Another thing about biochar that nettles me is the ongoing fuzziness about claims of nutrient retention by charcoal. The broad brush claim is that biochar reduces nitrogen leaching from soils, and so more is retained for use by plants. One claim I read was that nitrogen was held by biochar until released by enzymes secreted from plant roots.

It now seems that these claims were naive at best, that all charcoal does is attract cations (positively charged molecules such as ammonium), which is the same thing that soil organic matter and most clay particles do already. Soils deficient in such materials might benefit from charcoal, but no more than they would from unchared organic matter, and is as much a soil PH imbalance problem as anything else.

The problem for soil nutrient loss is anion repulsion. Cations such as ammonium, potassium, phosphorous and calcium are attracted to soil particles, and so are not mobile in soil. Anions such as nitrate are repelled and so are subject to leaching. This is a problem since all forms of nitrogen in soil are inexorably converted to nitrate by a combination of chemical and biological processes. Cations can be too strongly attracted in acid soils so that they are locked away from plants, and anions are too strongly repelled so that they leach away quickly.

It doesn't matter what form of nitrogen is used as a soil amendment, it all gets converted to nitrate and a chemist couldn't detect the origin of nitrate once converted. Neither can a plant. Nitrate is nitrate. The time it takes to do conversion varies, and losses along the way to volatilization can happen for some forms, but if it is all added at one time then a slug of nitrate will eventually be available to plants and leaching.

The trick is to have the nitrate available only when plants need it. This reduces the amount needed to grow a crop since more is used rather than just leaching away, repelled by soil and so mobile in water to travel down below the root zone into aquifers or be washed away by surface flows. Doing this takes knowledge of conversion rates and plant growth cycles. Conversion rates vary with soil temperature and moisture so it can be a challenge to get the timing right.

Worse, some forms of nitrogen such as manure (green or brown) consume soil nitrogen in the conversion process since soil microorganisms need it to do their work. Nitrogen lack can slow conversion, making timing more difficult, and can create an even worse nitrogen deficit until conversion catches up with consumption. The observed effect can be that adding manure actually retards growth, but it's just a timing problem.

One way that this problem is dealt with is to use "time release" capsules - polymer coated nitrogen prills. Fertilizer in this form is more expensive and so is used mainly for high value plants in greenhouses and pots - flowers and such. The polymer coating slowly dissolves in soil water releasing the "medicine", just like for human medications.

Some researchers, Norman Borlaugh for example, have advocated focused research to develop new forms of fertilizer and efficient manufacturing processes that address this issue. Little has changed since the beginning of the green revolution a half century ago when many current materials and manufacturing processes were developed. Other approaches are the use of "fertigation", dissolving nutrients in irrigation water so that they can be added a bit at a time with water, another is repeated applications of small amounts of fertilizer timed to plant growth cycles. Both of these methods have higher costs are are not suited to all crops.

A biological solution is "just in time" nitrogen fixation by soil microorganisms that live and work with the plants. This is the case with rhizobia that nodulate on legume roots and some other bacteria that are free living in soil or that have a loose association with non-legume grasses. Some fungi such as mycorrhizae help avoid losses by collecting and transporting mineral nutrients to plants which are then bartered for sweets. It may be that research into such biological approaches will be as useful as research into new materials and manufacturing processes, or more so.

Untimely nitrate wouldn't be such as issue if it didn't wash away. If the soil could hold more water then it could retain more nitrate. That's another thing that biochar and organic matter can help with. Soil rich in carbon can absorb nitrate in solution even if it can't adsorb nitrate directly through chemisorption, physical structure, van der Waals forces or whatever. The nitrate will still be consumed, denitrified, by microorganisms and result in emission back to the air as various gases (NOx, N2, N2O etc), but it won't leach as much unless the absorbent soil is saturated. Then it drains well.

You can't win, you can't break even, and you can't get out of the nitrogen game but you can play well and lose grudgingly, make a good showing and generally be a more worthy opponent for the enemies of untimely nitrate. This is better business, better stewardship and good management.