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Aug 102012

Proper soil preparation provides the basis for good seed germination and growth of garden crops. The steps of soil preparation include testing, amending, and tilling.

Testing the Soil

Contact your local County Extension Office to get instructions and sample bags for testing your garden soil. Soil tests are especially critical in a new garden plot but soil should be analyzed at least once every 3 years because conditions do not remain static. The soil test will tell you how much organic matter and fertilizer is needed. To get a relibale soil test, you first need to take a proper sample.

Rows of vegetables growingAmending with Organic Matter

Organic matter improves soil structure, increases water-holding capacity, improves fertility, and reduces problems with soil diseases. There is no replacement for organic matter in improving soil health and providing good growing conditions for vegetables.

Organic matter can be added in the form of plant waste (such as leaves or lawn clippings), composts, green manures, or aged animal manures. It is necessary to consistently  add organic matter (every year or two) because it rapidly breaks down in the soil.

Using compost is a preferred method of adding organic matter to the soil. Apply organic matter to the garden area by spreading a layer of compost 3 to 6 inches deep on the soil surface and tilling to a depth of 10 to 12 inches, if possible with available equipment.

Another method of adding organic matter to soil is to use green manures, which are any living plant material that is mixed with the soil while still green. Green manure crops include such crops as, for example, wheat, oats, clover, mustard and vetch. Grow the green manure in the part of the garden where you will later chop it while it is still green and immediately till into the soil.

Animal manures are best applied in the fall so they have time to completely decompose and salts can partially leach out of the root zone before spring planting. It is best to use animal manures that have been aged for one or two years before applying to a garden, or used composted manure.

To make your own compost, see Composting at Home for a detailed discussion of how to do this.


In this brief format, it impossible to provide fertilizer application recommendations that will apply to every gardening situation. But, whether you prefer traditional or organic methods, some concepts are universal. For one, nothing can replace a soil test for providing the information required to make appropriate fertilizer application decisions. Two, for vegetable gardens, additions of nutrients in some form will be required to consistently grow a good crop. Knowing the amount of fertilizer elements required will make it easier to choose an appropriate product to apply.

Generally, for most Idaho soils, a fertilizer that is relatively high in nitrogen and phosphorus, contains a moderate amount of potassium, and possibly some sulfur will work reasonably well. Follow the fertilizer label directions or contact your local County Extension Office about application rates and methods. Also, UI bulletin, Using Soil Test Results for Garden Fertilization, contains information on interpreting a soil test and how to determine the quantity of fertilizer product to apply.

Fertilizer requirements for vegetable crops vary widely, which complicates fertilizer application decisions. Some crops, such as peas and beets need very little fertilizer. Most long-season crops, such as corn and melons require fairly large amounts (see accompanying table).

Low nutrient-using crops High nutrient-using crops
Bean Broccoli
Beet Cabbage
Carrot Cauliflower
Chard Celery
Lettuce Collard
Parsnip Corn
Peas Cucumber
Potato, early-harvested Eggplant
Radish Kohlrabi
Rutabaga Leek
Spinach Melon
Tomato, short-season areas Onion
Turnip Pepper
Potato, late-harvested
Tomato, long-season areas


Deciding how to fertilize a vegetable garden should take into account the fertilizer amounts needed by “low-nutrient using” crops vs. “high nutrient-using” ones. If you are planting a garden for the first time, it is almost essential to have a soil test so you know the nutrient status of the soil to help determine how much, if any, fertilizer to apply.

If you have been growing a garden for several years in the same location and have been reasonably satisfied with the production, a soil test may not be essential. However, a yearly application of fertilizer is still likely needed. Using a complete fertilizer product, fertilize your garden with 2 to 3 lb N per 1000 ft2 and immediately till it into the soil at the beginning of the season just prior to planting. This amount should be adequate for the low-nutrient using crops. Then after the plants are up and growing, apply an additional 2 to 3 lb N per 1000 ft2 alongside the row—oftentimes referred to as a “sidedress” application—and water into the soil. For this application, you can use a product that is high in N and low in other nutrients. Use UI bulletin, Using Soil Test Results for Garden Fertilization, to determine actual amounts of fertilizer product to apply depending on the nutrient content of the product purchased.

Sandy garden soils, which do not hold nutrients as well as clay, silt, clay loam, silt loam, and other non-sandy soils, require some modification of the above fertilizer application practices. Only part of the total applied fertilizer should be put down before planting. The rest should be applied in small increments during the growing season. This provides the plants with the required nutrients while reducing losses due to leaching. A soil test will help you determine your soil texture.


The final step in soil preparation is tillage. Ideally, soil should be tilled to a depth of at least 10 inches. This can be very difficult with small garden equipment. If larger equipment cannot be used, it may be beneficial to occasionally fracture the soil to a greater depth by pushing a long-tine garden fork deep into the ground and pulling the handle backward to break up the hard lower layers.

It is important to avoid working soil when it is too wet. This is espcecially true for non-sandy texture soil. Working wet soil breaks down the soil structure causing it to become cloddy and hard. The negative impact of tilling wet soil may last for years. To determine if the soil is dry enough to work (till), take a handful and squeeze it tightly into a ball. If the ball breaks into granular pieces when pressed lightly between your fingers, it is dry enough to work. If the ball remains intact and feels sticky when you squeeze it, wait a few days before scheduling tillage operations.

For more information about soils, soil preparation, and fertilizers, study the Idaho Master Gardener Handbook, Chapter 5: Soils and Fertilizers.

 August 10, 2012