How Plants Work (2015)
3 What’s Essential
WE GARDENERS TEND TO FUSS about our garden plants’ nutrition. We worry whether we’re providing the right form of phosphorus, if organic forms of nitrogen are better than inorganic ones, and if we need to add chelated iron or kelp or humic acids or mycorrhizal fungi. Whew.
The fertilizer sections of nursery and garden centers that have exploded in the last decade are beginning to look like nutritional supplement stores. How in the world can you find out what exactly you should be buying? In other words, how can you determine what’s essential for your garden plants and what’s just a clever marketing strategy?
Strictly speaking, the essential elements are those that plants require to complete their life cycle. This chemical collection is constantly changing as we learn more about plant biochemistry. When I took my first class in plant physiology, I think there were fifteen elements labeled as essential. Now we recognize at least nineteen of them for most plants, and some specialized plants require still others.
The way researchers determine whether an element is essential to plant growth is through a nutrient exclusion experiment. Seedlings are grown hydroponically in solutions that contain all the known nutrients except for the one in question. Researchers can discover whether the plants reach maturity and reproduce (in other words, complete the life cycle), as well as observe what deficiency symptoms look like. As you might imagine, it becomes more and more difficult to run these experiments with nutrients that are required by plants in increasingly smaller amounts.
Although it’s easy to do nutrient exclusion experiments and see how a particular species responds to a missing element, it’s extraordinarily difficult to do this in the garden. Toxic levels of one nutrient can mimic deficiencies of another, and viruses and other environmental factors can cause symptoms similar to nutrient deficiencies. Instead, gardeners should rely on soil tests to discover which nutrients are deficient and, more importantly, which are in excess.
Yellowish leaves on this bean plant might indicate a nitrogen deficiency.
Mineral nutrients were historically divided into macro- and micronutrients, depending on how common they are in plant tissues relative to one another. But again, as our understanding of plants improves, we discover that species can vary dramatically in their mineral nutrient content, so it’s more useful to look at what function these nutrients play, rather than worrying about how much of each a particular plant might contain.
The Big Three: Carbon, Hydrogen, and Oxygen
Your garden plants, you, and all other life on Earth are carbon-based life forms. Carbon forms the scaffolding from which other elements are hung, in particular hydrogen and oxygen. These three elements together are the principal building blocks of organic compounds, which include carbohydrates, proteins, fats, and nucleic acids (DNA and RNA). (In a chemistry context, the word organic relates to the molecular makeup of a compound. It has nothing to do with the popular notion of equating organic with natural.) Carbon, hydrogen, and oxygen are obtained from the atmosphere and water. Therefore, they aren’t considered to be mineral nutrients, which come primarily from the soil.
The Other Three: Nitrogen, Sulfur, and Phosphorus
Nitrogen is an intrinsic part of amino acids, which means it’s needed for every protein and enzyme a plant produces. It’s also part of the chlorophyll molecule, is embedded in DNA, and is a major component of alkaloids, a group of protective secondary compounds. Soil microbes need nitrogen, too, so this element can become deficient in an actively growing garden as everyone competes for their fair share. If you notice the older leaves of your plants turning yellow during the peak of the growing season, it might mean your soil nitrogen levels are low. Plants scavenge nitrogen from their older leaves and use it to support new leaf growth, so the old leaves become chlorotic.
Root nodules contain bacteria that convert atmospheric nitrogen to a form that can be used by plants.
Some common garden plants, like peas and beans, avoid this situation altogether by producing their own nitrogen. These plants, which include all of the legumes and members of some other plant families, are nitrogen-fixers, meaning they strip nitrogen from the air and transform it into a solid form. Nitrogen-fixing plants rely on specialized bacteria to transform nitrogen gas into a solid form. The plants are landlords: they provide room and board for their bacterial tenants, who take up residence in the roots in customized nodules. Sugars and other necessities are routed through these nodules, so the bacteria are well fed as their nitrogenase enzymes crank out nitrogen compounds for the plant to use.
A fascinating part of this symbiotic relationship between plant and microbe is that nitrogenase requires a low-oxygen environment to work its enzymatic magic. The plant keeps its root nodules’ oxygen levels low by binding oxygen to a specialized protein called leghemoglobin. It’s a delicate balancing act, because the bacteria require oxygen for respiration: the levels of oxygen have to be low enough for nitrogenase to function, but high enough so the bacteria don’t die. Like the hemoglobin in your own blood, leghemoglobin is bright red when it binds oxygen, surprising many a gardener who has cut one of these nodules open.
Like the relationships between bacteria and ancestral plant cells that led to the development of mitochondria and chloroplasts, nitrogen-fixing bacteria in plant roots might be in the midst of a similar evolutionary process. Right now plants have to become inoculated with the nitrogen-fixing bacteria, but in time the bacterial DNA could become permanently entangled in the plant cells’ reproductive process. At that point we’d have a “nitroplast,” which, like chloroplasts and mitochondria, would have completely different DNA than the nucleus.
In some environments, nitrogen deficiencies are chronic and soils might not be hospitable to nitrogen-fixing species. These include some wetland soils as well as thin soils over bare rock. In such environments, plants have evolved another way to obtain nitrogen: by trapping and consuming insects and even larger prey. Carnivorous plants can also take up nitrogen from the soil when it’s available, but their traps provide a backup pantry just in case. Most of us don’t live and garden in areas with such nitrogen-poor soil, but even if we do it’s pretty easy to fix the problem with moderate additions of alfalfa meal or some other nitrogen-rich fertilizer.
Sulfur is also required for two amino acids, so it’s just as important for proteins and enzymes as nitrogen, though sulfur is not needed in as large a quantity. Because most soils contain sufficient sulfur, deficiencies are rare. When sulfur is deficient, however, the chlorotic yellowing can be distinguished from nitrogen deficiency because it occurs in the younger rather than older leaves. Once sulfur is in the plant it’s not very mobile, so it can’t be scavenged and reused in young leaves like nitrogen can.
Phosphorus is another building block needed for constructing membranes, energy-containing compounds, and genetic material. It’s unusual to find phosphorus deficiency in most home gardens and landscapes, and deficiency symptoms are not as clear-cut as they are for nitrogen and sulfur. Unfortunately, common gardening lore warns that red leaves are a sign of phosphorus deficiency, but red foliage can be caused by many factors, not just nutrient deficiency. In fact, many well-tended gardens often have too much phosphorus due to the overuse of so-called transplant fertilizers including bone meal, guano, chicken manure, and superphosphate. We’ll take a closer look at the unexpected effects of phosphate toxicity later in this chapter. But before you buy another box of bone meal, I want you to solemnly swear never to add phosphate to your gardens unless a soil test tells you to do so.
Table sugar is sprinkled on soil as an herbicide.
THE SUPPOSED BENEFITS
Sugar stimulates bacterial growth, which ties up nitrogen and prevents weed seed germination and growth.
HOW PLANTS RESPOND
Well, yes, this can work, but it’s nonselective. In other words, nitrogen deficiency is going to zap all of your plants, not just the weeds. Weeds are called weeds for a reason: they are aggressive and persistent. If a weed can’t tolerate your soil, your annuals and vegetables certainly won’t be able to either. There are better ways to control weeds, and you can save the sugar for more delectable purposes.
Cell Wall Strengtheners: Boron, Calcium, and Silicon
Boron, calcium, and silicon all add structural stability to plant cell walls. You can consider them to be the plant cell’s skeleton. Boron has been described as a cellular stapler needed to hold other chemicals together and stabilize cell walls. It might even function as a natural insecticide, because boric acid is toxic to many insects. Calcium not only strengthens cell walls but is important in cell membranes too; it’s both a structural component and used for communication between cells.
Every gardener, at some point, will have become intimately familiar with the importance of silicon in certain plants. This is the element that glass is made of, and if you’ve ever gotten a grass cut, bingo! That’s the silicon home protection system. Pioneers made use of another silicon-rich plant in the pre-Brillo pad era. Horsetails, or scouring rushes, are natural pot-scrubbers because of the abrasiveness provided by silicon.
In home gardens and landscapes, none of these three elements is usually limited. Because these minerals are associated with the cell’s skeleton, their absence usually causes young leaves and fruit to look lumpy, bumpy, or otherwise deformed. A soil test is the only way to know for sure if one of these elements is missing.
Gypsum is calcium sulfate, an inorganic fertilizer.
THE SUPPOSED BENEFITS
Gypsum improves plant health by improving soil tilth.
HOW PLANTS RESPOND
Once again, adding any nutrient to soil without knowing if it’s deficient can create mineral imbalances in the soil with negative effects on plants. The overuse of gypsum can cause soil deficiencies of other important plant nutrients, including iron, magnesium, manganese, phosphorus, and zinc, and it may negatively affect inoculation of roots by mycorrhizal fungi.
Boy, that soil test is looking better all the time, isn’t it?
Chemical Jugglers: Copper, Iron, Magnesium, Manganese, Molybdenum, Nickel, and Zinc
These nutrients are crucial to many enzyme activities, because they shuttle electrons between the chemicals in reactions. These metallic elements easily change back and forth from one charged state to another as a result of accepting and donating electrons, which are negatively charged. This may seem an esoteric bit of information, but many heavy metals in the soil are toxic to plants and to you.
The metallic nutrients are sometimes difficult to dissolve. You can put an iron nail in your garden soil and watch it slowly turn to rust, but this isn’t a very efficient way of providing nutrients to roots. Instead, iron is usually available in a chelated form. You might see containers of Fe-EDTA, which is shorthand for iron chelate. Chelate comes from the Greek word for claw, and this is exactly what chelating compounds do: they grab onto individual iron atoms and make them soluble in water. Once the iron has been transported into the appropriate plant tissue, the plant’s own chelating compounds trap the iron atoms so they stay soluble and functional.
Deficiencies of some of these metallic elements, such as iron, magnesium, manganese, and molybdenum, create an oddly artistic pattern of leaf yellowing. The veins remain green and the tissue between the veins turns yellow, a pattern called interveinal chlorosis. This is a common symptom in many landscape trees and shrubs. Acid-loving plants, like rhododendrons, show this clearly when they’re planted in alkaline soils or if they’re near newly poured concrete sidewalks or foundations. The lime from the concrete leaches into the soil, which raises the pH and prevents iron uptake by acid-loving species.
Yellow leaves with distinctly green veins create a pattern called interveinal chlorosis.
Much of the time, however, pH has nothing to do with interveinal chlorosis, and soil tests show plenty of available iron. It turns out that this type of leaf chlorosis is not caused by a soil nutrient deficiency, but rather by excess phosphate, which we’ll discuss later in this chapter.
Water Managers: Potassium, Chlorine, and Sodium
These three minerals move freely throughout the plant, managing water movement between the cells and throughout the plant. Potassium is most important in this regard. It’s partially responsible for opening and closing the stomata, which are the exit portals for water moving through the plant. This mineral also assists with water transport across membranes, which can cause cells and entire tissues to become turgid or flaccid, depending on which way the water is moving. Fortunately, potassium is rarely deficient in home gardens and landscapes. Chlorine is of secondary importance in regulating water movement, but is required in minute quantities; soil deficiencies are highly unlikely.
In some species, such as cacti and succulents, sodium can take the place of potassium. The two elements are similar to one another in size and charge, so it’s a convenient substitution for plants that often live in very salty soils. Plants adapted to salty soils are also found in coastal areas, where salt water makes a regular appearance. It’s possible that sodium deficiencies could occur in these and other salt-loving species when grown outside their native habitats and away from their normal soils.
Epsom salts are another name for magnesium sulfate, an inorganic fertilizer.
THE SUPPOSED BENEFITS
Epsom salts increase seed germination, improve nutrient uptake, and enhance overall growth.
HOW PLANTS RESPOND
Epsom salts are often used to treat magnesium deficiency in fruits, vegetables, and timber species. Magnesium deficiency commonly occurs in soils under intensive agricultural production, not your backyard vegetable garden. Most nonagricultural soils contain plenty of magnesium—sometimes too much—and adding more just makes matters worse. Excessive application of other nutrients, like potassium, can interfere with a plant’s ability to take up magnesium, making it appear that the soil has a magnesium deficiency when the problem is actually potassium toxicity. Test your soil before you try to diagnose and treat a nutrient problem.
Mineral Specialists: Cobalt, Selenium, and Aluminum
A handful of elements are required for growth by only a select group of plants. Probably the most common of these is cobalt. Members of the pea family and some other species can house nitrogen-fixing bacteria in their roots, and cobalt is required for nitrogen fixation. Whether the cobalt is required by the plant itself or its microbial guests isn’t clear, but in the absence of cobalt nitrogen-fixing species don’t complete their life cycle.
Selenium is another unusual nutrient, seemingly required only by plants like milk-vetch (Astragalus species) that colonize selenium-rich soil. In fact, these species are considered to be indicator plants, acting like green arrows pointing to high levels of selenium in the soil. The selenium that accumulates in their tissues is toxic to cattle and other grazing herbivores, explaining the origin of another common name for some species of Astragalus: locoweed. Selenium obviously protects accumulator plants from being eaten, but other possible functions aren’t yet known.
The variety of hydrangea colors is partially due to their aluminum content.
Many gardeners use aluminum tags to identify their plants. Some of these plants use aluminum themselves, but in a very different way. Hydrangeas are probably the best known garden-variety aluminum accumulator. The gorgeous blue hues some of them sport are due to aluminum in their tissues. Like selenium, aluminum is toxic to animals, and perhaps this is the reason we don’t see a lot of insect damage on hydrangeas. People unfortunate enough to experiment with hydrangea tea suffer the same type of neurological poisoning that you’d get from deadly nightshade. Occasionally, younger hydrangea leaves and stems can fall victim to deer and some insects, probably because they haven’t accumulated defensive chemicals found in mature tissues.
Future research will undoubtedly uncover other minerals required by specialized plants for their survival. But there are some elements that may never reach the essential nutrient standard because they are essentially toxic to plants, to animals, and to you.
I’m just throwing that title out there so you can figure out if you’re a music freak or chemistry geek. If your first thoughts were of Led Zeppelin or Metallica, my teenage son thinks there’s hope for you. But if you instead visualized lead, arsenic, or mercury, then read on!
Some of the essential elements we earlier labeled as chemical jugglers are heavy metals: iron, copper, manganese, and zinc are four of them. They all transfer electrons by alternatively accepting them from one chemical and donating them to another. There’s another group of heavy metals, however, that aren’t as willing to pass electrons back and forth. Instead, they effectively shut down the enzyme system that’s driving a chemical reaction. These are toxic heavy metals, including lead, mercury, arsenic, and cadmium, which can become an unwelcome addition to your plants’ tissues, as well as to whatever eats those plants, including you.
Heavy metals are everywhere. They are naturally occurring elements that happen to have bad effects on living systems. Because they are elements, they don’t break down. This is an important distinction to make, because other types of contaminants do break down and become less harmful over time. If your garden or landscape soil contains toxic heavy metals, they are there for good unless you have the soil removed. This is unfortunately a topic that causes gardeners distress, but understanding where metals come from and what you can do about them helps.
FOREWARNED IS FOREARMED
Unless you live in the middle of nowhere, it’s likely that your soil will contain heavy metal pollutants of some sort. Lead is by far the most common, as our reliance on lead-based paints and gasoline has deposited plenty of this element into urban and residential soils. Sadly, arsenic is also common, especially in land historically used for agriculture in the early part of the last century. It turns out that arsenic is a great pesticide, and it was used liberally in treating orchard crops and other agricultural fields. Until a few years ago, pressure-treated lumber was laden with arsenic and chromium, both of which slowly leach out of the wood and into the surrounding soil or water. Smelting operations, like the one in my hometown of Tacoma, Washington, dumped tons of arsenic and other heavy metals into the atmosphere, where they might settle out in soils miles away from the smelter. Tires and mulches made from recycled rubber break down to release a constant supply of zinc, cadmium, chromium, and selenium. Even unregulated topsoils and composts can carry heavy metal contaminants; it’s always best to purchase soils and compost that have been tested for metal content and certified.
Once in the soil, where do the metals go? Alkaline and clay soils tend to hold them pretty tightly, whereas acid and sandy soils are more likely to release them for root uptake. Plants vary widely in their uptake of metals, depending on the species, life stage, and tissue of interest. The bottom line is that it’s impossible to predict with any accuracy what plants are going to take up what metals into which tissues.
That being said, there are some general observations that can be helpful. Roots and leaves are the tissues most likely to accumulate a given heavy metal. So stem vegetables, such as celery, leeks, and rhubarb, are probably pretty safe. Botanical fruits, meaning anything containing seeds, tend to be safe from metal accumulation. This makes sense, because the fruit serves to attract fruit-eaters, who deposit the seeds elsewhere. The same goes for floral products like nectar and pollen. You’re not likely to eat much of these, but beneficial insect pollinators are. From a plant’s perspective, killing off bees and butterflies is not a great way to reward one’s reproductive matchmakers (or produce offspring, for that matter).
Obviously, you should have your soil tested if you are at all concerned about possible heavy metal contamination. Even some of the essential heavy metals, like zinc, can become toxic to plants if they’re too concentrated in the soil. Soil testing is the best way to sort out what minerals your soil holds so that you can take appropriate action. And what if those heavy metals do show up in a soil test? Let’s look at some constructive ideas.
First, let’s address a worst case scenario: your soil has unsafe levels of arsenic from pesticide usage almost a century ago. The fruit orchards razed in the 1940s to build suburbs may be a distant memory, but the arsenic remains an invisible hazard until your soil test brings it to light. You probably will make the decision not to grow vegetables in this soil, though you can still plant turf, trees, shrubs, and other ornamentals instead. If soil removal is not feasible, you should consider building some raised beds. These beds can be constructed from natural wood, plastic timbers, the new version of pressure treated lumber (which contains no arsenic or chromium), bricks, stones, or concrete blocks. Or you can try container gardening, which has been a staple of apartment and condominium living for years.
Once you have your new vegetable area designated, you’ll want to purchase certified clean topsoil and compost. Your nursery or garden center will probably carry both. You can also make your own compost, as long as you know that your feedstock is free of contaminants. Avoid planting near roadways; the lead from years of leaded gasoline lurks in road dust and can easily be blown into your vegetables. Finally, use only tapwater or water gathered in rain barrels for irrigation of edible plants; gray water can contain unwanted contaminants.
Hopefully you’ll discover that the heavy metals in your soils are at baseline levels—remember, they are naturally occurring elements—and that it’s perfectly safe to grow whatever you like. Now you can use your soil test to determine which, if any, nutrients need to be added through fertilizers. You may be pleasantly surprised that you don’t need to add much of anything.
Soil Testing: A Wellness Checkup for Your Garden
Let’s assume you’re feeling run down and suspect you might have a mineral deficiency of some sort. Would you run to your nearest supplement store, buy one of everything, and chug them all down? Of course not! You would probably go to your doctor first and have some blood work or other lab testing done to find out what might be missing. Your garden soils and plants deserve the same treatment. Before you spend a lot of money on fertilizers and other remedies, you need to find out what your soil already contains and what it might need. Otherwise, you could be creating nutrient toxicities by adding too much of some minerals. And it doesn’t matter whether the nutrients come from organic or inorganic sources: too much is too much.
It’s easy to take soil samples and send them off for analysis. There are many private soil testing labs available, but I prefer university labs. The prices are reasonable, and there is no hawking of soil remedies that you might find with some less savory private labs. Regardless of which lab you choose, you’ll need to follow their directions carefully in sampling the soil. You should take samples from several places in your garden area, avoiding the inclusion of any mulch, and then mix them all together before taking a final sample from the mixture. You’ll also need to identify what kinds of plants you’re interested in growing in any particular area, such as turf, vegetables, annual flowers, or trees and shrubs. If you are concerned about heavy metal contamination, you’ll need to ask for these specific tests to be done. They’re usually not part of a basic soil analysis procedure.
You may be tempted to use one of the little home testing kits sold at nurseries and garden centers. My advice is don’t waste your money. Though these kits may accurately measure pH, they are not sophisticated enough to tell you much more about nutrient content. Just like the medical labs you rely on for blood analysis and other tests, soil testing labs provide the most accurate results. Most importantly, they will provide information explaining what, if anything, you need to do to improve nutrition or address contamination.
Fertilizer Fun Facts
Gardeners are most familiar with fertilizers based on three elements: nitrogen, phosphorus, and potassium. It’s the NPK formula on nearly all fertilizer packages. An NPK ratio of 10-10-10 means that the fertilizer contains 10 percent of each of these elements by weight.
With the emphasis on NPK fertilizers, you might well assume that these three nutrients are most commonly deficient in your garden. Nothing could be further from the truth. In fact, only nitrogen tends to run low in garden soils, and then usually only in the summer when everything is growing like gangbusters. At that point, it’s easy to give a little dollop of a nitrogen-rich resource if your annual plants look like they’re slowing down. (The growth of trees, shrubs, and other perennials tends to slow naturally in the summer anyway, so they aren’t good indicators.)
The NPK label on this fertilizer bag shows that it contains 9 percent nitrogen, 1 percent phosphorus, and 0 percent potassium by weight.
So why the preponderance of NPK fertilizers in garden centers and nurseries? It’s a holdover from our agricultural past. When farmers grow crops, they are deliberately creating an artificial system to maximize production of the crop over a short period with lots of water and fertilizer. It’s like an intensive care unit. Nitrogen, phosphorus, and potassium tend to be the three nutrients that get used up the fastest by rapidly growing crops and need to be supplemented throughout the growing season. And when the crops are harvested, most of the vegetation is removed from the field, which also strips the soil of nutrients. So agricultural crops—and possibly your vegetable garden—will use soil nutrients quickly and may require additional fertilizing through the growing season.
But most of our home landscapes are not dedicated to growing crops. Instead, we have permanent plantings: our lawns, trees, shrubs, perennials, groundcovers, and bulbs. We’re not growing them at a frantic rate because we’re not harvesting them. When home landscape soils are tested, most of them have enough phosphorus and potassium—sometimes too much. With plenty of conventional and organic choices available to the home gardener, it’s smarter to avoid the traditional NPK formulations and choose something that fits the needs of both your soil and your garden. Soil test reports often recommend specific fertilizers, or you can choose your own by carefully reading the labels and only buying what you need.
THE PROBLEMS WITH PHOSPHATE
Although excess levels of potassium don’t seem to pose a problem—this element is fairly soluble and is quickly used by plants and microbes elsewhere—too much phosphate can wreak havoc on soil organisms, your plants, and, even worse, any nearby aquatic system. Let’s look at problems right in your garden.
Excess soil phosphate, whether from organic sources like compost, bone meal, and bat guano or an inorganic source like rock phosphate, creates one of the most common nutrient problems in landscapes. You’ll recall that iron deficiency in plants can be seen as interveinal chlorosis. It turns out that phosphate really does a number on iron, both in the plant and in the soil. First, it reduces the ability of plants to take up iron: the more phosphate in the soil, the less iron is taken up by the roots. Next, phosphate unfortunately combines with iron to create insoluble iron phosphate. This compound can’t be used by the plant, nor can it be easily broken down. In fact, most soils contain plenty of iron, but it can’t be taken up by the roots and it becomes unusable when there’s too much phosphate.
If this wasn’t bad enough, phosphate also inhibits the development of the mycorrhizal relationships between fungi and plant roots. These beneficial fungi are not able to penetrate the roots unless phosphate levels are low. So the gardener’s best friends, the miraculous mycorrhizae, are MIA whenever phosphate levels are higher than necessary. The result? Your plants will need to expend more energy for root growth than they normally would. In a sense, then, phosphate is stimulating root growth, but in a very bad way.
Interveinal chlorosis in this rhododendron was caused by high soil phosphate levels.
Ignore the seductive packaging that suggests supplemental phosphate will help you take home the blue ribbon for the biggest rose. As a rule of thumb, you should never add phosphate fertilizer to bulbs, groundcovers, perennials, shrubs, or trees unless a soil test states that phosphate is pretty much nonexistent. In your vegetable gardens, you may need to use it, especially if you are really pushing production. But follow the recommendations of your soil testing lab. Don’t assume anything.
There! I’ve now saved you a chunk of change, as you’ll never need to buy bat guano, rock phosphate, or bone meal again.
In those instances when you do need to provide specific nutrients to your plants, you’ll have choices of inorganic or organic forms. Organic formulations aren’t necessarily better, and many of these organic products, including bat guano, seaweed extracts, peat, and other exotic materials, deplete the natural resources of another ecosystem. For instance, the kelps routinely harvested for fertilizer use are literally the forests of the ocean. It’s like clear-cutting an old growth forest simply to make an unnecessary product. Peatlands are the world’s largest terrestrial repository of carbon, and degraded peatlands take centuries to regenerate after they are harvested. There’s really no defensible reason for using these materials when good-quality substitutes are available. Environmentally conscious gardeners should avoid these green-washed products.
Most of the nutrients that you might need to add to your garden or landscape can easily and naturally be provided through organic mulches. Composts, wood chips, pine needles, and other recycled materials are not only good uses of materials that might otherwise end up in the landfill, but they also provide a slow feed of nutrients to the soil. This is the way that nature provides nutrients, and gardeners would be wise to follow this approach.
DELIVERING THE GOODS
There are always people with products to sell who recommend alternative methods of doing things. Fertilizer injection, either into the soil or directly into the trunk of a tree, is one of these methods. The idea is that by delivering nutrients directly to plant tissues, deficiencies are corrected faster and less fertilizer is wasted than by soil application. Science just doesn’t bear this belief out. In fact, there are some serious problems with both methods.
Soil injection deliberately places fertilizer below the layers of fine roots found near the soil surface. Scientific testing has shown that this method is no more effective than soil application. It’s a waste of money. Jabbing needles into trees and shooting them up with fertilizer is not only useless, it breaches the tree’s protective barriers and creates nice pathways for insects and pathogens to enter the tree.
Another odd practice that seems to be gaining popularity is foliar fertilization, or spraying leaves with liquid fertilizer. There is some logic behind this notion, because plants can take up some nutrients through their stomata. Most commonly, fossil fuel gases containing sulfur and nitrogen can be taken in along with carbon dioxide and used in leaf proteins. The drawback to foliar fertilizers, however, is that some nutrients such as iron are immobile once they’ve entered a cell. These nutrients won’t be passed on to other parts of the plant. And other nutrients, such as nitrogen, are required in such large quantities that foliar absorption can’t possibly meet the entire plant’s needs.
Foliar treatment is useful for diagnosing nutrient deficiencies, because you can test a small part of the plant and confirm whether there’s something missing. Misting a chlorotic rhododendron leaf with iron chelate, for example, allows iron to be taken up. If the chlorosis is due to iron deficiency, the leaf will green up in a matter of days. But that’s the only practical benefit of foliar sprays.
Don’t substitute foliar fertilization for natural root uptake of nutrients. In many cases, foliar nutrient deficiencies are the result of an imbalance of soil nutrients, as we saw with phosphate and iron interactions. So applying fertilizer to the leaves doesn’t address the real issue, which is a soil fertility imbalance. It’s just a quick fix that treats the symptom and masks the underlying problem. It’s like putting a Band-Aid over a splinter in your finger. You can’t see the splinter anymore, but it’s still there!
TEA? NO THANKS!
I could hardly write a chapter on plant nutrition without addressing the increasingly popular use of compost tea. For those not familiar with the product, it’s made by adding compost to water and aerating the mixture, sometimes with additives like molasses, packaged microbes, or humic acids. It’s appealing because it makes us feel nurturing and turns plant nutrition into a recipe. Not surprisingly, compost tea is heavily marketed as a natural way to fertilize plants by soil or foliar application.
I’ve been reading the science on compost tea for well over ten years now, and there is nothing that would encourage me to recommend its use. Nutrient and microbe levels are quite low, many times lower than what you’d find in compost alone; the process simply dilutes the nutrients and microbes.
Compost tea recipes can best be described as artisan, with their esoteric ingredients that sound impressive but are nothing more than marketing hype. This is the ultimate green-washed product: it has the environmental bells and whistles, but it wastes resources (all those pointless artisan ingredients) and expends energy. The aeration must be continuous and it requires electricity. Enthusiasts must apply the brew often, because it doesn’t stick around. I’m a cheap-and-easy kind of gardener, so I can’t imagine using such a product without any beneficial results.
My advice? Use compost, and let nature make the tea. Water will percolate through layers of organic mulches, giving your soil and plant roots a slow feed of nutrients. Not only is this cheap and easy, it’s also natural and based on scientific evidence.
Ideally, our garden plants are well watered and full of essential nutrients. Now what? How does a plant take these inert chemicals and create sugars, which are the ultimate energy source for all those complex biochemicals? It’s time to let a little sunlight in on the discussion.