Though we are hydroponic grow enthusiasts, most of us also have gardens and landscapes around our home. Often, many of these plants struggle to thrive or grow vigorously. A motto I like to use when giving outside garden advice is “make a $100 planting hole for a $10 plant.” This attempts to put the focus for landscape plants on the soil and root system where it needs to be. In hydroponics we provide only the best grow media for our crops. Outdoor growing is almost always done in the soil or dirt that happens to come with our yard.

Soil is comprised of inorganic particles and categorized into three different sizes; clay, silt and sand. Of course, there are also other things that comprise soil: rocks, organic matter, water, air, and sometimes things that really don’t belong there. A good soil typically fits into the loam category (see soil pyramid chart) and also has a reasonable per cent of organic matter likecompost. This type of soil is friable, meaning it breaks up easily, and lends itself to growing root system expansion and health. A good soil does not have too much clay, nor does it have too much sand.

Fig. 1 (Above) — The soil particle known as clay is smaller than 0.002 mm in diameter. Silt is next in size at between 0.002 and 0.05 mm. Sand particles then go up to 2.0 mm in diameter. Clay particles, because they are so small, tend to prevent drainage and nutrient movement. A soil with a high per cent of clay will feel sticky and the shape will be moldable just like clay you would buy for making pottery.

Soil Composition

Just what the composition, or particle makeup is of your soil is a very important thing to know. There is a simple “jar test” you can do to find this out.

Take a clear plastic or glass jar, fill the jar a little more than halfway with distilled water, then pour in your sifted soil sample. You will want to remove the rocks first. Place and secure the lid, then shake thoroughly. When you set the jar down, time for 20 seconds and mark a line where the sediment has laid. This will be sand level and the larger sand particles will settle within this amount of time. Then at 1.5 minutes make another mark as this will represent the silt line. After 10 to 30 minutes the sample should have cleared, mark where the sediment is then, as this is the clay line. You will not be able to visibly see the lines demarking the various soil particle separations as in reality the size differences are very gradual.

After measuring the height of each line, you can then either calculate the percentage of each of these particles and find that combination on the soil pyramid chart, or click on the QR code at the end of this article and use the link there to access the jar test calculator.

If your soil is loam or silt loam the amount of amendments needed is not too great. Sandy soil can be improved with water-holding amendments, but high-clay soil will only tend to remain hard and poor draining. It is often best to grow veggies in a raised bed with imported soil if your native soil is more than 25 per cent clay. Tough soils either do not drain adequately or they drain too fast. Sometimes the soil composition is fine, but the chemical nature of the soil is hostile to plants if the pH is too high or too low.

Soil Drainage Too Slow — Too Fast

A common issue faced when growing in outdoor soil is the drainage rate. Though hydroponic solid grow media systems drain rapidly, this aspect is accounted for by the particular system set-up. In soil when the percolation rate is quicker than an inch per minute, soil moisture is gone long before the next irrigation. This is a very tough growing condition and, if not checked before planting, will very likely mean plant failure.

For sandy soil to hold moisture it is necessary to amend the planting hole by putting a moisture-retaining product like coir or peat moss under the plant and mixing an amendment like this with the soil that is placed around the plant. With rapid drainage, not only does moisture leach away quickly, but so do nutrients. Even when amending with a moisture retention product, it will be important to use slow-release nutrients and apply them more frequently. The symptoms of malnutrition are many and include yellowing leaves, sparse foliage, and very weak production of fruit.

A more common problem growing in tough soil is when there is high clay content and its associated slow drainage rate. When the percolation rate is slower than an inch in 20 minutes, we encounter a new set of problems. First, drip irrigation with a timer is common and this type of soil will require a run or application time sufficient to penetrate at least 12 inches. Though water will continue to penetrate after application has stopped, slow-draining soils will usually need 60 to 90 minutes of run time to sink to the depth needed. These soils also stay wet much longer. Secondly, wet soil does not allow for air or oxygen to get to the roots for respiration to occur. The technique to deal with this is usually amending the soil with sand in addition to the organic matter needed. Amending usually calls for mixing the native soil with some other material having a different structure. When clay is high, however, amending can bond these two materials forming an adobe-like soil that is still problematic. When the clay content is more than than 20 per cent, it’s best to replace some or all the native soil to reduce the clay content. Even here we may obtain a reasonably good soil within the planting hole, but still having this surrounded by poor draining soil. This is much like having a tub filled with sand — it still won’t drain.

Sometimes there’s a layer of clay soil that can be dug through to create drainage, yet often this is unknown and the attempt to dig a “stovepipe” through the clay can be fruitless. In most of these situations the best option, while still being able to plant successfully, is to create a raised bed, or at least a berm where the planting is above the mean-soil line and so will drain.

Another issue, in more extreme slow-draining cases, is anaerobic bacteria breed and the wet soil becomes toxic, causing root rot diseases. This creates a terminal situation for whatever plants are located there. Knowing the drainage of the soil before planting is critical when trying to grow in poor soil.

Soil Amendments

Materials like sand, compost, coir, peat moss, perlite, or vermiculite are among the amendments available for changing and improving the soil condition. These amendments not only help control drainage but are necessary for the microbial soil life which is so essential for plant health and vigor. Organic matter in the soil is key to sustaining soil microbes. Once bacterial or fungal life is introduced to the soil, it is primarily the organic matter that feeds this life. The reactions created when bacteria feed on organic matter are responsible for nutrient precipitation where it then becomes available for the plant. These reactions also produce acids that work towards lowering soil pH. Even with organics like this, balance is the key. Too much green organic matter is like putting too many logs on the fire and smothering the flames. If, for some reason, it’s necessary to add high levels due to poor native soil, be certain to use organic matter that has completely finished composting and contains low levels of green material.

The water being used will have a big influence on soil conditions. Where municipal or tap water is high in total dissolved solids (TDS) these “salts” will, over time, build up in the soil and on the roots causing decline in root function and ultimately malnutrition and even drought issues. Find out what the TDS is of the water being used so you can remediate the resulting problems accordingly. Depending on just how high the dissolved solids are, soil leaching may be required as often as once each year. Leaching will help push away these salts from the plant. For this reason, softened water is not recommended. Salt-softened water is very high in sodium. Using water softened by potassium replaces calcium and magnesium with potassium. All three of these nutrients are required by plants in the right proportions, and using potassium softened water, though less damaging to plants than salt softened, will still disrupt the proper balance of these nutrients which is likely to create plant stress issues.

Creating Healthy Root Systems

To grow and take up nutrients there are several soil conditions that are vital for success. Each of these conditions requires balance. An extreme in any of them will inevitable lead to abiotic stress and ultimately to disease and decline.

  • Balance in soil composition, where it is not too sandy or too saturated with clay, is vital.

  • Balance in soil moisture is also critical. Soggy or dry soils each create their own set of problems.

  • Balance in soil nutrient concentration, where required levels of macro and micronutrients are available and toxic levels of these are avoided, is a must.

With each of these areas maintained, it is possible to create a living soil filled with beneficial microbial life that works in synergy with the plant to boost health, vitality, and growth.

There are many other vital aspects to consider when growing in poor soil: soil chemistry, climate (rainy, dry, hot, cold, or windy), understanding and installing proper irrigation, and using plants capable of handling these growing conditions. If you have been having trouble keeping your landscape or outside garden healthy, you should know you have lots of company.