When and if you plant your beans too closely together (or if you plant in the shade) - they will REACH for the light, becoming very leggy. Planting too close together stops the light from hitting the sides of the plant (plants in the middle are effectively in the shade with exception to the top portion), only the top portion of the plant gets sunlight and therefore the plant reaches that way.... "up". You could, if you were planting only one row provided the row ran N/S plant closely together because 2 sides get full sun (if rows run W/E the plants in the same row tend to shade each other -- plant large plants on the North side of rows that run N/S; this is Northern Hemisphere). That's why spacing is usually given as, between plants (within a row that runs N/S) and then the distance between rows (running W/E). Of course if you are planting on a steep slope, than run your rows parallel to the slope as a general rule of thumb This is the above ground portion of plant spacing. There is also the below ground portion of plant spacing: roots, water and nutrition. If you were to just look at the root (and ignore for a moment the above ground portion): tap rooted plants can be planted closer together than fibrous root plants, because the tap root goes DOWN, and fibrous spreads out and down. Companion planting takes root and sunlight requirements into account: for example: you can plant carrots AROUND a tomato plant ; because carrots don't need much light (and the tomato does), and carrots being a tap root don't interfere with the fibrous roots of the tomato plant, which just go around the carrot. Mind you when you pull the carrot you do disrupt the tomato a little; but not so much that I would be concerned. You can get more vegetables in a smaller space when you learn which plants can work as "understory" plants to others. When it comes to plant nutrition; spacing ensure there is enough soil to source the appropriate vitamins and minerals a plant requires. When planting tightly; or using companion planting you need to take into account that you are planting tighter than the recommended spacing and therefore increase the nutrition (manure/compost) in areas where tight planting is happening.

- FaithCeleste Archer