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Organic SoilFertilizers add nutrients to the soil to encourage plant growth. They are composed of primarily three ingredients: nitrogen (N), phosphorous (P), and potassium/potash (K).  Nitrogen promotes leaf growth, phosphate stimulates root growth and flowering, and potassium is important to the overall plant vitality.

Organic fertilizers are derived from living things - examples include bone meal, fish meal and seaweed. Nutrients in organic fertilizer are released over time as microorganisms in the soil break them down. About half the nutrients in organic fertilizer will be available to plants the first season you use them. Synthetic fertilizers are an alternative, but if improperly used they can contribute to water pollution, kill beneficial bacteria needed in the soil, and weaken plants over time.

What to apply
Before applying fertilizer, perform a soil test to determine the levels of acidity and alkalinity in your soil. The results of the test will help you choose the best fertilizer suited for your soil.

In general, compost enriches and balances every soil - so if you use only one fertilizer, compost tends to be your universal best bet (and making your own compost is an environmentally-friendly practice as well as a wonderful exercise to do withe kids - visit the Composting section for procedures!).

You can buy organic fertilizers at your garden supply center. Ask for help selecting a product that fits your size and type of garden, and reflects your soil test results. You can also mix up your own fertilizer from materials you purchase from a garden center or feed store. Here is one organic fertilizer recipe adapted from Gardening for Dummies:

This recipe makes enough fertilizer for a 100 square foot garden. It yields the equivalent of 4-5 pounds of synthetic 5-10-10 fertilizer.

Materials:

  • A wheelbarrow - it will hold about two bushels
  • 2 1/2  bushels rotted manure (for nitrogen)
  • 1 quart of bone meal (for phosphorus)
  • 1 1/2 quarts of wood ash (for potassium)

Steps:
1.    Mix the ingredients together.
2.    Work the mix into the top 4 inches of soil, preferably in early spring before planting or in autumn. 

How to apply fertilizer
Measure out the amount recommended for the size of your garden, your soil type, and the type of plants you are growing.  Scatter evenly (broadcast) over the surface of raised beds, or in a band about 18 inches wide along the rows.  Then blend in using a hoe or spade.  If you are planting in hills, add extra fertilizer to each one.

You can also add fertilizer again as “side dressing” for your medium and high demand plants during their growing cycle.  About every four weeks, sprinkle a small amount of fertilizer onto the garden area where the plant roots are spreading into (away from the center of the plant).  Gently scratch the surface to mix in the fertilizer, being careful not to disturb the roots. Watch to see if the rate of plant growth increases after each application of side dressings.  If not, then the fertilizer wasn’t really needed, and you should stop adding fertilizer to those plants.

You may add fertilizer to the soil in early spring when you are preparing the soil for your garden. Plants also need fertilizer when they are at the peak of their growth and when they are producing flowers or fruits.  Additionally, you can put fertilizer on your garden in late fall for “dormant” feeding. This helps prepare the beds for planting in the following spring.

Not all vegetables have the same appetite - the following guide indicates the level of demand for nutrients among various vegetable plants.

Low-demand vegetables
Arugula (rocket), beans, beets, carrots, collard greens, most herb varieties, kale, parsnip, peas, Swiss chard, turnip greens

Medium-demand vegetables
Artichoke, basil, cilantro, broccoli, brussels sprouts (late), cabbage (large, late), celery, sweet corn, cucumbers, eggplant, garlic, lettuce, mustard greens (autumn), okra, parsley/root parsley, peppers (small-fruited), potatoes, pumpkin, radish (salad and winter), scallions, spinach (autumn), squash, tomatoes, turnips (autumn), watermelon, zucchini

High-demand vegetables
Asparagus, broccoli, brussels sprouts (early), Chinese cabbage, cabbage (small, early), cantaloupe/honeydew, cauliflower, celery/celeriac, Asian cucumbers,leeks, mustard greens (spring), bulbing onions, peppers (large-fruited), spinach (spring), turnips (spring)