By Bev Johnson, Otter Tail County Master Gardner
COMPOST DOESN’T REALLY “FEED’” THE SOIL
Then why the heck is Bunkey out turning over his overgrown compost pile? One of the first things a new gardener is told to do is start a compost pile. It’s good for your soil they are told. O.K., how good? Actually, the fertility of compost is quite low and very variable depending on what you fed it. In general, compost contains about 1 to 4 percent nitrogen and about the same amounts of potassium and phosphorus leaving you with the ratios of about 2-1-2. This is similar to NPK commercial fertilizers. The difference is that compost is very slow releasing.
A farmer may add about 150 pounds of commercial fertilizer per acre of corn. An organic farmer would add 10 to 20 tons of compost and that wouldn’t even totally cover all the soil. So why compost? It’s extra mass is a case of more equaling more. Compost is full of carbon microbes and fungi. The carbon provides food for the microbes living in both the soil and the compost. These now well fed microbes rev up the nitrogen cycling in the soil converting it from organic to the inorganic forms that are more readily used by plants. This also happens when you apply synthetic fertilizer, but the synthetic stuff can give the plants more than they can use. Since Nitrogen is water soluble, the excess washes down through the soil and can cause polluting problems. Biological conversion, in contrast happens quite slowly giving the plants a slow release and more consistent food throughout the growing season. Also, the carbon material in compost is a sort of sponge sopping up moisture and any unused N that is dissolved keeping it in the root zone where the plants can get at it. Compost helps shield crops from drought and nutrient stress longer and better than their synthetic brothers do.
And then the fungus. All plants use fungi. If you dig a plant or tree, be sure to take as much soil with the roots as you can. The plant has a colony of fungus specific to that plant creating a symbiotic relationship. The fungi take sugars from the roots and in return, send out hyphae extending the surface of the root. The more root the more food they can scavenge from the soil, particularly phosphorus that aids in photosynthesis. So, you say, what has this to do with compost? Compost is full of fungi. The active and well-fed fungus found in compost helps hold the soil components together and helps prevent eroding in a heavy rain. Fungi produce hyphae and carbon rich exudates that bind soil particles together into small clumps called aggregates. These clumps keep essential soil particles like carbon and minerals in place and available to plants roots. It also creates spaces between the clumps called pores that hold water and air close to the plants roots where it is needed.
Compost is the ultimate recycling machine. It takes your coffee grounds, vegetable skins, bad fruit, and all the stuff you sweep up that you, the kids, and the dog bring in the house every day and turns it into just picked peas, warm from the garden tomatoes and the best tasting corn of the summer. What a deal!! Sure, it takes a bit of energy to turn over and when finished, haul to the garden, but you need the exercise anyway to keep that schoolgirl/boy figure after eating all that corn on the cob. Thus ends this lesson.