Ideal soil mixture to start
I'm just starting a raised bed in the backyard and I'd like some advice on soil mixture. I already read all the downsides about raised beds, but we have chewing/trampling dogs in our backyard, and it's honestly the only way to make a go of it. That said, can someone help me with the right soil mixture? The raised bed is 8x4 and about 30" off the ground, made of hardwood. Should I get 1/2 fill dirt and 1/2 compost? And if so, what kind of compost do you recommend? I obviously want something organic, and I'm NOT a fan of anything made by Monsanto. I saw something called Gro-Well Organic Garden Compost - does anyone know anything about this? Any other recommendations? I'm also looking for organic alfalfa hay and haven't found that yet for mulch. Thank you.
Great article on the soil. Your comments on the importance of the carbon/nitrogen ratio in organic matter would have helped me a few years ago when I bought some compost that was made from partially-composted ground-up trees. It finished composting in my garden, sucking all of the nitrogen out of it in the process. I couldn't grow anything in that part of the garden that summer (my tomato transplants turned yellow and died, and even my zucchini wouldn't grow. I learned that it's important to understand what's going on at the molecular level. The best on-line resource I've found regarding composting is at http://www.soilandhealth.org/03sov/0302hsted/030202/03010200.html.
As for the ideal soil mixture to start with, I normally (except for the unfortunate incident mentioned above) use Omni compost, which I've had excellent results with for about five years. When I convert a new section of lawn into garden, I fill an 18 inch deep hole with a mix of 1/3 Omni and 2/3 dirt (I throw away the top 6 inches of bermuda grass contaminated soil, and use the bottom 12 inches). Sometimes I also mix in some sand, especially if I'm going to plant something that likes lots of drainage, like my strawberry patch. I cultivate more Omni compost into the top few inches of my pre-existing garden every year before planting. You can buy it by the bag (1.5 cubic foot for $4 at Lowes and Home Depot) or in bulk at Gro-Well (602-269-5784), where they make it. They are on the east side of 27th ave, just south of Lower Buckeye road, right before the entrance to the landfill. They charge $23.50 for one cubic yard (27 cubic feet). One cubic yard weighs about 750 pounds. Two cubic yards will fill the back of a full-sized pickup truck. It's a great price if you need lots of it, but you'll need a pickup truck (they dump it in the back using a bulldozer, and won't sell it to you unless you bring a tarp or buy one from them). The ingredients include "forest products" (probably ground up trees), gypsum, and chicken manure (which is high in nitrogen).
Here is my essay on Soil, Sorry for the length, but I couldn't figure out out to attach a do***ent.
WHERE DO VEGETABLES SHOP For GROCERIES
Most people go to the supermarket to get their vegetables. The question is, “Where do vegetables go to get their groceries?” The short answer is that Vegetables get their nutrition in two places. They get most of their nutrition from the air .The rest they get from the soil. When combined with sun light a plant has all it needs. In this paper I talk about the soil as the vegetable’s supermarket. Basically Vegetables send their roots to the “pore spaces” within the soil to obtain their nutritional materials. The Vegetable’s personal shopper is its root (especially little hair roots), the supermarket is the soil and the pore spaces within the soil are the aisles and shelves.
But first, just what is it that vegetables need in the way of nutrition since they don’t eat pancakes or roast beef? A little background is in order. Vegetables (along with all other living things) are made up of organic molecules. These organic molecules are mostly carbohydrates, proteins and fats.
The majority of a vegetable’s organic molecules are in turn made up mostly of the elements; Carbon, Hydrogen, Oxygen and Nitrogen. These are the main elements that vegetables need to consume in order to make up stems, roots and leaves as well as the tomatoes, onions, broccoli and so on that we are interested in.
For the most part carbohydrates and fats are made of just 3 elements; Carbon, Hydrogen and Oxygen. Proteins are made of the same three elements plus Nitrogen.
By way of an illustration, the primary material trees are made of is cellulose. (White cotton ***** are 100% cellulose). Cellulose is a carbohydrate and the tree uses it to make cell walls. Each and every cellulose molecule is made up of repeating groups of 6 carbon atoms, 10 hydrogen atoms and 5 oxygen atoms (C6H10O5). Lignin is the second most common component of a tree and its chemical formula is (C9H10O2, C10H12O3, C11H14O4).. Again, there are lots of C, H, and O’s.
Neither of these carbohydrates provides nutrition for people. From the human perspective we call these carbs “fiber”. Fortunately plants also make starch and sugar carbohydrates that people can digest which is why we eat potatoes, kale and tomatoes and NOT wooden fence posts. Plants make a lot of other molecules like proteins and vitamins which are interesting to people and again mostly C’s H’s, O’s and a few N’s. The trick for the plant is to obtain these C’s H’s, O’s and a few N’s from its environment. So nutrition for a vegetable is largely related to obtaining these basic elements.
“But back to shopping.
As to shopping for the carbon and oxygen (the C's and O's) plants don’t need a personal shopper (roots). Vegetables simply breathe in carbon dioxide (CO2) from the atmosphere. There is a lot of carbon dioxide out there and more is being created every day. Good for the plants but maybe not so good for us :-( As for energy the plants use sunlight to create sugars (C’s, H’s and O’s).
Although the bulk of the raw material making up plants come from the air, plants still need hydrogen, small quantities of other elements and water to grow and prosper. These are the nutrients the vegetable needs to shop for in the soil pore spaces.
The hydrogen comes from the vegetable’s supermarket in the form of water. Water is H2O after all so it has lots of hydrogen. The oxygen part of water is released by the plant (with the help of the sun) as waste product. The remaining nutrients like nitrogen, potassium and phosphorous, along with a little over a dozen other elements that a vegetable needs to grow are dissolved in the water in the soil or stuck to the walls of the pore spaces.
STORE LAYOUT AND ACCESSIBILITY: TILTH
A Vegetable’s roots do best when the soil (the supermarket) has the nutrients they need but also is dependent on the texture/structure of the soil. The aisles and shelves need to be big enough to allow room for the nutrients, the flow of air and water and to provide efficient root access. In soil we call the texture “Tilth. Tilth defines how light the soil is, how much pore space there is, how permeable it is to water and air flow and how easy it is for the roots to expand and grow into the vegetable’s supermarket.
When talking about dirt gardening, (as opposed to hydroponic gardening or manufactured soil gardening) vegetables are generally expected to do best in soil with the following gross characteristics:
1.About 50% rock particles by volume. Rock particles are called sand, silt and clay with sand being the largest particles, silt being smaller particles and clay being tiny particles. Sand and silt are can be made of almost any mineral but are mostly made of irregular shaped pieces of quartz (SiO2). Clay is made up of tiny clay minerals and though tiny, are basically flat lattices with lots of internal spaces.
Generally soil with something like 60% sand, 30% silt and 10% clay makes ideal garden soil and is called "loam" A little clay goes a long way.
2.About 50% pore space by volume. The spaces between the rock particles are called pore spaces. Healthy roots need to have both water and air in the pore spaces with a 50-50 mix seen as about optimal. In fact oxygen is needed at the roots to facilitate the movement of nutrients out of the pore space and into the root. If there is NO oxygen at the root, the plant won’t suffocate but it will starve to death.
3.About 5-6% Organic mater. More about organic material later.
BUILDING THE SUPERMARKET
As we said, for a vegetable, soil is the supermarket. The pore spaces are like the aisles and shelves in the market. The plant can send roots down into the soil and the roots, especially the little hair roots, suck up the water and nutrients found in the pore spaces. These nutrients travel up into the plants leaves (and sometimes stems) where, with the help of energy from the Sun, the plant starts the process of making simple sugars which power the plant to make all the more complex organic molecules we are interested in harvesting.
If the is soil is made of a lot of tiny clay particles, then they naturally pack so tightly together that there is no aisle space and roots find it hard or impossible to get to the shelves. (Clay minerals are flat and thin, think of deck of playing cards). Even water and air find it hard to get down there if the aisles are all clogged up.
Technically “porosity” is the amount of space between the particles and both clay and sand can have a high porosity. But clay always has a low “permeability” which is the soil’s ability for a liquid or air to flow through. Clay particles make great shelves and can hold a lot of nutrients (and lots of water) which is fine as long as there isn't so much clay that every thing gets clogged up. If the root can’t get down the aisle and water and air can’t get down the aisle, it does not matter how many nutrients are on the shelf.
I am told that even 20% clay is enough to make a soil act like a clayey soil and make it tough for plants to grow well.
If the soil is made of big sand particles we have the opposite problem. Sand does not have much surface area which effectively means that it doesn't have many shelves. The poor spaces are large but there are very few nooks and crannies to hold the water and nutrients. Even when watered liberally, most of the water flows right through the supermarket and out the back door taking with it what ever water soluable nutrients were there to begin with. (We call this leaching)
There is also another problem with using rock particles as the supermarket. Even though they are tiny they are still made of rock and that means they can be heavy. And if compacted by vehicle traffic or foot traffic, most of the pores (shelves and aisles) can be crushed out of existence. Concrete makes a very poor supermarket.
STOCKING THE SHELVES
Once we have the market built the question is, “how do the shelves get stocked?” This is to say “where do the nutrients come from that end up on the shelves?”
The thing to remember is that the water, nitrogen, phosphorous and potassium, along with smaller amounts of a number other elements need to be dissolved in the water sitting in the pore space and nooks and crannies of the minerals (especially clay) and organic material before it can be sucked up by the hair root. The roots don’t have teeth or a stomach so the nutrients need to be predigested, so to speak.
One place the minerals come from is from the rocks themselves. Granite, for example is made of silicon, potassium, sodium and calcium for the most part. As it gets eroded and broken down into sand and silt those elements become available in the water in small quantities. So sometimes the stocking just happens chemically.
But it turns out that not all the nutrients, like nitrogen and phosphorous, are just laying around in the rocks in sufficient quantities for optimal vegetable growth. The good news is there are a lot of things going on that make stocking the shelves easier and more efficient.
The first thing that helps is organic matter. Organic matter is simply stuff that was once alive and is now dead.
By its very nature organic material is lighter than rocks so it naturally lightens the soil when mixed in with the rock particles and improves tilth. The presence of organic material also keeps rock particles from packing together so tightly and as a result creates even more pore spaces. This makes it easier for the roots, air and water to get up and down the aisles. Since organic material is a little spongy it can also absorb and hold water and nutrients that might slide right off the shelves (sandy soil).
With organic matter, some earlier plant or animal went to all the trouble of collecting and concentrating nutrients to form its body (in the form of complex molecules like carbohydrates, protiens and fats) and now its body is available as it decomposes in the soil. The vegetable doesn't have to wait for a bazillion years for the granite or basalt to decompose. This is a win-win situation for the vegetable. Not only does organic matter lighten the soil (expand the aisles) it stocks the shelves with material that eventually turns into plant food.
But we have to remember that the organic matter needs to decompose into its basic elements (like nitrogen, potassium calcium etc.) for the vegetable to get any nutrition value. Plants don't eat carbohydrates, they need the base elements. So the root is not the only thing that needs to be growing in the soil. Little bacteria and fungus (aka microbes) are eating and decomposing the organic matter and breaking big complex molecules into little molecules that the plant needs to eat. Technically the plant isn't eating the organic material, the microbes are. While the microbes are eating (decomposing) the organic material they are freeing up plant nutrients at the same time.
Originally a plant used energy from the sun to create these complex molecules. When fungus and bacteria "eat" these molecules they obtain the Sun's energy that was originally stored by the plant. Of course sometimes the energy that went from the Sun into a plant got eaten by an animal before the stored energy ended up in the soil as manure or as animal remains.
When the microbes feed on the fats, proteins, carbohydrates and other organic molecules they free up the individual carbon, oxygen, hydrogen and other elements and makes them available to the vegetables. In good soil there are lots of shelves (nooks and crannies) that can accept and hold these minerals until the plant needs them.
These microbes need energy and nutrients to build their own little bodies and to do their own growing and multiplying. Generally they get their energy from oxidizing (burning) the Carbon in the organic material (pretty much the same process we use) but they also need some Hs, Os, and N’s to grow their tiny cell walls and such.
Most organic material has more Carbon than Nitrogen in it. If the organic material that we put into the soil has a lot more carbon than nitrogen (like over 25-30 times more) the little microbes will eat the carbon, grow like crazy and use up all the nitrogen themselves.
The result is that too much of the wrong kind of organic material will actually reduce the amount of nitrogen sitting on the shelves and nitrogen is especially important to vegetables. Shopping will be easy but there won’t be much nitrogen on the shelves.
For example if you are going to use sawdust as an organic material you will need to augment it with more nitrogen if you want your vegetables to do well. That’s because sawdust has 300+ carbon atoms for every nitrogen atom. It’s pretty hard for the microbes to grow on sawdust let alone leave any nitrogen for your vegetables.
On the other hand, alfalfa has 12 Carbon atoms for each Nitrogen atom so it makes a good organic material for releasing nitrogen. Well composted cow manure has 15-20 atoms of Carbon for every Nitrogen atom. Both of these organic materials result in the net addition of nitrogen (and other nutrients) to the soil.
If there are no healthy bacteria and fungus populations there is "No" decomposing going on and the organic material is just lightening the soil without adding to the nutrients.
In addition these microbes also create acids and in other ways help decompose the rock which release some of their nutrients without having to wait a bazillion years.
Microbes help out in other ways as well. Fungus found in the soil “Mycorrhizas” form a mutually beneficial relationship with the roots of most plant species. This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant. The carbohydrates are moved from their source (usually leaves) to root tissue and on to fungal partners.
In return, the plant gains the benefits of the mycelium's higher absorptive capacity for water and minerals. Think of the mycelium as little root extensions for the hair roots. This can greatly expand the area from which the plant can derive nutrition and improves the plant's mineral absorption capabilities.
Another cool thing mycorrhial fungus can do is help with the up take of phosphorus. Plant roots alone may be incapable of taking up phosphate ions that are demineralized, for example, in soils with a basic pH. The mycelium of the mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonize.
Symbiotic Nitrogen-fixing bacteria, such as the Rhizobium species, detect a compound secreted by the roots of leguminous plants and then produce nodular factors which signal to the plant that they are present and will lead to the formation of root nodules. In these nodules are bacteria which are sustained by nutrients from the plant and which convert nitrogen gas to a form that can be used by the plant. Non-symbiotic (or “free-living”) nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses), and similarly “fix” nitrogen gas in the nutrient-rich plant rhizosphere.
I wouldn’t be surprised to find out that they are doing a lot of other things to help vegetables as well.
Sometimes the little microbes combine little rock particles into bigger rock particles which also increases pore space. And finally, over time, the microbes come to the end of their life and then their bodies become organic matter as well.
So a healthy dirt soil is literally alive and has its own cycle of growth. Adding organic material provides more food for the microbes. More microbes release more plant nutrients. As the organic material is consumed the microbe populations decrease. At that point even the microbes are decomposed by other microbes.
Then there are the bigger critters.
Bigger critters in the soil also help make nutrients available. These are the guys like earthworms and gophers. While gophers are not generally considered good for a vegetable garden they do open up the aisles, mix and aerate the soil and add their own organic poop material. Earthworms aerate the soil and convert a lot of organic material into plant available nutrients and so do a great job of stocking the shelves as well as widening the aisles. Over time worms can completely till a garden. (I am talking about decades here). Before Darwin was famous for his theory of evolution he was famous for his study of earthworms. (or maybe it was after :-)
So that’s what is going on in the soil supermarket.
ALTERNATIVE TO THE DIRT BASED SUPERMARKET
Of course plants don’t absolutely need a dirt supermarket. In hydroponic gardening the gardener obtains all the needed nutrients and dissolves them in water for the vegetables. The plants still need someplace to stand so usually some inert mater like plastic pellets or rock wool or even just some plant supports provide a place for the plant to stand. The plant still needs light and water and the roots still need oxygen but the other nutrients are just in the water. That’s a whole other way of gardening.
We also have manufactured soils. The square foot Gardner recommends using a soil mixture of 1/3 peat moss, 1/3 vermiculite and 1/3 of a mixed variety of composts. Other folks suggest using the Western Organics premium potting soil which consists of forest compost (moderately composted sticks, twigs and leaves) and some other kinds of compost. This is mixed with a little peat moss and vermiculite and some organic 5-5-5 fertilizer. As long as the tilth is good, water and nutrients are available and the necessary nutrients are there, this should work just fine.
I know folks that use it on their homemade beehives inside & out. I've used it when laying down the concrete form for a cistern install & raised beds. Inside & out. It does take several applications on the wood for it to be of any protective use. The wood will darken over time.
Red Wood looks better but it is pricy. I have built a couple of beds using the 6 foot cedar fencing as well but that does require corner posts and pretty much determines the bed to be no longer than 6 feet. Too early to tell how long they will hold up.
After reading Zotero's great posting on sunken gardens and wicking beds I may be digging them up again, decisions decisions :-)
A cheaper option than Cedar or Redwood is Douglas Fir. They sell them in different sizes but I like using the 12 inch thick boards. It usually comes out to $50 for a 1ft high 4ftx8ft bed. I use 4x4's for the corner posts. Douglas Fir like Cedar repels insects, but you probably want to wipe it down with linseed oil to weatherize it.
In Australia they just use straight compost for these systems in addition to adding worms to the grow bed & a feeding station for the worms. They've been using these systems for over 30 years w/no problem.
You have a lot of options for the grow medium in a traditional raised bed. I prefer using a soil-less potting mix, because it retains water better & it allows the roots to develop really vigorously. You can either buy it in bags or make it yourself. It most likely will be cheaper to make your own mix, especially since the bed you have is pretty deep. Your raised bed totals out to 80 cubic feet or 3 cubic yards. Most bagged mixes come in 1-3.5 cubic feet increments. They run anywhere from $20-40 a bag.
If you do want to buy the mix, Fox Farm makes a Potting Soil bag that is really good for container gardening. It's the only bagged mix that doesn't need any type of amending. You can use it straight out of the bag. Sea of Green sells this product. I've only heard of the Grow Well products, but their not available where I live, so don't have any direct experience with them. I would definitely talk to people who have used it in a raised bed & see how it's worked for them. Especially since it's going to be so expensive to buy all the bags. BUT if the Grow Well company sells a planting mix by the truckload that would be a pretty good deal too. It should be cheaper than buying the bags.
If you do want to make your own mix you can use Peat Moss or Fine Pithe COIR (shredded coconut husk). COIR is more sustainable since there is 3 coconut tree's for every human being & if we don't use the husk it gets thrown into the landfill. Peat Moss is mined from Peat Bogs which take thousands of years to form. Home Depot has the best price on it & free shipping. It's actually cheaper than Peat Moss. Here is a link to the product:
You will need 13 of these 3 cubic foot bricks to fill your raised bed less than halfway. That's about $120. You'll want to mix that with a bag of Vermiculite & Perlite, about another $40. Then you want to add compost that can be picked up in a truck. It usually costs anywhere from $30-60 for a truckload of organic based compost. So we're talking anywhere from $190-220 to make your own mix on the cheap!
This is why I like the Australian method where they use straight compost. It saves a lot of money. You can also build the wicking beds with pretty much anything, saving you on the cost of wood.
Also, you're probably going to have a very hard time finding a non-chemically grown & GMO free alfalfa. In Tucson there is only 1 place in town that grows organic alfalfa & they sell it for $12 a bale. It's a smoking deal, but I haven't been able to find anything like that in Maricopa county. I just bring it up from Tucson for my mom's garden.