Pros and Cons of Each Approach
First, let's revisit why it is desirable to move the soil at all. Conventional wisdom was soil is tilled is to loosen it so oxygen and water can reach the area where roots will grow. However, many passes over a field with equipment and even heavy animals such as oxen or horses, much less heavy equipment, further compacts the layer of soil just beneath that of which was tilled. Additionally, the broken-up soil is very prone to being compacted by rainfall. its ability to create a more natural soil that retains nutrients and water, prevents soil erosion, and compacts less.
Plowing, it is claimed, incorporates fertilizers and crop residues into the soil, making nutrients readily available to the roots of the plants. Turning organic matter under also has the benefit of preventing planters from becoming fouled with surface trash. In no-till farming, crop residues are left on the surface, where the nutrients that result from their decay can leach into the soil. This leaching process is far more thorough than you might imagine. Fertilizers—including anhydrous ammonia, phosphorus, and potassium—are at least as effective on the no-till fields where they've been dispersed as on the plowed plots. These ingredients can be placed directly into the planting trench, where they're most needed, during seeding. As for the potential problem of the planter fouling with residues, it is recommended to cut a slot through surface trash.
Though the soil does stay cooler until a little later in the spring because of the insulating layer of residue, the day/night soil temperature fluctuations are smaller. A no-till field rapidly makes up its deficit in growth rate as the weather turns warm. And if the summer should be really hot and dry, no-till yields will nearly always exceed those of plowed ground. Since soil moisture levels can be more than 10% higher in late July in an unplowed piece of ground, it's not surprising that plants are happier in a field covered with mulch.
One of the significant downsides of plowing is it tills the entire area, where doing so by hand will only create the trench needed for seeding. Both plowing and tilling can be done in a way to help minimize the amount or water runoff, but a big benefit is that no-till soils stay more moist than those in tilled fields. The surface residues trap water and protect the earth below from the evaporative effect of the wind. In areas, where adequate spring moisture depends largely on spring rains and/or snowmelt, the vegetative cover helps keep the snow from blowing away and water runoff. (For more on permaculture practice click here). The advantages don't end there, though. If you grimaced when you read "anhydrous ammonia", consider this: One of the main problems with conventional agriculture's heavy use of nitrogen fertilizers is the leaching of these compounds into surface water during runoff. By retaining rainfall, the untilled field also better holds the chemicals that have been applied to it, thereby decreasing their pollution potential.
Our Findings in the Backyard Garden
When the climate is drier, we strongly prefer a minimal till approach as described on the Building Soil page. This might involve taking a spade and lifting up the soil where the plant will go and perhaps 12 inches of space on all sides. Then as the plant grows, we will hand pick the weeds that grow up around it. This is very desirable and doable during periods of low rain fall because a minimal amount of soil is turned over to release carbon and dust in the to atmosphere, the bacteria and beneficial creatures in the slow are minimally disturbed and the weeds - with some regular diligence, and especially when a drip irrigation system are used - are easily kept under control.
However, we've also found that in optimally rainy conditions, no till seemed like a futile attempt to do the right thing for the environment. Weeds quickly out-paced our ability to pull and pests arrived in droves, often taking advantage of the long stems and leaves of the weeds around the crops to hide themselves and gain advantageous access. Our attempt at minimal tillage in a year like this left us breathless, exhausted and with a minimal yield of crops. One of our take-aways was we needed to over-turn more of the soil around the plants and we needed to weed more often. Unfortunately, tiime is not in endless supply, so here are some other things we learned: