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The Sixteen Necessary Plant Nutrients

Exploring fertilizers and plant fertility:

The human body requires over a hundred different nutrients for growth and health, but plants only need sixteen. People, and other animals, unlike plants, cannot make food from sunshine…alas!

Most fertilizers, especially the lower priced ones, only have three nutrients, and are lacking in the micronutrients.

First of all, let’s get clear on one thing…. fertilizer is not really “plant food” at all, no matter what it says on the bag! The real food of plants is sunshine, light…and plants are constructed so that they can make their own food, sugars and starches, through the process called photosynthesis.

The macro and micronutrients act more like vitamins and minerals for the plants…they are needed in addition to the food the plant gets itself from light. Also, in order for a plant to produce lots of food for itself, in order for a plant to best utilize that magic process, photosynthesis, these nutrients are necessary. Photosynthesis, by the way, is Latin, and means “making things from light.”

The Macronutrients:

Nitrogen (N), phosphorus (P), potassium (K), are the three common macronutrients we would buy in a bag of fertilizer. These three are always expressed in the same order, N, P, and K. On any and all commercially sold fertilizers, organic or inorganic, the amounts of K,P, K will be expressed as number, and these numbers are percentages.

Thus: a bag of fertilizer that has on it 20-10-30 will have, by weight, 20% nitrogen, 10% phosphorus, and 30% potassium.

A hundred pound sack of 20-10-30 would have in it exactly 20 pounds of nitrogen, 10 pounds of phosphorus, and 30 pounds of potassium.

Nitrogen (N) is abundant in some soils (soils rich in humus), and is very lacking in other soils. Often, but not always, soils in the Midwest will have abundant nitrogen, and soils in the south and in the west will be low in nitrogen.

Nitrogen leaches from a soil when it is watered, or when it rains. If we irrigate plants often, we probably will need to supplement them with some additional nitrogen. Plants grown in containers will always need supplemental nitrogen.

Nitrogen makes plants grow fast, it makes them grow green and tall. Plants that are lacking in nitrogen grow slowly and their older leaves turn yellow before they should. Too much nitrogen can kill plants, quickly, but the right amount can help them grow strong and fast. Too much nitrogen can encourage leaf and stem growth, at the expense of fruit. Tomato plants, for example, that get too much nitrogen, will grow very large, but will have very few tomatoes. Good sources of organic nitrogen are chicken manure, blood meal, and rabbit manure.

Phosphorus (P) is needed for the growth of roots, flowers, seeds and fruits. Plants lacking in phosphorus will make few flowers, and their fruits will be small. Root crops like potatoes, carrots, radish, turnips, beets, and parsnips require abundant phosphorus for optimum production. Bulb plants, such as daffodils and tulips, also require plenty of phosphorous. Unlike nitrogen, phosphorous does not move quickly through the soil, and it does not leach out fast as does nitrogen. This means though, that in order to get phosphorous down to the roots of plants, sprinkling phosphorous fertilizer on the top of the soil may not get the job done. Phosphorous is best applied pre-planting.

Good organic sources of phosphorous are bone meal, compost, and all types of manure.

Potassium (K) is sometimes sold as potash. Potash is a substance that is high in potassium. Potassium in adequate amounts if necessary for all types of plant growth, but in particular potassium is needed for strong stems. Adequate or high levels of potassium also tend to impart some extra winter hardiness to plants. Potassium makes it possible for plants to over-winter when the temperatures drop real low. Potassium is found in all complete fertilizers (as the last number expressed) and can be supplied by making good use of wood ashes. Like phosphorous, potassium doesn’t travel well in the soil, and is useful for pre-planting.

Calcium (Ca) is another macronutrient, but one that is sometimes called a secondary nutrient. Calcium is abundant in lime and in gypsum. If the soil pH of your soil is acidic (6 or lower) then it will pay to add some lime to your soil to add calcium, and to bring your soil pH closer to neutral (7). If your soil is already neutral, or slightly alkaline, if it has a pH of 7 or above (more common in the west than in the east) then instead of lime, use gypsum to add calcium to your soil. Gypsum is also useful for breaking up tight clay soils. Soils that are lacking in calcium will not grow good crops. Lemons grown on calcium deficient soils will get a dark soft bad spot on the ends of the fruits. Much the same thing will be seen with tomatoes grown on calcium deficient soils…the tomatoes will rot on the ends, a condition called “blossom end rot.”

Crushed up eggshells added to your compost is also a good source of calcium.

Magnesium (Mg) is another necessary plant nutrient often called a secondary nutrient. Magnesium is part of the chlorophyll in green plants and is essential for photosynthesis. Fertilizers with trace elements, manures, compost, and dolomite limestone are good sources of magnesium for plants.

Sulfur (S) is needed for root and flower growth, and adequate sulfur in the soil will also make plants more winter hardy and resistant to cold. Some fertilizers contain sulfur, and sulfur can be purchased as soil sulfur…usually used to make a soil more acidic. A good source of inexpensive sulfur is gypsum.


The micronutrients plants need to grow are boron, copper, chloride, iron, manganese, molybdenum, nickel, and zinc. These micronutrients are best supplied through good compost, fish emulsion fertilizers, and most types of composted manures. Only very small amounts of these nutrients are needed, but needed they are. Too much of any of them, especially copper, chloride or boron can also cause problems. Sometimes water from some wells in the west will have too much boron for good plant growth, and tap water in many areas will have too much chloride for optimum plant growth. Always save and use rainwater.

Other nutrients needed: For plant growth also needed are oxygen and carbon dioxide (C02) By supplying our soils with plenty of organic matter, with humus, we make sure that there is enough carbon dioxide in the soils. In order to get enough oxygen into our soils, we need to take steps to make sure our soils are friable, that is, that there are enough air spaces in the soil for good root growth.

In tests done on midwestern cropland, it was shown that pumping extra air into the root zone could double yields of corn. Air itself is 78% nitrogen and 20% oxygen, so the more air into the root zone, the better. Besides adding gypsum, compost and organic matter to our soils…all of which will help make the soil more friable, thus will add more air to the soil, besides that we can and should use mulch. Mulch is proven to improve the friability of soils. Mulch encourages the growth of earthworms, and the tunnels the earthworms make through the soil provide space for additional air. The worm castings from the worms add to the general fertility of the soil. A good thick mulch holds in water, keeps down weeds, keeps the soil cooler when hot and warmer when cold.

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