8.1: Organic Fertilizers

Organic Approved (Or Not)

A fertilizer is, technically speaking, any substance that is added to soil to provide the nutrient elements that plants need in order to grow. The difference between organic fertilizers and non-organic is mainly that organic fertilizers are naturally occurring substances, while non-organics are either manufactured or synthesized. One could argue whether some fertilizers currently classified as organic meet the organic definitions. For example, though “blood” is a naturally occurring substance, blood meal fertilizer requires collecting and drying large quantities of animal blood from a slaughterhouse–certainly not a naturally occurring phenomenon. The definitions are discussed and debated long and hard in the organic community. Substances are classified as organic (or not) by the National Organic Standards Board (NOSB). This board meets continuously and is informed and influenced by industry, product developers, consumers, and farmers before they come to a decision on “approved for organic use” or “not allowed for organic use”.

Also, remember that another technical definition of organic is that it is a carbon-containing compound. We are not using that organic definition here when we talk about fertilizers that supply essential plant elements. Here we mean organically approved fertilizers--since all compounds that do not contain carbon are technically inorganic.

Fertilizer Grades or Guaranteed Analysis

Fertilizers that contain three of the macronutrients, N-P-K are said to have a guaranteed amount of these elements in them. Most fertilizer bags or spec sheets will list some analysis like 10-10-10. Although this fertilizer does have 10% elemental nitrogen, it really has 10% phosphorus in the form of P₂O₅ and 10% potassium as k₂O. When soil fertilizer recommendations are made, they are given as lbs. of each combined component needed, not as lbs. of the element needed. The fertilizers listed below show the % of P₂O₅ or % k₂O in the material rather than the % of the singular element.

In this section, we explore three macronutrient fertilizers: phosphorus, potassium, and nitrogen. We will not examine sulfur and calcium fertilizers, because there is only one organic source for them, and that is naturally-mined gypsum. Sulfur used to be supplied adequately by recycling animal manure on the land or through precipitation when acid rain was a problem. The University says calcium is not ever needed as a fertilizer. When magnesium is needed, it is simply spread as dolomitic limestone, which, by definition is lime with Mg embedded in it. Both gypsum and dolomitic limestone are approved organic fertilizers.

Phosphorus (P)

Phosphorus enters the plant as P₂O₄^- (phosphate ion). However, because this ion reacts very quickly with other ions, it is delivered in fertilizers in a stable form and standardized as, P₂O₅ (phosphorus pentoxide, more simply called phosphate).

 

Organic Approved

 

1. Rock Phosphate (Calcium phosphate). The chemical formula of this naturally occurring ore is 3[Ca₃(PO₄)₂]. It is found in extensive deposits that were formed 10-15 million years ago in the ocean when dissolved phosphorus precipitated out of solution with calcium. In the United States, phosphate rock is mined in Florida, North Carolina, Tennessee, Utah, and Idaho and ground into a fine powder. Some predict that we will run out of this resource in the next 50 years. Hard rock phosphate contains about 33% P₂O while colloidal (contains clay) phosphate contains 22%_. Some growers like Tennessee brown rock phosphate because of the other trace minerals it contains. In a few million years, check back on the Bay of Green Bay. It may be a phosphorus mine since we are loading the Bay with it now because of intensive cropping here.
2. Bone meal. Bones have a lot of phosphorus in them, but they normally decompose too slowly to be effective fertilizers. In the mid-19th century, entrepreneurs were gathering bones and treating them with sulfuric acid to make superphosphate. Bones became scarce commodities. Today, bones are ground into fine powders and applied as “meal”. It is a relatively high-priced fertilizer. Reasons not to use this are the same as with blood meal--if the source of the animal bones is unknown, the bones are not technically organic. Contains 17-30% P₂O_5.
3. Poultry Manure. This fertilizer is discussed more fully in the nitrogen fertilizer section. It is listed here, for reference, as containing about 1% P₂O_5. If your soil test results suggested that you add 30 lbs P per acre, you would have to add about 2 tons of poultry litter, or equivalently, 70 lbs of litter in a 1,000-square-foot garden.

Not Organic Phosphorus

Commercial phosphates. There are several non-organic phosphorus fertilizers including superphosphate, Triple super phosphate (TSP), mono-ammonium phosphate (MAP), and di-ammonium phosphate (DAP). These synthetic, commercial fertilizers are made from rock phosphate that is treated with sulfuric acid or phosphoric acid to extract and concentrate the phosphorus. When the phosphoric acid is then neutralized with ammonium, MAP and DAP are created. These synthetic fertilizers are good sources of both phosphorus and nitrogen with the P being very soluble. They typically contain 45-53% P₂O_{5 .} They have in the past been relatively cheap to manufacture.

Nitrogen Fertilizers (N)

There are many sources of nitrogen fertilizers. Most commercial nitrogen fertilizers are made with synthetic processes and are therefore not qualified as organic. These synthesized fertilizers have concentrated amounts of nitrogen. They have been made at a relatively inexpensive cost in the past, which made them more attractive to most farmers than organic sources of nitrogen. However, it takes significant energy to manufacture concentrated nitrogen fertilizers and the price has risen quite steeply. In our area, farmers started considering organic nitrogen sources around 2008, even if they are not organic farmers, simply because the cost of manufactured N was getting out of hand and was not cost-effective. Now that fracking has made cheap energy available, the price of N fertilizers has fallen again. Table \(\PageIndex{1}\) shows the relative nitrogen concentrations of various fertilizers.

Blood Meal

This fertilizer supplies nitrogen and iron. It is just what it sounds like, dried animal blood. Most organic people shy away from blood meal because we do not like the source; probably confinement, GMO-corn & soybean-fed pigs and cattle. When they are slaughtered, the blood is collected after stunning, while the animal bleeds out, into a trough on the floor.  The blood is collected and dried.  The animal does not need to be organic for its blood to be considered organic.  Some buyers, like Organic Valley Cooperative, will not buy milk from farmers who use bloodmeal even if it is allowed by the National Organic Program.  Growers should check their product buyer as well as OMRI (organic materials review institution) to stay current on whether blood meal is allowed, or not allowed.

 

Poultry Litter

 

This has been a favorite source of nitrogen for organic farmers for many years. Especially if a farmer does not have their own livestock to provide manure, poultry litter can be a good choice. Recently, however, there has been some debate over poultry litter. If the chickens producing the manure/litter are fed GMO corn and soybeans, it is likely that their manure contains residual glyphosate (Roundup herbicide’s active ingredient). It might just be that the only way to obtain glyphosate-free poultry litter is to source it from poultry farms that do not feed conventional corn & soybeans.  Right now, that source is not readily available.  Although poultry litter also supplys phosphorus, it is mainly used for its N contribution.

Green Manure

This is not really manure at all. It is living, green plant material that is plowed down into the soil. Many organic farmers grow a green manure cover crop over the winter on bare soil to protect the soil from wind and water erosion. Then, in the springtime, the green manure is plowed down. The decomposing plant material feeds the microbes and soil life. Green manures can cause population explosions of soil micro and macro-biota. These life forms can tie up other nutrients in their bodies and actually set back a seeded crop for a few weeks until they themselves die and the nutrients that were immobilized in their bodies become released or mineralized once again. Most green manures are plowed down at least a month before seeding to give this microbial uptake, growth, and dieback cycle adequate time to take place.

Dairy Cattle Manure

Manure is excrement from cattle. In Wisconsin, manure has been a problem when too many animals are concentrated in too small of a space, and the nutrients in the manure either leach or volatilize. A sustainable goal for livestock farmers is typically to match the land base with appropriate animal numbers so that manure can be used as a fertilizer asset rather than having it become a liability. Cattle manure from dairy farms usually is managed in one of three ways.

1. The first, and previously most typical way most livestock farmers handle manure has been a “daily haul” in which farmers run a barn cleaner in a gutter to collect manure behind cows that are tied in stalls. Manure is semi-solid and hauled daily to adjacent fields and spread with a manure spreader. This becomes difficult to accomplish once all the fields have crops growing on them in the summer. It is also difficult in winter when deep snow and frozen ground make access to fields or snow melt and runoff very real possibilities. A daily haul is becoming a thing of the past as manure pits come to dominate the landscape.
2. In order to avoid the problems with spreading at inappropriate times and to deal with the sheer volume of manure, many dairy farmers have gone to manure pits. In this system, manure is mixed with water to transport it into a pit. It is typically accumulated for six months and then spread after crops come off in the fall, and again in spring before planting. The trouble with this system is that the nitrogen in liquid manure may volatilize under anaerobic pit conditions. Also, adding water adds to the cost of moving the water/manure slurry out to fields. And lastly, the free N in liquid manure is great for causing a flush of weed seeds to germinate. Conventional farmers then deal with the weed problem they have created by applying herbicides. I do not recommend liquid manure in organic soils because at the volumes it is often applied it overwhelms and “drowns” the aerobic soil micro-organisms.  Liquid manure does not add to soil health because it doesn't add the organic matter of a semi-solid manure.
3. Some organic dairy and beef farms compost their manure as a way to stabilize and store nitrogen. This is the system we use on our farm. We spread a layer of straw through a bale processor in a three-sided shed and let the cows move about freely in this building. This is called a bedded pack. Each day the manure and straw bedding mix together and accumulate, layer by layer until the bedded pack is several feet high. Every six weeks or so, the bedded pack is hauled out with a skid steer, placed in a long windrow, and then left as either a static pile or composted in place by periodic turning. This system has drawbacks also; it is time and machinery intensive and if not managed well, N can be lost from the compost.

 

Horse Manure

 

This can be a good, free source of nitrogen. Some horse owners may actually pay you to haul away this “waste” from their barns and stables. One thing to consider, however, is that most horse owners de-worm their horses with Ivermectin. Residual Ivermectin in horse manure may affect dung beetles and other soil micro-organisms. More information on this is currently needed, especially whether composting the horse manure reduces the Ivermectin.

Fish Emulsions

Fish emulsions are a slurry of ground-up fish, usually an oily ocean fish called menhaden, that is cooked and stabilized and sold in smaller volumes usually only feasible for backyard gardeners. The NPK analysis of fish emulsions is often 5-1-1 or 5-2-2. Check the label of how the emulsion is made.  Some phosphoric acid is allowed to reduce the fish odor, but it must not be more than 1% of the final weight to be considered organic. In Northeast Wisconsin, we have a couple of companies that use fish waste from Lake Michigan to manufacture local products, some of which are available in bulk for bigger organic enterprises.

Table \(\PageIndex{1}\): Nitrogen fertilizers and % Nitrogen on a dry matter basis (other sources list different amounts of nitrogen in these fertilizers – they do vary widely on their content – but this list seems reasonable to me)

Organic Fertilizers	% N	Synthetic Fertilizers	% N
Cow manure	.5	Fish emulsion	5
Horse manure	.6	Tankage	6.5
Pig manure	.6	Blood meal	13
Sheep manure	1	Chilean nitrate	15
Poultry Layer litter	1.5	Ammonium nitrate	33
Poultry Broiler litter	1.3	Ammonium sulfate	20
Compost	1.2	Urea (synthetic)	43
Fish meal	9

From R. Parnes, Fertile Soil, published by agAcess, Davis, CA.