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Organic Fertilizers for Horticulture

By Louise Placek

If you are growing commercially, no matter what you are growing in containers or what soil mixture you use, you will need to fertilize. This is especially important during active growth when nutrient needs rise sharply.

In addition, during the warm/hot months, increased watering washes nutrients out of the soil and these nutrients need to be replaced in the form of a soil drench or foliar application. Feeding your newly planted plugs regularly is important, at least until they have an established and healthy root system. Plants whose root sys­tems have been even slightly disturbed in transplanting will need extra nourishment until the new roots have formed.

Commercial growers most often use chemical fertilizers because it is the easy way to get nutrients to the plant. These fertilizers come in many formulations of NPK (nitrogen/phos­phorus/potassium) and are easy to apply as a liquid soil drench, in time-released pellets (“prills”), or granular forms that are mixed into the soil. Some are formulated to “force” specific types of growth such as foliage or flowers. They are also designed to move growth along at a quick pace to get plants out to the nurseries in a timely manner. Fertilizer is especially important when the grower is using a soil-less mix.

Call me an organic purist, but I tend to equate chemical fertilizers with heroin. The plants will be fine as long as they are getting the steady supply of calculated N-P-K. In fact, they will respond quite well. Several things bother me about this process. One, the fertilizer was made in some chemical-manu­facturing lab and it is totally artificial. Two, I have a problem force-feeding any living thing. Three, a plant grown this way is like building a house with cardboard rather than bricks; the plants just don’t hold up in the long term if exposed to any stress.

A model of a tank fertilizer system.
A model of a tank fertilizer system. Courtesy of Made from Scratch.

When a plant is allowed to grow at a genetic rate (rather than a chemically calculated rate) it will have a stronger overall constitution. When you give your plants a natural, earth-made fertilizer it is like sitting down to dinner for the plant. If it is hungry, it will use what is available. If not, it won’t. Simple, gentle and sensible. Folks, I could pay my July electric bill if I had a dollar for every time someone has said to me, “Your plants look so healthy.” That’s because they are. Anyone can grow plants, but to grow sturdy, vigorous, vital plants takes a different frame of reference and mind—it becomes a thing of beauty.

Fertilizers and Soil Amendments You Need

The following is a basic list of fertilizers and soil amend­ments that can be used in organic horticulture. It is not intend­ed to be comprehensive; it is meant only to acquaint you with some of the most common products and how they are used. I will not even mention N-P-K numbers as people depend too heavily on these when trying to decide on a product. Natural products such as these generally do not have high N-P-K num­bers, but offer a wealth of readily available nutrients to plants in a living soil.

Alfalfa Meal

Alfalfa is a perennial legume that is used as fodder for animals, green manure for crops and, when cut, dried and ground into a meal, it makes a great soil amendment. Some of the many nutritional benefits alfalfa meal offers are nitrogen, phosphorus, potassium, calcium, magnesium, trace minerals, triacontanol (a growth stimulant), sugars, starches and a bank of amino acids. It can be mixed directly into your soil or made into a “tea” and used as a soil drench or foliar spray. If you make the tea in warm weather, don’t let it sit more than a couple of days. The smell will knock your socks off.

Blood Meal

This is dried, slaughterhouse blood. Very mal­odorous and rather expensive. High in nitrogen. Probably bet­ter off in your garden than in your soil mix.

Bone Meal

A by-product of the meat industry, bone meal is animal bones that have been pasteurized, dried and ground into a powder. Used as a calcium and phosphorus amendment, but also contains some nitrogen. Can mix directly into your soil or dip your plug roots that need extra calcium into the meal before planting.

Note: In an organic setup, both bone meal and blood meal should be used only if there is nothing else available that will provide the same benefits. There also should be a full disclosure of where the bone or blood meal is from and how it was processed to avoid contamination of organic plants with products that are not organic.

Calcium Sulfate

Gypsum, as it is commonly called, is a mined or industrial by-product material used to correct calci­um deficiency (especially in alkaline soils) and to loosen up tight clay soils allowing better drainage, which can release excess sodium if present. Would be used in garden soil for the most part, but is good to know about.

Colloidal Phosphate

Often referred to as soft rock phos­phate, this is mined, crushed phosphate that has been sus­pended in clay. Good, long-term source of calcium and phos­phorus. We dip our flowering-fruiting plug roots in the powder before planting.


Although many different organic substances are used to make compost, the fundamental nature of it is the same. The end product is the result of digestion of these sub­stances by microorganisms (bacteria, fungi, etc.) and some macro-organisms such as earthworms. The process releases nutrients from the original material and creates a soil condi­tioner rich in humus, humic acid, vitamins, minerals and nitro­gen.

Final analysis of any compost depends on what original materials were used. Some commonly used are manure, cotton burr, vegetable and other plant waste (hay, grass, leaves, tree twigs/bark, etc.), mushrooms, rice hulls, paunch manure (cow stomach contents), and even the hulls of certain nuts. Compost can be worked into the soil, used as a mulch/top-dressing, or made into a “tea” and used as a soil drench or foliar spray. (For more information on compost see Chapter Six.)

Cotton Burr Compost

Another by-product of the cotton industry, this is the wickedly sharp calyx of the cotton flower in which the boll rests. There is only one company I know of who uses only organic cotton burrs and aerobically composts them into a wonderful, earthy soil amendment. As with the seed meal, it is a good, slow release source of nitrogen, phos­phorus and potassium. I’m in love with the smell. I use the fine-screened product in my soil mix.

Cottonseed Meal

A by-product of cotton ginning, the seed is ground into a meal and used as a soil amendment. It is a slow-release nitrogen, phosphorus and potassium source. This is a good product, but due to the large number of chemicals used to grow cotton (pesticides, herbicides), one should be certain that the source of the meal is only from organically grown cotton. Good luck.

Earthworm Castings

Earthworm poop is finely digested organic matter. It has all the same basic benefits of compost, but in a more compact, easy-to-use form. This is a great addi­tion to potting soil and even can be used safely in plug mix­tures. Commercially available from small earthworm farms.

Fish Emulsion

As the name implies, it is an emulsified fish by-product in a concentrated form. When mixed with water it can be used as a soil drench or a foliar spray. It is valued for its nitrogen, phosphorus and trace minerals. When I first started using fish emulsion, I was not sure I was going to be able to get past the smell. I found that not only did I get used to the smell, it was virtually gone by the next day. Most importantly, my plants love it.


This coarse-grained, light-colored, hard igneous rock is often used by landscapers in crushed (gravel-like) form as mulch and in garden pathways. In organic horticulture the sand or meal is mixed with soil as a source of potassium and other trace minerals.

It also has paramagnetic properties, which as you recall, helps other nutrients become more avail­able to the roots. If possible, try to find the partially decom­posed granite.

Green Sand

Mined from ancient ocean beds, this silica-based material—officially called glauconite—is greenish colored sand that is loaded with potassium. It also contains iron and other elemental nutrients. Best if used in a soil that has good microbial activity.


This is aged, dried poop from bats and sea birds. Most of what you see commercially is bat guano, but some garden supply catalogs have droppings from birds that live on sea cliffs. They are all high in nitrogen, humus (a good soil builder), microorganisms, vitamins and minerals.

It is good stuff, but has a very strong urine-like odor. I would avoid top dressing the soil of pots with it because it has an odd, almost greasy consistency when wet and just doesn’t seem to work its way down like compost. It is better worked into the soil ahead of time. It is a fine powder so you should wear a mask when mixing it.

Lava Sand

Generally a combination of crushed volcanic rock often including basalt. Can be used as a soil amendment for drainage, minerals and paramagnetic properties.


This is a general term referring to the various white, powdery materials containing a substantial amount of calcium carbonate. Some also have a generous amount of magnesium, so before using as a garden soil amendment do a soil test and see what you really need. Often used to adjust an acid soil pH higher.

Caution: do not buy lime intended for industrial use as it may have toxic heavy metals.


Any animal manure can be used as a nitrogen source in soil mixes as long as it has been well aged, pasteur­ized and/or composted. Some might be considered mild enough to use directly, but why take the chance? Different manures have varying levels of nitrogen.

I would check out what the animals are being fed. If the manure is full of hor­mones, antibiotics or other chemicals, you will not want to grow plants in it. Always wash your hands thoroughly after handling manure of any kind—better yet, wear gloves.


This sweet, thick, black syrup is a by-product of the cane sugar industry. It comes in various grades, but the blackstrap grade retains the most nutritional components and is good for people as well as plants. Molasses can be added to foliar sprays or to soil drenches and adds iron, sulfur, potassi­um and other trace elements as well as sugar to feed the ben­eficial micro-organisms in the soil and on plant leaf surfaces. Can be used as a “sticker” instead of soap or oil when spraying botanicals for insect control.


This product is kelp that is ecologically harvest­ed, dried and ground into a powder. It comes to you in either powder or liquid concentrate. Mixed with water, it can be used alone or with fish emulsion as a foliar spray or soil drench. This is the “black gold” of organic fertilizers. Packed with trace min­erals and natural hormones, this product not only fortifies overall health but assists in the uptake of other nutrients.


This yellow powder is a natural mineral that binds with calcium in garden soil to bring an alkaline pH down. Often called elemental sulfur or flower (flour) of sulfur.

Source: Made from Scratch