How can you tell if your soil is providing the right nutrients for your plants? Plants can suffer all kinds of problems and even die if they aren't in fertile enough soil. Different species have different requirement, but across the board there are two critically important factors anyone can track (primary macronutrients and pH), and two other factors that need to be measured professionally, but are less likely to cause you problems (other nutrients and cation exchange capacity).
NPK - The Holy Trinity of Plant Nutrients
Nitrogen (N), Phosphorus (P), and Potassium (K) are the three primary macronutrients plants receive from the soil. These three nutrients aren't more important to plants than the other nutrients we'll discuss later, but they are needed in much larger quantities. Nitrogen is used in amino acids, chlorophyll, DNA, RNA, and many more critical molecules. Phosphorus used in DNA, RNA, cell membranes, protein modification, and cell signalling. Potassium is critical in water regulation, cell signalling, and complex chemical building.
N, P, and K are also the three nutrients fertilizer manufacturers are required to measure and report on their packaging. If you look at a fertilizer label you will see somewhere on the front three numbers listed with dashes - 22-0-4 in the bag pictured on the right - and these numbers will tell you what percentage of the product is nitrogen, phosphorus, and potassium, in that order.
Before applying a fertilizer to your soil, you need to test to see what nutrient levels you already have, and what you need to add. There are home soil testing kits you can pick up at garden centers and hardware stores to test N, P, and K content on your own. These are generally pretty accurate as long as you use them properly, but professional soil testing is not terribly expensive. If you are testing your soil for the first time, you can learn a lot by getting a full professional test done. I'll give you more details on that later. Fixing deficiencies in the big 3 is as easy as applying a fertilizer that contains the nutrient you're low on. In many states, there are legal restrictions on applying phosphorus fertilizers since run-off can lead to algae blooms in our waterways. Fortunately, we rarely have problems with low phosphorus levels in Indiana. Don't add it unless you know your soil needs it.
pH - Soil Acidity Changes Nutrient Availability
Your soil can have the perfect balance of nutrients, but if the pH is out of a plant's optimal range the plant's roots can't absorb those nutrients. Most plants are happiest with a soil pH that is neutral or slightly acidic. This can be a problem here in central Indiana where our soil is often alkaline because of its limestone bedrock. The chart on the left illustrates how different nutrients are more or less available to plants depending on soil pH.
When your soil is too alkaline, you will often see problems with low iron and manganese. Deficiency in either one disrupts a plant's ability to build chlorophyll, so you end up with yellowing between plant veins. Not all plants will be equally affected. Anything native to your region will probably be fine (unless it naturally grows in a specific environment with an unusual soil type for your region). Many non-native plants will also do just fine. This chart gives information for dozens of trees, shrubs, perennials, herbs, and vegetables.
Soil pH can be measured using home test kits (pH strips are often included with kits that measure N, P, and K). If your soil is too alkaline (pH above 7), you have a few options for acidifying it. You can incorporate rich compost annually either as a top dressing or actually till it into the soil (never do extensive tilling under a tree). This will also help improve another soil quality called cation exchange capacity (CEC) which influences the soil's ability to keep nutrients in place rather than having them leached out into the groundwater. Learn a little more about CEC (and a lot more about soil in general) by checking out our post on soil chemistry and biology.
Another option is routinely applying aluminum sulfate, which can be found in garden centers. If you want to acidify just one area - for example if you absolutely must grow rhododendrons or azaleas, or if you want blue flowers on your hydrangeas - there are options to quickly reduce pH. Holly-tone is one popular acidifying product, and aluminum sulfate can work here too. Some people swear by dumping their daily coffee grounds around plants that need acidic soil.
Micronutrients - Quantity Isn't Everything
Micronutrients (and the secondary macronutrients) are also critical to plant health, but we usually don't monitor them closely. There are three secondary macronutrients (magnesium, sulfur, and calcium) needed in significant amounts, but not as much as nitrogen, phosphorus, and potassium. There are nine micronutrients considered essential for all plants that are only needed in very small amounts: boron, copper, zinc, iron, chlorine, manganese, molybdenum, nickel, and cobalt. These elements contribute to plant health in a wide range of capacities. They strengthen cell walls, power photosynthesis, build proteins, signal plant stress responses, and so much more.
When we do see deficiencies in these nutrients, it's often a symptom of pH problems that keep plants from taking the nutrients in rather than a problem of the nutrients not being present. The best way to deal with this is to pick plants that are adapted to your soil pH, or you can refer to some of the remediation suggestions in the previous section.
If you do need to increase micronutrient levels, your best bet is to pick a general fertilizer that includes micronutrients. Many already do, and they should be listed on the label. This information may be in smaller print than the NPK amounts, but if a company has included micronutrients they will want you to know.
Soil Tests - Knowledge Is Power
Before you add any fertilizers or amendments to your soil, you really should take the time to test it yourself or have it professionally evaluated. This will help your plants by alerting you to problems you may not have known about, and it will help your wallet by ensuring that you spend money only on the products you need. As I mentioned earlier, most garden centers will sell test strips to check for nitrogen, phosphorus, potassium, and pH. If this is your first time taking a close look at your soil, or if there is one spot in your yard where everything seems to die for no reason you can identify, it's a good idea to send in soil samples to a lab. It only costs about $10 per sample for most tests, so it's not an expensive process. To begin with, you'll probably want to have a few samples from different areas of the yard tested, especially if you live in a recently built subdivision. If you are dealing with a problem spot, you just need a sample from that one area. Check out this Purdue extension bulletin for details on properly preparing a soil sample for analysis (some labs may provide their own instructions, so follow those if provided). Click here for a list of labs in different states. Allow a few weeks to get your results back, unless otherwise noted on the lab website.