Often times when people find out I'm vegan, they look at me with astonishment and say, 'But how do you get any protein!? Don't you have to take special pills or injections?' The answer of course, is a laughable no. A reasonably healthy diet will provide plenty of protein without any special measures. Likewise, reasonably intelligent gardening methods will ensure your organic garden stays fertile and healthy. In fact, you can do so with little or no extra expense.
First a brief overview of plant nutrition: There are three main nutrients every plant needs: nitrogen, phosphorous, and potassium (N-P-K). When you see three numbers on commercial fertilizer (like 10-10-10, or 20-5-5), these are just the percentage of N, P, and K. So 5-5-5, 10-10-10, and 20-20-20 contain the same equal proportions of N-P-K but in increasingly strong concentrations. Nitrogen is used primarily in green leafy growth. Phosphorous is used mostly in root growth. Potassium is used primarily in fruit formation. Non-organic fertilizers provide an abundance of N-P-K, but there are a few problems: the nutrients leach away quickly, and go into the groundwater. Furthermore, a strict N-P-K diet is a little like a diet of sugar and vitamins - it will keep you alive and growing, but it is essentially junk food. There are many dozens of other nutrients and symbiotic micro-organisms that make plants healthier and more robust. Organic fertilizer sticks around longer instead of leaching away with the rain, and it provides those extra trace nutrients and micro-organisms that growing plants need.
I'll provide three levels of effort in terms of fertilization: Easiest, Easier, and Easy. If you are a serious gardener, you'll want to do all three. Each of these methods provide a good, well-balanced mix of N-P-K, so you don't have to worry about using too much of one nutrient. Don't use much fertilizer on baby plants or seeds, as this can be harmful to them.
Easiest: Liquid fertilizers can soak into the soil quickly, providing a boost to established plants. Liquid fertilizers are all around you: old coffee and old tea are some of the most readily available. Just dilute with water and spread around the base of your plants. Here's what I do: I take all my households' tea bags (about 20 a week), tear each of them into two pieces, and dump them in a large watering can. This gets all the leafy bits inside to mix around with the water in the watering can. Every Saturday I empty the tea-filled watering can on my tomatoes and corn, and it works beautifully. You can also water ('tea') your houseplants this way.
Easier: Start composting. Take all your garden waste and stick it in one pile. You can use an enclosed plastic bin, an open wooden structure (easy to make with old pallets), or just leave it loose. If you are using an enclosed plastic bin, you can stick food waste in there as well as the garden waste. Turn the mix every month or so with a shovel/spade/digging fork. After 6 to 12 months, you'll have beautifully rich topsoil. Dig it into your garden in the off-season, and the nutrients will last all year. Supplement with liquid fertilizers for extra hungry plants like corn.
Easy: Get a Wormery! This is a special bin that holds tiger worms (aka red wrigglers). You just dump your food waste in the sealed bin (any food waste - veggies, fruit, meat - even bones), and the worms eat it. The liquid waste the worms leave behind is called castings, and these drain down to the bottom of the bin, where there is a tap. Turn the tap, and the liquid castings drain out the bottom. These castings are chock full of every trace nutrient and micro-organism a plant could want - very powerful stuff. I bottle these castings in 2 litre bottles throughout the year, and use them in the growing season. Every week or two in June, July, and August, I take 1 litre of worm castings, dilute it in water 1 to 10, and our this mix around the base of my plants. They love it. When the wormery gets full (about once a year) and the contents resemble top soil, empty the contents into a wheelbarrow and mix it into the rest of your garden (best done in November when the growing season ends). This adds plenty of organic material which improves the overall condition of the soil. Make sure at least a few worms are still in the wormery and start the process all over again. The great thing about this system is that it also eliminates your food waste from going into a landfill - which typically comprises 1/3 of all landfill waste.
There are plenty of other fertilizing methods. The fine people at Shillingford Organics use a nettle tea system (basically nettle plants rotting in water) to fertilize all their poly-tunnel plants, and they produce the best vegetables I've ever had (better than mine!) - and they manage to feed a few hundreds local households this way. Comfrey and seaweed also work well in this method. I also compost my pet rabbits' litter waste (shredded paper and rabbit droppings) which is odourless. In the late autumn, I put my bunnies in the garden, and they eat all the old plants to the ground, 'fertilizing' as they eat. Growing beans, clover, or other legumes boosts nitrogen in the soil. You can usually get free compost from your local government. You can also be lame and just buy compost or other organic fertilizers. There is no limit to the methods creative organic gardeners can use to keep their soil not only fertile, but healthy as well.