Twenty nutrients have been identified that are required by plants. Of these, nitrogen, phosphorus, and potassium are required in relatively large amounts. Nitrogen is associated with lush vegetative growth, adequate phosphorus is required for flowering and fruiting, and potassium is necessary for durability and disease resistance. Calcium, sulfur, and magnesium are also required in comparatively large quantities. These six nutrients are referred to as macronutrients.
The other nutrients, referred to as micronutrients, are required in very small amounts. These include such elements as copper, zinc, iron, and boron. While both macro and micronutrients are required for good plant growth, over-application can be as detrimental as a deficiency. Over-application of plant nutrients not only may impair plant growth, but may contaminate groundwater by leaching through the soil or pollute surface waters by washing away.
Soil testing Testing your soil for nutrients and pH is important to provide your plants with the proper balance of nutrients while avoiding over-application. If you are establishing a new lawn or landscaping, a soil test is strongly recommended. The cost of soil testing is minor in comparison to the cost of plant materials and labor. Correcting a problem before planting is much simpler and cheaper than afterward.
Once your yard is established, continue to take periodic soil samples. While many people routinely lime their lawns, this can result in raising the pH too high. However, since many fertilizers tend to lower the pH, the pH may drop below desirable levels after several years, depending on fertilization and other soil factors.
Home tests for pH, nitrogen, phosphorus, and potassium are available from garden centers. While these may give you a general idea of the nutrients in your soil, they are not as reliable as tests performed by the Cooperative Extension Service at land grant universities. University and other commercial testing services will provide more detail and you can request special tests for micronutrients if you suspect a problem. In addition to the analysis of nutrients in your soil, they often provide recommendations for the application of nutrients or on adjusting the pH.
The test for soil pH is very simple--pH is a measure of how acidic or alkaline your soil is. A pH of 7 is considered neutral. Below 7 is acidic and above 7 is alkaline. Since pH greatly influences plant nutrients, adjusting the pH will often correct a nutrient problem. At a high pH, several of the micronutrients become less available for plant uptake. Iron deficiency is a common problem even at a neutral pH on such plants as rhododendrons and blueberries. At very low pH, other micronutrients may be too available, resulting in a plant toxicity.
Phosphorus and potassium are tested regularly by commercial testing labs. While there are soil tests for nitrogen, these may be less reliable. Nitrogen is present in the soil in several forms and the forms can change rapidly. Therefore, a precise analysis of nitrogen is more difficult to obtain. Most university soil test labs do not routinely test for nitrogen. Home testing kits often contain a test for nitrogen which may give you a general idea of the presence of nitrogen, but again, due to the various transformations of nitrogen, the reading may not be reliable.