There is no real "best" natural fertilizer because what you might use depends on what your soil needs and what is most readily available. Any lawn will grow better growing in soils that are well endowed with organic matter that are evenly moist but well drained.
What does your soil need?
How much organic matter is in that soil?
How well does that soil drain?
What does that soil smell like?
What kind of life is in that soil?
How workable is that soil?

Perhaps these simple soil tests may be of some help.

1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

This post was edited by kimmsr on Fri, Jan 17, 14 at 7:10

I just read an article that said human urine was highly effective--and of course you can get it for nothing. Seriously! Donations should be from people not on medications.

Here is a link that might be useful: Modern Farmer article on human urine as fertilizer

The use of human waste (urine and excrement) has long been a means of fertilizing, but it does have major concerns because in areas were these "waste" products flow freely diseases are rampant. Cholera is one of many diseases.
While urine may provide some Nitrogen it will not supply an organic matter that the soil needs to grow strong and healthy plants.

I'm thinking on using coffee grounds to fertilize the lawn. Can someone tell me the right amount of it?
Thanks

Coffee grounds could be a good food for your lawn, but how much depends on a number of factors such as the amount of organic matter in the soil. I have seen nothing definitive about how much except some, not too scientific , recommendations to apply 1 to 2 cubic yards of coffee grounds per 1,000 square feet.

Here is a link that might be useful: coffee grounds as lawn food

Thank you.

About the 1 to 2 cubic yards, can you tell me the amount in pounds or kg? I'm not familiar with this measures.

Roughly one cubic yard weighs about 2,000 pounds.
A cubic yard is a 3 foot, by 3 foot, by 3 foot pile of material. That converts to about 909 kg.

I prefer alfalfa meal over the pellets. The pellets will sometimes have seeds in them and then you end up with alfalfa growing in the lawn... which isn't horrible...just annoying.

I typically use blood meal in the early spring, then alternate between alfalfa meal/corn meal/cottonseed meal, throughout the year. Typically will do a meal spread around memorial day and another around labor day.

Joe, Blood meal, aka tankage, comes from slaughter houses, Confined Animal Feeding Operations, where the animals are fed large amounts of antibiotics to help promote weight gain and also to help keep those animals kept in very deplorable conditions kind of more or less healthy until they are slaughtered. This is one, of many, that European meat producers are objecting to the importation of meat, and meat products, from the USA.
I am not so sure blood meal, today, should be considered an acceptable organic product.

This post was edited by kimmsr on Sat, Jan 25, 14 at 7:06

Blood meal comes from the slaughter house and NOT all of the animals killed at the slaughter house are from CAFO. Though I'm sure a good many of them are. The blood meal is a by-product of the demand for meat.... I don't mind providing uses for by-products.

I am not buying certified organic alfalfa meal or any other certified organic crop meal, either. Its too expensive and not readily available. I don't think buying those non-certified organic crop meals are any worse than buying blood meal.

"organic" is in the eye of the beholder.

I think the most organic thing one can do is to minimize the amount of lawn that is being maintained which minimizes the amount of any meal that one needs to buy.