Plants can be harmed not only by not having enough but also by having too much of a particular nutrient. For this reason, soil tests are often conducted to determine how much of a particular nutrient is needed, according to the University of Tennessee. While testing facilities often charge fees, the fees are usually worth it, since gardeners can spend even more money on fertilizer if they purchase too much or the wrong nutrient combination.
Plants need the right amount of nutrients and cannot thrive with excessive or deficient nutrients. Plants that do not receive enough nitrogen have thin stems and their old leaves will die, while plants that get too much nitrogen are weak and susceptible to pests. Plants that do not have enough phosphorus have a difficult time forming flowers and fruit, according to NASA. Plants that don't get enough potassium develop leaves that appear burned and cannot survive the winter.
Soil testing and analysis is usually done at a laboratory that specializes in these kinds of tests. Some colleges and most municipalities conduct soil test and analysis, according to the University of Tennessee. The grower merely needs to send samples of the soil to be tested. Different laboratories have different methods of testing soil, so they might give somewhat different results. Some growers send samples to more than one laboratory to get multiple opinions, according to Colorado State University. The results of a soil test are usually printed and mailed to the address of the grower with recommendations on what should be done to the soil to improve it, according to the University of Tennessee. Oftentimes, the grower is told exactly how many pounds of a given nutrient need to be added to the soil.
Calcium is sometimes tested for whenever tomatoes and peppers suffer blossom end rot. Tomatoes, cabbages and grapes sometimes need a magnesium soil test, according to the University of Tennessee. When lime is applied to the soil of corn and snap beans, a zinc test is sometimes needed. Soybeans sometimes need a manganese test in areas with a pH above 7.
Soil testing and analysis is mostly important in the areas of nitrogen, potassium phosphorus and lime testing, since these are the nutrients that are most likely to be deficient in the soil, according to the University of Tennessee. Soil tests for these nutrients should be performed annually.
Soil tests are conducted by adding a solution to the soil that extracts a particular nutrient in the same amount in which the plants extract the nutrient. How much the solution ends up extracting determines how much of the nutrient is in the soil and whether or not a plant will be able to extract sufficient amounts of this nutrient to thrive, according to Colorado State University.