Organic fertilizers are all the rage these days, and with good reason. They can provide plants with a long, slow feed, and their manufacture reuses waste products while putting less demand on natural resources.
Many gardeners, though, make the mistake of approaching the use of organic fertilizers in the same way as they might use a chemical fertilizer.
For instance, the other day a gardening expert on the radio was touting the benefits of guano, or bat droppings. He was right about guano being rich in nutrients. It has almost 20 percent nitrogen in a form that can be taken up quickly by plants. And guano surely is natural: It's merely scooped out of caves where bats hang out, then bagged up for sale.
The problem is that guano is not much different in its effects on plants and soil from any quick-acting, chemical fertilizer. The same could be said for blood meal, poultry manure, and other concentrated, quick-acting — albeit organic — fertilizers.
Bulk up. The thing missing from all chemical fertilizers and from concentrated organic fertilizers is bulky organic matter. Yes, plants benefit from bulk in their diet, just as we do.
Like the bulk in our diet, bulk's benefits to plants are indirect. These benefits include getting soils to hold more air and water, making nutrients already in the soil more available to plants, and helping plants fight off certain diseases.
Bulky organic material that is good for plants is familiar stuff. Raw organic materials include straw, autumn leaves, sawdust, hay, and manures. When these raw, organic materials decompose, they become such things as compost, leaf mold, old manure, and humus — all of them somewhat richer in plant nutrients than the original organic materials that went into them.
Note that most of the bulk associated with manures comes from the sawdust, straw, or whatever other material was used for bedding for the animal. What comes out a chicken, for example, has little organic material and much of its benefit to plants comes from the wood shavings or straw that is scooped up along with the poop from the floor of the chicken house.
In fact, the more concentrated any organic fertilizer is, the less bulky organic material it offers.
Too much of a good thing. This is not to say that plant growth cannot suffer from the opposite extreme, that of applying too much raw bulk and not enough actual nutrients. Raw organic materials low in nutrients, such as sawdust and straw, can cause temporary nutrient deficiencies if mixed into the soil.
More concentrated fertilizers do have their uses. With houseplants, for example, which have no room in or on their pots for bulky organic materials. And quick-acting, concentrated fertilizers are needed for emergency feeding of neglected plants.
A balancing act. The lesson here is: Generally, don't seek out the most concentrated — the "richest" — organic fertilizer. If you do use concentrated fertilizers, also regularly enrich your soil with plenty of organic materials, either digging them into or just laying them on top of the ground.
An alternative to hauling all that bulky material into your garden is to grow it in place.
Set aside a different part of your garden each season, or part of a season, in which to grow so-called "cover crops." These are plants such as oats, rye, or buckwheat that you grow specifically to enrich the soil with organic matter.
A third alternative is to avoid any concentrated fertilizer and annually give your soil all the food and bulk needed at the same time by spreading on top of the ground an inch depth of compost or rotted manure. Make it a practice to fertilize with your pitchfork rather than your granular fertilizer spreader.
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