By Kim Robson:
Gardening is a never-ending learning curve. Every year I learn something new, mostly through trial and error. We’ve all heard of adding coffee grounds, crushed eggshells, Epsom salts, and more to supplement soil. But why? And are those supplements appropriate for your soil and your plantings? It could actually be a waste of time, money and effort; and it could even harm your crops.
Testing Your Soil
Who knew? It’s kind of the 1st rule of gardening. Soil can be tricky. It’s all about balance. Different groups of plantings will have different needs. Whether or not you use compost will change the soil’s needs, too. So, if you have fruit trees, berry brambles, perennials, a lawn, a vegetable garden, or raised beds, their soil should all be tested separately.
You may have noticed three numbers on the labels of most packaged fertilizers, like 15-30-15, for instance. Those numbers are important, and different fertilizers will have different formulations for different uses. They correspond to the letters N, P, and K, which are the chemical designations for Nitrogen, Phosphorus, and Potassium. The NPK ratio of your fertilizer can make or break your crops.
Nitrogen (N) makes your plants grow. According to the Division of Agriculture and Research Extension at the University of Arkansas, “Nitrogen is normally the most limiting nutrient for optimum plant growth. Soil tests that estimate soil N availability are not currently used because soil N exists in many forms which may change with time and influence plant availability.” P.S. Corn gluten has a lot of nitrogen, making it great for combating crabgrass.
Phosphorous (P) helps the plant harness the energy from photosynthesis to drive its metabolism. A phosphorus deficiency can lead to impaired vegetative growth, weak root systems, poor fruit and seed quality, and low yield. Excess phosphorous can lead to algae blooms in the nearby lakes and streams. According to University of Massachusetts at Amherst, “Excessive soil phosphorus levels are a concern due to the potential negative impact on surface water quality. Most phosphorus losses occur with runoff, but where soil levels are extremely high, subsurface losses can occur. Phosphorus enrichment is a leading source of water quality impairment of many lakes, streams, and rivers in New England.”
Potassium (K) ions help the plant utilize the nitrogen and water. If the soil doesn’t have enough potassium, then the plant will be more susceptible to disease. The slow release of potassium from native soil can replenish some of the potassium lost by growing crops, but it’s limited and variable. Fertilization often is necessary to maintain optimum yields. In 2005, about 93% of world potassium production was consumed by the fertilizer industry.
Of course, NPK aren’t the only nutrients your plants need. There are scores of micro-nutrients like calcium, magnesium, sulfur, and others.
pH is the measure of the acidity or alkalinity in a solution. A pH of 7.0 is neutral. Most vegetables like a pH between 5.8 to 6.5. Blueberries like a much lower pH than other fruit-bearing bushes at 5.5. The Division of Agriculture and Research Extension at the University of Arkansas states, “Roses, turf-grasses, fruits, and nuts like a lower pH of 5.5 to 5.8. Based upon your results, you will need to add lime to increase the pH or elemental sulfur to decrease the pH.” Applying a leaf-based compost will increase your pH over time.
How Do I Test My Soil?
The best time to test your soil is in the fall. That way, you can prepare your beds ahead of spring planting time. The easiest way is to get a soil test kit from your local nursery. Very common is the Rapitest kit, which tests for nitrogen, phosphorus, potassium, and pH. It’s simple, inexpensive, and easy to use.
For more detailed results, you can buy a test kit online that gets sent to a lab for chemical analysis. Michigan State University has a great soil test program.
This short video shows the best method for taking a soil sample. You should take soil from different areas of the planting bed you are testing, then combine all that soil into one sample. It’s also a good idea to take soil tests every few years, as soil conditions can change.
As for interpreting your results into useful information, I recommend this page from University of Massachusetts at Amherst’s website.