Anything other than a tree or a bush can be grown on a bale. Plants that produce their edible sections above ground are easy. Thus peas and beans, cabbages, salad crops,pumpkins,and marrows for example are prime candidates for straw bale cultivation. Sweet corn will send its long root down into the depths of the bale to absorb nutrients. Onions and leeks also root shallowly and produce near crops to the surface, so there are no problems. Potatoes, parsnips and carrots,all root vegetables, can also be grown, for they are able to push down into the decaying straw. However,if you grow members of the Brassica family [cabbages, kale, cauliflower, broccoli, sprouts calabrese,turnips] you will need to add some lime, as this family requires a pH higher than other vegetables do, so if you do want to grow a range of vegetables,different straw bales for vegetables with difering requirements might be a good idea.
Some plants require staking, such as beans, and this can cause a problem, as stakes cannot be easily driven into a straw bale. The solution can be to fix stakes into the ground near the bale and fix the plants to them. This is likely to be the case with beans, peas and marrows. If the ground is too hard placing the bale near a mesh fence allows the plants to be tied to the fence
Certain vegetables are greedy feeders, such as pumpkins,so they will need a large dose of fertilizer. But the up-side of this is that members of the pumpkin family are well suited to straw bale cultivation. One large pumpkin per bale can be quite a nice little project that can allow you to specialize with different plants for different bales.
Herbs and salads can have the soil spread a few centimetres deep across the top of the bale, and these two kinds of easy-to grow-vegetables will happily thrive. However, some, especially root crops, will benefit from a small compost filled hole specially dug out of the straw surface into which the seedling can grow. Potatoes, especially,require some depth of soil and an application of manure,or failing that potato fertilizer. The manure applied can be in the form of liquid manure or pelleted manure, which can be purchased from garden centres and a variety of other outlets. Compost can also be liquidized and applied to the bale.