Answer: I've included a variety of information below on improving the soil and fertilizing during the growing season. Vegetable crops are heavy "feeders" and no matter how fertile the soil, it must be continually improved to maintain it. Add compost or other organic matter, such as manure, straw, leaves, etc. You can also make your own compost out of organic matter.
Add a 2-3 inch layer of compost to your soil several weeks before planting to greatly improve it's fertility. If using organic matter that isn't broken down, try to incorporate it several months in advance.
As for the fruit trees, research now shows that it's better not to improve the backfill in the planting hole. Tree roots will expand about 1.5 to 4 times past the tree's canopy, so improving a tiny area around the trunk doesn't do much good. It may actually harm the tree, as the roots may wrap around themselves, staying within that area, rather than expanding. I'll also include some tree planting info below.
Continue to add lots of organic matter each year to garden beds, which over time will not only improve your soil's fertility and drainage, but will also increase it's ability to retain moisture and nutrients. It also provides food for earthworms and microorganisms that do the soil-building process. You can never add too much compost!
In sandy soils, compost improves soil fertility, water and nutrient retention. In clay soils, it improves drainage. Add a 4-6 inch layer of compost and incorporate it about 12-18 inches deep. Each planting season, add more compost. You may want to incorporate a balanced fertilizer (e.g., 10-10-10) or add organic fertilizers such as fish emulsion, bone meal, and seaweed/kelp before the initial planting. Follow package instructions. If you prefer organic fertilizers, you may need to use three different sources, since they seldom come mixed together the way non-organic fertilizers do.
Side dressings of fertilizer are often beneficial during the growing season, but you shouldn't have to fertilize as frequently as you water. Perhaps once every two weeks at most. As your soil fertility improves, this won't be needed. Examine your plants to see what might be deficient. Slow growth and/or yellowing leaves is often a sign of lack of nitrogen. No flowers or fruit set means phosphorous is missing. Always ensure that the soil is moist before fertilizing, and then water the fertilizer in well afterwards. This helps prevent "burn." If you use a granular fertilizer, scratch it into the soil at least 4 inches to the side of the plant to prevent burning roots.
Here's a little background on fertilizers for your info: You probably noticed that fertilizers have 3 numbers on the container. These numbers refer to the percentage of nitrogen (N), phosphorous (P), and potassium (K) in the fertilizer. These 3 elements are referred to as macronutrients because plants need them in fairly large (i.e., macro) amounts to thrive. How these elements interact is complicated but in general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers (and eventually fruit), and potassium keeps the root system healthy.
Here are some organic sources of nutrients:
Nitrogen: alfalfa meal, blood meal, coffee grounds, cottonseed meal, fish emulsion, seabird guano.
Phosphorous: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp
After planting, add a 1-2 inch layer of mulch. Mulch is great to help retain soil moisture, reduce weeds, and as it breaks down it provides nutrients to the soil. Any organic matter can be used as mulch. Try compost, bark, wood chips, straw, or pine needles.
Tree Planting Techniques
Trees develop shallow, spreading root systems in the top two feet of soil and have few deep or ?tap? roots. Till or loosen an area of soil that is five times as wide and only as deep as the tree?s root ball (or container size). Starting with a wide section of aerated soil provides roots with oxygen and allows them to spread easily.
In the center of this area, dig a planting hole that is twice as wide as the root ball and no deeper. The top of the root ball should be level with the ground?or just slightly above to allow for sinkage.
Do not amend the backfill with organic matter. In over 30 studies on trees, no advantage was found to incorporating amendments into the backfill. Ensure that the tree is securely upright but do not heavily tamp or pack the backfill, which compacts soil and impedes water and oxygen flow.
Form a circular berm, or rim, to make a water well on the outside of the root ball. The goal is to keep water away from the trunk to discourage disease.
Add a three- to five-inch-deep layer of mulch around the tree?s entire planting zone. Mulch conserves water by keeping soil temperatures cooler and reducing evaporation. Keep mulch about six inches away from the trunk to help prevent disease. Fertilizer isn?t needed for a tree?s first year.
Water the extended planting zone slowly and deeply. Soil should remain moist but not too wet during the first year of growth. Always water deeply (two feet) to encourage root growth and to flush salts below the roots? active growing area. Deep, infrequent irrigation is preferable. Frequent, shallow sprinklings do more harm than good. To determine how far water has penetrated, poke a soil probe (any long metal rod or screwdriver) into the soil. It will move easily through moist soil, stopping abruptly where soil is dry. As trees mature, expand the watering zone somewhat beyond the tree?s canopy (or dripline), which is where roots are actively growing.
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