When the seed packets start getting delivered in the spring it is very hard to not start throwing everything into the ground at once. I want peas! I want carrots! I want tomatoes! Every plant however, has its own personal space and will not perform if that space is not given.
On the back of those seed packets there is some information that is very important to read. The first is the planting depth. The rule of thumb is you plant the seed as deep as it is large: for a carrot seed this is just below the surface, for a bean this is about an inch down.
The second piece of information to look for is the plant spacing. It will read something like, "sow 1 seed every inch in rows spaced 12" apart" that would be a safe bet for a carrot. Often after this line reads something like "thin to 6 plants per foot" which is also very important. Thinning is just as important as proper spacing!
Spacing is important because plants require water, light and nutrients. When carrots, for example, are planted but not thinned they are competing for the same root space, the same water and the same sunlight. This yields the same, very small, carrot roots. When provided with a few inches to grow it roots and shoots, and far enough away from the next plant so that it has its own water supply, it can thrive.
Every crop has its own requirement, and you will learn these with experience. Tomatoes cannot be planted a foot from each other. That is how diseases happen.
Companion planting is a great concept, but when it is poorly implemented your garden turns into a mess. There are books about companion planting that can help you be successful. Allow me to use the title of the book "Carrots Love Tomatoes" as an example. Carrots are slow growing, and do not like to be crowded. Tomatoes also grow relatively slowly and like to grow vertically. When planted at the same time, the carrots get ample light and water while the tomato is young. By the time the tomato reaches maturity the carrot is done growing and ready to harvest as well.
Now allow me to use the fictional book "Tomatoes hate to be planted 6 inches from peppers" as an example. Tomatoes and peppers need to be planted on at least six square feet per plant. This is a roughly a square of 2.5 x 2.5. Unknowing practitioners of square foot gardening may think, "a square for a tomato, a square for a pepper, a square for a tomato, a square for a pepper." More than likely the peppers will not produce and the tomatoes will die from diseases if this is done.
So read your seed packets and trust them!
And now you know!