When someone tells me they have a black thumb, one of the first things I ask is what type of soil they have in their garden. Almost inevitably, they either give me a blank stare or a shrug. But for any garden to find success, you must have quality soil...and the best way to know how good (or bad) your soil is, is to run a test and amend accordingly. That is the key to a green thumb.

Why Testing is Important 

In order to thrive, plants require certain things, including irrigation, so many hours of sunlight...and the right nutrients and acidity in the soil. Soil is, after all, food for plants. Just as you wouldn't expect to thrive if you only ate junk food, so you can't expect plants to thrive when they don't have a nutritious "diet" from the soil. 

It's smart to test your soil before beginning a garden, and then perhaps every few years or so. If you notice growth or productivity problems with your plants, that also indicates a good time to test your soil.

The best time of year for testing is the fall, but it's acceptable to test in the winter (as long as your soil isn't frozen) or even in spring or summer. However, it takes time for soil amendments to do their work; the sooner you test, the sooner the amendments you add to the soil can do their thing - and the sooner you can have a thriving garden. 

This soil test shows a pH of 6.0 - acceptable for most gardening.

First, test the pH of your garden soil - how acidic or alkaline it is. If the pH is too high or too low, your plants will not be healthy. For example, potatoes grown in soil that's too alkaline tend to get scab and other diseases. And while potatoes do like slightly acidic conditions, if they are grown in soil that's too acidic, they simply don't thrive - and could even potentially die. 

A pH of 7.0 is considered neutral; 0 means the soil is highly acidic; 14 means it's highly alkaline. In general, food crops prefer soil that has a pH of 6.0 - 6.5, but a range of 6.0 - 7.5 is considered acceptable for most vegetables.

Other Soil Tests 

Testing the soil for basic nutrients, commonly referred to as NPK, is also important. "N" stands for nitrogen, which is the nutrient that makes plants grow rapidly, putting on many leaves. Lack of nitrogen in the soil results in plants that grow slowly, turn yellow, and drop leaves. Too much nitrogen in the soil causes over-rapid growth that results in weak, spindly shoots. 

Testing NPK.

"P" stands for phosphorus, which helps plants grow a healthy root system and is especially beneficial during blooming and seed setting periods. Too little phosphorus leads to purplish stems, dull green or yellow leaves, and potentially no blooms. Too much phosphorus reduces a plant's ability to use micronutrients (especially zinc and iron), which leads to poor growth and even plant death. 

"K" stands for potassium (sometimes called potash). It helps plants form chlorophyll and can aid in fighting disease. If soil lacks adequate levels of potassium, plants may appear generally sick, have small fruit, and/or older leaves that turn yellow. Too much potassium in the soil reduces a plant's ability to use other nutrients. 

How to Test Your Soil

There are two main was to test your garden soil's pH and NPK. One is to send samples off to a laboratory, which typically costs $40 - $100; you can find regional labs that will do garden soil tests through your local extension office. (Find your local extension office here.) Another method is to use a home testing kit. For about $25, you can buy test kits at local garden centers or online. Kits give you everything you need to test your soil - usually multiple times. 

Generally, professional laboratory testing is considered the most accurate, but for the average gardener, testing with a home kit is accurate enough, as long as you follow the directions. If you do choose to send soil samples off to a lab, be sure to gather them by following the lab's instructions to a T. 

The soil testing kit I use.

What to Do About Imbalances 

Whether you send your soil to a lab for testing or you use a DIY test kit, instructions should be included that will tell you how to amend your soil according to the test results. But here are some general guidelines. 

To make soil more acidic: Amend with sphagnum peat, iron sulfate, or elemental sulfur (a.k.a. "flowers of sulfur” or "micro-fine sulfur"). Do note that sulfur can kill beneficial microbes in the soil. After adding sulfur to the soil, re-test in 40 - 60 days. You may also wish to include the following, which will, if added over a period of time, increase acidity to the soil: pine needles, woodchips, and rotted leaves or leaf mold. 

To make soil more alkaline: Amend with garden lime; after adding it to the soil, re-test in 40 - 60 days. Over time, if periodically added, the following will also help make the soil more alkaline: bone meal, ground eggshells or clamshells, and small amounts of hardwood ashes. 

To increase nitrogen: Amend with alfalfa meal, blood meal, shellfish meal, or ammonium sulfate. 

To increase phosphate: Amend with bone meal, shellfish meal, or rock phosphate. 

To increase potassium: Amend with green sand, rock phosphate, or potash-magnesia ("Sul-Po-Mag"). 

Always check your soil test instructions for details on how much of any given amendment you should apply to your garden soil. You can add too much of a good thing! When re-testing soil after adding amendments, expect only small changes in pH - typically, 0.5 to 1 unit, tops. Wait 5 - 6 weeks before testing and, if needed, amending again. 

Optimal Soil pH for Some Common Edible Plants 

Apples 5.0 - 6.5 

Blackberry 5.0 - 6.0 

Blueberry 4.0 - 6.0 

Lemon 6.0 - 7.5 

Orange 6.0 - 7.5 

Peach 6.0 -7.0 

Pear 6.0 - 7.5 

Pecan 6.4 - 8.0 

Plum 6.0 - 8.0 

Raspberry (red) 5.5 - 7.0 

Asparagus 6.0 - 8.0 

Bean, pole 6.0 -7.5 

Beet 6.0 - 7.5 

Broccoli 6.0 - 7.0 

Brussels sprouts 6.0 - 7.5 

Cabbage 6.0 - 7.0 

Carrot 5.5 - 7.0 

Cauliflower 5.5 - 7.5 

Celery 5.8 - 7.0 

Chives 6.0 - 7.0 

Cucumbers 5.5 - 7.0 

Garlic 5.5 - 8.0 

Kale 6.0 - 7.5 

Lettuce 6.0 - 7.0 

Pea, sweet 6.0 - 7.5 

Pepper, sweet 5.5 - 7.0 

Potatoes 4.8 - 6.5 

Pumpkins 5.5 - 7.5 

Radishes 6.0 - 7.0 

Spinach 6.0 - 7.5 

Tomato 5.5 - 7.5 

Clay, Sandy, or Loam? 

In addition to running a pH and NPK test on your soil, you should understand whether your garden soil is clay, sandy, or loam. 

Clay soil is made of tiny, densely packed particles. It's not ideal for gardening because it won't allow water to drain well and may prohibit plants from spreading their roots. Sandy soil has, of course, lots of sand in it. This can be beneficial, except that pure sand has no nutrients and, since water drains away quite quickly in sandy soil, plants may not get enough to drink. Loamy soil is a mixture of silt (particles that are between the size of sand and clay), sand, and clay. It's ideal for gardening since it retains the right amount of moisture and nutrients for plants. 

A simple test for discovering the makeup of your soil is to sprinkle water on the ground, making the soil moist, but not wet. Scoop up a handful, squeeze it, and open your hand. Does it crumble when gently poked? Then the soil is loamy. Does the soil retain its squeezed shape even after a gentle poke? It is clay. Does the soil crumble the moment you open your hand? It is sandy.

Or, dig a hole one foot deep and about 6 inch wide and fill it with water. Allow the water to completely drain. Fill the hole with water again, but this time, pay attention to how long it takes for the water to completely drain from the hole. Well-draining soil drains 1 or 2 inches of water per hour. If the soil drains more slowly, it either has rocks blocking water drainage or is high in clay. If the latter is the case, work compost and other organic matter into the soil. If the soil drains more quickly than an inch an hour, it's too sandy and adding organic matter will also help.