We just finished rebuilding the last two sections of raised beds in our garden, and we hope it will be for the last time (or at least a long time). Our goal has been to practice "no-till" farming. However, since we started started making biochar last fall, and we knew that we wanted to get a lot of it into the ground, we figured that we would have to till the ground one more time. Theoretically the biochar could have been dug in by shovel, but since we don't have young, strong backs anymore, or enough time to dig all the beds, we opted to till in the biochar. Also, since our topsoil is a little heavy (ie. "clayey"), we added some sand to be tilled in at the same time. Our biochar was pre-mixed evenly with compost (ie. 50/50), so we got organic matter in at the same time. We calculated how much of the biochar/compost mix we needed to add to get the tilled soil to about 10-15% biochar (by volume). We dropped it on the area, raked it out, did the same with the sand, and then tilled it.
Almost all agricultural plants like to be in well-drained soil. They don't like "wet feet". Rice and watercress are a couple exceptions. Even lettuce and celery, which both like lots of water, don't like to be standing in it.
We don't have very good soil drainage in our area. A couple or more feet below the surface, there is an almost impenetrable hardpan in the subsoil of clay and stone. Even though our farm is on the highest elevation around us, water doesn't run off very well. After a rain, any depression has standing water in it. One of our projects for this summer is to put in a drainage system around the garden plots and in the orchard. This problem is accentuated in spring, during "mud season", which has just recently ended here. Most people who are unfamiliar with life where the ground freezes hard in winter (and even some who are), think that mud season is caused by all the melting snow. Not really. When the ground begins to freeze, the frost works its way lower and lower into the soil. The amount and duration of the cold determines how deep it goes. This winter it was very deep. When the weather starts to warm, the ground-frost starts to melt, again from the top down. Because the ground is still frozen underneath, any snow-melt or rain can't penetrate, and the water that isn't able to drain away just stays in soil, now mud.
One of the things we have been doing that helps ameliorate a lack of drainage is to build our raised beds higher than normal. "Raised beds" generally bring to mind an image of above-ground planting beds encompassed by lumber or masonry. That certainly looks decorous and may work for small gardens, but in a farm setting, or even a micro-farm or large garden, it tends to be counterproductive, not to mention costly. Besides for installing it, it is a lot of extra work digging and raking up against a fixed border than in a bed without one. In fact, most raised beds on small-scale or micro-farms aren't raised much at all---only a few inches. After we finish tilling the plot, we stake out our beds and aisles and then shovel the loose soil in the aisles onto the beds. Then we rake out the tops of the beds and are ready to plant. The aisles end up having a look of trenches. After we put down a weed-barrier mulch in the aisles, they don't look quite as deep. One drawback to this design is that to keep the beds from falling into the aisles, we have to slope the sides. This reduces our planting area for the bed. This is a tradeoff we accept for the definite improvement in soil drainage. We've noticed a definite benefit to the plants grown in beds built like this, versus those grown on level ground.
The reason we hope to not have to till again is that biochar is a one-timesoil amendment. Unlike compost or other organic matter, or fertilizers, or almost anything else, biochar is virtually forever. Its lifespan in the soil is measured in centuries and millennia. The crystalline carbon matrix that is biochar is so stable that it will outlast all of us. The only reason to put down more would be to increase the percentage in the soil. Although research is on-going for the exact amount, there appears to be a point of diminishing returns for biochar in the soil. That's good news, because essentially a little goes a long ways. And lasts a long time.