Beyond Soil Tests

Though soil tests are useful for identifying nutrient deficiencies as well as soil pH, they do not tell the whole story. In addition to issues related to water and sunshine here are some factors that can affect plant growth that are not due to nutrient deficiencies or pH.

• Too much phosphorus: Many soils that have been fertilized for a number of years may have phosphorus levels that are quite high. Too much phosphorus can interfere with the uptake of some micronutrients such as iron, manganese and zinc. High phosphorus soils should only be fertilized with fertilizers that have relatively low amounts of phosphorus.
  
  
• Poor soil physical characteristics: Roots need oxygen as much as they need water. A tight clay soil can restrict soil oxygen levels as well as make root penetration of the soil difficult. Increasing the organic matter content of clay soils can help break them up. Add a 2-inch layer of organic matter and till it in.
• Walnut trees: Walnuts give off a natural herbicide that interferes with the growth of some plants such as tomatoes. Vegetable gardens should be at least 50 feet away from walnut trees if possible.
  
  
• Tree roots: Trees not only compete with other plants for sun but also for water and nutrients. Extra water and nutrients may be needed.
  
  
• Shallow soils: When new homes are built, the topsoil is often stripped off before the soils are brought to grade. Though the topsoil should be replaced, it sometimes is not or is not replaced to the same depth as it was originally. You are left with a subsoil that usually does not allow plants to grow well due to a lack of soil structure. Adding topsoil to a depth of 8 to 12 inches would be best but this often is not practical. In such cases, try to rebuild structure by adding organic matter and working it into the soil.