Plant Nutrients 101 – The effects of essential nutrients to plants

Plants should be fertilized because most soil does not provide the essential nutrients necessary for optimum growth. Even if you are lucky enough to begin with great garden soil, as your plants grow, they absorb nutrients and leave the soil less fertile. Remember those tasty tomatoes and beautiful roses you grew last year? It took nutrients from the soil to build those plant tissues. By fertilizing your garden, you replenish lost nutrients and make sure that this year’s plants have the food they need to flourish.

Well-fed plants are healthier, less susceptible to disease and insect problems, more productive and more beautiful.

There are 6 main nutrients that plants require. Plants obtain the first 3 main nutrients (carbon, hydrogen and oxygen) from air and water. The other 3 are nitrogen, phosphorus and potassium.

Nitrogen helps plants create the proteins they need to produce new tissues. In nature, nitrogen is usually in short supply so plants have evolved to take up as much nitrogen as possible, even if it means not taking up other necessary elements. If too much nitrogen is available, the plant may grow abundant foliage but not produce fruit or flowers. Growth may possibly be stunted because the plant isn’t absorbing enough of the other elements it needs.

Phosphorous stimulates root growth, helps the plant set buds and flowers, improves vitality and increases seed size. It does this by helping transfer energy from one part of the plant to another. To absorb phosphorous, most plants need a soil pH of 6.5 to 6.8. Organic matter and the activity of soil organisms can also increase the availability of phosphorus.

Potassium improves overall vigor of the plant. It helps the plants make carbohydrates and gives disease resistance. It also helps regulate metabolic activities.

There are 3 additional nutrients that plants need, but in much smaller amounts:

1.      Calcium is utilized by plants in cell membranes, at their growing points and to neutralize toxic materials. Additionally, calcium improves soil structure and helps bind organic and inorganic particles together.

2.      Magnesium is the only metallic component of chlorophyll. Without it, plants can’t process sunlight.

3.      Sulfur is a component of many proteins.

Finally, there are 8 elements that plants need in tiny amounts. These are called micronutrients and include boron, copper and iron. Healthy soil that is high in organic matter usually contains adequate amounts of all these micronutrients.

Organic vs. Synthetic

Do plants really care where they get their nutrients? Yes, because organic and synthetic fertilizers provide nutrients in another way. Organic fertilizers are made from naturally occurring mineral deposits and organic material, such as bone or plant meal or composted manure. Synthetic fertilizers are made by chemically processing raw materials.

In general, the nutrients in organic fertilizers are not water-soluble and are released to the plants slowly over a period of months or even years. For this reason, organic fertilizers are best applied in the fall so the nutrients will be available in the spring. These organic fertilizers stimulate beneficial soil microorganisms and improve the structure of the soil. Soil microbes play an important role in converting organic fertilizers into soluble nutrients that can be absorbed by your plants. Generally, organic fertilizers and compost provides all the secondary and micronutrients your plants need.

Synthetic fertilizers are water-soluble and can be taken up by the plant quickly. In fact applying too much synthetic fertilizer can “burn” foliage and damage your plants. Synthetic fertilizers give plants a quick boost but do little to improve soil texture, stimulate soil life, or improve your soil’s long-term fertility. Because synthetic fertilizers are highly water-soluble, they can also leach out into streams and ponds. Synthetic fertilizers do have some advantages in early spring. Because they are water-soluble, they are available to plants even when the soil is still cold and soil microbes are inactive. For this reason, some organically-based fertilizers, such as All-Purpose Fertilizer, also contain small quantities of synthetic fertilizers to ensure the availability of nutrients.

For the long-term health of your garden, feeding your plants by building the soil with organic fertilizers and compost is best. This will give you soil that is rich in organic matter and teeming with microbial life.


Law of the Minimum – Liebig’s Law

Justus von Liebig, generally credited as the “father of the fertilizer industry”, formulated the law of the minimum: if one crop nutrient is missing or deficient, plant growth will be poor, even if the other elements are abundant.

Liebig likens the potential of a crop to a barrel with staves of unequal length. The capacity of this barrel is limited by the length of the shortest stave (in this case, water) and can only be increased by lengthening that stave. When that stave is lengthened, another one becomes the limiting factor.

Image 1 soil_food_web

The Soil Food Web

“Feed the Soil to Feed the Plants” The Soil Food Web is the basis of soil fertility in organic gardening and farming. Organic gardening relies on the interconnected web of soil organisms—from bacteria and fungi to protozoa, micro arthropods, and earthworms—to supply nutrients to your vegetables.


feed the soil

Feed the Soil to Feed the Plants

Improving garden soil takes two things: lots of organic matter, and lots of elbow grease (or a tiller).

At the extremes—a sandy soil that drains too quickly and a clay soil that doesn’t drain—you have to fix the drainage issues first, or nothing good can happen. But even at the extremes, adding organic matter is still the best way to improve garden soil. This is what “Feed the Soil to Feed the Plants”, the mantra of organic gardeners everywhere, means. When you “feed the soil” by increasing soil organic matter, the soil organisms that break it down multiply, and their waste products provide a “micro-manure” organic fertilizer that’s deposited right at the root hairs of your plants. See The Soil Food Web for more information.

In Clay soils, organic matter improves the pore structure of the soil, increases drainage, and increases soil biodiversity by allowing aerobic processes to take place deeper into the soil horizon. See Improving Clay Soil for structural fixes for drainage problems and other issues of clay soil.

In sandy soil, organic matter slows the rate of drainage, improves moisture retention, and increases the biodiversity of the soil ecosystem. See Gardening in Sandy Soil for solutions specific to sandy soil frustrations. In acidic soil or alkaline soil, organic matter “buffers” soil pH, reducing swings in pH caused by adding soil amendments. See Changing Soil pH for more information.

If your garden is in the Goldilocks zone, a loam soil, consider yourself lucky. Loam soil has a nice structural balance between sand, silt, and clay. Even if it had no life in it, loam soil would retain moisture—just by the way its particles stack. But because it retains moisture, loam soil usually has a rich and diverse soil ecosystem. Whether you have clay soil, sandy soil, or anything in between, improving garden soil always involves adding organic matter. At a minimum, you have to replace some of what you take out of the soil by harvesting vegetables.