Tomatoes can be and are grown in all soil conditions -- but is there an optimum soil for tomatoes? Yes, actually there is! Researchers have discovered that for consistent, successful tomato growing, you do want to pay close attention to your tomato soil. It's important because good soil helps to grow healthier tomato plants -- and healthier plants lead to better disease and insect resistance, and better fruit.
I take the easy way out and mix my own soil for tomatoes in raised beds and containers. I take inspiration from Mel Bartholomew's best-selling gardening book, All New Square Foot Gardening, in which he recommends dispensing with the hard work of evaluating and amending your soil. Instead, he's a proponent of creating your own perfect mix of compost, peat moss and vermiculite that is guaranteed to be just right for your garden. I'll give out Mel's formula later in this article. The main drawback to creating your own mix is the expense and impracticality of creating your own soil for all but smallish raised beds or container gardening.
First, let's consider the basics. You'll need to plant your tomatoes in a spot that has good drainage, yet where rain water doesn't just run off -- as on a slope. Tomatoes don't like "wet feet". A good way to test your soil drainage is to dig a two-foot hole next to the plot. Fill the hole with water. If the water has drained away in under two hours you're probably OK. If not, your soil is too clayey and needs to be amended with more sand. Or, you should choose another garden location, build raised bed gardens, or grow tomatoes in containers.
Soil consists of three components -- sand, loam, and clay. Sand is rough, comparatively large particles that allow for good drainage and good air, but doesn't retain nutrients or water very well. Loamy soil is the best kind for growing and is fairly easy to work with. Roots can penetrate loamy soil and it drains well. Clayey soil is characterized by tiny particles that cause it to have poor drainage and air penetration.
Most gardeners will tell you that the best soil for tomatoes is sandy loam. This type of soil is a loam containing a lot of sand and just enough clay to hold it together. Compare your soil with this test: squeeze a handful of moist soil together in your hands -- it should just barely hold together without falling apart. If the soil holds together firmly this means it has good moisture retention. Moisture retention is important for minimizing blossom end rot in tomatoes. But if the soil holds together and can't be crumbled (isn't friable) the soil may be too clayey. To improve clayey soil, you'll want to mix in organic matter each year (more about that below). Finally, if you're uneasy about judging the quality of your soil, I'd advise you to get your soil tested at the county agricultural extension service or other lab.
The pH scale is used to characterize the acidity or alkalinity of your soil. When starting a new garden site, it's best to start with a soil test. Most soils are slightly acidic, that is, slightly less than pH 7, which is neutral. Very rainy areas, such as the Pacific Northwest where I live, tend to have very acidic soils, which is great for rhododendrons but isn't the best soil for tomatoes. Tomatoes grow best in soils that are just slightly acidic, from a pH of 5.5 to 6.8. Garden centers sell home pH testers that are adequate for determining your soil's pH level, if you don't want to send it out for testing. It's not difficult to modify your soil's pH level.
If your soil is too acidic (most common) you can apply agricultural or dolomitic lime, which is readily available where lawn fertilizers are sold. Follow the application recommendations on the bag. It's best to rectify overly acidic soil in stages, because overliming can be harmful to plants.
If your soil is too alkaline, there are mulches that can be used: oak leaf mold, composted bark, sawdust, and pine needles, to name a few. These can be applied on top of or blended into the soil.
All soil types will be improved by mixing in organic matter every year. It helps soil hold important plant nutrients. By adding organic matter to sandy soil, you improve the ability of the soil to retain water. In a clay soil, it will loosen the soil to make it more friable (crumbles easily). You can increase the organic matter in your garden by mulching, cover crops, or mixing in organic soil amendments.
Mulching is simply spreading a protective layer of a material on top of the soil. Mulches can either be organic -- such as leaves, grass clippings, straw, bark, and similar materials -- or inorganic -- such as stones, brick chips, and plastic. For long term soil improvement use organic mulches, which have numerous benefits. It retards weeds, helps retain moisture, and most importantly improves the soil condition as it slowly breaks down, loosening the soil and adding nutrients.
Cover crops (aka green manure) are called such because they protect the soil against erosion and improve soil fertility. They should be planted at the end of the tomato growing season then plowed under in the spring. Good cover crops include annual grass seeds, rye, buckwheat, oats, red clover, and vetch. There are a number of benefits of sowing cover crops:
Another way to improve your soil organically is to mix organic matter such as leaves, grass clippings and manure into the soil. Run leaves and other yard waste through your lawn mower to speed up the breakdown time. After a few years of mixing in organic material each fall your garden soil should become an excellent soil for tomatoes no matter how poorly it started out.
Admittedly improving your garden soil involves some arduous work. If you don't have the time or inclination, below are some possible alternative approaches that will still ensure great soil for tomatoes.
If you want to mix your own perfect tomato soil, follow Mel Bartholomew's simple recipe from All New Square Foot Gardening. As author of the best-selling gardening book of all time, Mel probably knows a thing or two.