Subject: Improving desert soil with green manure crops
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Jack,
10 inches of water year, on average, is not a lot to work with. If you are able to get something established after a series of good years, it can all dry up during the next series of dry years. That might be fine. I just depends on what you are ready to deal with over time. How much acreage are you working with? Can you be a little more specific about the location, elevation, slope, vegetation (native or otherwise)? What was the land used for previously? I mentioned microcatchments because that is an easy way to amplify the water you do get. If you can concentrate the water that falls on the property onto smaller prepared plots, your 10 inches becomes 40 or 100 inches. A microcatchment can be as large as several acres or as small as a few square inches. I mentioned soil crust because that is a proven method for both stopping porosity at the surface and increasing water retention at the surface. A biological crust is often associated with grasses when stabilizing dunes. The crust creates a mini-microcatchment, holding water at the surface, allowing the grasses to establish themselves. In a desert environment, the crust fills the space in a pinyon-cedar forest, and between mesquite, sagebrush or bunchgrass. In a truly dry area, it may be the only plant life for miles and miles. A biological crust is made up of algae, mosses and lichen. It can stay dormant for months or years, then quickly rejuvenates as soon as water hits it, swelling to seal the gaps between soil particles in the first few millimeters. The downside of biological crusts is they are very fragile and do not recover well after being disturbed, so they don't do well with heavy animals (including people) or machinery. You could use them to establish grasses and shrubs, and hope that the root mass is then sufficient to intercept and retain water. You could also use them to seal large microcatchments that supply water to storage tanks, trees and garden plots, but you need to protect them from being disturbed. You could also use structural or chemical crusts, but I have not read much about them, though I remember seeing a photo in National Geographic, many, many years ago, showing a dune stabilization project in Iran, using crude oil (it may also have been a waxy byproduct of the refining process, which would be more benign) to seal the sand for seeding. I wouldn't recommend doing anything like that without researching it thoroughly first. |
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Subject: Improving desert soil with green manure crops
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I don't agree. There is some run off, it is arroyo country after all. The soil is pourous and deep. I need to observe a few rainstorms, but the topography and soil tell me the majority of the water soaks deep into the water table (greater than 80 inches) and is not plant available, especially for new young root systems. I need the capulary action of the soil (currently mostly gravel and sand) to bring the water back up and keep it in they soil structure in the top 18 inches to make it available to most plants. However, I would rather loose the water to deep soakage than have it run off and take my soil with it. I can bring it back up with a well and drip lines; but would prefer the soil to have what it needs to do its job properly. I just need something to help the humus factor in keeping the top 12 inches moist. Of course it will need mulch to KEEP it in the top 12" and not evaporating, so much is just as important to organic matter. |
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Subject: Improving desert soil with green manure crops
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| If your problem is high soil porosity, you might consider encouraging soil crusting (structural, chemical, biological), to create microcatchments. | |
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Subject: Improving desert soil with green manure crops
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I have a lot of wind. So why not not mow? Then it's still rather stuck in place, right? Just gets blown over to cover the ground. |
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Subject: Improving desert soil with green manure crops
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Your logic about extra water retention only works if there is not already runoff. Where I live, we get about 8" of precip each year, but the silty-loam soil means that there is no runoff, except on the steepest slopes, or where vegetation has been eliminated (crop fields). Increasing organic matter is not going to get me any more water through reduced runoff. It will however aid in reducing evaporation. Just keeping the soil covered is extremely effective as well. In answer to your original question - how long to increase organic matter 1.5% in desert soil... I've seen studies of no-till agriculture in arid climates where the organic matter might increase that much in 40 years. Nothing happens fast in a desert. If you can import mulch, that will go a long way. In these dry climates, managing cover crops is pretty tricky. My own observations are that if you terminate the cover crop too late, it will transpire much more water, leaving the soil bone dry, possibly affecting the next year's growth. If you terminate it too soon, you won't have enough residue and might get higher evaporation. It's a tricky balance. My current thinking though, is to leave living vegetation as long as possible, then mow them to the ground to reduce fire risk. And hope it doesn't all blow away in the wind. |
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Subject: Improving desert soil with green manure crops
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Have you considered Hugle-berms? As the point is to keep what rain does fall, the berms keep it from running off and the Hugle not only soaks in that water but is decaying. I have installed these in my High Desert land that grows "weeds" very well but not "crops." If you haven't watched the Greening The Desert (1 and 2) I recommend it as that is what convinced me to try. If he was successful in the Jordan Desert (gray lifeless salty sand) with only 2" average rain fall, it would seem our 6" to 10" ought to work too. I dug 1 1/2' deep trenches so I could use that dirt to cover the woody stuff. Big wood last longer but if you want more rapid decomposition you could use just what is around. I have a mix of old tree parts and the 'sticks' from our weeds. The land seems to want to shade itself and grows a wide of variety of stuff.
My open land is covered by gamma grasses and where that is there are less tumble weeds (Russian thistle?) which takes over the disturbed areas. Those seems to make a great chop and drop plant, leaving the woody sticks behind but they collapse easily when stepped on. The chickens really like the young plants. I have enlisted the help of a Native Plant knowledgeable friend for my first plantings, but the trees actually went in first though I have been told the cover crops should have that honor, especially the nitrogen rich ones like alfalfa. But the whole thing is about catching the rain we do get. I will be following your progress and, of course, share my results. So far the Sun chokes have survived. They were 'stunted' the first year but did grow some nodes. Then this year they have sprouted whole new plants. I didn't know they would do that but that should make good shade so I'm excited to see how they do this year. |
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Subject: Improving desert soil with green manure crops
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Hi Jack,
I also like to grow my own mulch; cheaper and easier, but slower. I use bird seed mix as a base for multi-species cover/bird food crop. The one I like has; black oil sunflowers, millet, milo and wheat. To this I add as many natives as I can round up, alflafa, clover, vetch and whatever extras I have in the garden like carrots, parsnips, barley, corn etc., anything that produces tons of seed. All this goes in a bucket and is broadcast Sepp Holzer style. Problems still arise when there is no shade, since the sun can cause a lot of that carbon to convert to CO2 and leave. On these areas, I start with cardboard, straw and compost, then go into that with some tough shrubs and trees and when they are established enough to make a little shade, then plant out. When I plant the trees and shrubs, I dig a water harvesting depression first and then plant and mulch. This directs the runoff from the cardboard into the tree wells, helping to keep them alive. If it's a large area, I will just make little islands like this and then try to connect them. |
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Subject: Improving desert soil with green manure crops
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Thank you both for the feedback.
As far as being able to only store what falls, that is true. My thinking is this. On acre of ground, a half inch rain event drops 13,577 gallons of water. My job is to keep as much of that as possible. 1% increase in the organic matter of the top 12 inches of soil is said to increase retention by 16,500 gallons. First problem solved. Of course the next challenge is how to keep it from evaporating from the effects of sun and wind. That can be combated with cover crops and living mulch. The plants further retain water and then create the organic matter to chop and drop to improve soil. Rinse and repeat. 10 inches of annual rainfall puts 271,500 gallons of water on the soil per acre. That is a lot of water to work with. If as you say, you can only grow what water is given to you; then I think the imperative is to retain all you can. It sounds like the way to do that is amend the soil with organic matter and put life on it to hold it in the soil, rather than back into the atmosphere. The only question in my mind is how long it will take to grow the material necessary for this system to take hold. |
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Subject: Improving desert soil with green manure crops
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| From west Texas myself, one good source of organics is wood chips, they're a problem for every city and most would be happy to give your name to the tree trimming companies that are always looking for a place to dump chips. Kochia, tumble weeds and sunflowers are very common weeds that will produce a lot of plant material for chop and drop, my chop and drop involves a riding mower and it drops wherever it lands after it discharges. | |
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Subject: Improving desert soil with green manure crops
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There are fields in my village that have been planted into alfalfa for as long as anyone now living can remember. They are at their highest and best use as-is... Perhaps if the alfalfa needs to be rejuvenated the field will be planted into wheat, rye, or barley for a year or two, but it inevitably returns to alfalfa. These fields are not suitable for growing other crops... The limiting factor is not organic matter, but water (9" to 14" per year). We are desert, and can expect to grow desert adapted crops. Organic matter can't retain moisture that isn't available to be retained. The best dry-farm hay-fields around here are clay. It grabs the winter moisture and holds on to it long enough for an early summer crop of alfalfa to reach maturity. However, a few feet away inside the boundaries of the irrigation district, everything is radically different... The clay is still the best place to grow alfalfa, but we can get 3 gorgeous crops every year instead of one scraggly crop most years. We can grow any vegetable or tree that survives the winter cold and matures in the available frost free days. |
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Subject: Improving desert soil with green manure crops
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I have a question that I am hoping those whom deal with this sort of soil can answer, hopefully from first hand experience; but open to all whom would like to respond.
How long would it take to develop a desert soil to say 2% organic matter (which I think could be fairly productive soil) from say .5% using green manures in a chop and drop or cell grazing system. Let's take a subject property as follows West Texas desert country. 10 inches of rainfall spread over the calendar year, but weighted heavier in the months of June through September. Seldom freezes, but winter average lows in the low 30's. Hot summers; but mean annual temp of around 75 degrees F. Soil is a gravelly loam. Deep with no restrictive layers. Ph is on the high side but not too bad. Mostly in the 8 with a little in the 8.5 range. Organic matter in the upper 12 inches about .7% and the layers below around .5% down to 8 feet. Cover crops might included Comfrey (bocking 14) Alfalpha with limited irrigation. Multi seed cover crop mixes. Other suggestons welcome. What are some high tonnage per acre cover crop plants to consider? I live in a place that has endless tons of free organic matter for the taking (grass clippings, leaf litter, rice hulls, shreded wood chips, etc...) but the fuel to haul them to the subject property in sufficient quantities is prohibitively expensive. I think the only practical solution is to grow the compost in place. Am I talking a few years for the soil to improve enough to hold enough water to grow food? Are we talking 10 years, a lifetime? I know I can grow edibles that like low rainfall areas and drought resistant varieties; but I am more concerned the soil to be able to retain the moisture between limited rainfall events, so I am not irrigating too much. I have access to ground water. I don't know what restrictions I will have on irrigation rights. Does anyone know Texas' policy on how much water one can draw for ag purposes? I am sure it varies from County to County, but would be interested in anyone's experience whom has faced this permitting process in Texas in the past few years. The soil is mostly alluvial deposits of gravel and sand. I have not done a perc test, but imagine the water runs through it like poop through a duck. I need to improve the soil structure or it will be like filling a glass with no bottom. How much mulch will be enough to raise organic matter 1.5% or so? Does anyone have any thoughts on the often quoted study that says 1% increase in Organic matter equates to 16,500 gallons of water retention in the upper foot of soil? Do you find that too be true or only under limited test conditions? Thank you for your input. |
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