Plant Nutrients In-depth (Guide to Backyard Farming)

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In the past couple of sections we have discussed the major macronutrients, soil structure, cover cropping, crop rotation, and other soil-building practices in a fair amount of detail. But this level of information is not sufficient in and of itself. It's a lot like saying that a human needs carbohydrates, fats, proteins, air , and exercise--but forgetting to mention vitamins.

Plants make their own vitamins, which is one reason why they are so nutritious. But to make them, or sometimes even survive at all, they need a wide array of both macronutrients and micronutrients.

Most of the micronutrients take care of themselves between using various compost ingredients, specifically adding kelp or seaweed to compost once in awhile if available, and using commercial liquid kelp fertilizers such as Neptune's Harvest or Root Boost in the beds.

In spite of this, deficiencies sometimes develop. In my garden, because I grow a lot of cabbage-family plants, boron can especially be an issue because with inadequate boron, broccoli grows with hollow stems. So I specifically add a teaspoon of borax (mixed with a cup of blood meal for easy dispersal) to every garden bed at the beginning of each gardening season.

------ These hollow stalks are the result of boron deficiency.


Calcium: Calcium stimulates rhizobia bacteria, is critical for healthy cell walls, assists with movement of carbohydrates within plants, aids in the absorption of trace elements, and aids the activity of enzymes. Adequate calcium (or its even uptake) is necessary to prevent potato scab, blossom end rot, black lesions on carrots, and other diseases. Adding lime (which contains calcium) to the hole when transplanting brassicas can help prevent a fungal disease called "club foot. " Bioavailable calcium is necessary for proper utilization of nitrogen.

Plants use large amounts of calcium, so it is important to ensure sufficient levels in the soil. Most often, this is done through adding lime of various sorts. Calcium can be added in the form of regular garden lime; dolomitic limestone, which also contains magnesium; and pulverized oyster shells. All of these take time to work and become bioavailable, so I generally recommend adding them to beds in the fall so they have time to work before the garden season starts in spring. In general, you should add eight pounds of lime yearly to every 100 square feet of garden beds. I generally recommend using dolomitic lime as magnesium and calcium work in concert in a way that benefits the plants.

Nitrogen: Nitrogen is an essential element for life as, combined with carbon and hydrogen, it is a building block for amino acids, which combine to make proteins and even the basic data of life:

RNA and DNA. Nitrogen is critical for every metabolic process in a plant and for the formation of important compounds such as chlorophyll. Predictably, plants need a lot of it. We covered nitrogen sources and quantities in a previous section.

Phosphorus: Phosphorus is an ingredient in the protoplasm within cells and is important for all cell division and growth. It is the single most important element for the germination and growth of plants, followed shortly by nitrogen. Almost all soils are deficient and need supplementation as described in a previous section . The most obvious place where phosphorus deficiency will become evident is in seedlings, because starting media is usually nutrient deficient. In these cases, the leaves will develop a purplish color on the underside. For seedlings, a complete liquid fertilizer containing phosphorus such as Neptune's Harvest (if growing organically) or Miracle Gro otherwise will head this off at the pass. Apply fertilizer to seedlings after the first set of leaves appear and every two weeks thereafter if using a nutrient-free growing medium. For crops in the ground, use the information from section 4.

Potassium: Plants require a large and continuous supply of potassium to help in the metabolism and movement of carbohydrates, protein synthesis, growth, and cell division.

Adequate potassium supplies ensure better shape, color , and flavor of vegetables and fruits. The chief sign of deficiency can be seen in leaf edges that appear "scorched. " Root crops, especially, require adequate potassium and perhaps even increased levels for best production. Section 4 contains information on the proper levels of potassium to maintain in your beds.


Boron: Though classified as a micronutrient because of the small amount required, it is absolutely crucial to practically all life processes of plants including hormone activity, ion exchange, nutrient movement, and water metabolism. Every plant shows boron deficiency a bit differently, depending on the severity. In beans, the beans are deformed. In broccoli, the stems are hollow.

Because deficiency symptoms can be easily confused with deficiencies in other elements, the best bet, short of a professional soil test, is to ensure adequate supply routinely. Because excess boron can be toxic to plants, and plants will absorb it in excess if too much is available, you have to be careful to only add what is needed. You can add three teaspoons of borax per 100 square feet of bed space once a year , and this will generally work out fine. You should mix it thoroughly with something like powdered lime or bone meal before broadcasting to ensure even distribution, and then work it into the top couple of inches of soil.

Copper: Copper plays an important role in root metabolism, photosynthesis, and enzyme activation, as well as protecting plants from adverse effects of excessive nitrogen. Copper , like boron, is required in only small amounts and is toxic in excess. It is available in the form of copper sulfate crystals and should be added to the beds yearly at the rate of 1-1/2 ounces (four tablespoons) per 100 square feet of bed. Because it is hard to distribute such a small quantity evenly over such a large space, it should be thoroughly mixed with something being added to beds in a larger quantity, such as lime, bone meal, or other fertilizer.

Iron: Though required in only small amounts, iron is essential for the chlorophyll cycle in plants. Without it, plants start to appear "bleached out" (called chlorosis) and suffer from stunted growth.

Toxicity isn't a big issue with iron, as plants tend to self-regulate how much they take from the soil. Nevertheless, because it is required in only small amounts, excess shouldn't be supplied. Iron can be added incidentally through the use of blood meal as a nitrogen source. Because I use blood meal in my garden, iron deficiency has never been an issue. However , some may prefer not to use blood meal, in which case preventively adding six ounces of iron (or ferric) sulfate per 100 square feet of garden bed every year should do the trick. Another more organic alternative is to deliberately include nettle plants, which are very high in iron, in the compost pile.

Magnesium: Magnesium is an important element throughout the garden. It speeds up composting, makes nitrogen more readily available, stimulates rhizobia bacteria, assists in root development, and is necessary for carbohydrate motility within plants. It interacts in balance with calcium in a variety of ways and reduces the effects that could arise (however unlikely) from excess quantities of aluminum, manganese, or iron in the soil. Magnesium coexists with calcium in dolomitic lime, and you will have no problems from magnesium deficiency as long as you use dolomitic limestone as a source of calcium in your beds. Barring this, magnesium sulfate--also known as Epsom salt--is available at grocery stores or pharmacies and can be used at the rate of 24 ounces yearly per 100 square feet of garden bed.

Manganese: Though manganese is required only sparingly, its presence in adequate quantities can have an enormous positive effect on crop yields, especially for root crops. Deficiency is seen as a uniform yellowing in new leaves. This makes sense, because manganese is essential for the production of chlorophyll. Most soils have adequate naturally available manganese, but if the pH of the soil is higher than 6.5, supplementation may be needed. As well, over-farmed soils may be deficient. If you suspect deficiency, you can add 12 ounces of manganese sulfate per 100 square feet to your beds once every three years.

Molybdenum: Molybdenum is an essential catalyst for the formation of enzymes and synthesis of amino acids and proteins.

Although it is essential, in excess it is extremely toxic to plants. If you accidentally add too much molybdenum to your beds, treat them with copper sulfate, which will reduce the bioavailability of the molybdenum so it will be less toxic. Most soils today, especially those in agricultural usage, are deficient in molybdenum. If you supplement with manganese when molybdenum is already deficient, that makes matters worse-so I recommend always using molybdenum and manganese together . Supplementation can be in the form of molybdic acid, ammonium molybdate, or sodium molybdate at the rate of 1-1/2 ounces per 100 square feet of garden bed mixed with something else to facilitate even application.

Molybdenum disulfide, used as a lubricant, isn't suitable because it is so stable that it has no effective biological availability.

Sulfur: Sulfur is an important ingredient of essential amino acids required for building plant proteins. In essence, without sulfur , plants cease to exist. On the other hand, sulfur exists in many different forms, and some are more useful to plants than others while yet others are absolutely toxic. In general, the sulfate forms of sulfur are healthful to plants whereas the sulfide forms of sulfur are deleterious. Because sulfur compounds tend to acidify the soil a bit, you should keep an eye on the soil pH and add lime if needed to offset the effects of the sulfur in terms of pH. Sulfur is usually applied in the form of plain elemental sulfur , known as "flowers of sulfur . " Evenly distribute over the beds at the rate of 24 ounces per 100 square feet once yearly.

Zinc: Zinc is crucial for seed production, metabolism, and regulation of the water and carbon dioxide equilibrium in plants.

Zinc deficiency is not particularly common, but when it shows up, it can be identified in the form of chlorotic bands within the leaves of a plant. Zinc is much more available in acidic (pH less than 7) than in alkaline (pH greater than 7) soils. Thus, the 12 ounces of zinc sulfate per 100 square feet of bed that are needed to correct or prevent deficiency would need to be multiplied by five for a total of 60 ounces if the soil in the beds is alkaline.

Making a Micronutrient Mix

Once your mini-farm has been up and running for three years or so, this will probably be unnecessary for established beds because the soil fertility practices will conserve a lot of plant nutrients. But, when just getting started, it may be needed. For enough for 300 square feet, combine the following ingredients in a bucket, and dry mix thoroughly:

Borax: 1-1/2 ounces

Copper sulfate: 4-1/2 ounces

Ferric sulfate: 18 ounces

Magnesium sulfate: 72 ounces

Manganese sulfate: 12 ounces

Sodium molybdate: 4-1/2 ounces

Sulfur: 36 ounces

Zinc sulfate: 36 ounces

Nutrient Conservation

Other than boron, we've never had to supplement with a micronutrient on my farm. The primary reason is because the biochar in the soil holds nutrients so they don't leach, the use of cover crops keeps the nutrients from becoming mobile, and conscientious composting of all plant matter from the garden preserves the nutrients that have entered the plants from the soil so they can be returned to the soil.

The other reason is variety of inputs. When we use organic fertilizers, we use a variety. One time, we might use alfalfa meal for nitrogen, but another time we might use blood meal. For potassium we might use greensand at the beginning of a season but wood ashes at the end. And all throughout, the compost bin collects a wide array of materials ranging from crop debris and lawn clippings to stale eggs and the entrails of slaughtered animals. As a result, because the animals largely receive feed from off the farm, the compost contains more nutrients than were taken out of the soil in the first place, making it an incredible fertilizer.

This is important. Everything that you can't make yourself or conserve on your mini-farm becomes something that will eventually cost you money. Keeping costs down is the secret that makes mini farming an economic activity that is more beneficial than simple gardening.


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Updated: Monday, February 15, 2021 8:54