations of phosphorus are in manure, dairy compost (2%), bone meal (15%) or vermicompost (1%).

It takes a few months for microbes to convert bone meal phosphorus into plant-accessible nutrients.

The high phosphorus concentrations in swine or poultry manure requires the addition of nitrogen to soil.

Worm castings contain about 5% phosphorus.

Aside from human urine, other organic phosphorus sources include bat guano, bone meal, alfalfa meal, shrimp or crab waste, burned cucumber skins, hair or mushroom compost.

Inorganic phosphorus: The highest concentrations of phosphorus from inorganic sources include colloidal phosphate (2%) and rock phosphate (3%).

Synthetic phosphorus: Phosphorus fertilizer is manufactured from either sedimentary or igneous rock phosphate to produce calcium phosphate for plants. Some phosphorus concentrations in synthetic fertilizers are:

RELATED LINK: Phosphorus: Putting the P in Plants

Potassium

In the olden days, plant ash was soaked pots to create fertilizer. As a result, the name “pot ash” survived until the industrial era when potash was synthesized from potassium-rich salts and potassium carbonate (K2CO3).

Potassium is a soft alkali metal that turns into potassium peroxide flakes (KO2) in seconds of exposure to oxygen. Add water and it turns into heat and potassium hydroxide (KOH). Cool for a lab experiment. Of course, potassium is in the air, the soil, and in fertilizers.

Without potassium, plants cannot move carbohydrates, nutrients, and water (turgidity) from roots to stems and tissues. Plants also need potassium to produce protein, starch, sugar and ATP (adenosine triphosphate) for photosynthesis.

In the air: Any potassium in the atmosphere cannot be absorbed by plants. However, legumes use potassium to absorb nitrogen from the air.

FACTOID: Plants also absorb hydrogen, oxygen, and carbon from the air.

In the soil: During the earlier stages of growth, plants absorb potassium (K2O) from the soil.

Potassium in the soil comes from weathered minerals such as mica and feldspar orthoclase.

Potassium is highly soluble and therefore won’t stay in sandy, chalky, or peaty soil as well as in soil high in lime or low in clay.

In fertilizers: The most common organic sources of potassium include plant residue such as wood ash (5-8%) and kelp meal (2.5%).

In inorganic fertilizers from rock powder, potassium in the form of K+ ions easily dissolve in water. These are usually sold as granite meal (4%), greensand (7%), sulfate of potash magnesia or langbeinite (22%), or muriate of potash or potassium chloride (60%). Synthetic options with the highest concentrations include:

  • 50% – sulfate of potash
  • 43% – potassium sulfate
  • 37 to 44% – potassium nitrate
  • 22% – sul-po-mag
  • 18% – sulfate potash magnesia
  • 10% – kainite

PRO TIP: When buying, choose the appropriate potassium concentration.

Calcium

Since the Roman Empire limestone was used to build roads, bridges, and aqueducts, and buildings. In fact, “calcium” comes from calx, the Latin word for lime. Yet, calcium was produced only in 1808 when electricity became available for experiments.

Aside from developing strong cell membranes, proteins, roots, and leaves as well as resisting pests and pathogens, calcium is an important backup messenger when physical or biochemical stress happens to plants.

Although soil contains a lot calcium, it’s not useful to plant for two reasons: they’re either too hard (limestone, aragonite, marble) or not soluble in water (chalk, calcite, gypsum, dolomite).

Fortunately, microbes create a soluble nutrient (Ca2+) that plants use for dividing, structuring, and elongating their cells. In fact, in the entire plant structure, calcium pectate (C6H10O7) holds all the cell walls together. Thank you, calcium-hungry microbes!

FACTOID: The cell walls of two adjoining plant (lamellum) are only held together by calcium and magnesium pectates.

In the air: Although calcium doesn’t turn to gas, plants use their leaves to absorb calcium solution that’s sprayed in the air. The only water-soluble calcium that plants can use is calcium nitrate.

In the soil: Calcium deficiency can be induced by excess sodium and high acidity in soil. However, since natural water sources contain calcium that leaches into soil, calcium deficiency in plants is rare. In addition:

  • Plants absorb calcium in the form of Ca2+.
  • Soil with higher organic matter and clay content can hold more calcium.
  • The ability of plants to absorb calcium lessens as soil temperature decreases.
  • Plants can absorb more calcium in alkaline soil but less in acidic soil.

PRO TIP: In alkaline soil, excess calcium can induce phosphorus deficiency in plants.

  • Too much nitrogen and potassium in the soil can induce calcium deficiency in fruiting vegetables such as cucumber, squash, or tomatoes.
  • The ideal calcium concentration in soil is no less than 5 milliequivalents or meq/100g.

In fertilizers: Fertilizers use either organic or inorganic calcium in various forms and blends.

Inorganic calcium is made from lime, gypsum, calcium nitrates, calcium chloride, as well as calcium chelates that are not made by organic activity.

  • Lime is sold as burnt lime (quicklime), slaked or hydrated lime.
  • Calcium sulfate (gypsum) doesn’t affect soil Ph – great for plants that prefer acidic soil (e.g., rhododendrons).
  • Limestone provides calcium carbonate (CaCO3) for plant nutrition and neutralizing soil acidity

PRO TIP: Limestone (calcium carbonate) is different from hydrated lime (calcium hydroxide).

Organic calcium in various concentrations can be from organic sources such as:

  • oyster shell flour (33.5%)
  • bone meal, wood ash (24%)

Magnesium

John Black identified magnesium in 1755 but it took about 200 years until chemist Humphry Davy used electricity to decompose two chemicals and created magnesium in 1808. Short story: without electricity, we won’t have magnesium alloys, flares, tablets, or fertilizers.

Even so, green plants are green because of magnesium. They use magnesium to absorb potassium and create flavor in fruits, seeds, and vegetables. Without magnesium, plants can’t process nucleic acid, breathe or use carbon dioxide in photosynthesis.

FACTOID: Magnesium makes up 6.7% of each chlorophyll molecule.

Plants need very little magnesium, but it means a lot. For instance, lack of magnesium makes plants look anemic due to lack of chlorophyll. However, only about 20% goes to capturing solar energy. The rest goes to processes such as:

  • production of oils and fats
  • metabolizing carbohydrates
  • distributing phosphate compounds throughout the plant
  • translocating sugars and carbohydrates
  • absorbing nutrients such as potassium
  • stabilizing cell membranes
  • regulating phosphorus metabolism (e.g., germination, producing pollen, buds, seeds)

Fortunately, magnesium can be found formed in the atmosphere, in growing media, and mixed in manufactured fertilizer.

In the air: Burning magnesium metal reacts with oxygen to form magnesium oxide, which is blended in fertilizers to create a rich and stable source of magnesium.

In the soil: When soil contains clay and organic matter, plants can absorb soluble magnesium (Mg2+).

  • Plants can absorb more magnesium when the soil contains high levels of calcium, potassium or sodium.
  • In heavy rainfall or during garden irrigation, every square meter of acidic or sandy garden soil loses up to 8 grams of magnesium each year, causing plants to absorb potassium instead.
  • Magnesium is generally immobile in soil with pH 6.5 or more. In acidic soil, magnesium is more mobile and leaches easily.
  • Magnesium is naturally abundant in unweathered soil, in medium-weight and heavy garden soil with more clay, but less so in older, weathered, light, sandy, and acidic soil.

PRO TIP: A bit of magnesium fertilizer can fix a phosphorus deficiency.

In fertilizers: There are no organic sources of magnesium. In fertilizers, magnesium is produced from inorganic rock powder. Examples:

  • 40% magnesium carbonate in dolomite (dolomitic limestone) for acidic soil
  • 55% – magnesium oxide or magnesia
  • 10% – magnesium sulfate or Epsom salt for neutral or alkaline soil
  • 11.2% – sulfate of potash magnesia

Sulfur

Can you imagine a world where onion, mustard, garlic, coffee and tobacco are tasteless? That’s a world without sulfur.

Some 2,000 years before the Bible was written, civilizations were already using sulfur as medicine, incense, fumigant, and bleaching agent. Later, it went into gunpowder, matches and fireworks. Today, sulfur is used to make rubber, detergents, batteries, and insecticides.

FACTOID: Many still spell it as sulphur. However, scientific journals and the International Union of Pure and Applied Chemistry (IUPAC) spell it as sulfur.

In its purest form, sulfur is a light yellow crystal found in sediments and in volcanic deposits. Without sulfur, plants can’t form roots or seeds, can’t resist pathogens, and can’t survive low temperatures.

Plants also use sulfur to produce vitamin B1 (thiamine, biotin) and proteins, oil seeds and fruit sugar, starch and fats, protoplasm, enzymes and amino acids.

PRO TIP: Some countries consider sulfur as a macronutrient and use NPKS instead of NPK labels).

In fact, without sulfur, oilseeds won’t have oil, cereals won’t have enough protein and starch for milling or baking, and forage won’t fatten herds. Plants need sulfur to use nitrogen to grab sunlight and create chlorophyll. That’s why sulfur-starved plants are yellow.

Although sulfur is odourless, sulfur gas in swamps and paddy smells like rotten eggs. And while sulphur is tasteless, it creates flavour in fruits, oils and vegetables.

FACTOID: About 90% sulfur in each plant goes to the building blocks of protein: the amino acids cysteine, methionine and cysteine.

RELATED LINK: The Secrets of Sulfur

Since sulfur does not dissolve in water, plants absorb it from the atmosphere, the soil, and from fertilizer.

In the air: Sulfur excess can be caused by air pollution, which is difficult to control.

  • Through their leaves, plants can absorb very low concentrations of sulfur dioxide (SO2) from the air. This is not enough for plant nutrition.
  • When sulfur dioxide turns into liquid, it’s an acid that can damage plant leaves.
  • Sulfur candle contains 98% sulfur, which can kill insects in a (3m x 2m) room.
  • Sulfur in gas form can smell septic.
  • Sulfur burned in oxygen becomes gas (sulfur dioxide) that kills mosquitos, flies, ants, wasps, rats and mice, and even fungus.

In the soil: The highest concentrations of sulfur in inorganic minerals include 30 to 99% in elemental sulfur, and 17% in gypsum.

  • Sulfur-deficiency in soil is rare.
  • Organic sulfuric minerals contain potassium sulfate, magnesium sulfate, and calcium sulfate.
  • Plants absorb 99% of their sulfur nutrients through roots.
  • Sulfur comes from acid rain that leaches into soil. Humus in soil also provides sulphide gas (elemental sulfur).
  • Elemental sulfur is often found near hot springs and areas around volcanos.
  • Without sulfur, plants cannot absorb nitrogen from the soil.
  • In sandy soil, water leaches away sulfur sulfates (SO4-S) and nitrates (NO3-N).
  • Unlike other sulfate forms, elemental sulfur in soil does not leach easily.
  • It takes months or years for sulfur to acidify soil.

In fertilizers: The highest concentrations of sulfur in inorganic fertilizers are 24% in ammonium sulfate, 17% in potassium sulfate, and 12% in superphosphate.

  • Sulfur is mostly used to manufacture sulfuric acid (H2SO4) in phosphate fertilizers.
  • Natural sulfur fertilizer comes from plant residue, animal manure or from irrigation water.
  • Organic sulfur fertilizers include potassium sulfate, magnesium sulfate, ammonium sulfate (AS), and single superphosphate (SSP)
  • Plants can absorb sulfur nutrients from sulfate sulfur (gypsum, ammonium sulfate).
  • Elemental sulfur must be oxidized before it can be used as a plant nutrient.

FACTOID: Sulfur fertilizers also contain nitrogen.

RELATED LINK: Sulfur – Plant Answers

Main sources: Organic or natural sources of sulfur include calcium sulfate in gypsum, magnesium sulfate in Epsom salts, iron sulphide in iron pyrites, and lead sulphide in galena. Gardeners use organic wettable sulfur as spray against fungus and mites. Other options are:

Sulfate fertilizers (SO4-2) are ready to use: gypsum, ammonium sulfate (AS), single superphosphate (SSP), potassium sulfate, or potassium + magnesium sulfate.

Elemental sulfur fertilizers (S⁰) are cheaper but must be converted to sulfate-sulfur: sulfur bentonite, elemental sulfur-fortified NP fertilizers, sulfur-coated fertilizers.

Liquid sulfur fertilizers such as ammonium thiosulfate or potassium thiosulfate are not easy to dissolve in water and must be oxidized by soil microbes for plant absorption.

FAQs and Answers

Are yellow leaves signs of lack of nutrition? That’s possible. However, there may be other causes such as drought, stress, pests, or pathogens (plant diseases). Check this article on nutrition deficiency.

What’s the best fertilizer for general use? For general applications, choose a general, balanced fertilizer. Any NPK label with the same numbers (for example: 10-10-10) is good for soil enhancement.

Should I use the same fertilizer for a plant from growing stage to maturity? As your plants grow, their nutritional needs change. For instance, different nutrients are required for germinating, rooting, flowering, and fruiting.

Takeaways

Great! Now you know the essentials of plant macronutrients. Dazzle your gardening and farming friends with your new knowledge. Here’s a shortlist of key takeaways to help you remember.

  • The nutrients that plants consume in the largest quantities are called macronutrients.
  • The 3 primary macronutrients are nitrogen, phosphorus, and potassium.
  • The 3 secondary macronutrients are calcium, magnesium, and sulfur.
  • These nutrients are available to plants from the air, from soil, and from fertilizers. However, only a few plants can grab nutrients from the atmosphere; most feed using roots in the soil.
  • Fertilizers may be sourced from natural, synthetic, organic, or inorganic materials and may be sold as a powder, pellet, or liquid concentrate.

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Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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