Farming Rotations, (Sister crops, etc.) and selective crop breeding.

[calkhi]

I am well aware that farming rotations in game might make it a bit too easy with the current system, but it's not exactly a bad idea if the current systems of replenishment are toned down. Here's my suggestion.

Peas, Peanuts, Soybeans, or any sort of legume, really, has a special sort of bacteria which allows the nitrogen in the atmosphere to be collected and converted into a useable form by the plant. When this plant is harvested, the roots containing this bacteria continue to work and produce useable nitrogen in the soil for other crops. Other plants do sequester other materials into the roots, but nothing really comparable to the action of legumes. That said, grass spread, or the addition of a hay crop or clover should be allowed onto farm blocks, with grass and hay slowly restoring the soil to its max fertility limits at current rate of renaturation, while producing animal feed specifically designed to maintain weight and animal health.

So essentially, for rotation, plant legume, let grow, harvest, plant grain, (Rye, wheat, oats, what have you.) let grow, harvest, let field rest/plant grass. Rinse and repeat for a basic three field rotation, or add in another vegetable to allow for a four field rotation. Likewise, on HIGH FERTILITY SOIL ONLY, as biodiversity increases, the field eventually, this is after years of farming, using fertilizer, etc, becomes the same strength as terra preta essentially. This is basically an end game idea, and just something I threw in last minute to add an option to say, increase field yields on small, centralized servers without having to explore every nook and cranny.

For crop breeding/selective harvesting. The idea is that you start with, say, wheat. Each crop field, assuming this maintains scale with the other block game, is about 1x1 meters. Assuming that there's about 10-20 plants per square meter, with early heads of wheat you'd achieve about 3 kernels of wheat with a reminder that we ARE basically cave men here, higher quality wheat seeds could be found in ruins, but the idea is that this would be sparse, as individual farmers in the end basically had to grow what they could, even lower quality seed. You'd get about 30-60 grains of wheat, which seems about right for the current harvests. However, in modern wheat, there can be a WHOPPING 30 kernels per STALK of wheat, which means that in ideal conditions, you'd get a wheat harvest per meter squared of farmland of about 300-600 grains of wheat. This would be something that'd take years, and I do mean YEARS of work in game, but each grain crop should absolutely be able to get to the point that, with the right conditions, you'd be slamming out wheat enough to sustain yourself on a relatively minuscule farm. Which, of course, you already can do, and just as a minder, IRL you'd need about 1/2 meter to 1 meter squared to make a loaf of whole grain bread, assuming you're only going to eat one loaf for months at a time. Which realistic growth times do seem to be scaled in game, so that's practically already in place.
Finally, did you know that cabbages, broccoli, kale, cauliflower, and brussels sprouts are actually the same plant? Literally, they're the same plant! Just a different cultivar, but otherwise, LITERALLY THE SAME PLANT! Where's our Brassica oleracea so that we can selectively harvest them until they produce yummy, yummy brussels sprouts?! It'd just take time to say, control right click with an empty hand to inspect the plant, with a text box responce of "This plant seems to have very large, spread out leaves." (Kale) "This plant seems to have forgone most of the production of flowers to produce a tight bulb of densely pack leaves." (Cabbage.) "This plant seems to produce a long stalk with many, MANY flowers, but fewer leaves." (Broccoli.) "Oh boy, the flowers are real dense on this bad boy. And they're (Insert color choice here, they come in white, purple, green, and orange.)!" (Cauliflower.) "Boy howdy, those are some plump fresh sprouts on this bad boy." (Brussels sprouts.)
Essentially, it's literally domestication for plants. First, man domesticated the wolf, then he looked to his fields and said "I can domesticate those too." Then he looked at the bovine species and added "And: I can MILK you."

Alright, humor time over. I figured after a long wall of text, a bit of funny might help massage the tightened wrinkles within your head to allow them to relax once more and go about your day with a smile. I'm roughly familiar with how things work in the game, but I haven't taken an in depth look at the code, but I imagine it'd be simple as adding a depletion rate in the negative value for adding nutrients, and as for crop breeding, that's really up to the individual to have a percent chance of random mutation in crops, or something that needs to be done via support structures made by the player. IE: Stunting the growth height of the plant deliberately and forcing it to put out low sitting heads of leaves, etc.