Plants need numerous elements to grow. Generally, all but 3 of these elements exist in the air and the soil in quantities sufficient to sustain plant growth. The other 3 elements, namely, nitrogen, phosphorous, and potassium are consumed by the plants in big quantities and, therefore, should be added to the soil in the form of fertilizers.
Organic fertilizers include animal and poultry manure, green manure, bone meal, granite dust, phosphate rock, and wood ashes.
1. Manure: Manure (cow, horse, pig, rabbit, chicken, etc.) is the main organic fertilizer. It is sold in different forms: raw, dehydrated, and composted. Raw manure is messy and smelly and, therefore, does not appeal to many gardeners. It also attracts flies and other insects. Furthermore, raw manure often contains a considerable amount of undigested weed seeds. These seeds germinate and grow weeds fast because they are in a nitrogen-rich environment. Controlling these weeds can be a difficult and time-consuming task.
Fresh raw manure should not be allowed to get in contact with seeds, because it inhibits their germination. Instead, it should be worked into the soil and watered frequently for a week before sowing the seeds. The advantage of raw manure is that it is cheap. Dehydrated and composted manures are more convenient to use because they are less smelly. The price of dehydrated manure is considerably higher than that of composted manure. The nutritive value of dehydrated and composted manure is written on the bag. Dehydrated manure should be watered several times before it is allowed to get in contact with the seeds. Composted manure can be applied safely at any time.
2. Green Manure: Green manure is a crop that is grown specifically for the purpose of plowing it under in order to add nutrients and organic matter to the soil. Some of the green manure crops are alfalfa, clover, red clover, buckwheat, Sudan grass, and oats. The former 3 crops are legumes. Their roots add nitrogen to the soil, but they take longer to grow.
Green manure crops may be planted in the summer before fall planting or in the late fall before spring planting. At least 3 weeks must pass between plowing a green manure crop under and planting a crop in its place. Green manure crops are not suited for areas that freeze in winter.
3. Bone Meal: Bone meal is crushed animal bones and, as such, it is an organic fertilizer. It contains little nitrogen and potassium but is rich in phosphorous. Bone meal is used as a starter fertilizer because its high phosphorous content promotes the growth of plant roots. Its advantage is that it does not burn the seeds. Its disadvantage is that it is expensive. Adding bone meal to the potting soil for containers improves the growth of vegetables and herbs.
4. Granite Dust: Granite dust contains 4% potassium and is devoid of nitrogen and phosphorous. It takes years for granite dust to release its nutrient to the plants.
5. Phosphate Rock: As the name implies, phosphate rock is very rich in phosphorous but devoid of nitrogen and potassium. It takes years for phosphate rock to release its nutrients to the plants.
6. Wood Ashes: Wood ashes are an excellent source of potassium. They also have a considerable amount of minerals, especially phosphorous. Wood ashes are devoid of nitrogen because burning the wood evaporates all the nitrogen they contain. Their nutrient content varies according to the wood. Wood ashes are alkaline and therefore can be added to reduce the soil's acidity.
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