al's mix and square foot gardening

luv_figsMarch 20, 2010

hi,

i just started reading about square foot gardening, and it sounds like container gardening, except that its a whole raised bed, instead of containers. it also asks for a different soil mix: 1/3 vermiculite, 1/3 compost, 1/3 peat moss.

how come it is so different from al's mix? they are both essentially in containers, albeit the sfg container is a huge bed.

how can people swear by sfg if the soil mix is so different? aren't they concerned about drainage?

i've already posted this in the sfg forum, but am curious to see what people say here.

thanks,

jen

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jojosplants(9/ Tucson, Az.)

Hi Jen,
I used to use the SFG method and really liked it.

It's not really a container, it has no bottom. So because it is in direct contact with the soil, it drains just fine. The soil creats a wick to pull out all excess water.

If it were to have a bottom, and be raised, then it becomes a container.

SFG was designed to get the max harvest from a small space, and for those with small yards who don't want to till up an area.

I did very well with it. The only reason I no longer use it, is I put a huge aviary in my back yard to raise birds, small pool for my son, and lost my space...LOL!

Now I garden in the front, mostly in containers.

JoJo

    Bookmark   March 20, 2010 at 9:54AM
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tapla (mid-MI z5b-6a)

We can use wicks to help drain away the water that occupies the PWT in containers, but in raised beds, the entire earth is employed as a wick; therefore, the water in raised bed soils quickly moves down into the earth, so raised bed soils can be as fine as regular garden soil. We do, however, usually use a good amount of organic matter in raised bed soils to help keep compaction from top-watering so frequently to a minimum.

I use a variation of the 5:1:1 mix in my raised beds and it's been extremely productive. It's very rich and full of all kinds of soil life. You can see the excellent tilth at a glance, too:

.... see the grains of Turface and sand in the soil?

Al

    Bookmark   March 20, 2010 at 10:47AM
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luv_figs

thank for the clarification, it makes sense now.

al, you mentioned that you use a good amount of organic matter in raised beds to counteract the affect of top watering compaction. how come that principle doesn't hold in container mix? because i thought we wanted to stay away from compost in container mix because it decomposes too quickly.

can you clarify what happens if my container mix is basically compost and perlite? if the compost decomposes, will it just be perlite left? and it can't happen that fast, within a season, right?

thanks! and i just learned with tilth was too after reading about it!

always learning from you posts al.

-jen

    Bookmark   March 21, 2010 at 1:03AM
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tapla (mid-MI z5b-6a)

Good. I often use words I know people wo9n't be familiar with because those who are most enthusiastic and interested in learning will look them up. Invariably, they will learn much more than the meaning of the word. A good deal of what I know came as a result of problem solving or trying to acquire the information to head off a problem. Sometimes you guys ask questions, my answers to which I need to make sure are accurate, so I'll check them against a reference. Again, invariably I learn something along the way. It keeps me off the streets, too. ;o)

FWIW - I don't remember exactly what I started with in my RBs, but it was probably something like
5 pine bark
2 Michigan (reed/sedge) peat
1 sphagnum peat
2 play sand
2 Turface

I included some micro-nutrient powder and a charge of 27-3-3 fertilizer, too.

The more organic matter you use, the greater the shrinkage. Compost will literally disappear to nothing over 5 years or so, you'll need to keep replenishing it. There are other problems associated with almost all fine organic matter too, so I think you should be thinking in terms of at least a 50-60% mineral fraction (topsoil, sand, Turface ......), unless you use pine bark - then you can use less.

In raised beds, the organic matter promotes lots of soil life, which helps with aeration and guards against compaction. In containers, soil biotic populations are tenuous, and you water far more often than you do in raised beds. Also, if you use the fine particulate matter in containers, it supports high PWTs that kill off the soil organisms, so you don't get the same benefit.

If your container mix is compost and perlite, it will be extremely water retentive, unless the perlite is at least a 60-70% fraction of the mix. That's why I say you can't amend compost to make it drain properly. Once compost is less than 50% of the mix, you're using compost to amend the other ingredient(s).

Finished compost is primarily lignin and some lipids, neither of which decompose all that quickly. The problem with compost in containers isn't necessarily that it breaks down so quickly, it's the small particulate size that supports excess water retention. If you decide to water in small sips to compensate for the water retention issue, you guarantee that soluble salts will accumulate in the soil unless you take regular steps to flush it.

Again, it's not that you CAN'T grow in a soil based on compost, but it's much more difficult, your decisions are much more critical, and the margin for grower error is much narrower than if you were using a durable and well-aerated mix.

Al

    Bookmark   March 21, 2010 at 8:57AM
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