Welcome to Book of the Week – a weekly feature offering you a glimpse between the pages of an Acres U.S.A. published title. Get the Book of the Week email newsletter delivered directly to your inbox! This week’s Book of the Week feature is Advancing Biological Farming, by Gary Zimmer. 

There are a lot of different fertilizer sources out there — some that work well in a biological farming system and others that do not. Knowing the benefits and drawbacks of different fertilizer sources can help you make the right choice for your crop and for your farm. The following table and discussion covers the most common fertilizer sources and should help answer your questions on the pros and cons of using each.


Manure and compost are excellent sources of nutrients because they provide a blend of minerals in a form that is tied to biology. The difference between these sources of nutrients is how quickly they become plant available. Compost is a slow-release source of nutrients while manure is soluble, meaning it is quickly available to plants.

A farmer I work with found out the downside of applying a lot of soluble nutrients in the spring when he went out and applied 8,000 gallons of liquid manure to his fields and then planted soybeans. He was overrun with weeds. Some people say if you don’t compost manure the weed seeds in the manure will germinate and cause problems, but in my opinion that is only a small part of it. The bigger problem is the soluble nutrients in liquid manure that cause weed seeds already in the soil to germinate. I have problems with weeds on my farm when I put raw manure on the land in the spring, but I see fewer weeds after I apply compost. I don’t believe this happens because I have a problem with weed seeds in my livestock manure. Rather, raw livestock manure is full of soluble nutrients, which sets up conditions for weeds to germinate and grow. The nutrients in compost are stabilized and less soluble so fewer weeds pop up right after applying compost than after applying raw manure.

Green manure crops (or cover crops) and crop residues are also excellent sources of nutrients. Not only are green manure crops a means for holding onto nutrients so they can’t leach, tie up or erode, as the plants decompose they feed soil life, which releases nutrients in a very plant-available form. Similar to the comparison made of manure and compost in the previous paragraph, green manure crops and young, succulent plants are a source of soluble nutrients, while mature plants and crop residues are slow release. A young green manure crop worked back into the ground breaks down right away and immediately releases nutrients into the soil. You won’t find a trace of that green manure crop two weeks after it is worked into the soil. In contrast I can go out to a field and find corn stalks two years after a corn crop was harvested and the stubble worked into the ground. Mature plant residues break down much more slowly, and the nutrients in them take a long time to become plant available.

Nitrogen Sources

Like other fertilizers, nitrogen is sold based on solubility. If you look at the Fertilizer Sources table, you will see that anhydrous ammonia, ammonium nitrate and urea are all very soluble sources of nitrogen. As I’ve already discussed, I don’t recommend using anhydrous ammonia because I don’t believe it fits on a biological farm where the goals are to increase soil organic matter and humus over time.

I don’t usually recommend urea because it is unstable and can release ammonia gas into the soil, which is toxic to roots and soil life. Applied urea also needs to be kept at least six inches away from the seed so it does not inhibit root growth. However, using a small amount of urea is not always a problem. It is often found in small quantities in foliar sprays, and in that form I think it works well.

My preferred nitrogen sources are ammonium sulfate and pelletized chicken manure. They both have some soluble and some slow release aspects to them.

I can no longer use ammonium sulfate on my farm because it is certified organic and ammonium sulfate is not allowed by the organic rules, but I would use it if I could. It is excellent for spring application on corn, small grains and alfalfa because it has a warming effect on the soil, which extends the growing season. The other thing I like about ammonium sulfate is that the nitrogen source (ammonium) is hooked to sulfur, which is a needed element in a fertilizer program.

Chicken pellets from laying hens are high in nitrogen (from five to eight percent), and provide nitrogen in a form that is easily digestible by soil microorganisms. Next to manure and cover crops, chicken pellets are the source of nitrogen I use most on my farm.

If more nitrogen is needed on a biological farm, I often recommend ammonium nitrate (liquid 28 percent). Since we want to use as little nitrogen as possible to get the job done, placement, timing and add-ons like thiosulfate, humates or molasses can improve efficiency and allow a reduction in quantity. Another good option for an efficient nitrogen source that can save future trips over the field is polymer-coated urea, labeled as ESN (Environmentally Smart Nitrogen). The nitrogen in ESN is coated in a substance that breaks down from moisture and temperature, slowly releasing nitrogen into the soil. Farmers I know who use ESN have been very satisfied with its performance.

Fish meal, feather meal and animal byproduct fertilizers are also excellent sources of nitrogen, but they are very expensive. They work well as a supplement to other nitrogen sources, but are usually not practical as the sole source of nitrogen for a crop.

Legume cover crops are another excellent source of nitrogen, working well either as a stand-alone cover crop, or when interseeded into other crops. On my farm, I have had good success interseeding clover into my corn crop. Some legumes, like alfalfa and clover, can provide up to 200 pounds per acre of nitrogen per year. A legume cover crop will provide nitrogen two ways: first, as it is growing and fixing nitrogen in its root nodules, and second, when it is worked back into the soil and becomes food for microbes. Also don’t forget that cover crops have more benefits than just supplying nitrogen; they also build soil structure, prevent erosion and feed soil organisms.

Phosphorus Sources

Orthophosphoric acid, or orthophos, is a liquid phosphorus source used as an ingredient in many high quality liquid fertilizers. It is a readily available source of phosphorus for plants. However, because of its chemical make-up, it ties up quickly with other elements in the soil and may become unavailable within hours of application. Polyphosphoric acid, or polyphos is produced by dehydrating orthophos. This process makes it more stable so it stays in the soil longer before tying up with other elements.

MAP and DAP (monoammonium phosphate and diammonium phosphate) are highly soluble dry phosphate fertilizers. Both also contain nitrogen in the ammonium form. MAP has a lower pH and less ammonium than DAP, making it a better source of soluble phosphate and is easier on soil life. The commercial fertilizer industry makes soluble phosphorus fertilizers like MAP and DAP by taking insoluble rock phosphate and mixing it with an acid, like sulfuric acid, to create orthophosphoric acid. The phosphorus is then purified out, which means calcium, sulfur and other beneficial elements found in the rock phosphate are removed.

The final step is to mix the purified orthophosphoric acid with ammonia to create MAP (monoammonium phosphate) or DAP (diammonium phosphate). This process makes a highly soluble phosphorus source, but all of the other elements of the rock phosphate have been removed. I generally do not recommend DAP. It has a high pH which can damage root hairs, those fine hairs on roots that take up most of the water and nutrients plants consume. DAP is also high in ammonia and can release ammonia gas into the soil, which is hard on soil life.

My preferred phosphorus source is a blend of rock phosphate and a commercial soluble phosphorus source such as MAP. I like to include rock phosphate in the blend because I want to keep the calcium, sulfur and trace elements found in the naturally mined rock. I also don’t want to overdo application of soluble nutrients, which in the case of phosphorus ends up being a waste of my money since much of the phosphorus from a soluble source will tie up quickly in the soil. By applying a mix of rock phosphate and commercial phosphorus, I get a good blend of soluble and slow-release phosphorus.

If I have acidic soil that needs phosphorus and calcium, that is the perfect time to add a rock phosphate soil corrective. The acidity in the soil will speed up the breakdown of the rock phosphate, and I get as much calcium out of it as I would if I put lime on. If I don’t have an acidic soil, it will take a long time for the phosphorus to become plant available unless I have abundant soil biology. Regardless of soil pH, phosphorus uptake is tied to soil biology. Planting a cover crop like oats, rye or buckwheat can stimulate soil biology and help plants access phosphorus in the soil. Plants with more acidic roots, like oats and buckwheat, can extract more phosphorus from the soil reserve and from rock phosphate. These plants hold that phosphorus in their tissues, putting the nutrient into a biological cycle. This interaction is a vital part of the system. If you put rock phosphate on a hard, dead soil without any life in it and no green plants growing, the opportunity for that phosphate to show up is pretty minimal.

Calcium Sources

High calcium lime (close to 35 percent calcium) and dolomitic limestone (close to 20 percent calcium and 12 percent magnesium) are mined calcium sources that are very slow release. They are a good source of calcium for acidic soils. Just as acidity helps release the nutrients from rock phosphate, acidity breaks down high calcium lime or dolomitic lime. On soils that are neutral or higher pH, these sources will not supply much plant-available calcium. To get more calcium on soils that are not acidic, a source of calcium that’s more soluble is needed.

When I started working as a farming consultant I went in search of a soluble calcium source. I found a source of lime (calcium carbonate) that was finely ground, had been burnt in a kiln, and then hydrated to remove the caustic effect of burnt lime. At the time I had no idea that by putting calcium carbonate through a kiln the carbon was burned off and what was left was soluble calcium. In addition, being a natural, mined material and a byproduct of manufacturing meant this calcium source also had some sulfur and other beneficial materials in it. When I applied the hydrated burnt lime to the ground, I got a calcium response in the plant right away. It worked wonders on my alfalfa crops. Later my partners and I developed a product from the hydrated burnt lime called Bio-Cal. Over the years I’ve seen wonderful responses from the application of Bio-Cal, especially on legumes. Unfortunately, I can no longer use Bio-Cal on my organic farm because it is burned and thus it is considered synthetic. We therefore developed OrganiCal to take the place of Bio-Cal on organic farms. It is a soluble source of calcium similar to Bio-Cal, but rather than burning the limestone it is finely ground and blended with acid binders and sulfur. This makes it more plant available than straight limestone, and because it is not burned or processed, it is approved for use on organic farms.

HumaCal is another calcium product my colleagues and I developed. It is a blend of finely ground limestone and gypsum with humates. Humates are large, complex molecules that have a low pH and contain a lot of sites that hold on to nutrients like calcium. This means that humates can help break down rocks like limestone into a plant-available form, and can also hold on to the plant-available nutrients so they don’t leach or tie up. This makes humates an excellent material for blending with a lot of nutrients, including calcium. I have done quite a bit of research on HumaCal demonstrating that it provides plant-available calcium, and I’ll talk more about this in the next chapter. Gypsum, which is calcium sulfate, is more soluble than lime. I like to use gypsum on my land when the soil is high in magnesium because gypsum is not only a source of calcium, it also supplies sulfur. The sulfur will hook to magnesium in the soil and form Epsom salts (magnesium sulfate) which is very soluble. That means it makes the magnesium more plant available but it also leaches, so it washes some of the excess magnesium out of the soil. Calcium nitrate and calcium chloride are both very soluble sources of calcium.

Calcium nitrate is often used as a foliar on high value crops because not only is it a good source of available calcium, it also supplies soluble nitrogen. However, it is a very expensive way to provide calcium, so it is generally only used on high value crops like potatoes and other vegetables. Calcium chloride is better known as road salt. It is also used as a foliar spray, but less often. Even though it supplies soluble calcium, it does have chloride, which has some negative side effects.

About the Author:

Gary Zimmer is an organic dairy farmer, an accomplished speaker, a sought-after farm consultant and president of Midwestern BioAg, a biological farming products and services company. He is also the author of The Biological Farmer, the prequel to Advancing Biological Farming.

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Save money and order the Advancing Biological Farming/The Biological Farmer Combo here. 

Be sure to check out the Gary Zimmer audio collection for a complete selection of his previous Eco-Ag Conference seminars!

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