I can't see any flaws in this argument. Can anybody else? Can we somehow solve the flaws of irrigation, at least partially? Even the most dedicated permie sometimes wants to add some water to things. (If anybody here NEVER waters and lives in a climate with less then 15 inches of rain, let me know what you do!)
Gilbert Fritz wrote:This leaves the salt in the soil. Eventually, you have disaster. I can't see any flaws in this argument. Can anybody else? Can we somehow solve the flaws of irrigation, at least partially? Even the most dedicated permie sometimes wants to add some water to things. (If anybody here NEVER waters and lives in a climate with less then 15 inches of rain, let me know what you do!)
It would seem that the argument breaks down when you consider that the fruits, grains, and vegetables that are removed from the farmland contain salt. Continually removing salt from the soils without some sort of replenishment would probably also lead to problems.
Gilbert Fritz wrote:Let's say that you ate the vegetables etc. yourself and didn't export them, and recycled all humanure. Then of course, you would be adding even more salt from your diet.
That's what most of east and southeast Asia have been doing probably since they started using agriculture, several thousand years. And many parts of east and southeast Asia terrace and irrigate most everything. So I don't think it can be a general rule that irrigation ruins the land.
My desert community has been irrigating the same fields for 150 years. There is no evidence of salt buildup on our land. The amount of rain that we receive is approximately the same as the amount of irrigation water that we apply. Runoff from farmland around here only happens during the most severe thunderstorms. I live in a mountainous area... There is more than enough water on our land for it to sink all the day down through the soil. Our village is on an escarpment above the flood plain of a river. Springs are abundant along the length of the escarpment. I figure that's because the irrigation water is hitting a clay layer that underlays the village and it's escaping where the river cut through the clay layer.
I don't believe this part of the scenario presented above: "there is not enough water to add enough irrigation water to seep all the way down through the soil". Where I see salt buildup around here is in closed basins, where the water table is near the surface of the soil, and it can only leave through evaporation... An open basin, which is drained internally doesn't build up high concentrations of salt. As a reference point, our irrigation system is designed to apply one inch of water per week for 12 to 15 weeks. Also, our irrigation water is mostly snow melt. It hasn't had a chance to acquire a lot of water soluble salts... And the mountains around here have been thoroughly scrubbed of salts, because the rain scours away every bit of salt that it can find, and carries it down to the salt flats.
Salt does not just disappear. It can be moved, concentrated or diluted, but it is always a consideration. Many arid soils are already highly saline (without human input). Improper irrigation can draw salt up from lower layers in the soil which can build up on the surface.
Much of the water available for irrigation in arid regions carries a lot of salt in it, either from runoff through saline soils, or passing through salt deposits. You have to keep the salt in solution and moving on. Fortunately, here in the Great Basin, the salt doesn't have to go far before it finds a salt lake, pan or flat.
The problem is that you need enough water to support plant growth AND enough water to keep salts from building up to dangerous (for the plant) levels (to keep the salt moving on). I discovered this dilemma when experimenting with container gardening (no drainage). My plants grew like gangbusters early in the season, but then became poisoned with salts about halfway through the Summer. I think I can control it with better fertilizing choices and mulching to keep evaporation to a minimum. Leaching the salts out of the potting soil during the off season would also help (but you still need a good balance of fertilizer to start the next planting). In Utah, the Spring runoff flushes salts out the top layers of soil wherever there is at least some buildup of snow.
There are plants, like salt grass, that will draw salt from the soil. There are also many salt tolerant crops. Some conventional crops have been grown successfully with brackish estuarine water in the Sunderbans, with the addition of potassium.
Irrigation is not destined to ruin cropland, but as with most things, it must be done with care.
Yeah, even that one...
Soil ecology and the extremely complex water cycle that goes with it pretty much precludes any one size fits all statements.
There are plenty of places where irrigation has run horribly amok and ruined thousands or millions of acres.
And plenty of places where appropriate practices have included irrigation for well over 1,000 years.
Be thoughtful, pay attention, keep learning and watching and reading. You'll do fine.
Paul has often claimed that you can get 100% away from irrigation if you do it right.
But I think irrigation has it's place as a useful tool.
A simple response to Lawton's hypothesis is that the mulching made the difference in the effectiveness of the Winter rains (concentrated by the swales) flushing salt out of the soil, by significantly reducing evaporation. Also, keep in mind that Lawton uses drip irrigation under the mulch, rather than relying solely on soil moisture from Winter rains. I would be interested to know what the salt concentration was beyond the mulched growing areas.
One fungus may produce wax, but another will be decomposing it. Soil microbiology is a fascinating subject. There is still much to be learned and applied.
I suppose there is a certain amount of rainfall that will make salt concerns a non issue. In Create an Oasis with greywater, Art Ludwig said that he does not worry about the salts in greywater and human waste when the rainfall is above 30 inches. Also, the addition of gypsum and heavy flushing irrigation with rainwater can remove salt below the rooting zone, assuming that the water is available and the the soil is permeable enough to allow this.
Rebecca and Joseph; how many inches of rain do you each get? And where does your irrigation water come from?
I am curious about this because I am trying a mental experiment, imagining a long term sustainable Denver Colorado. It is certainly leading me into a lot of interesting research.
The rain gauge in my east field recorded 12" of precipitation in 2013, and 17" in 2014.
I just found an experiment performed in Israel where they used sea water to irrigate fruit trees in the desert, by using drip lines to put the water deep in the soil. The salt stayed down there and the trees could draw up water.
Also, I am starting to wonder about the salting effects of salt brought into an area for consumption, in pre industrial times when all human waste was composted on site.
My fields are about 3 miles and 5 miles from the storage reservoir. The longest tributary to the reservoir is less than 10 miles long. We get salt dust winds sometimes from the desert. Around here a lot of salt is applied to the roads during winter. None of the road-salt runoff makes it into the intake of my irrigation system, but the next town downstream gets it. We are on septic tanks here. I'd guess that the culinary salt ends up dissolved in the spring water that flows from the escarpment below town, and that it affects the water of the next town downstream.
I notice salt buildup on potted plants that have a drip-pan underneath them. It doesn't seem to be much of a problem on pots that are free draining and have no contact with the leachate.
I'm mostly focusing this on drylands, where we don't get enough rain. And in most cases irrigation water leaves by evaporation or transpiration, leaving salts behind. If enough water is applied, it moves down through the soil. This tends to require a lot of water, which is often hard to find in drylands. Efficient means of applying water, such as drip irrigation, minimize the movement of water through the soil.