Just Dirt? 
There is a recent study that puts together statistics on soil erosion from more than 125 sources. It says “the US is losing soil ten times faster than the natural replenishment rate while China and India are losing soil 30-40 times faster than the natural replenishment rate”. The study goes on to say “worldwide, as a result of wind and water erosion in the past 40 years 30% of the world’s arable land has become unproductive”. The study also says that the soil that is washed away harms other ecosystems, including our oceans and atmosphere. Obviously a food crunch is on the horizon.
Food production has been artificially propped up by the use of concentrated fertilizers which will soon be priced out of the market as critical fertilizer elements are nearing exhaustion. Not so incidentally, such fertilizers predispose the soil to erosion and eventually soil collapse. The organic agriculture movement addresses soil loss by calling for the increase of the organic content in soils, which helps hold the soil against wind and water erosion. However only 1% of the farms in the US are organic farms. Unfortunately organic agriculture has not yet shaken the practice of tilling which exposes even carbon supplemented soil to the ravaging effects of wind and water, as it destroys stable humus which is the most erosion-resistant organic faction.
To rebuild our soils so they are inherently fertile (not requiring supplemental fertilization) will require huge subsidies difficult to obtain in these economic times. For some time there has been a program in the US to pay farmers to plant the most eroded soils (collapsed in many cases) to native soil-building grasses and forbs and leaving them untilled. But it is the soils not yet fully degraded that can respond much faster to restoration measures and there is no program of soil restoration for this group. Instead of serious soil restoration programs we have focused on finding new lands to bring into production and of course proceed to degrade via tilling and fertilizing. 80% of these new lands are presently being carved out of tropical forests. This deforestation has huge implications regarding global warming.
We need to replace status quo agriculture with an agriculture that keeps the soil intact and perpetually covered. The steel we pull through soils is not sensitive to the myriad life forms it encounters but we who pull that steel must become sensitized. For cover crops, legumes have particular value in fixing nitrogen but need to be widely spaced for best root growth and nitrogen fixing. Grasses are best to provide maximum organic matter and can in many cases rely on free nitrogen-fixing organisms for their nitrogen needs. Other cover crops, particularly those in the brassica family, are good for root penetration of the subsoil and suppression of soil pests. In terms of spacing they fall somewhere in between legumes and grasses. I see no benefit in using a cover crop mix (with exceptions) due to the need for different plant spacing.
One cannot garden well focusing on these matters alone. The larger environment must also be managed in a fashion that Nature demonstrates. Predator habitats are needed within or surrounding the gardens and in these habitats plant diversity is essential. Frogs have always been an integral part of my gardens and I have provided habitat for them (miniature ponds and bogs) in my temperate zone gardens. Here in the tropics they seem to do quite well on their own.
We need a change in how we study ecosystems. Our conventional approach has been to find the thing that causes problems and then fix it. Recently, however, a new approach is being made that studies systems that are working well and learning about the various interactions that make them work in a balanced manner. Of course it is up to us gardeners to apply this knowledge.
In studying working systems it becomes ever more evident that Nature is on our side. This is in fact a planet perfectly suited for human habitation. We are protected here but Nature cannot protect us from ourselves.
Using nitrogen fertilizers suppresses these free nitrogen-fixing organisms and tilling destroys mycorrhizae fungi that also supplies nitrogen obtained through its decomposition of organic matter, as well as a vital companion to nitrogen – phosphorous.
