Urine as fertilizer

The most important nutrient in urine is nitrogen (N) followed by phosphorus (P) and then potassium (K). Other nutrients are contained in small quantities only and there are other substances like sodium and chloride, that are not considered nutrients, but still can impact plants positively or negatively.

The actual concentration of nutrients depends on many factors. These values (based on a western vegetarian diet) give some basic orientation:

nitrogen: 300 mmol/l - in terms of ammonia this would be 5.1 g/l.
phosphorus: 10 mmol/l - in terms of P2O5 this would be 0.73 g/l. (In terms of PO4 this would be 0.95 g/l.)

Using the right quantity

Depending on the crop, there are big differences in how much of which nutrient it needs. E.g. pumpkins need huge amounts of nitrogen, while peanuts (and all peas actually) need very little nitrogen but still need phosphorus.

Just as too much water can drown plants, too many nutrients can also be harmful. Besides being wasteful and a source of pollution overuse of any fertilizer can actually reduce the harvest. This is called fertilizer poisoning.

The nutrient requirement depends on the local climate and the crop variety used. In many countries the government or some farmers association provide recommendations including during which phases of crop growth fertilizer should be applied. Data from other countries might be used as rough first estimate.

There are some methods, to get an rough estimate yourself or to fine tune an existing estimate:

Circularity theory

Most nutrients can not be created or truly destroyed. Any phosphorus in a plant will end up as part of a humans body or urine, after the plant is eaten. When urine is applied to the soil, the phosphorus will enter the soil and remain there until taken up by a plant or washed away be the rain or blown away by the wind.

In the long term a field of a size, that can feed one human, should also get the urine of one human to maintain its nutrients.

Three patches method

Let's say one has an estimate, how much fertilizer might be appropriate. This method allows to verify and fine tune the estimate.

Pick a uniform (same soil, shadow etc) test field and plant it uniformly: same density and crop mix. Then divide the field into three areas of about equal size. In the first area little or even no fertilizer is used. In the second area some low estimate is used - and in the third area some high estimate. Also the areas should be picked in a way so that the seconds area provides a buffer between the first and the third.

If some area performs clearly better than the other, then this gives the new estimate. If several areas perform the same, then the area where less fertilizer was used gives the new estimate. If the new estimate is different from the old one, then the process should be repeated with the new estimate on a new field.

It is also possible to test for two different fertilizers at the same time. (Could be fresh urine and urine sludge, or compost and urine, or urine and ash, or anything really.) In this case areas are arranged in a cross pattern: Three areas for the first fertilizer in one direction and on top of that three areas for the second fertilizer given nine areas in total.

When one is very unsure about the initial estimate of fertilizer demand, using four areas instead of three might help: In this case the first area gets not fertilizer at all and acts as reference. The third area gets the estimated fertilizer, while the second area gets half and the fourth area gets double the estimate.

There is one thing to look out for: If the test field has very poor soil that hasn't seen enough fertilizer in a long time, then this method might give to high fertilizer estimates. This is, because part of the fertilizer is used up regenerating the soil and not everything is available for plants. When one suspects this to be the case, then the method can be repeated on the same test field with the next crop in the rotation and the areas arranged in a cross pattern like discussed above. That way the influence of the previous years fertilizer application on the current years demand can be investigated.

What in the soil happens: The mobility of nutrients and substances

Plants don't pick up all available nutrients at once. Rather they draw nutrients from the soil as they grow. Therefore the soil has to store the nutrients until they are needed. There are big differences how much of each nutrient a soil can store.

Putting more nutrients into the soil than it can store, usually gets them lost. If there is excess, that can't get lost quickly, it is likely to harm the plants. Some plants (like tomatoes) are more resistant to excess fertilizer then others.

Loss usually happens either via washing out or evaporating. The speed of washing out is related to the so called "soil mobility" of a substance. That is: If water moves one meter in the soil - how far does the substance move at the same time. That is: How much does the substance "stick" to the soil?

Information about important nutrients or substances:

Nitrogen

At the time of application urine usually contains nitrogen in the form of ammonia. Ammonia is a gas - albeit with good solubility in water. If the soil is too dry, ammonia will just evaporate and get lost that way.

Ammonia has medium mobility in soil. What ammonia isn't picked up by plants or soil micro-organisms in a few days, is converted into nitrate.

Nitrate is also a viable as nitrogen source for plants. It has the advantage that it does not evaporate, but also has medium mobility in soil. Also it acts like a salt and thus easily harms plants in higher concentrations.

Phosphorus

Phosphorus has rather low soil mobility. With many soils it builds solid particles, which reduces mobility even further and reduces the availability of free phosphorus for plants. This means, that plant pick up phosphorus only slowly and phosphorus poisoning is unlikely to happen.

Anything can be too much, but with urine nitrogen poisoning usually occurs long before phosphorus becomes a problem.

Potassium

Potassium has rather high soil mobility. Any potassium that can't be used by plants immediately is quickly washed away and thus lost.

Sodium and Chloride

They are not considered nutrients, but present in urine in rather high quantities from edible salt. Since they both have very high soil mobility, so that concentrations shouldn't build up. Some plants (tomatoes are a well known example) are quite resistant to chloride and might even gain a good taste from it. Most crops are at least somewhat resistant, so there rarely are problems. But some crops (e.g. grapes) are very intolerant to chloride and should only be fertilized with specially processed urine.

Applying the urine

The most important consideration is: Ammonia easily gets lost by evaporation. There are several tricks to avoid this.

Ideally the soil is already somewhat humid (but not wet) before distributing the urine.
The urine should be put directly onto the soil instead of touching the leaves of plants. Anything that sticks to the leaves will just evaporate.
Not too much urine should be distributed at once. Better to distribute smaller amounts several times during the growth period.
Directly after distributing the urine a second pass with fresh water flushes the ammonia a few centimeters into the soil, where it is protected from evaporation.

An alternative strategy is to dilute the urine 1:10 or 1:20 with fresh water and use this for watering the plants regularly - but not every day. Always watering with 1:20 would be way too much and lead to fertilizer poisoning. (Unless it rains a lot and little watering is actually needed.)

Generally fertilizers shouldn't be applied shortly before strong rain, because the rain would just wash the nutrients away before the soil can store them.

Also applying fertilizers in the last few weeks before the harvest probably is pointless, unless one wants to prepare the soil for the next crop already.