Nutrient Deficiency Guide

This guide is written for farms operating in Thailand. It covers fourteen mineral nutrients: what a shortage of each one looks like on the leaf, why it usually happens here, how to confirm it before spending money on a correction, and what type of product corrects it. Every figure is marked as either sourced or unverified, and there is a section at the end listing what could not be verified at all.

The single most useful habit is this: a leaf symptom tells you which nutrient the plant cannot reach. It does not tell you whether that nutrient is missing. Those are different problems with different fixes. Measure first.

Quick symptom finder

Start here. Find the row that matches what you are looking at, then read that element's section below. The last column is the measurement to take before you change anything.

Measure before you treat

Four numbers separate a real shortage from a pH or salt problem. None of them need a laboratory. Take them in this order.

1. pH of the water going in, after nutrients are mixed. This is the number most feed charts quote, and it is the least informative of the four on its own.
2. pH of what drains out of the pot or bed. The difference between this and the feed pH tells you what the root zone is actually doing. A drain reading far above the feed reading is the usual explanation for iron and manganese symptoms on new growth.
3. EC of the water going in. Measured after mixing, so it includes whatever the raw water already contained.
4. EC of the runoff. Rising runoff EC over several days means salts are accumulating faster than the crop is using them. That restricts water uptake by osmosis and produces symptoms that look like several deficiencies at once.

There is a fifth number that farms skip and should not: the EC and pH of the raw water, before anything is added. Without it you cannot tell whether a problem is coming from the fertiliser programme or from the well. In Thailand this matters more than in most places, for reasons set out in the water section below.

Published target ranges, by growing system:

Published EC targets are crop-specific rather than general. Oklahoma State Extension gives, for hydroponic production: tomato 2.0–4.0 mS/cm, cucumber 1.7–2.0 mS/cm, lettuce 1.2–1.8 mS/cm, pepper 0.8–1.8 mS/cm. There is no single correct EC; there is only the EC that suits the crop, the stage and the root-zone volume.

pH and what the plant can actually absorb

This is the part a new grower needs most, and also the part where the popular teaching material is least reliable. The familiar coloured bar chart showing each nutrient's availability against pH comes from a 1946 paper by Emil Truog. That paper contained no data and no references, and Truog himself described the chart as generalised, tentative and partly based on assumptions because data were lacking.

What is well supported, and worth memorising, is a much shorter list:

In a container or soilless bed the two failure directions are symmetrical and both are common. Above roughly pH 6.8 the trace elements — iron, manganese, boron, zinc, copper — become less available and the plant shows shortage symptoms on new growth. Below the target range the same trace elements become more mobile and are absorbed in excess of what the plant needs, which produces toxicity instead. Correcting a low-pH problem by adding more micronutrient makes it worse.

Sourced: University of Georgia Extension B1256.

The pH absorption chart below is the CANNA version, translated and published by AZ Growshop. It is a teaching aid for the direction of each effect, and carries the caveat above.

Download the pH absorption chart (PDF)

Water sources in Thailand

Nutrient problems that a farm blames on fertiliser often start in the water. Thailand has a wider spread of irrigation water quality than most growing regions, and the published data is thin outside Bangkok, so the practical answer is that every farm has to measure its own source.

FAO thresholds for irrigation water

These are the standard guideline values, from FAO Irrigation and Drainage Paper 29 (Ayers & Westcot, 1985). Read your own water against them.

Municipal tap water

For the Bangkok supply area the Metropolitan Waterworks Authority publishes annual averages: pH 7.38, total dissolved solids 161 mg/L, total hardness 90 mg/L as calcium carbonate, chloride 16 mg/L, sulfate 20 mg/L. That is benign water by the FAO table — no restriction on any axis. It also means the water is supplying a small but real amount of calcium, which matters when you later switch to rainwater and it disappears.

The average hides a range. An independent survey of 32,711 samples from 2,354 Bangkok locations recorded tap water conductivity from 120 to 2,374 µS/cm and dissolved solids from 72 to 1,424 mg/L, with the 95th percentile around 500 µS/cm. The top of that range crosses into the FAO slight-to-moderate band. No equivalent public dataset was found for Provincial Waterworks Authority supplies outside Bangkok, so for a farm in the provinces the figures above are context, not a substitute for a meter.

Well water, and the Isaan salt problem

Thailand has about 2.302 million hectares of salt-affected soil, of which 1.904 million hectares are inland rather than coastal. The cause is geological, not agricultural: the Cretaceous Maha Sarakham rock-salt formation underlies the Khorat Plateau, and saline groundwater has been located by resistivity survey at 5 to 30 metres depth in the Khon Kaen basin, with the salt source itself deeper than 100 metres. This is not something irrigation practice created and it is not something irrigation practice can remove.

How bad it gets locally: a 2024 survey of 75 surface-water samples around salt workings in Non Thai and Phra Thong Kham districts of Nakhon Ratchasima found salinity from 0.5 to 30 parts per thousand, with sodium and chloride exceeding standards and the water rated unsuitable for irrigation in several areas. Thirty parts per thousand is roughly seawater. A farm drawing from a shallow well anywhere on the Khorat Plateau should treat a conductivity meter as basic equipment, not an optional extra.

High sodium in the root zone also interferes directly with potassium uptake, which is why a potassium symptom on a farm using well water is a reason to test the water before increasing the potassium feed.

Harvested rainwater

Rainwater is the default second source through the rainy season and it has two properties worth understanding. First, it is close to zero in dissolved minerals, so it supplies no calcium, no magnesium and no sulfur at all. A farm that runs on tap water in the dry months and rainwater in the wet months has quietly removed part of its calcium and magnesium supply at the same time as the rain is leaching those elements out of the ground. That combination is a common route into a calcium or magnesium symptom that has nothing to do with the fertiliser being wrong.

Second, very low salinity is itself listed as a hazard. FAO Paper 29 rates water below 0.2 dS/m as a severe infiltration hazard at low sodium adsorption ratio, because water that clean disperses soil structure. On a low-clay Isaan sand this matters less than on a heavier soil, but it is a real, cited reason not to assume clean water is automatically good water.

A national study of 152 Thai rainwater samples collected across six regions between November 2022 and February 2024 found 80% of samples acidic and every sample failing at least one drinking-water standard. That study does not publish conductivity, dissolved solids or hardness values, so the exact figures for Thai harvested rainwater remain unmeasured in the literature reviewed here.

Canal and surface water

No national dataset of canal or irrigation-channel conductivity for Thailand was located. The only Thai surface-water salinity figures found are the salt-mine-affected samples above, which are an extreme case rather than a baseline. Canal water also carries a variable sediment and nutrient load from upstream use, so its EC can shift within a single season. Measure it at the point of use and measure it more than once.

The three Thai seasons and what each one does to nutrition

The Thai Meteorological Department defines three seasons, not four: hot from mid-February to mid-May, rainy from mid-May to mid-October, cool from mid-October to mid-February. Each one changes nutrient behaviour in a different direction, and a guide written for a temperate four-season calendar will point you the wrong way on at least two of them.

The measured size of the hot-season effect: tomato grown at a vapour pressure deficit of 2.22 kPa, against 0.95 kPa, showed leaf calcium down 10.7% and stem calcium down 19.0% in one cultivar. A second cultivar in the same trial went the other way, leaf calcium up 7.2%. So the effect is real, measured, and cultivar-dependent — which is worth knowing before attributing every hot-season symptom to it.

Sourced: Yu et al. (2022), Environmental and Experimental Botany 195:104786.

A correction that matters, because the common advice has it backwards

What waterlogging does, with numbers

Oxygen in a flooded soil is consumed within the first hours to days. After that the soil chemistry changes in a fixed order. Manganese converts from its reducible form to its water-soluble form at a redox potential of +200 to +300 mV at pH 6–8, and at around pH 5 — which is the normal pH of an Isaan sand — almost all soil manganese makes that conversion. Iron reduces at +300 mV down to +100 mV at pH 6–7. The highest ferrous iron concentrations occur at 2 to 15 centimetres depth, which is the main rooting zone. Plant manganese and iron content rises to two to ten times the drained-soil level. The ammonium to nitrate ratio shifts to about 10:1 in waterlogged soil and above 30:1 in chronically flooded soil.

The practical consequence is short: in the rainy season, fix drainage before you fix nutrition. Adding a trace-element product to a waterlogged acid bed adds to a problem that is already an excess.

Sourced: Li et al. (2025), Frontiers in Plant Science 16:1653008.

What Thai soils start from

A deficiency guide written for a temperate soil assumes the soil will hold a season's worth of applied nutrient between applications. Most Thai soils will not, and the difference is large enough to change practice rather than just refine it.

An exchange capacity of about 2 cmol/kg is an order of magnitude below the temperate baseline. On that soil, a single large application of potassium, calcium or magnesium is not stored for later use; most of it leaves with the next heavy rain. Split applications are not a refinement in the Northeast, they are the only approach that works.

Which shortages are actually documented in Thailand

A 2022 study compared 65 paddy soil samples from the Northeast, North, Central Plain and Bangkok Plain against 1960s baselines at the same sites. Available copper, zinc, sulfur and boron were below deficiency thresholds in the dominant soils, most severely in the Northeast. Manganese and iron were adequate in all fields. Total potassium, manganese and silicon were significantly lower than in the 1960s.

That ordering is close to the reverse of what temperate guidance prepares a grower for. In Thailand the micronutrients to watch are copper, zinc, sulfur and boron. Manganese and iron are far more likely to be a rainy-season excess than a shortage.

Sourced: Hirose et al. (2022), Tropical Agriculture and Development 66(1):33–43.

A trap specific to Thai sandy soils

In a six-year trial at Nakhon Ratchasima on soil with an exchange capacity below 2 cmol/kg, the soil received an acid load of 6.3 to 7.6 kmol of hydrogen ions per hectare per year — and the pH did not fall. Kaolinite dissolution buffered it. Over the same period the clay fraction itself was being consumed: kaolinite fell from 88% to 78% of the clay fraction by one measure, and from 82% to 34% by particle count, while smectite rose. A flat pH reading on a Thai sand does not mean nothing is happening. It can mean the soil's capacity to hold nutrients is being spent to keep that reading flat.

Sourced: Lesturgez et al. (2006), Agriculture, Ecosystems & Environment 114:239–248; Dur et al., FAO.

Nitrogen (N)

Where the symptom appears

Nitrogen is mobile in the plant. When supply runs short the plant withdraws it from old tissue to feed new growth, so the oldest, lowest leaves go first while the top of the plant stays green for a while longer. Sourced: Cornell NRCCA; University of Florida IFAS HS1373; Montana State Extension.

What you see in the field

A relatively uniform yellowing of the lower, older leaves, and the whole plant looks lighter than it should. The key word is uniform: the yellowing spreads evenly across the leaf blade rather than picking out the spaces between the veins. That is what separates nitrogen from magnesium and iron, which both leave the veins green. In a bad case the lower leaves go yellow-white, drop, and stem diameter and leaf size fall. Leaf stalks may turn purple.

Common causes in Thailand

• Feed strength genuinely too low for the growth stage.
• Heavy rain through the monsoon. Nitrate carries almost no charge interaction with the soil, so it leaches more readily than any other nutrient, and a sandy Northeast soil with an exchange capacity near 2 cmol/kg holds very little of it between rains.
• Substrate containing a lot of fresh, undecomposed organic material. The microbes breaking it down lock nitrogen up in their own tissue and the crop cannot reach it.
• Waterlogging, which shifts available nitrogen towards ammonium rather than nitrate. In chronically flooded ground the ratio exceeds 30:1.

Confirm before you treat

Check the runoff EC first. A low runoff EC alongside uniform lower-leaf yellowing supports a real shortage. A high runoff EC with the same symptom points somewhere else — usually salt accumulation restricting water uptake, which mimics several deficiencies at once. Confirm on tissue analysis if the block is large enough to justify it.

How it is corrected

Raise the strength of the base nutrient feed, or add a dedicated nitrogen source. Through the rainy season, split the same total into more frequent smaller applications rather than fewer large ones, because the soil cannot store what it is given. Foliar application of a nitrogen-containing solution acts faster than root feeding for an acute case, but supplements root feeding rather than replacing it.

Phosphorus (P)

Where the symptom appears

Phosphorus is mobile, so symptoms show on the older leaves first. One caveat worth knowing: UC IPM reports the opposite leaf position for woody broadleaf plants, where young foliage becomes abnormally dark green. Sourced: Cornell NRCCA; Montana State Extension; UC IPM.

What you see in the field

The plant is weak and stunted with delayed maturity, and the older leaves turn an unusually dark green, then dull purple or reddish-purple. Leaf tips can look burnt. Over two to three weeks dark spots appear on the older leaves, which curl, shrivel and die with an ochre-purple colour. Stunting without obvious yellowing is the signature — a phosphorus-short plant is small and dark, not pale.

Common causes in Thailand

• Root-zone pH away from about 6.5 in either direction. Above pH 7 phosphate binds to calcium and forms compounds the root cannot take up; below 6.5 it becomes insoluble in a different way.
• Low root temperature. Phosphorus uptake is temperature-sensitive, which makes the Thai cool season the more likely window for it.
• In an acid soil, high iron and aluminium fix phosphate out of solution. Thai Ultisols and Acrisols are exactly this kind of soil.
• Poor mixing. A granular phosphate that is not distributed through the medium reaches only the roots that happen to find it.

Confirm before you treat

Measure root-zone pH and root-zone temperature before anything else. Phosphorus rarely runs out in a fed crop; it much more often becomes unreachable. If pH and temperature are both in range, a soil test for available phosphorus is the next step, and it is worth doing because the opposite error — too much phosphorus — causes its own zinc and iron problems.

How it is corrected

Use an inorganic phosphate source, which is immediately available, and mix it thoroughly through the growing medium rather than placing it. In a liquid feed, a phosphate-containing base nutrient covers it. If pH is the cause, correct the pH and the existing phosphorus becomes available again without adding more.

Potassium (K)

Where the symptom appears

Potassium is mobile, so the older leaves show it first. Sourced: Cornell NRCCA; University of Florida IFAS HS1373; Montana State Extension.

What you see in the field

Mottled or chlorotic areas on older leaves with the leaf burning at the margins, usually leaving the midrib alive and green. That combination — dead, dry, brown edges around living central tissue — is the signature and is hard to confuse once seen. Early on the tips of younger leaves may show grey edges. The yellowing then runs inward from the margin towards the veins, picking up a rusty colour, and the leaf curls and eventually drops.

Common causes in Thailand

• Sodium in the irrigation water. Sodium in the root zone slows potassium uptake directly. On the Khorat Plateau, where 1.9 million hectares of inland salt-affected soil sit over the Maha Sarakham formation, this is the first thing to rule out — not the last.
• Leaching. Potassium is held on exchange sites, and a soil with an exchange capacity around 2 cmol/kg has very few of them.
• Competition from calcium or magnesium applied heavily, or from excess ammonium.
• A potassium-fixing potting mix that binds it out of reach.

Confirm before you treat

Test the irrigation water for sodium and conductivity before increasing the potassium feed. Adding potassium to correct a sodium problem raises the total salt load and makes the crop worse. Then check runoff EC: if it is already high, the answer is to flush, not to feed.

How it is corrected

If runoff EC is high, flush the medium with clean water first, then resume feeding at a measured strength. If the shortage is real, a potassium-containing base nutrient or a dedicated potassium source corrects it. On sandy Northeast ground, apply it split across the season rather than in one dose.

Calcium (Ca)

Where the symptom appears

Calcium does not move in the phloem. It travels upward in the transpiration stream and cannot be recovered from old leaves, so symptoms appear on new leaves, the growing point and the fruit, while the old leaves stay perfectly green. Sourced: Cornell NRCCA; Mosaic; Montana State Extension.

What you see in the field

New leaves come out distorted or irregularly shaped, and leaf tips can stick together. Yellow or brown spots appear surrounded by a sharp dark brown outline, often at the leaf edge, spreading over weeks until the tissue dies. On fruit it shows as rot at the blossom end; on lettuce and similar leafy crops as tipburn on the enclosed inner leaves. Root development slows and roots may darken.

Common causes in Thailand

Calcium arrives with water movement, so anything that reduces transpiration or competes for uptake causes a shortage even when calcium is present.

• The hot season, mid-February to mid-May. High vapour pressure deficit is the measured risk window. At 2.22 kPa against 0.95 kPa, leaf calcium fell 10.7% and stem calcium 19.0% in a tomato trial.
• High salinity. Calcium uptake in tomato fell from 143 mg/day at 3 mS/cm to 88 mg/day at 15 mS/cm — a measured, linear decline.
• Competition from ammonium or potassium applied heavily.
• A calcium-poor growing medium, or a switch to rainwater, which supplies none. Coco coir binds calcium and magnesium, and reverse-osmosis and rainwater contain neither.
• Leaching in acid sandy soils, where calcium is stripped out. Mean exchangeable calcium in the Khon Kaen dataset was 0.81 cmol/kg.

Confirm before you treat

Check EC and the calcium content of the raw water. Then look at the growing conditions rather than only the feed: a calcium symptom appearing in the hot season on fast-growing plants in small pots is usually a transport problem, not a supply problem, and more calcium in the tank will not fix it. Tissue analysis is worth doing here because the leaf and the fruit can differ.

How it is corrected

If salinity is the cause, flush with clean water. If supply is genuinely short, a cal-mag supplement or a calcium nitrate solution supplies it in an available form, and a properly limed potting mix or a pre-buffered coco substrate prevents it recurring. Where transport is the limit, the correction is environmental — reduce the extremes of vapour pressure deficit, keep water available, avoid root-restricting pot sizes for fast crops.

Magnesium (Mg)

Where the symptom appears

Magnesium is mobile and easily moved from old tissue to new, so it shows on the middle and older leaves first. Sourced: University of Florida IFAS HS1373; Montana State Extension; Mosaic.

What you see in the field

Yellowing between the veins with the mid-ribs staying distinctly green, starting at the leaf margins and working in. Rusty brown speckles appear within the yellow areas, and cloudy, vague yellow patches form between the veins on medium-aged leaves. Leaf margins may turn yellow or reddish-purple, leaves become brittle and can cup or curve upward. Young leaves and developing fruit stay unaffected at first, which is a useful confirmation.

Common causes in Thailand

• Potassium in excess. This is the most thoroughly documented antagonism in plant nutrition: high potassium in the soil solution leads to preferential potassium uptake at the expense of magnesium, and magnesium is the cation most strongly affected.
• A wet, cold or acid root environment. Magnesium availability falls below about pH 5.8.
• Leaching. Exchangeable magnesium in Thai paddy soils has fallen significantly over fifty years, and magnesium leaves acid sandy soil as a counter-ion with mobile anions during heavy rain.
• Rainwater irrigation, which supplies none.
• High calcium, or high ammonium, competing for uptake. Above about pH 7.4 excess calcium can override magnesium uptake entirely.
• A restricted root system, or elevated EC.

Confirm before you treat

Look at the potassium level in the feed before assuming the magnesium is short — in a fed crop the antagonism is more often the cause than the supply. Check root-zone pH and root temperature. A magnesium symptom on middle leaves with young growth clean is consistent; a symptom on new growth instead points to iron or manganese.

How it is corrected

A 2% Epsom salts solution applied as a foliar spray every four to five days for about a week is the standard fast correction, with the root-zone cause addressed at the same time. At the root, a magnesium-containing cal-mag supplement covers it in a soilless system. Correcting pH and root temperature, and keeping root temperature in the 20–25 °C band, often resolves it without extra magnesium.

Sulfur (S)

Where the symptom appears

Sources disagree, and the disagreement is worth knowing. University of Florida IFAS and Montana State Extension classify sulfur as mobile; Mosaic states it is immobile and does not move from old to new growth. What all of them agree on is the field observation: symptoms show on the new, younger leaves first, sometimes followed by the older ones. Use the symptom position, not the classification.

What you see in the field

A general, even yellowing — not interveinal — of the younger leaves, with the plant stunted. It looks very like nitrogen shortage, and leaf position is what separates them: nitrogen starts at the bottom of the plant, sulfur at the top. A strong purple colour in the leaf stems, from anthocyanin, often accompanies it. As it progresses the leaves go deep yellow and growth and flowering decline.

Common causes in Thailand

• Sulfate leaches readily from surface soils, so the rainy season is the risk window on light ground.
• Available sulfur measured below deficiency thresholds in Thai paddy soils in the 2022 national comparison, most severely in the Northeast. This is one of the four elements that survey flagged.
• Root-zone pH too high, or calcium levels too high, in a potting mix.
• A base nutrient line that supplies little sulfate, combined with rainwater, which supplies none.

Confirm before you treat

Check that the symptom really is on the young leaves and really is even rather than interveinal. Then check root-zone pH. Because sulfur and nitrogen look alike, a tissue analysis that measures both is the reliable separation when the leaf position is ambiguous.

How it is corrected

A sulfate-containing fertiliser corrects it — magnesium sulfate in a soilless system, or a sulfate form of a nutrient already being applied. Where the cause is a high root-zone pH, lowering it addresses the uptake problem directly. Composted organic sources work but release slowly, which makes them a prevention rather than a correction.

Iron (Fe)

Where the symptom appears

Iron does not move in the plant, so it appears on the newest leaves and growing shoots and nowhere else at first. Sourced: University of Florida IFAS HS1373; Montana State Extension; Penn State Extension.

What you see in the field

Strong yellowing between the veins of the youngest leaves, with a sharp distinction between the green veins and the yellow tissue. That sharpness is the single most reliable way to separate iron from manganese, which produces a blurred, spotty version of the same pattern. In a severe case the leaf goes almost white, then develops dead tissue, and growth stops.

Common causes in Thailand

• Root-zone pH above about 6.5. Penn State Extension names high pH as the most common cause of iron shortage. Iron is largely unavailable much above pH 7.0.
• Waterlogged growing medium and cold root temperature.
• High zinc or manganese competing for uptake.
• Excess phosphorus, which depresses iron accumulation.
• Damaged roots, which cannot acquire it.

Confirm before you treat

Measure the runoff pH, not just the feed pH. An iron symptom on new growth with a runoff pH above 6.5 is a pH problem, and adding iron to it is spending money to treat the wrong thing.

In Thailand there is a second check. The 2022 national soil survey found iron adequate in all sampled fields. In field soil here, a true iron shortage is less likely than a pH or drainage problem, and during the rainy season iron excess is the more probable direction.

How it is corrected

Correct the root-zone pH first; in most cases the symptom resolves without adding iron. Where iron genuinely needs supplying, a chelated form is used because the chelate protects the iron from being locked out by pH — EDDHA chelates hold at higher pH than EDTA. Improve drainage and raise root temperature where those are contributing.

Manganese (Mn)

Where the symptom appears

Immobile, so symptoms appear on the young, upper leaves. Sourced: University of Florida IFAS HS1373; Montana State Extension.

What you see in the field

Yellow striping between the side veins of the larger leaves at the top of the plant, with no sharp distinction between vein and yellow tissue — the appearance is spotty and blurred rather than cleanly patterned. Small yellow to brown dead specks form within the affected areas. Leaves, shoots and fruit are reduced in size. In palms the equivalent is the deformed frond disorder known as frizzle top.

Common causes in Thailand

• High root-zone pH, where manganese precipitates as manganese oxide and becomes unavailable. The fertiliser consequence is measurable: about 4 lb/acre satisfies the requirement at pH 5.5 against more than 17 lb/acre at pH 7.0.
• Excess phosphorus interfering with trace element availability.
• Low substrate temperature.
• High iron, copper or zinc competing for uptake.

Confirm before you treat

Measure root-zone pH. Then, before treating, rule out the opposite problem, which is the more likely one in Thailand.

How it is corrected

Where the cause is high pH, lowering the root-zone pH into the target range restores availability without adding anything. Where manganese genuinely needs supplying, a trace element blend covers it, and a foliar application acts faster than root feeding. Note that molybdenum is involved in manganese transport within the plant, which is why trace element products supply the group rather than single elements.

Zinc (Zn)

Where the symptom appears

Immobile. Montana State Extension adds a useful nuance: the interveinal yellowing starts on the middle leaves, with young and old leaves following in later stages, rather than strictly on the newest growth. Sourced: University of Florida IFAS HS1373; Montana State Extension.

What you see in the field

The signature is not the colour, it is the shape. Internodes shorten and the terminal leaves bunch together into a rosette, so the top of the plant looks compressed. Yellowing between the veins accompanies it on the new leaves, and the foliage becomes uniformly pale between the veins. Leaves are often smaller than they should be. Where a plant looks stunted at the top with crowded small leaves, zinc is the first thing to consider.

Common causes in Thailand

• Excess phosphorus. This is the most thoroughly documented cause: high phosphorus forms insoluble zinc phosphate, and a phosphorus-to-zinc ratio above about 100:1 in corn is associated with zinc shortage. High phosphate also impairs the mycorrhizal uptake pathway the plant uses to acquire zinc.
• High pH. Zinc solubility falls roughly a hundredfold for each unit of pH increase.
• Liming, or high calcium.
• Thai context: available zinc measured below deficiency thresholds in the dominant paddy soils in the 2022 national comparison, most severely in the Northeast. This is one of the four elements that survey flagged.

Confirm before you treat

Check the phosphorus level in the soil or feed first. In Thai paddy soils, available phosphorus rose from 3.19 mg/kg in the 1960s to 42.8 mg/kg by the 2010s — a thirteenfold increase — which makes phosphorus-induced zinc shortage a plausible and growing problem rather than a textbook curiosity.

One honest caveat: a Kasetsart University study on jasmine rice found a slightly negative yield response to applied zinc across 3–42 mg/kg, which does not sit comfortably with the survey finding of sub-threshold available zinc. Treat zinc as a genuine open question in Thai soils and confirm by tissue analysis before applying at scale.

How it is corrected

Where excess phosphorus is the cause, reduce the phosphorus rather than chasing it with more zinc. Where zinc is genuinely short, a trace element blend supplies it, usually in chelated form, and manganese is commonly applied alongside it. Correcting pH downward into the target range restores availability without any application.

Boron (B)

Where the symptom appears

Every extension source treats boron as immobile, so the symptom is on the growing point. The primary literature qualifies this: boron is phloem-mobile in species that transport polyols, and forms mobile complexes with sucrose in wheat and canola. For most crops the practical rule stands — look at the newest growth. Sourced: Montana State Extension; Mosaic; Stangoulis et al. (2010), Plant Physiology 153(2):876–881.

What you see in the field

Death of the main growing point. That is the defining feature and it separates boron from everything else on this page. Young leaves go chlorotic and may develop dark brown, irregular lesions; new leaves are thick, brittle and deformed rather than simply discoloured; terminal buds die and the plant throws out side shoots in a witches'-broom form as apical dominance is lost. Fruit set and seed development fail before any leaf symptom is obvious.

Common causes in Thailand

• Leaching in the rainy season. Undissociated boric acid and borate anions move freely in water and leach readily from the upper soil layer in high-rainfall regions. Less than 5 to 10% of total soil boron is plant-available, and under 2% is freely soluble.
• Low soil organic matter, which is the condition of roughly 60% of Thai agricultural land.
• High pH reducing availability.
• Competition from phosphate, which shares an absorption and transport system with borate, and from zinc.
• Reduced transpiration, since boron moves in the transpiration stream as calcium does.
• Thai context: available boron measured below deficiency thresholds in the dominant paddy soils in the 2022 national comparison.

Confirm before you treat

How it is corrected

A trace element blend containing boron, applied at a measured rate, or a dedicated boron source at a rate calculated from a soil test rather than estimated. Through the rainy season, split small applications rather than applying once, because the element leaches. Where reduced transpiration is the limit, the correction is environmental rather than nutritional.

Copper (Cu)

Where the symptom appears

Immobile. The sources differ on where exactly it shows: Montana State Extension puts it on the younger leaves, the University of Arizona describes it as affecting the whole plant rather than a specific leaf age. Sourced: University of Florida IFAS HS1373; Montana State Extension; University of Arizona Extension AZ1106.

What you see in the field

The two main sources describe the leaf colour differently and both are reported here. Montana State Extension describes chlorotic younger leaves, stunted growth, delayed maturity, excessive tillering, lodging and sometimes brown discoloration, with plants more prone to disease. The University of Arizona describes leaves that are dark green with the plant stunted. What both agree on is the structural picture: weak stems that fall over, too many side shoots, wilting leaf tips, and a plant that looks disorganised rather than simply discoloured.

Common causes in Thailand

• High pH. Copper solubility falls roughly a hundredfold per unit of pH increase, the same as zinc and manganese.
• High organic matter or peat substrates, which bind copper strongly.
• High potassium or high zinc competing for uptake.
• Thai context: available copper measured below deficiency thresholds in the dominant paddy soils in the 2022 national comparison, most severely in the Northeast. This is one of the four elements that survey flagged, and it is the one least expected by growers trained on temperate material.

Confirm before you treat

Because the two published symptom descriptions conflict on colour, copper is a poor candidate for visual diagnosis alone. Confirm by soil or tissue analysis before applying. Excess copper intensifies shortages of molybdenum, iron, manganese and zinc, so a mistaken application has consequences beyond the wasted product.

How it is corrected

A trace element blend supplies it. Where pH or organic matter binding is the cause, correcting that restores availability. Rates published by Mosaic for field application are 3 to 10 lb/acre, which indicates the order of magnitude rather than a recommendation for any particular crop or soil.

Molybdenum (Mo)

Where the symptom appears

Disputed. University of Florida IFAS classifies molybdenum as immobile, with symptoms on new growth; Montana State Extension classifies it as mobile; the University of Arizona places the symptoms on the older, lower leaves. Where sources conflict this directly, treat leaf position as unreliable for molybdenum and confirm by analysis.

What you see in the field

Light green colour overall, with necrotic spotting on the leaves. Pale leaves that are sometimes scorched, cupped or rolled. On legumes it mimics nitrogen shortage closely, because molybdenum is required for nitrogen fixation, and the University of Arizona notes that molybdenum problems are quite rare in practice.

Common causes in Thailand

• Low pH — the opposite direction to every other trace element on this page. Molybdenum is less available under acid conditions and more available as pH rises. A Thai acid soil at pH 5.5 is therefore the one place where a trace element shortage can be caused by pH being too low rather than too high.
• High sulfur. Sulfate and molybdate compete, and high sulfur reduces molybdenum uptake, which matters for nitrogen fixation in legumes.
• Excess copper or zinc.

Confirm before you treat

Check pH, and note that the correction direction is the reverse of the usual one. Then rule out nitrogen, which it resembles.

No Thailand-specific evidence of molybdenum shortage was found in this review. It did not appear in the 2022 national element survey and no Thai study of it was located. Treat it as the least likely of the fourteen in a Thai context until a tissue analysis says otherwise.

How it is corrected

Raising a very low pH towards the target range increases availability, which is usually the whole correction. Where an application is needed, the quantity is very small — published sufficiency levels in corn are 0.1 to 2.0 ppm in tissue, the lowest of any nutrient on this page — so it is supplied through a trace element blend rather than alone.

Silicon (Si)

Where the symptom appears

Silicon is not classified as an essential nutrient for most plants. It is treated as beneficial or quasi-essential; IPNI upgraded it from omission to beneficial substance in 2015. Only a few species — horsetail, some algae — cannot survive without it. Phloem mobility was not found in the sources reviewed. Sourced: Rutgers NJAES FS1278; Pavlovic et al. (2021), Frontiers in Plant Science 12:697592.

What you see in the field

There is no described visual shortage symptom. Rutgers states plainly that symptoms of silicon shortage are generally not visually apparent in the field. The indicators are indirect: increased susceptibility to powdery mildew, and lodging in grain crops. Anyone offering a photograph of a silicon deficiency leaf is going beyond what the literature supports.

Common causes in Thailand

Because there is no defined deficiency, there is no defined cause. What is documented is the concentration plants reach: grasses commonly near 1% silicon, some species such as horsetail as high as 10%. For wheat, a flag-leaf concentration of 1% or more has been recommended for disease suppression and yield.

Confirm before you treat

Tissue analysis is the only way to know a silicon level, and standard sufficiency tables do not include silicon — it is absent from the reference ranges used for the other twelve elements on this page. Thai context: total silicon in paddy soils measured significantly lower in the 2010s than in the 1960s, and the Northeast was identified as deficient in available silicon.

How it is corrected

Silicon is supplied as a separate additive rather than through a base nutrient, because it can react and precipitate out of solution if mixed directly with concentrated nutrients — mixing order matters. One property makes it unusual and worth remembering: silicon availability does not change much across the pH range used for crops, so it is the one element on this page that a pH problem does not lock out.

Nickel (Ni)

Where the symptom appears

Nickel was the last element added to the list of essential plant nutrients, late in the twentieth century. It is a component of the urease enzyme and is therefore required to convert urea to ammonia in plant tissue. Montana State Extension classifies it as immobile. Sourced: Mosaic Crop Nutrition; Montana State Extension.

What you see in the field

In field crops, no nickel shortage has been observed under normal growing conditions. Experimentally it produces chlorosis of young leaves and dead meristematic tissue. The one well-attested field disorder is in tree and nursery crops: mouse-ear, marked by small curled leaves and stunted growth, documented in pecan. Montana State Extension describes necrosis of leaf tips with an adjacent dark green zone, and thickened, curling leaves.

Common causes in Thailand

Not established for field conditions. Nickel is required in extremely small amounts and the critical tissue level appears to be around 1.1 ppm — low enough that ordinary contamination of fertiliser and water usually supplies it. No antagonisms were found in the sources reviewed.

Confirm before you treat

Nickel is absent from the standard sufficiency range tables used for the other elements here, so a routine tissue analysis will not report it. If mouse-ear symptoms appear in a tree crop, a specific nickel analysis is required. No Thailand-specific nickel data was found in this review.

How it is corrected

It is included here for completeness rather than as a practical field correction. A crop showing symptoms that suggest nickel should be investigated by analysis before anything is applied, because nickel is toxic at concentrations not far above the requirement.

Leaf tissue analysis: how to do it so the result means something

A meter tells you about the root zone. Tissue analysis tells you what the plant actually took up, and it is the only way to settle the cases where two elements look alike. Most tissue results are wasted because of how the sample was taken, not how it was analysed.

Take three samples, not one

This is the single most important procedural point and the one most often skipped. Collect one sample of whole plants from the worst-affected area. Collect a second from a marginal area where growth is only slightly reduced. Collect a third from plants that are normal and healthy. A single sample from a sick plant gives a number with nothing to compare it against; three samples show you the gradient, which is what identifies the limiting element.

Which leaf, and when

The general rule is the most recently matured leaf, taken at a defined growth stage, from a defined number of plants. Published examples, to show the shape of the rule:

How to read the result

Results are compared against published sufficiency ranges. Three cautions come with them. First, concentration alone is often not enough to diagnose a problem, and calculating nutrient uptake is better. Second, anything that severely reduces growth — hail, drought, waterlogging — concentrates the nutrients in what tissue remains and produces falsely high values. Third, a tissue test carries no fertiliser recommendation with it: it is a diagnostic, not a prescription.

For reference, the sufficiency ranges for corn at tasseling and bloom, from the Southern Cooperative Series Bulletin #394, which is the standard regional reference in the United States. These are for corn at that stage and are not transferable to another crop or another stage without its own table:

Nickel, silicon and chlorine are absent from that table. Where a laboratory reports them, there is no corresponding published sufficiency range in this reference to read them against.

What this guide could not verify

This section exists because a reference document that hides its gaps is worse than one that admits them. Everything listed here was looked for and not found, or was found to be disputed.

• The claim that most apparent deficiencies are really pH or EC problems rather than true shortages. This is repeated everywhere in grower material and no published figure quantifying it was found. What is supported is directional and narrower: high pH is named as the most common cause of iron shortage specifically, and zinc, copper and manganese solubility falls about a hundredfold per unit of pH increase. Those are real and useful. A percentage covering deficiencies in general is not something this guide can put a number on.
• The term nutrient lockout. It does not appear in academic, extension, FAO or peer-reviewed sources. It is a useful shorthand for three separate documented processes — pH changing solubility, ions competing at the root, and an excess of one nutrient inducing a shortage of another — but it is not a technical term and should not be treated as one.
• The numeric bands of the familiar pH availability chart. The chart's per-nutrient pH ranges could not be verified from any source. The commonly circulated values are reproduced from one another rather than from data, and the original 1946 paper contained no data and no references.
• A root-zone pH range specific to coco coir. No academic or extension source giving one was found. The soilless container range of 5.4–6.8 is published; a coir-specific figure is not.
• Phloem mobility of sulfur, molybdenum and copper. Extension sources contradict each other directly. Where they do, this guide reports the disagreement rather than choosing a side.
• Thailand-specific molybdenum data. None was found, of any kind.
• A Thai critical level for boron, or a Thai boron deficiency survey beyond the single 2022 finding that available boron was below threshold.
• Water quality data for Provincial Waterworks Authority supplies. Only the Bangkok metropolitan dataset was found. For a farm in the provinces there is no published baseline to read against.
• Conductivity or dissolved solids figures for Thai canal water or Thai harvested rainwater. The national rainwater study exists but does not publish those values.
• Quantified leaching losses in kilograms per hectare for Thailand. The mechanism is well documented and the Thai soil and rainfall conditions are measured, but the loss rates themselves were not obtained.
• Silicon phloem mobility, and any visual silicon deficiency symptom. Neither is described in the sources reviewed.

If a figure matters to a decision on your farm, measure it on your farm. That is the honest conclusion of this section, and it applies more in Thailand than in the places most of this literature was written.

Downloads

Files hosted here rather than linked to another site, so they stay available.

pH absorption chart — the pH range across which each nutrient is taken up. Original by CANNA, translated and published by AZ Growshop.

Download the pH absorption chart (PDF)

Related pages

Terms used on this page are defined in the glossary:

• Grower's Glossary — definitions of the technical terms used here
• Fertilizers — base nutrients and the functional groups referred to above
• Measurement tools — pH and EC meters, the equipment this guide assumes you have

Sources

Every figure in this guide traces to one of the following. Where two sources disagree, both are named in the text.

Plant nutrition and deficiency symptoms

• Cornell University NRCCA, Nutrient Management CA1 — nutrient mobility
• University of Florida IFAS HS1373, Movement of Plant Nutrients
• Montana State University Extension, Soil Fertility — Nutrient Deficiencies
• University of Arizona Extension AZ1106, Guide to Symptoms of Plant Nutrient Deficiencies
• UC IPM, Common Nutrient Deficiency Symptoms
• Penn State Extension, Hydroponics Systems and Principles of Plant Nutrition
• Mosaic Crop Nutrition — secondary nutrients, micronutrients, nickel
• Stangoulis et al. (2010), Plant Physiology 153(2):876–881 — boron phloem mobility
• Long & Peng (2023), Genes 14(1):130 — boron interactions with other elements
• Xie et al. (2019), Frontiers in Plant Science 10:1172 — phosphorus, zinc and iron
• Rutgers NJAES FS1278, Silicon Needs of Soils and Crops
• Pavlovic et al. (2021), Frontiers in Plant Science 12:697592 — silicon interactions

pH, availability and analysis

• University of Maryland Extension FS-1054, Soil pH Affects Nutrient Availability
• University of Georgia Extension B1256, Essential pH Management in Greenhouse Crops
• Oklahoma State University Extension HLA-6722, Electrical Conductivity and pH Guide for Hydroponics
• Clemson Land-Grant Press, Interpreting Routine Soil Tests
• Jensen (2010), IPNI Plant Nutrition TODAY Fall 2010 No. 2
• Hartemink & Barrow (2023), Plant and Soil 486:209 — criticism of the pH availability diagram
• University of Missouri Extension G9069, When More Is Less — nutrient antagonisms
• University of Minnesota Extension, Understanding plant analysis for crops
• Southern Cooperative Series Bulletin #394, Reference Sufficiency Ranges for Plant Analysis

Thailand — soils, water and climate

• Land Development Department of Thailand, via FAO Global Soil Partnership — soil orders and problem soils
• Funakawa et al., in FAO, Management of Tropical Sandy Soils for Sustainable Agriculture
• Yanai et al., in the same FAO volume — Northeast Thailand soil fertility
• Vityakon, in the same FAO volume — organic matter loss under cultivation
• Dur et al., in the same FAO volume — clay mineral dissolution
• Lesturgez et al. (2006), Agriculture, Ecosystems & Environment 114:239–248
• Hirose et al. (2022), Tropical Agriculture and Development 66(1):33–43 — Thai paddy micronutrient status
• Yanai, Tanaka & Nakao (2022) — fifty-year change in Thai paddy fertility
• Arunin & Pongwichian (2015), Land Development Department — salt-affected soils of Thailand
• Arjwech, Everett & Wanakao (2019), Songklanakarin Journal of Science and Technology 41(5)
• Terakulsatit et al. (2024), Environment and Natural Resources Journal 22(5) — salinity around salt workings
• Metropolitan Waterworks Authority, Consumer Confidence Report 2025
• Kordach et al. (2018), E3S Web of Conferences — Bangkok water survey, 32,711 samples
• Soontornpipit et al. (2025) — Thai harvested rainwater quality
• Ayers & Westcot, FAO Irrigation and Drainage Paper 29 Rev. 1 (1985)
• Thai Meteorological Department season definitions, via the Thai government portal
• Kyuma, JIRCAS Tropical Agriculture Research Series 15:105–117 — tropical versus temperate soils

Climate, transpiration and waterlogging

• Yu et al. (2022), Environmental and Experimental Botany 195:104786 — vapour pressure deficit and calcium
• Saure (1998), Scientia Horticulturae 76(3–4) — tipburn and night humidity
• Adams & Ho (1993), Plant and Soil — environment and calcium uptake
• Kabir & Díaz-Pérez (2025), Horticulturae 11(7):807 — calcium transport
• Li et al. (2025), Frontiers in Plant Science 16:1653008 — ion toxicity in waterlogged soils
• Lehmann & Schroth, Nutrient Leaching, in Schroth & Sinclair (eds.), CABI
• Atique-ur-Rehman et al. (2018), Agronomy for Sustainable Development 38:25 — boron leaching

Supplementary material

• CANNA deficiency guide articles and Info Courier leaflets, used for cross-reference and for the leaf images on this page. Supplementary, not a primary source.

Questions about a symptom on your own farm are welcome. Send photographs of the affected leaves together with your feed pH, runoff pH, feed EC and runoff EC — those four numbers answer more questions than the photographs do.