Manure isn't a useful source of nitrogen for crops. There is a small amount of nitrogen in it, but little of that small amount is in a mineralized, plant available, form. Worse, it is already spoken for by the bacteria that have the job of decomposing the manure - composting it in effect - to recycle it back to soil. Putting manure on your fields reduces available nitrogen and retards plant growth. There is a small net benefit after time has passed and all the workers have done their jobs to release and reform the organic nitrogen, but it is a mistake to equate manure with fertilizer.

For example, a typical load of dairy manure - which would have some urine in it and that's where most animal nitrogen is excreted as urea - might give you 10 pounds of nitrogen per ton. A ton of urea fertilizer - same stuff - such as you can buy would give you 900 pounds of nitrogen, nearly 100 times as much.

But only some of the nitrogen in manure is available to plants, somewhere between 10% and 50% of it depending on age, handling and spreading method. That means that you need to apply 2 to 10 times as much manure as the raw nitrogen numbers would indicate. And you need to water or plow it in right away since 10% of its already meager nitrogen will evaporate each day it sits dry on the surface.

In an example application of this for a typical rye grass and clover pasture you would want 3 applications of 30# actual nitrogen per acre - early spring, early summer and early fall. To get that with manure you'd need between 6 and 30 tons of manure per acre for each application. That would just smother the grass. To get that with urea, which is 45% nitrogen, you'd need about 65# per acre for each application. It also needs to be watered in or incorporated to avoid loss.

Worse yet, soil bacteria would consume the meager nitrogen in the manure and more besides to rot the manure. The composter's rule of thumb is that you need a 1:30 ratio of nitrogen to carbon to support a thriving bacterial community and compost the organics. When all was said and done the organic nitrogen would at last be released and be plant (or bacteria) available, but your crop would already be harvested, or plowed under, by then.

In the process of composting the manure the bacteria emit GHGs - either methane or CO2 depending on conditions and community composition. They not only eat the nitrogen they pollute the air. There ought to be a law against that! Actually, a technology is a better idea than a law, but only if used. If the manure was composted in an anaerobic digester the methane produced could be captured. It could be an energy source and hydrogen feedstock (CH4) for a Haber-Bosch process to make nitrogen fertilizer. It would take added energy to do that, there's no free lunch, but the compost and nitrogen would be a far better soil amendment than raw manure.

The pasture in the example actually can use more than twice as much nitrogen as applied. It also gets a lot from the clovers and some from rain. About 7% of the earth's nitrogen budget is fixed by lightning and falls in rain. I specified rye grass in the example because it responds well to soil fertility. It will live after a fashion, and produce a little, without adding nitrogen, but the yield will be pathetic by comparison to a fertile pasture. There are other grasses and plants that won't respond that way. Fertile or not they grow at about the same slow rate, just a little better when fertilized.

Growing cultivars capable of utilizing increased fertility gives greater yields. Said another way, they use less cultivated land, leaving more wilderness, for the same yield. Finding efficient ways to increase fertility, and using such cultivars, can help achieve the doubling in food production needed to feed the growing population. We can do better than just flinging manure on our fields and growing heritage cultivars, and we need to do ever better in the coming decades.