When Compost is the Fertilizer

Years ago, I somehow got the idea that a product wasn’t a fertilizer unless it had a guarantee of its N-P-K ratio (nitrogen – phosphorus – potassium) on the label.  This idea totally contradicted my personal experience of growing vegetables in mounds of nearly pure compost.

An early experience with “composted humus and manure”

Back in the early 1990s, when Joe and I first got serious about gardening in Georgia, the red clay in our yard was a major obstacle to success. I had been reading about gardening — mostly information from up North — that said: if you can grow radishes, you can grow tomatoes.

The problem, of course, was that we couldn’t grow radishes. We added our own yard-compost to the garden, which included bags of leaves and grass clippings from our neighbors’ yards, to make more compost. However, improving the garden-soil/clay to a more loamy texture was slow.

At the time, we had small children and a tight budget. Buying enough amendments to improve the whole garden in one season was not really an option.

About the third year (could have been the fourth), I bought some commercial “composted humus and manure”, in bags, at a garden center. There was no N-P-K information on the label. The stuff was stinky, which means it was not completely composted, and it contained some large, suspicious-looking lumps (wads of cow poo?), but it was cheap.

My method was unorthodox, I am sure: I dug shallow holes, like saucers, a foot-and-a-half or more wide, in the garden, then dumped on about a half a bag of the “composted humus and manure” per hole for big plants like tomatoes and squashes, and about a third of a bag per hole for smaller plants like peppers and cucumbers. Luckily, the garden at that time was small, so not too many bags were involved.

The holes were shallow enough that the composted stuff made low mounds. I pushed my spading fork through each mound a few times, to break up the interface between the compost and that red clay, then, I planted right into those stinky mounds.

Our plants grew, and they made plenty of vegetables. Around the middle of July, I dumped about a quarter bag of the same “composted humus and manure” around each plant as a side-dressing to provide additional nutrients, to keep that productivity going.

At the time, buying the compost meant that additional fertilizer didn’t fit in our budget. All the nutrients for plants in the garden came from the yard-compost, which was distributed throughout the whole garden, and the mounds of composted humus and manure.

What counts as fertilizer?

The definitions of fertilizer vary, but there is general agreement that a fertilizer contains elements that improve plant growth and productivity; they are given to plants to help them grow.

I usually think of fertilizers as being products that are concentrated sources of plant nutrients. Examples are sul-po-mag for potassium or blood meal for nitrogen.  You can buy these in little bags or boxes, and a cup or two in a small garden can be enough to support good plant growth, if potassium and nitrogen are in too-low amounts in the soil.

Composts, though, are typically added by the wheelbarrow-load, instead of being measured in cups. They do not seem to be concentrated sources of nutrients.

Nutrients in compost

Information from the University of Massachusetts says that most finished composts have an N-P-K ratio of about 1-1-1.  The numbers tell the percent of that nutrient in the product, so a product that has an N-P-K of 1-1-1 contains 1% each of nitrogen, phosphorus, and potassium.

These are low amounts, considering that a standard chemical fertilizer recommended for home gardens is 10-10-10, which contains 10% each (ten times more!) of the three major nutrients.

In addition, Iowa State University’s article on the Do’s and Don’ts of Composting cautions that compost should not be considered a fertilizer, because the release of nutrients from compost is slow. Again, though, my experience was that the compost was enough.

One advantage of composts is that the nutrients N-P-K are not the only nutrients they provide. Since composts are made of broken-down plants, they also can contain magnesium, zinc, boron, iron, and other essential plant nutrients — that the original plants were made of — that support plant growth.

Is compost safe?

Safety for people

Composts made from animal manures, if they are not completely composted, can contain pathogens that are dangerous to humans, like E. coli and Listeria.  Most home-made composts will not contain manures, which lets most of us cross that off our lists of potential hazards.

However, home compost piles will contain a wide range of bacteria and fungi that can cause allergic reactions in some people. Cornell University’s Waste Management Institute recommends that susceptible individuals wear dust masks when turning the compost on dry/windy days.

Safety for the garden

Composts made with bio-solids (treated sewage sludge) can contain more heavy metals than are safe, especially when compost is used in very large amounts (from report posted on Cornell University website). Home-gardeners who are filling raised beds with purchased, all-compost mixes may want to check the origins of the compost, to see if bio-solids are in it.

Composts can also contain fairly high amounts of soluble salts (See the U. Mass article on compost use and soil fertility). This is not normally a problem in gardens and fields that receive adequate rainfall, but in greenhouses or high tunnels where the soil doesn’t get good, drenching rainfall, the salts can be a problem. Mushroom compost, especially, can be high in soluble salts (see table in Oregon State University’s pdf about organic fertilizers). 

If your garden has received many loads of compost, but productivity seems to be dropping, consider having the soluble salts level in the soil checked through your local Cooperative Extension office.

Interesting bits I learned while looking for good resources about “compost as fertilizer”

1. The odor/aroma of finished compost, like a damp log that is decaying in the forest, is caused by the action of actinomycetes, which are a kind of beneficial bacteria. (From a chapter summary on this webpage from ScienceDirect.)
2. Lettuce, spinach, and celery are three crops that can accumulate heavy metals in higher amounts than other crops.  (From a Cornell University paper about compost guidelines.)
3. In some places in the U.S., composting food scraps aboveground is not allowed. (University of Maryland article about how and why to make compost.) 
4. Compost is known to suppress some plant diseases, but if nutrient levels are high, the ability of compost to hold down diseases is reduced. (Chapter summary from ScienceDirect.)

Why am I thinking about this now?

I have a bag of Soil3, a cubic yard of compost, that I’ve been using to amend my garden with this summer. It reminded me of the olden days, when I grew some of my crops in little mounds of compost. The good news is that the stuff in my pile of Soil3 is not at all stinky!