When to Add Compost and How Much?

Compost being made by Ponderosa Dairy in Amargosa
Nevada by the windrow method

Q. I hope you remember me from the last time you were in Kingman teaching.  I am a Master Gardener.  My question involves composting in the home garden for spring planting.  There seems to be no definitive answers as to when is the best time to add compost to the garden, and exactly how much compost to apply.  I hear that it may be better to apply the compost before the first frost so that it has time to break down before spring planting, and then again I hear that you should add it maybe a month or so before you plant.  The latter is normally the approach I take.  So far, so good, but can you share your thoughts with me on these two topics?  Thanks.

A. I do remember you, Ron. Regarding the compost, I usually look at the color of the soil to determine how much compost to add. For instance, a dark rich soil that crumbles easily will need less compost than one which is hard to dig and does not crumble easily. A well prepared vegetable bed should be so friable that you can dig it nearly with your hands, unaided by a shovel. This kind of soil structure we call "crumby" like cake crumbs.

Compost being added to the vegetable
plots at The Orchard in North Las Vegas

Once we achieve this state in a vegetable garden soil we just need to replenish the organic matter which was lost during the growing season. Generally we figure that we lose about 1/3 of our organic matter content of our soil each year. So the first year it is 1/3 of our total organic matter content. The second year it is 1/3 of our remaining organic matter content, etc. So you see it will not run out
in three years to zero. It will continue to diminish annually but at a different rate as the total amount of organic matter diminishes.

When preparing a raw (never amended) desert soil for a garden it will take about three years of applying a heavy amount of compost to the soil each year. These applications of compost must also be accompanied by growing vegetables in it. Just putting a compost into a soil and doing nothing else and waiting for three years will accomplish absolutely nothing. Water and microorganisms must be in the mix as well. I like to compare it to making bread or a cake. You can add all the dry ingredients to a bread or cake mix you want but without adequate liquid they will sit there and do nothing.

In the raw desert soils of the Las Vegas Valley we have less than 1/10 of 1% organic matter. That is amazingly low. In other rawvdesert soils the organic matter may reach 3 /10 or 4/10 of 1%, still not much. We want our soils to get to 5 to 8% organic matter. This is 50 to 80 times higher amounts than we have. To achieve this I like to add a minimum of 50% compost to our raw desert soils. I would even push it higher, closer to 75% the first year. Every time the soil is prepared for a new planting, organic matter or compost should be added. It does not matter the time of year in the warm climate of Las Vegas. In colder climates when soil temperatures drop into the 40's, most compost will sit there and not do much. As soon as the soil temperatures hit the mid 50's the microorganisms will kick in and start to work.

Yes, this is my foot on top of a desert soil typical
of what we would use for vegetable production.
It is modified heavily with compost over a three
year production cycle.

Compost by definition has finished its decomposition. It is then ready to release all the goodies that was built up into it during the composting or rotting process. So if a compost is a finished compost it will not continue to decompose. This is not true of unfinished composts or manure. These will continue to "rot" or decompose and they create their own heat when they do so and are piled together into a pile. Heat builds in the center of the pile to over 160F which is what you need to start killing weed seeds and bad plant pathogens.

So add a finished compost every time you plant. The amount varies with the color of the soil. Darker, rich soils need less than lighter colored less well developed garden soils. But if you add compost every time you plant and you continuously garden for three years in it, then the garden soil will be sustainable with small amounts of compost every time you plant. I hope this helps.