The Heart of It

Why children are different

The single most important idea in this guide: a level of contamination that is genuinely fine for an adult can still be harmful to a child. Children are not small adults.

Why a child's body responds differently

• Smaller body, bigger dose. The same amount of a contaminant is a much larger dose per pound of body weight in a child than in an adult.
• Still-developing systems. The brain, nervous system, and immune system are still being built through childhood. Metals like lead and mercury interfere with that construction in ways that don't happen in a finished adult body.
• They take in more soil. Children put hands to mouth constantly, play close to the ground, and simply ingest and inhale more soil and dust than adults do — the EPA's own exposure figures account for this.
• Higher metabolism and breathing rate. Pound for pound, children eat, drink, and breathe more than adults, so more of what's in the soil and dust gets into them.
• A lifetime to develop consequences. A harm that starts at age eight has seventy years to express itself; many effects (like cancers) have long latency.

Why "below the regulatory limit" is not the reassurance it sounds like

Most soil numbers people cite were built for a different question than "is it safe for a child to grow and eat food here?" They were built for cleaning up contaminated industrial sites, on the body weight and behavior of adults. Two clear examples:

• Lead. There is no known safe level of lead for a child. The CDC sets a “blood lead reference value” of 3.5 µg/dL not because that level is safe, but to flag the children with the most exposure. Even low levels are linked to lost IQ points and attention and behavior effects. In 2024 the EPA lowered its residential soil screening level for lead from 400 ppm to 200 ppm (100 ppm where several sources are present) — an acknowledgment that the old number wasn't protective enough.
• Pesticides in food. Congress recognized this difference in law. The Food Quality Protection Act of 1996 requires an extra 10-fold safety margin specifically to protect infants and children unless data prove it's unnecessary. The principle is already settled in one arena — it simply hasn't been carried over to garden soil.

The Exposure

How children are actually exposed

To understand the risk, picture the real day of a child in a school garden — not a one-time visit, but years of hands-on contact with the same ground.

Four pathways at once

• Skin contact. Hands, arms, knees in the soil for hours, repeatedly.
• Hand-to-mouth. Children touch their faces and mouths constantly; soil on hands becomes soil ingested.
• Breathing dust. Digging, raking, and dry beds put soil dust in the air right where small lungs are working hard.
• Eating the harvest. The food is grown in, and pulled from, this exact soil — and some plants concentrate metals from the soil into the parts we eat.

The part that changes everything: time

A child who takes part from kindergarten through twelfth grade may be in repeated contact with the same soil system for thirteen years. Meanwhile, compost and amendments are added to that same ground season after season. This is a chronic, cumulative, repeated-loading situation.

Ordinary agricultural guidance and contaminated-site cleanup standards were never designed for this scenario. They picture a one-time decision about a site, for an adult. The school garden is the opposite: the same children, the same soil, year upon year, with more material added each season.

The Inputs

What goes into the soil

Here is where good intentions and incomplete information meet. Several popular soil practices — taught widely and used sincerely — can quietly add contaminants or move them into forms children can take up.

Practices worth a second look

Fermented plant extracts sprayed on growing sites

Fermentation is, by definition, an anaerobic (low-oxygen) process. The same oxygen-starved conditions that make a ferment also favor the survival of anaerobic pathogens — which then get sprayed directly onto soil and onto the crops children handle and eat.

“Super-accumulator” weeds fermented and sprayed back on the soil

Some common weeds are excellent at pulling heavy metals out of soil and concentrating them far above normal levels — that's real, and it's why they're used to clean contaminated land. But fermenting that metal-rich biomass and spraying the liquid back onto the garden is the opposite of cleaning: the acids of fermentation put those stored metals back into more soluble, more available, and sometimes more toxic forms, sprayed right across the bed where children work.

Compost teas brewed and sprayed on crops

When a tea is aerated and fed a sugar (molasses, kelp, guano), the whole point is to grow the microbe population — and that growth window is exactly where human pathogens can multiply too. USDA Agricultural Research Service scientists documented regrowth of E. coli O157:H7 and Salmonella in fed compost teas even when the starting compost tested below pathogen limits. Two things matter in a school garden: a vigorous plant response tells you nutrients moved, not that the brew was safe; and microscopy (the tool many growers trust to “check” a tea) can spot fungi but cannot tell dangerous E. coli from a harmless look-alike — that needs DNA testing.

“Free” compost from the ornamental world

Donated compost from a botanical garden or landscape source can be a genuine gift — or a hidden problem. Many chemicals used routinely in ornamental horticulture are not permitted in food production, and a number of them do not break down in composting. As the pile shrinks, they concentrate, and then keep releasing in your beds.

It's not the amendment — it's what went into it

Here is the mindset that ties all of this together: compost is a process, not an ingredient. The word “compost” tells you how something was made — not what it was made from. And what was composted is the whole question. The same holds for every amendment: judge the feedstock and the source, not the label or the story.

Take worm castings — a genuinely excellent amendment, and one we recommend highly. But worms are heavy feeders, and their castings carry whatever the worms were fed. We have clients across the country whose castings tested high for cadmium, traced back to cardboard used as the worms' food source. The castings weren't the problem — the feedstock was. Know where your castings come from and what the worms eat, and they stay one of the best things you can add.

The Hidden Chemistry

How “locked-up” contaminants get released

This is the deeper science, and it's the piece almost no one explains. A soil can look fine, test fine on the basics, and still release stored contaminants when you add the wrong thing. Here's how.

The phosphorus trap

It's common for compost to test 3,000–5,000 ppm phosphorus, while conventional and organic programs broadly agree soil phosphorus should usually sit under about 150 ppm. Beyond the direct harm of that overload, heavy phosphorus does something subtler and more serious: competitive desorption.

Phosphate competes for the very same binding sites on iron and aluminum oxides and clay particles that hold other compounds. When you flood the soil with phosphate, it elbows those other compounds off their binding sites and releases them into the soil solution. This matters most in gardens amended heavily with manure or manure compost — themselves a major source of the very compounds listed below.

What gets released by phosphate competition

• Arsenic — arsenic is the chemical “twin” of phosphate and the priority case. Adding phosphate to soil carrying legacy arsenic (old lead-arsenate orchard residue, or natural Coast Range background) pushes it off its binding sites and into the soil water. One honest complication: arsenate also enters roots through the plant's phosphate transporters, so heavy phosphate competes at the root and can reduce uptake even while it raises the amount in solution. Which effect wins depends on your soil. Treat heavy phosphorus on arsenic ground as a strong reason to test — not as a guaranteed outcome in either direction.
• Glyphosate and its breakdown product AMPA — bind to the same sites and are displaced the same way; AMPA is as persistent as the parent.
• Glufosinate and phosphonate fungicides (Fosetyl-Al, phosphite products used heavily in vineyards, citrus, avocado, and nurseries) — same binding, same release.
• Not lead, and not fluoride. You will see both listed under this heading elsewhere, including in earlier drafts of our own material. Neither holds up. Phosphate immobilizes lead — it converts it to pyromorphite, a lead-phosphate mineral that stays sparingly soluble across soil pH and even in stomach acid, which is why phosphate amendment is an established technique for lowering lead bioaccessibility. And phosphate rock is fluorapatite, so phosphate fertilizer is a source of fluoride (and, depending on the ore, of cadmium and uranium) rather than something that sets fluoride loose.
• Veterinary antibiotics from manure (tetracyclines, fluoroquinolones) — survive digestion and composting, bind to the same sites, and are freed by phosphate and organic-acid amendments; they also drive antibiotic-resistance in soil microbes.

Released by other common amendments

• Cadmium — chloride is the clearest case: it forms soluble cadmium complexes and raises plant uptake, so muriate of potash and chlorinated wash water work against you. Sulfate salts are implicated as well, though less consistently. Note what this is and isn't: it describes what a material does in the soil, not what is in it. Gypsum is still a clean input by composition — it simply isn't inert. Cadmium is readily taken up by leafy greens.
• Mercury, copper, lead — pulled into solution by dissolved organic carbon from heavy compost-tea and humic/fulvic programs, which chelate the metals.

The “killer compost” class

A group of persistent pyridine herbicides — aminopyralid, clopyralid, picloram, triclopyr — bind to organic matter rather than mineral surfaces. They survive an animal's digestion and composting (12–36 months for some), then release slowly as the compost breaks down in your beds. They are the single largest compost-contamination story of the last twenty years, and they destroy broadleaf vegetables at parts-per-billion levels.

The other side of the coin — healthy soil protects

Here is the hopeful part, and it is just as real as the rest. The same living, organic-matter-rich soil you work so hard to build is itself a protective barrier. Organic matter and clay carry negative charges that grip positively charged metal ions — lead, cadmium, copper, zinc — and hold them tightly, so less moves into the water around the roots and into the plant. Keeping your soil rich in organic matter and near a neutral pH measurably lowers how much of any metal a plant can take up.

It isn't a cure-all — it doesn't bind every contaminant equally (arsenic, for example, behaves more like phosphate), and it never replaces testing. But it means your stewardship is not in vain: a well-built, living soil is quietly working for you and the children every single day. One caution about how far to carry this, because the point is worth making honestly: organic matter is not a universal binder. Radiocaesium is the standard counter-example — it is held by clay minerals rather than by humus, and post-Chernobyl studies consistently found the highest soil-to-plant transfer in organic and peaty soils. Cation exchange is real and it is working for you every day on lead, cadmium, copper and zinc. It is not a blanket shield, and it is not a substitute for knowing what you put on.

The Metals

The metals, one by one

Eight metals do most of the work of this concern. Here's what each one is, where it comes from, and why it matters — especially for a child.

Keep Perspective

Aluminum — a high number is not a crisis

Aluminum deserves its own plain explanation, because it's the number most likely to alarm people for no good reason.

Why the big number doesn't matter

Aluminum readings vary enormously depending on how the lab measures them. A routine extractable soil test reads far lower than a total digestion. So a large value on a soil or compost report — even several thousand ppm on a compost — is not, by itself, a problem. The bulk number is not the number that matters.

When aluminum actually matters — two conditions

• Low pH. Below about pH 5.5, aluminum starts to come into a form plants can take up. Above that, it stays locked away and harmless.
• Certain feedstocks. Alum-treated poultry litter (alum is aluminum sulfate, added to control ammonia — it both adds aluminum and lowers pH) or water-treatment residuals.

What to actually do

Only when pH is low or one of those feedstocks is in play do you test the part that counts: exchangeable aluminum by a 1 M KCl test. It's a routine, inexpensive test (about $7–$25 per sample, offered by university labs and by UC Davis's lab in California). Ask for it by name — a standard metals panel won't give it to you.

And here's the reassuring part: pH is both the warning sign and the fix. A simple pH test tells you whether you even need to worry, and if you do, liming the soil toward pH 6.5 turns the available aluminum back into the harmless, locked-away kind.

The Standard

The standard we recommend

Rather than invent numbers, we adopted a tested, decades-old framework — the German soil precautionary values — and apply them at the point of use, where the children actually are.

The targets (mg/kg dry soil)

Values come from the German BBodSchV 2023 precautionary values, by soil texture. Sandy soils get the strictest (lowest) numbers because metals are more available there.

Aluminum is handled separately — see the Aluminum tab. It is our own addition, not part of the German framework, and only as a plant-available (1 M KCl) test.

Two adjustments that make it fair

• Texture. Sandy soils use the lowest values (metals move more freely); heavier soils allow higher.
• pH. Below pH 6.0, cadmium, nickel, and zinc are judged one texture-class stricter; below pH 5.0, lead shifts too — because low pH makes metals more available.

The response ladder — targets, not verdicts

These are conservative management targets that tell you when to look closer — not lines that mean “this soil is poison.” Exceeding a target is a signal to investigate, not a declaration of harm.

1. At or below the target → carry on. Production proceeds.
2. Above the target, below the action value (three times the target) → pause the inputs and trace the source. Something is loading the soil; find it.
3. At or above the action value → stop and remediate before growing again.

You're Not Alone

Who else already does this

This isn't a fringe idea or a foreign import. Protecting food-growing soil is settled practice across the developed world — and one U.S. state already does it as enforceable law.

New York already has numbers in the same range

New York's enforceable soil cleanup objectives for “Unrestricted Use” (6 NYCRR Part 375-6.8), co-developed by the state's environmental and health agencies on child-exposure scenarios and reaffirmed effective December 31, 2025, line up closely with our recommendation:

Chromium is identical and arsenic close. New York is slightly stricter on mercury (0.18 against our 0.2), and our values are below New York for every other element. New York's numbers are cleanup objectives for land with no use restrictions, including farming. They don't require anyone to test a garden, but they show that numbers in this range are already law in a U.S. state.

East and West alike

We didn't pick the German model in a vacuum. We reviewed the major frameworks and chose Germany's because it's precautionary — built to prevent build-up rather than react to it — and because it adjusts for soil texture and pH. But the wider point is that nearly every developed economy regulates this:

• European Union and its member states
• United Kingdom — CLEA soil guideline values
• Canada — CCME Soil Quality Guidelines
• China — GB 15618-2018 for agricultural land

Protecting the soil children grow food in is normal, mainstream, and long-established — across both the West and the East.

Why No One Caught It

The gap — and why it isn't anyone's fault

If this is a real problem, why hasn't someone already handled it? The answer is a structural gap, not negligence by any person or agency.

What existing guidance does and doesn't cover

Current guidance suggests testing a site once, before you plant, to see if it's “suitable.” It sounds responsible — until you try to actually do it. Because here is the part that makes no sense: there are no numbers built for this question to compare your results against. You collect the soil, pay the lab, and get back a column of figures — and there is no child-food-growing standard anywhere that tells you whether those figures are fine or a problem. You're handed a result you can't interpret. A test with nothing to measure it against isn't really a test at all.

The only soil numbers that do exist are the contaminated-site cleanup screening levels — and, as the next point explains, those were built for a completely different question and were never meant to say a soil is safe to grow and eat food in.

And even setting that aside, the one-time test looks at the single moment before any build-up has occurred. It says nothing about the next ten or twenty years, as compost and amendments are added season after season. The one moment it covers is the one moment nothing has accumulated yet.

Cleanup numbers aren't food-growing numbers

Where soil numbers do exist — the EPA's and California's screening levels — they were written to decide when to clean up a contaminated industrial site. They flag a site for further investigation; they were never meant to certify soil as safe to grow and eat food in, and they're built on adults. A real example makes this vivid:

After the January 2025 Eaton Fire, Pasadena Unified School District hired a licensed firm (Verdantas) working with accredited labs to test soil across its campuses. The firm — choosing freely — tested for almost exactly the eight metals we recommend. But the only benchmarks it had to compare against were cleanup screening levels (like DTSC's 80 ppm for lead). Even a rigorous, credentialed, well-funded testing effort had no child-food-growing standard to measure against. That's the gap, in real life.

“Organic” and “compliant” don't mean “tested for metals”

It's natural to assume a certification already covers this. It doesn't. Organic certification governs how a crop is grown — it does not test the soil for heavy-metal accumulation. A compost can be labeled “EPA-compliant” or meet a facility standard and still carry metals, because those rules were written for other purposes. Several agencies each touch a piece — EPA Region 9, CalRecycle, the Department of Pesticide Regulation, and California's Office of Environmental Health Hazard Assessment (OEHHA) — but none requires the operator of a school garden to test the soil children grow food in. Every label answers a different question than the one that matters here.

You Can Handle This

A practical guide for garden staff

Here is the whole thing turned into practical, affordable steps for the people who run the garden. None of it requires you to stop your program or start over. It just makes you the person who's actually looking.

In the print window, choose "Save as PDF" as the destination to get a PDF, or pick your printer for a paper copy for the garden binder.

1. Who does what

Name one person as the Soil Safety Coordinator: a garden coordinator, teacher or facilities lead. That person:

• Has the authority to stop food growing in any bed while a soil question is sorted out.
• Schedules testing and keeps the results.
• Logs every delivery of compost, soil or amendment: date, supplier, batch, amount, and which beds it went on.
• Reports to the principal or administrator, who decides how to communicate with families.
• Leads the yearly review (see the checklist at the end of this tab).

Keep these records permanently: site history notes, sampling maps and photos, lab reports, compost certificates and your own compost tests, delivery logs, water tests, and anything done in response to a high result. Records protect the children and the program.

2. Before you plant: know your site's history

Answer these before you choose tests. Ask your facilities department for records.

• What was here before? Orchards (arsenic from old pesticides), vineyards (copper), auto shops, gas stations or dry cleaners (petroleum, solvents, metals).
• Old buildings nearby? Buildings from before 1978 can shed lead paint into the soil along their walls and drip lines.
• Treated wood? Decks, fences, utility poles, railroad ties or play structures from before 2004 can leach arsenic, chromium and copper. Don't build beds from old treated lumber or railroad ties.
• Roads and parking lots? Old leaded gasoline left lead in roadside soil for decades. Keep beds well back from busy roads (about 100 feet where possible).
• Fill soil? Was soil brought in, and from where? Unknown fill is high risk.
• Runoff? Does water from roofs, parking lots, roads or uphill properties drain onto the garden?
• Past chemical use? Turf and ornamental pesticides, termite treatments, rodent control.
• Underground tanks? Current or removed fuel tanks. County environmental health keeps tank records.

3. Know your local baseline

Every area has its own natural levels of metals and minerals in its soil and water. Some rock and groundwater are naturally high in arsenic, nickel, chromium or other elements. A result means little until you know what is normal where you are, and what your inputs add on top of it.

• Soil: your first test of each bed is its baseline. If you can, also test a nearby spot that has never been gardened or amended, to see your area's natural level.
• Water: test your irrigation water to learn its baseline (see section 11).
• Compare every later result to the baseline. A rise from year to year tells you something is adding to it, even while the result is still below the targets.

Your county environmental health department, local water supplier, testing lab and neighboring farms may know the natural levels in your area. Ask them.

4. Testing your soil

5. Getting started before you have funding

Many gardens begin with a site being explored and no testing budget. You can learn a lot before you spend much, and what you learn tells you where to spend later.

• Free: CDFA's fertilizer product database (apps1.cdfa.ca.gov/fertilizerproducts) for any bagged product; your water supplier's annual water quality report; county environmental health records on wells, tanks and known contamination; and local farms, which often have years of soil and water tests and know the area's problems.
• Low cost: an agricultural lab water and soil test (for example, Wallace Laboratories) gives you the full ion picture: pH, salts, sodium, chloride, bicarbonate, boron and nutrients. A broad mass-spectrometry (ICP-MS) metals scan covers dozens of elements at once for a modest cost. It is less precise at low levels than a certified test, but it shows what is present and what deserves a closer look.

For your grant budget, plan these testing lines, and get written quotes from your labs:

6. Reading your results

Find your soil texture on the lab report, then compare each metal to the matching column in "The standard we recommend." If you're not sure of your texture, use the sandy column: it is the most protective, and raised-bed mixes are usually sandy.

Three zones:

• Below the target: OK. Keep testing every year, and compare to your baseline.
• At or above the target, but less than three times it: investigate.
• Three times the target or more: stop growing food in that bed until it is resolved.

7. If a result is high

8. Know your inputs

Compost is only one way metals reach a garden bed. Every material you add, including manure, meals, rock dusts, potting mixes and mulch, can bring something in with it. The rule is simple: document every input, and test the inputs of concern. Trust the analysis, not the label. "Organic," "natural" or "regenerative" describes how something was made, not what is in it. know your feedstock test what's risky

Document every input: what it is (name and brand), the supplier, the batch or lot number, the date, the amount, which beds it went on, and any certificate or test result. For bagged products, look them up in CDFA's fertilizer product database; some registrations list metals data.

Inputs of concern: test each batch before use

Chicken manure is a prime example of an input of concern. It can bring in several problems at once. Where it is a regular input, test every batch, track zinc, cadmium, aluminum and phosphorus together, and keep soil pH up. Cadmium and aluminum both become more available to plants in acidic soil, and leafy greens, the most common school-garden crop, take up cadmium readily.

Low concern: document it, no testing needed. Gypsum, agricultural lime, oyster shell flour, kelp meal at normal rates, and other materials with a long safety record, especially in small amounts. Check the specific product in the CDFA database. Clean is not the same as inert: gypsum carries no metals of concern, but sulfate and especially chloride (from muriate of potash or chlorinated water) can make cadmium already in the soil more available to plants.

What to ask a supplier for. For every batch of an input of concern, ask for a batch-specific Certificate of Analysis covering that batch, not an annual average: a full metals panel (plus aluminum for poultry litter or alum-treated materials); for compost and manure, pathogen tests (generic E. coli and Salmonella at minimum), physical contaminants (plastic, glass, metal) under 1 percent by weight, and a maturity test; and where the ingredients came from, with time and temperature records for compost.

• If a supplier can't document where a material came from, keep it off the food beds.
• Don't use compost made from sewage sludge (biosolids) in food beds.
• Test compost and manure for persistent herbicides. Some weed killers (aminopyralid, clopyralid, picloram) pass through animals and survive composting, then damage garden crops. Before using a new source, plant peas or beans in pots of it next to pots of plain potting mix. After about 3 to 4 weeks, cupped, curled or twisted new leaves mean don't use it.

9. Test the compost you actually received

A facility's lab report does not describe your compost. Permitted facilities test one composite sample for about every 5,000 cubic yards they produce. A school bed might get one or two of those yards. The facility's results show trends across the season, not the levels in your load. Take a sample from your delivery and send it to the lab yourself. Ask for two things:

• Metals, the same panel as your soil, to check the load is safe to use.
• Nutrients and salts: nitrogen, phosphorus, potassium, calcium, magnesium, sodium, carbon-to-nitrogen ratio, pH and salts (EC). You need these numbers to decide how much to apply. Compost often carries 3,000 to 5,000 ppm of phosphorus or potassium. Potassium washes out of the soil fairly readily, so it rarely builds up for long, but each heavy dose still shifts the balance with calcium and magnesium. Phosphorus stays. Conventional and organic programs broadly agree that soil phosphorus should usually sit under about 150 ppm. One inch of compost at 3,000 to 5,000 ppm phosphorus, mixed into the top 6 inches, adds roughly 180 to 300 ppm of total phosphorus; three inches adds roughly 480 to 800 ppm. Not all of it shows up on a soil test at once, but it stays and builds. Account for it before you apply, and apply only what your soil test calls for.

10. Two starting points for managing your soil chemistry

To set rates you need targets to aim for. Here are two published examples, depending on the path your program takes. These are not the answers. They are base targets for a community to start from and refine with its own soil tests, crops and experience. Both assume a Mehlich-3 soil test.

• Many California labs report phosphorus by the Olsen method, not Mehlich-3, and the numbers are not interchangeable. Ask your lab which method it uses.
• Watch phosphorus on either path. Higher phosphorus targets mean more phosphorus that can wash off-site and release arsenic in the soil.
• Neither set is a safety standard. Metals are judged against "The standard we recommend."

11. Water

Test your garden water before the first season and once a year after that. Water sources can also change with the seasons. A well can draw from deeper water late in a dry summer, a creek or pond changes with rain and runoff, and city suppliers sometimes switch or blend sources during the year. If your source changes seasonally, test it in each season you irrigate, at least for the first year, so you know its range. Test for two reasons:

• Safety: generic E. coli, arsenic, nitrate, and the same metals you test in the soil. On city water, your supplier's annual water quality report covers most of this. Ask for it.
• Ion management: pH, total salts (EC), sodium, chloride, bicarbonate (alkalinity), calcium, magnesium and boron. Water adds these to the soil every time you irrigate. Over time they can raise soil pH, build up salts, and upset the mineral balance, which changes how much metal plants take up and stresses soil life. City water needs this test too: the supplier's report is written for drinking, not for soil.

Keep water results with your soil results, and look at them side by side.

• City water: safe for food crops, but the chlorine or chloramine that makes it safe to drink is there to kill microbes, and it can harm the living soil too. Chloramine lasts longer than chlorine, and chloride makes cadmium already in the soil more available to plants. Keep organic matter up, since it buffers the soil. If the irrigation system connects to drinking water, it needs an approved backflow preventer; your water supplier typically requires this, along with annual testing.
• If your program chooses to dechlorinate garden water, the decision belongs to your district, which owns the plumbing. Get approval from district facilities staff and your water supplier first. Treat only a dedicated garden irrigation line, downstream of an approved backflow preventer (a carbon filter, or vitamin C, which neutralizes both chlorine and chloramine). Never use dechlorinated water for drinking, handwashing or washing produce. Apply it to the soil, not to the leaves of crops eaten raw. Don't store it: without chlorine, bacteria can grow in standing water. Maintain filters on schedule and log it.
• Well, pond, creek or canal water: before using it on food crops, run the full water test above and talk to your county environmental health department. Natural levels of arsenic and other elements in groundwater vary a lot from place to place, and whatever is in your water ends up in your soil every season.
• Any water that isn't drinking water: leave at least 24 hours between the last watering and harvest.

12. Watch your pH, and keep building living soil

Keep beds near pH 6–7. It keeps metals (and aluminum) locked in their harmless forms, and it tells you whether you ever need the extra aluminum test. Keep building organic matter: a rich, living soil grips metal ions and lowers how much any plant can take up. Your soil-building is part of the protection, not separate from it.

One caveat worth knowing before you lime. Where a soil is high in cadmium relative to zinc, raising pH lowers zinc availability, and because cadmium rides in on the plant's zinc transporters, liming on its own can increase cadmium uptake in leafy greens. Lime paired with zinc is the combination that has been tested and works. If your panel shows cadmium high and zinc low, deal with them together rather than liming alone.

13. Practices to avoid

• Planting food straight into a thick layer of municipal or industrial compost. A common practice is to spread about 3 inches of municipal compost on the bed and plant lettuce in it. That is roughly 400 cubic yards an acre. The roots grow in the compost itself, not in soil, so nothing dilutes it. California's legal metal limits for compost are many times higher than the targets in this guide: lead up to 300 mg/kg, against 40 for sandy soil. Lettuce and other leafy greens take up cadmium readily, and compost splashes onto the leaves. Municipal compost also often contains large wood chips that are not fully composted; as they break down in the soil, they tie up nitrogen the crop needs. Look at what you are getting before it goes into your soil, and test it.
• Fermented plant sprays on food crops. Fermentation happens without oxygen, the same conditions that let harmful bacteria survive and grow.
• Fermenting weeds and spraying them back on the bed. Many common weeds pull metals out of the soil and concentrate them. Fermenting them and spraying the liquid back puts those metals back on the bed in a more available form.
• Sugar-fed compost teas sprayed on leaves. If you use compost liquids, use a plain extract (compost soaked in water, nothing added) and apply it to the soil, not the leaves. A microscope can't show a tea is safe; only a lab test can.
• Piling on manure and compost. Too much phosphorus can release arsenic and other compounds already held in the soil.
• Untested fill or bagged soil of unknown origin in food beds.

14. Keep what you add in the garden

Monitoring your inputs also protects the land and water around the garden. What plants don't use, and the soil can't hold, moves off-site: phosphorus washes off and feeds algae blooms in creeks, ponds and lakes; nitrogen from heavy compost or manure leaches as nitrate into groundwater, including wells people drink from; potassium, sodium and other salts leach out of the bed; and metals move with eroded soil, dust and splash.

• Apply only what your soil and compost tests call for.
• Don't leave compost or manure piled uncovered where rain can wash it off. Cover the piles, and keep them away from storm drains, ditches and creeks.
• Keep beds covered with plants or mulch over the rainy season, and keep inputs and bare soil back from drains and waterways.
• Log every input, so you can always say what went on and how much.

15. Harvest basics

• Never set harvested produce on bare soil. Use clean food-grade containers from the start, and keep them for produce only, never for compost or soil.
• Wash all produce with drinking water before it goes to the kitchen. Rinse leafy greens three times. Visible soil on food carries the very metals this guide is about.
• Students wash hands after working the soil and before handling produce.

16. Season checklist and yearly review

Where to get help

• Your county environmental health department: contamination questions, well water, underground tanks, compost facility concerns.
• Your testing lab: which tests and methods to use, and help reading the report.
• Local farms: what they've found in the area's soil and water, and which labs they use.
• ORCA: the full technical manual this guide is based on (ORCA-SOP-F2S-001), free on request, plus the ORCA blog. David King, Executive Director · calorcaprogram@gmail.com · 707-397-5159 · orca-ca.com

Why farm to school can lead

The gaps this guide works around are not unique to school gardens. Across the country, every kind of farm faces the same problem: metals arriving with compost, manure and other inputs, building up season after season, with no one measuring the trend.

It has to start somewhere. The farm to school community is one of the most inspiring parts of American agriculture: teachers, garden coordinators, food service staff, parents and growers who care deeply about the children they feed. That makes it the right place to start. Schools that know their baselines, log their inputs and track their soil and water year after year will build something no rule requires yet: real experience in managing soil for long-term safety. That experience can then be shared with every other part of agriculture and conservation. Farm to school has the chance to lead.

Go Deeper

References & further reading

Everything in this guide can be checked. Here are the main sources, gathered in one place.

Children's vulnerability & health

1. CDC, Childhood Lead Poisoning Prevention — Blood Lead Reference Value (3.5 µg/dL; no safe level in children).
2. U.S. EPA, Updated Residential Soil Lead Guidance (200 ppm screening level), January 2024.
3. U.S. EPA, Exposure Factors Handbook, Ch. 5 — Soil and Dust Ingestion.
4. U.S. EPA, Summary of the Food Quality Protection Act (1996) — 10× children's safety factor.
5. ATSDR Toxicological Profiles — Lead, Arsenic, Cadmium, Mercury, Chromium, Copper, Zinc, Nickel.
6. IARC Monographs — Arsenic and Hexavalent Chromium (Group 1 carcinogens).
7. NASEM, Exploring Linkages Between Soil Health and Human Health (2024).

Soil chemistry & inputs

8. Peryea, F.J. & Kammereck, R. (1997) — phosphate-enhanced arsenic movement.
9. Creger, T.L. & Peryea, F.J. (1994) — phosphate fertilizer and arsenic uptake.
10. Thiele-Bruhn, S. (2003) — antibiotics in soils.
11. Sarmah, A.K. et al. (2006) — veterinary antibiotics in the environment.
12. Ingram, D.T. & Millner, P.D. (2007); Duffy, B. et al. (2004) — pathogen regrowth in compost teas (USDA ARS).
13. Butler, J., Garratt, M.P.D. & Leather, S.R. (2012) — Fertilisers and insect herbivores: a meta-analysis, Annals of Applied Biology (over-fertilization / excess nitrogen and insect-pest pressure).
14. Datnoff, L.E., Elmer, W.H. & Huber, D.M., eds. (2007) — Mineral Nutrition and Plant Disease, APS Press.
15. Scheckel, K.G. et al. (2013) — Amending soils with phosphate as means to mitigate soil lead hazard: a critical review of the state of the science. J. Toxicol. Environ. Health B (US EPA/NRMRL) — phosphate immobilises lead as pyromorphite.
16. Meta-analysis of phosphorus–zinc interaction across 51 studies (2020), Agronomy for Sustainable Development — the effect is mediated by mycorrhizal suppression and root uptake capacity, not by reduced soil zinc availability.
17. Zhang, W. et al. (2015), Plant and Soil — phosphorus-induced zinc deficiency in wheat is mycorrhiza-dependent; absent in autoclaved soil and nutrient solution.
18. Chaney, R.L., Forte, D., Seyfferth, A.L., Smith, R.F. et al. (2025). Factors Affecting Cadmium Accumulation and Mitigation: A Literature Review to Inform Spinach and Carrot Producers. HortScience 60(11):1908–1917 — soil Cd:Zn ratio governs uptake; zinc reduces spinach cadmium. Smith, R.F. & Hartz, T.K., UC Cooperative Extension, Monterey County — field trial: ~40% reduction in spinach cadmium after zinc sulfate.
19. IAEA Technical Reports Series — radionuclide transfer in terrestrial environments; radiocaesium is fixed by clay minerals, with the highest soil-to-plant transfer in organic and peaty soils.

California's own published work

13. Chang, A.C., Chen, W., Bar-Yosef, B., Page, A.L. & Kim, S. (2004). A Generalized Trace Element Mass Balance Model for Cropland Soils: Arsenic and Cadmium. Final report to CDFA — hundred-year accumulation scenarios with Monte Carlo uncertainty analysis.
14. Chang, A.C., Page, A.L. & Krage, N. (2004). Role of Fertilizer and Micronutrient Applications on Arsenic, Cadmium, and Lead Accumulation in California Cropland Soils. Final report to CDFA — benchmark soils compared 1967 vs. 2001.
15. CDFA Fertilizing Materials Inspection Program — Notice to Industry, Heavy Metals Analysis (all nine metals required from January 1, 2021).

Standards & precedent

16. German Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV 2023), Annex 1, Table 1.
17. German Environment Agency (Umweltbundesamt) — precautionary soil protection.
18. New York 6 NYCRR Part 375-6.8 — Soil Cleanup Objectives (Unrestricted Use), effective Dec. 31, 2025.
19. UK Environment Agency — CLEA soil guideline values.
20. CCME — Canadian Soil Quality Guidelines.
21. China GB 15618-2018 — agricultural-land soil standard.
22. Aluminum testing: University of Minnesota RAL; UC Davis Analytical Lab; Kansas State Soil Testing Lab (exchangeable Al, 1 M KCl).

ORCA field observations & recommendation

These are ORCA / Surprise Valley Agroecology's own practitioner field observations and professional recommendation — not peer-reviewed papers or government publications. They are the source of the field experience in this guide (the input practices, the worm-castings/cadmium finding, and the proposed targets and their point-of-use application). Every scientific and regulatory claim above rests on the independent government and peer-reviewed sources in the sections before this one.

20. ORCA / Surprise Valley Agroecology — Heavy-Metal Target Management for Child-Occupied Food Production (Germany enquiry brief, 2026).
21. ORCA / SVA — Farm-to-School Soil Safety: Field Observations (CDFA Science Advisory Panel briefing, June 2026).

Growing Healthy Food

Balance & excess — when more becomes a problem

Soil safety isn't only about contaminants. It's also about balance — and this is where protecting children's food and protecting the land become the same task.

Nutrients work in ratios, not piles

Plants take up nutrients in relationship to one another. Drive one too high and it “locks out” others — an effect called antagonism. A soil that tests “high” in something is not automatically good news; it can be the very reason a plant is starving for something else. Some well-established examples:

• Excess phosphorus drives zinc, iron, and copper deficiency. A plant swimming in phosphate can still show clear micronutrient shortage. It is worth knowing how, because the common explanation is wrong: the effect works through the plant — suppressed mycorrhizal colonization and reduced root uptake capacity — not by tying zinc up in the soil. It disappears in sterilised soil and in nutrient culture, and it largely does not apply to crops that form no mycorrhizas, which includes the brassicas, spinach, chard and beet. Lettuce it does.
• Excess potassium blocks magnesium and calcium. Too much of one cation crowds out the others.
• Calcium and magnesium balance each other — and that balance also governs soil structure and how water moves through the ground.

Why balance is a food-quality question, not just a yield one

An imbalanced plant is a weaker plant. It builds less complete nutrition, so the food that reaches a child's plate is less nourishing than it looks. And a stressed, imbalanced plant tends to make the simple compounds that pests thrive on — so it becomes more attractive to pests, which drives more spraying. This is not folklore: modern meta-analyses find that over-fertilized plants — especially those pushed with excess nitrogen — carry heavier insect-pest loads, and mineral balance is now a recognized tool in managing plant disease. Getting the balance right means better food and fewer inputs. Plant health comes first, and balance is how you get there.

Phosphorus: from garden overload to environmental hazard

Phosphorus is the clearest example of “too much of a good thing.” Compost commonly tests 3,000–5,000 ppm phosphorus, while soil phosphorus should usually sit under about 150 ppm. That is a massive overload, and it causes trouble in two directions:

• In the bed: excess phosphorus drives zinc, iron and copper deficiency in mycorrhizal crops (above), and — as the Hidden Chemistry tab explains — it can move bound-up contaminants such as arsenic off their binding sites.
• Beyond the bed: phosphorus does not stay put. Excess soil phosphorus runs off with the rain and leaches toward groundwater. In creeks, rivers, lakes, and the ocean it feeds algal blooms and oxygen-starved “dead zones.” What feels like generosity to a garden bed becomes a pollutant to an entire watershed.