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News from the Edible Demo Garden

December 2021: Composting & Testing Your Soil

UC Marin Master Gardener Stephanie Scarpullo gathering composted straw from our straw bales. L Stiles
UC Marin Master Gardener Stephanie Scarpullo gathering composted straw from our straw bales. L Stiles
November and December are clean up times in the garden, as well as assessing the state of our garden after growing all of those heavy feeder summer vegetables.  We have recently removed our straw bales and like last year, we wanted to use the composted straw as a fertilizer.  Last year, a simple home test told us that our composted straw was very high in nitrogen.  In our horseradish bed, we set up a side-by-side comparison of the use of this decomposed straw as fertilizer versus the regular compost that the farm purchases.  On the composted straw side, the horseradish leaves sprouted way before the ones on the farm’s  compost side did.  The leaves were fully-grown when the other side was half as high.  By the summer though, the farm’s compost side had caught up and there was no difference between them.  We wanted to use this super fertilizer again but first we had to test the nitrogen level. 


Soil Testing for the Home Gardener

Every Master Gardener in our training class learns how to use a do-it-yourself soil testing kit that will give you a pH reading and the level of Nitrogen (N), Phosphorous (P), and Potassium (K) in your soil.  The kits we use are inexpensive (under $20).  The process involves dissolving pills in distilled water with a soil sample and after the soil settles, comparing the color of the water against a key that tells you the level of a particular nutrient.  Are these tests totally accurate?  Probably not, but they can give you a rough estimate of your soil fertility for the primary nutrients that are required in larger amounts than other soil nutrients. One piece of advice, do this with a buddy because the directions are complicated and it helps to have one person read them while the other one actually does the testing.  It is more fun that way too.

If you don’t feel like being a chemist, there are also nutrient testing kits you can purchase that just require you to send a soil sample to a lab for testing.  You get a list of the results and also some recommendations for amending any deficiencies.   These test kits are also under $20 but you have to pay the postage for shipping to the lab. 

Choose which method suits you best.  For Master Gardeners, getting your hands dirty and experiencing science up close and personal can feel quite rewarding. 


Soil Testing Results

We tested the pH and the nitrogen level in our straw compost and found out that the pH was perfect for growing vegetables. The Nitrogen level, however, was between low and very low.  So what was the difference between last year and this year?  In October, we had substantial rain:  17 inches for the reservoirs in Southern Marin and 7.6 inches for Stafford Lake in North Marin.  We left our bales in place during October because we were still harvesting winter squash and the rains came down hard on them. 


At the end of the growing season, the leftover nitrogen is mostly dissolved in water held in the pore space between the soil particles.  It is very mobile and probably was depleted by the onslaught of heavy rains as it was washed into the soil below.  So what we hoped would fertilize our soil will now be used as mulch which will improve the soil structure but not fertilize our plants. 


Compost to Increase Fertility for your Spring Garden

As a result of our testing, we will now go back to the tried and true use of compost to solve our garden fertility problems.  Even though our straw compost was a faster way to start our horseradish growing again, by the spring, the farm’s compost had proven to be just as effective even with the slower start. 

The nitrogen in compost is in a form that is not available to plant roots.  But it is slowly converted over time to a form that can be used by the plant.  Now is the time to apply compost to your winter garden plants and the also the parts of the garden that will be left fallow.  Also, adding a mulch like straw, for example, on top of the compost will help to conserve soil moisture just like it does in the summer. 

Let’s hope that we get more storms this winter but with the compost’s slow process of converting organic nitrogen to a usable nutrient, your garden will be all ready to go come spring.  As we Master Gardeners love to say, "compost, compost, compost"!