The challenge in creating a rotation that balances the nutrient demands each crop makes on the soil depends on classifying the crops in terms of the nutrient demands. For high yields and high quality, all your crops will require a range of nutrients, available in proper proportion, and at the proper times during the growth of the plants.
It is a basic principle of organic gardening that plants need a wider range of nutrients than the simple N-P-K formula offered by most bagged synthetic fertilizers if they are to be healthy enough to resist disease and insect attack without the crutch of pesticides, and to deliver the full flavor and nutrition that was the motivation for growing them in the first place. The slow, continual release of nutrients from compost, manure, and the ongoing biological processes of a living soil is superior to an occasional shot of concentrated fertilizer. Still, certain groups of plants like more or less of various nutrients, especially the Big Three—nitrogen (N), phosphorus (P), and potassium (K)—and we can work out a system that arranges the crops according to their relative need for each.
For this purpose, we’ll divide our crops into four types, called:
- roots, and
Plants grown and harvested for their leaves thrive on nitrogen. Phosphorus is especially important to the flowering and fruiting of plants, so if we are looking to harvest them for their fruits, we need to make sure there is ample phosphorus available. Root crops are potassium-lovers, but make relatively light demands on the soil for nitrogen or phosphorus. Legumes, through a symbiotic relationship with rhizobia soil bacteria, are able to draw nitrogen from the air and store it in their roots. Because of this they are known as soil-builders; after harvest a portion of this stored nitrogen is released by their decaying roots for use by succeeding crops.
This descending hierarchy of nutrient demands suggests one way to arrange crops in rotation. After initial fertilization, whether by compost, manure or bagged fertilizer, the first season is devoted to leafy salad crops like lettuce, chicory and other greens, plus the nitrogen-loving members of the cabbage family: cabbage, broccoli, cauliflower, Brussels sprouts, and kale. In the next season, this same section of the garden would be planted to fruiting crops like tomatoes, peppers, eggplants, squashes, melons, cucumbers, and pumpkins, that can do well with the slightly reduced levels of nitrogen and will thrive on the phosphorus levels remaining. The third season a nutrient rotation would put root crops like carrots, beets, turnips, radishes, and the whole range of crops in the onion family,including leeks, garlic, and shallots in the plot, since their relatively low fertility requirements (with a preference for potassium) are ideally suited to the state of the soil at that point. In the fourth season of this rotation, legumes such as beans and peas are planted to help rebuild nitrogen levels in anticipation of the next round of the planting cycle.
Within each of these four groups, half of the crops are somewhat greedier than the others. The greens can use more nitrogen than the broccoli (and its relatives); the squashes and cucumbers like a richer soil than the tomato family; and onions, leeks, shallots, and garlic benefit from more nitrogen than do the other root crops. So if we to increase the subtlety of our plan, we could subdivide the plots further and feed the hungry half a bit extra before spring planting to make sure they get all the nutrients they need to reach their productive potential. This supplemental feeding could take the form of additional compost or, if compost is short, an application of organic fertilizer. While the fourth group, peas and beans, have similar nutrient requirements, they grow at different seasons. That makes it possible to take half of that plot, give it the additional compost, and plant corn and/or potatoes, which are often left out of kitchen gardens because of their large space requirements, but have an important function to play in that their required cultivation helps clean out weedy plants.
This four-square, two-part plot rotation is easy to understand if you draw yourself a plan. In a classic four-square garden the contents of each plot is switched in an orderly version of musical chairs, keeping the hungry crops in one half and the less demanding crops in the other. At the end of each four year rotation, the two halves can be switched, functionally creating an eight-part plan, without adding much in the way of complication.