Farm Fresh: Getting the Most from Manure

With great expectation you put your spring seeds in the ground and wait for seedlings to appear. But instead of coming up bright green, they're the yellowish pale color that spells doom for your crop. What happened? Nitrogen, or rather lack of it.

Without nitrogen, plants can't make the chlorophyll they need for photosynthesis and thus life. Nitrogen, in short, is essential to a plant's existence. For horse farmers seeking ways to use as few off-farm products as necessary, the biological production of nitrogen is an important issue.

As we are taught in school, 78% of the earth's atmosphere is nitrogen, so how can the soil have a deficit?Well, the atmosphere's nitrogen is in the form of a gas (N2). The two nitrogen atoms in this gaseous form are bound together with a triple bond that's so tight plants lack sufficient energy to break the bond. To grow, plants need soluble nitrogen in the form of ammonium (NH4+) and nitrate (NO3-) ions.

Okay, do we really need to know chemistry just to grow cabbage and corn? The short answer is no. The long answer is: if you wish to obtain maximum production and sustainability with minimum input, you do need to know where your nitrogen comes from. Well, you say, all I have to do is run down to the farm store and grab a couple bags of 19-19-19. You could do that, of course, but not so long ago farmers had to make do without 19-19-19. So let's look at how nitrogen gas is converted into the soluble ions necessary for plant growth.

Nitrogen becomes available for plant growth only through a process known as biological nitrogen fixation, making this process an important biochemical reaction for life on earth. The players in this cycle are legumes, bacterial microorganisms called Rhizobium, and an enzyme called nitrogenase.

For energy and as a source of food, Rhizobium use a plant's carbohydrates, which the plant produces by photosynthesis. While using the plant's carbohydrates as a food source, the Rhizobium infect the plant's roots, causing nodules to form. These nodules use the enzyme nitrogenase to break the triple bonds of gaseous nitrogen and create ammonia. The ammonia combines with organic acids to form amino acids and eventually proteins, thus creating a food source the plant can use.

The Rhizobium bacteria and the plant thereby enjoy a symbiotic relationship, in which both the plant and the bacteria benefit. Plants in the legume family that are particularly efficient with this symbiotic relationship are alfalfa, clover, peas, and beans. In one year, such legumes supply an average of 280 pounds of nitrogen per acre.

Once the nitrogen has been taken out of the gaseous form by nitrogenase and tied up in plant proteins through the symbiotic relationship, it can be made available for use by other plants through one of several methods. Manure, a byproduct of animals eating plants, contains the next important step in nitrogen availability.

Animals produce copious amounts of manure. For most livestock growers, including horse farmers, manure disposal or utilization is therefore of concern. Using manure as a source of fertilizer is an important aspect of sustainability for most small farmers. One of the main objects of this fertilization is to introduce nitrogen into the soil.

To get the nitrogen we typically apply manure with little thought to what we're putting on the soil. We know the nitrogen is in there somewhere, and the rest, well, is organic matter. Right?

Only to a degree. Without knowing what is actually in the manure, we can't possibly judge the efficacy of applying it to the soil. To maximize the benefit of your manure applications, you need to know both its carbon-to-nitrogen ratio and its concentration of phosphorus.

Manure's carbon-to-nitrogen ratio is a key factor in making nitrogen available to plants, because it drives microbial decomposition. Microbes typically need to ingest eight times as much carbon as nitrogen. However, because they metabolize only one-third of the carbon (losing the rest through respiration as carbon dioxide), to maintain a balanced diet they need a food source with 24 parts carbon and one part nitrogen.

Microbes attempting to decompose organic material with a carbon-to-nitrogen ratio greater than 24-to-1 have to scavenge nitrogen from other sources. They will continue to scavenge until all the available nitrogen is used up. This tying up of nitrogen and making it unavailable for plant use is called immobilization. If you put manure on your field that has a carbon-to-nitrogen ratio higher than 24-to-1, you will therefore see a nitrogen deficit rather than an increase. In other words, you're shooting yourself in the foot.

So the worst thing you can do is put fresh manure on your field, as it will cause you to lose precious nitrogen from your soil.What, then, can you do to take advantage of the nitrogen in manure? First, before putting manure on your soil, make sure it is well decomposed. Let the microbes fight it out while the manure sits in a steaming pile. Well decomposed manure has a carbon-to-nitrogen ratio of between 15-to-1 and 20-to-1, depending on what the animal has been eating Pig manure has a lower ratio of 12-to-1.

Once the carbon-to-nitrogen ratio drops below 24-to-1, more than enough nitrogen is present to break down the carbon. As the nitrogen level climbs and the carbon level drops, the microbes go nuts, causing decomposition to occur more rapidly.

Nitrogen in decomposing manure is held by protein molecules. Nitrogen that it is not needed by the microbes responsible for the decomposition of organic matter can be used by another set of microbes that take protein structures apart to create ammonium, a form of nitrogen plants can use. The process of taking nitrogen from the organic molecule where it is bound to protein, and thus unusable by plants, and changing it to the plant-available form of ammonium is called mineralization.

So now your manure is in a usable form and you know the carbon-to-nitrogen ratio is low enough for the nitrogen to be available for your plants through mineralization. Now what?

Manure is comprised of many other nutrients besides carbon and nitrogen. One such nutrient is phosphorus, which can be present in significant concentrations. Manure that is only slightly decomposed has approximately three times as much phosphorus as nitrogen needed for plant growth.

When you apply manure to your soil, the organic phosphorus binds to soil particles and is immobilized. Then, gradually, it becomes available to plants as phosphate. Because of this gradual release, excess phosphorus is initially not a problem. Applied year after year in large quantities, however, it can become an environmental hazard.

Because phosphorus binds tightly with the soil, few ways exist to remove it. One good way to remove it is to grow a crop that requires high phosphorus amounts. Corn is such a crop. Pastures, and even alfalfa, require much less phosphorus than corn, thus removing less of it from the soil.

Applied without consideration of the potentials for losing nitrogen and building up excess phosphorus, manure can become a hazard to your soil. On the other hand, due to its low cost and availability, manure used wisely is an excellent fertilizer for the small farmer.