Wednesday, May 21, 2014

Caution on Compost . . . Can It Be Too Much of a Good Thing? by Patti O'Neal

Amend, amend, and amend.  It is the mantra we all chant when managing our Colorado soils.  We here at CSU are constantly recommending that you add organic materials to your soils to improve water and nutrient holding capacity if you garden in sandy, gravelly or decomposed granite soils and to improve soil structure, drainage and filtration of water and nutrients in clay soils.  Improving the soil is still important for good plant growth and production of fruit and flowers.

But can you have too much of a good thing?  Much is being made, and justifiably so, of phosphates these days and their adverse effect on our groundwater supplies.  As a result many states are adopting laws to prevent the addition of phosphates to many products for household and outdoor use.
It is very likely that one day we will see the content of our fertilizers change as a result of these laws, and hopefully the labeling of our compost products so we know what we are getting. As with all gardening products, look for the OMRI seal.  The Organic Materials Review Institute is a non profit organization that certifies gardening products as being truly organic.  The exception is seeds, which become food, so are under the auspices of USDA and therefore will have the USDA Certified Organic symbol on them.

Photo University of Minnesota
 First of all, let’s clarify what landscape soils on the Front Range do, for the most part, contain. Though varied in textures, our soils have a good mineral and plant available nutrient content.  Plant available nitrogen and iron are the only properties that we lack sufficient quantities of.  With that said, let’s look at the results of a recent study performed at CSU through the Department of Horticulture and Landscape Architecture and the Soil, Water and Plant Testing Laboratory.  The study used randomly selected bagged compost products which were analyzed for plant based nutrients.  Here are some of the significant results of the study and recommendations for how we should use them.

NOTE:  The results of this study do not reflect either an endorsement or a condemnation of any particular product.

*Chemical properties of a given product may vary from bag to bag, both within a season and from year to year.  The analysis represents just a “snap shot” in time, and do not represent a definitive assessment of any particular product.
*Composted manure products tend to be very salty, and often contain high concentrations of plant available nutrients.
*Nearly all the products analyzed, both plant and animal based contained 2-15 times more plant-available phosphorous and 3 to 110 times more plant available potassium, than are needed for good plant growth.
*Plant available nitrogen contents were highly variable in the products analyzed.  Some contained nitrogen in nearly entirely organic form and contained little if any plant available nitrogen while others contained very high to excessive plant available nitrogen in ammonium and nitrate forms.

The moral of the story is that you may be doing something you believe to be wonderful for your soil, when in fact, you are not.  You may believe you are gardening completely organically, when in fact, you are not.

Oh great, you say?  So now what am I supposed to do?   The good news is that there are several things you can do.

*First, be conservative in the amount of organic amendment you apply.
No more than 2-3” of plant based compost mixed into the top 6-8” of soil or 1” of manure based compost worked into 8” of soil (especially if the salinity of the product is not known).
*Leach an organic product before use to help lower salinity.
*Use mulch as directed to conserve organic matter by lessening irrigation needed.  The more you conserve, the less you need to add in subsequent years.
*Try using green manure instead (cover crops like buckwheat, winter rye or vetch) to improve the organic matter in areas where it is practical to do so like the vegetable garden. 
*Make sure that there is a certified organic seal on any organic product that you purchase to insure that the contents are genuinely organic. 

Probably the single best thing you can do is to make your own homemade plant based compost.  This way you are assured of what goes in and what will come out.  Amendments are a good thing if used with discretion.  Until the labels are legally mandated, use caution and good sense for best results.