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Garden Soil Information
A discussion of the following topics- What is soil?
- What do plants take out of the soil?
- How do plants move nutrients from soil into their roots?
- How plants get enough nutrients from the soil to grow well
- Identifying and preventing nutrient deficiencies in plants
- Fertilizers: what they are; why to use them; synthetic vs. natural fertilizers
- What tests are commonly done on soils to determine if they are “good” for gardening?
- What do soil tests say about the soils in Eagle Heights and University Houses Gardens?
- Formal descriptions of the soils in Eagle Heights and University Houses Gardens
Summary
- Soil is about half empty space and most of the rest was originally rock. Only about 2-5% is "organic," meaning that it is composed of decomposing plants and animals. Both the rock portion and the organic portion provide needed nutrients to plants. Most soils that are "good" for gardening can provide the nutrients for many years any fertilization.
- Plants need many elements from the soil. Nitrogen (N), phosphorus (P), and potassium (K) are needed in the largest amounts by your garden plants.
- Fertilizers provide substantial amounts of N, P, and/or K. They are typically sold on the basis of their N, P, and/or K content, with a 20-10-15 fertilizer containing approximately 20 kg of N, 10 kg of P, and 15 kg of K per 100 kg of fertilizer.
- Leaves, compost, and composted manure contain too little N, P, and K to be considered fertilizers.
- Fertilizers used at Eagle Heights Gardens should be "organic," meaning that they must be made from animal or plant products (bone and blood meal, cottonseed meal, manure, etc.). Fertilizers used at University Houses Gardens can be either organic or "conventional" -- conventional fertilizers are not made from animal or plant products and usually have higher N, P, and/or K content than organic fertilizers.
- In most cases, it is difficult to know just by visual inspection of your plants what nutrient levels are like in your soil. If you wish, you can have soil tests done on your plot for a small fee.
- For historical reasons, Eagle Heights and University Houses garden plots have high levels of potassium. Of the major three nutrients plants need, you should only fertilize with nitrogen and potassium on a regular basis.
- For plants like tomatoes, fertilizer can be placed in each hole at planting time, scratched into the soil around the base of the plant after the plant has grown for a couple of weeks, or both. For crops like lettuce, work fertilizer into the top few cm of soil in the entire bed where the crop will be planted.
- See the Garden Manual for more detailed recommendations on how to take care of your soil and fertilize particular crops.