The question of how far human hands can reach into the middle of beds for harvesting is easily answered. The average person can reach 2 feet from a squatting position easily, and so the beds cannot be wider than 4 feet across.
How wide the aisles must be depends on the machinery to be used in tillage and the width of the people tending the beds. A human can easily walk and kneel in a space 2 feet wide, and so this becomes a minimum width of the aisle. A shovel is about 1 foot wide – smaller than the minimum width.
But what if shovels are not the primary form of tillage? Two horses pulling a plow are 6 feet wide – if this is the primary form of tillage, than the aisle must be at least 6 feet wide. Our tractor is 6 feet wide, but sometimes we use shovels. So, sometimes we have 2 foot wide aisles, sometimes 6 foot wide.
Based on ergonomic analysis, it would be good if the plants can reach at least 2 feet from the middle of the beds to the aisles.
The question is tested by tilling a wedge shape into untilled ground. The dimensions of the triangle shall be a length of 20 feet and a base width of 20 feet.
Plant the seeds of the crop in question in a line down middle, from the apex to the base every 12 inches so that the first seed is 0 feet from the hard ground, and the last (20th) seed is 10 feet from the hard ground, so that every seed from the apex has more tilled ground surrounding it.
You will notice that the seeds closer to the base are larger. This is because they have more tilled soil to eat: their roots can penetrate further, eat better and have more fertile soil to eat. Measure the yield or biomass of the vegetation as it travels down the line. You will notice that at a point, there is no longer an increasing rate of return for more soil. This point represents the greatest width you’ll ever want to have your beds.
Depending on soil conditions and species, we have found this ideal bed width distance to lie somewhere between 4 and 6 feet, on average. Clearly, the 2 feet the plants must traverse is within this maximum bed width.
But how wide should the aisles be?
Examining the same data with different objectives, you can observe that each plant does best when it has about 4-6 feet of soil to eat. This suggests that the aisle should be 4-6 feet wide for maximum yield, for most crops.
There are exceptions: some crops (especially sprouts!) need less soil to eat, and others need much more. Experimentation must be done on each crop to be grown if the exact data is to be learned. A good rule is, however, aisles of 4-6 feet wide, and beds of 4-6 feet wide, and if human hands must reach the middle easily, the beds should be limited to 4 feet wide.
We prefer beds 4 feet wide and aisles 6 feet wide because our tractor is 6 feet wide and we can till easily in the aisles then, and reach into the beds.
Is this an inefficient use of space?
Using the same data gained in the experiment described before, you can calculate anticipated yields based on aisle width and bed width. You will find that the point of decreasing marginal returns is the point at which you will earn the most money. Well fed plants are the best use of space: it is better to have fewer well fed plants than many starving plants – so long as you don’t have too few plants per bed. Aim for the maximum number of plants you can feed well – the point of decreasing marginal return for aisle and bed with… 4-6 feet per bed and aisle.
THE 2 FOOT AISLE 4 FOOT BED
On some of our lands we maintain a 2 foot aisle and 4 foot bed. Why do we till less than the ideal width?
We combine those plants with condensed root systems so that we can maximize the use of land. All those plants that have extensive root systems enjoy the 6 foot aisles, but for many crops, this is more than necessary. Spinach, radishes, lettuces, peas, sprouts, onions, tomatoes, peppers, and potatoes, among other high-water crops, need less root space than plants like squashes, beans, small grains, large grains, lambsquarter, and other “xeric” or low-water crops.