Healthy Soils Grow Healthy Plants

Healthy soils grow healthy plants. So what constitutes a healthy soil and what can you look for in a mix to ensure optimal results through a range of conditions? There are many existing articles, including some written by yours truly that have provided some insights, but let’s take a closer look.

First, let’s have a look at the soil structure. A generalized way to view this area would be to say that on one end we have a coarse soil, and on the other end we have a finer soil. Each extreme would offer its own distinct advantages and disadvantages in terms of soil and crop management.

Finer soils are those comprised of many smaller particles in a given volume, versus fewer but larger particles in the same volume. Finer soils therefore have more surface area for root contact.

There is a greater volume of micro pore space in finer soils for holding air and moisture. That is why finer soils are slower to drain, conserving moisture. However, the drawback is that these micro-pore spaces are quickly filled with moisture, displacing air space in the soil. Good aeration is important for root health and beneficial soil life (microbes), and becomes most critical in warmer growing conditions. Also, for indoor growers who like to push their crops with as many feedings as possible, the opportunity to do so will be lessened due to the finer soil’s moisture retention characteristics.

Capillary movement of moisture and nutrients is improved with a finer soil, particularly where lateral (sideways) movement is concerned. This is of significance to growers using raised beds for crop production. Even moisture levels make for an even plant canopy. Ideally, the upper two-thirds of the soil/soilless bed would comprise of finer soil particles, while the lower one-third would be coarser to improve drainage. Perlite works well to improve drainage, but only to a point. It will tend to rise from the bottom of beds or containers to the upper portion. Heavier materials such as LECCA (grow rocks) will tend to stay put. They also have an affinity for holding excess salts, helping to avoid toxicity at the roots which is very useful in non-draining applications. A drawback to having a drainage layer is that if the beds are re-used, the coarser aggregate gets mixed in with the rest of the media between crops when removing old root debris, mixing in amendments, etc.

Plants types that tend to develop finer root systems where the roots tend to be thinner, but create a fine web-like structure are better suited to finer soils. Also note that even plants that do not normally tend to produce finer roots, will do so (although only slightly) when grown in a finer growing media. Just for kicks try rooting some cuttings in a fine media such as straight, pH buffered peat VS cuttings rooted in coarse washed sand. The difference in the type of roots that are developed is noticeable. Finer roots have more surface area for water and nutrient absorption, but tend to be more delicate and subject to damage from over-watering, etc.

So what are some examples of finer growing medias? At the consumer level for indoor gardening applications they will be devoid of, or contain lesser amounts of, perlite and/or pumice. Peat based growing medias will typically be finer than coconut coir based mixes. Blends containing relatively higher additions of organic materials such as composts, worm castings, guanos, etc., will also tend to be finer in structure and offer higher moisture holding capacities and more surface area for finer, more delicate root systems.

There tends to be less compaction over the course of a crop with finer soil/soilless growing medias. If excessive moisture levels are maintained for longer periods, fungus gnats will thrive. Their larvae can be harmful to crops.

Coarser soils/soilless mixes have a greater volume of macro-pore space. This means that they drain more freely, and have larger pore spaces that will remain open for root atmospheric oxygen and faster drainage, even after heavier waterings. However, they have less overall surface area for root growth, and have less capillary (water movement) capacity. These soil/soilless blends are better suited for container crops, and are especially well suited to mechanical watering systems such as flood/drain, drip, etc. The additional airspace will promote healthy microbial growth, although microbial counts will likely be higher in a healthy, well managed, finer organic based growing media. The open pore space will make roots slightly more sensitive to air temperatures as finer medias can better insulate the roots.

For growers who attempt to “push” their crops with more frequent feedings, a coarser soil/soilless organic based medium is the natural choice. Compaction of the growing media, particularly with re-use, is greater in a coarser soil containing more macro pore space. The weight of the crop and the break down of the particles due to mechanical movement or through erosion (saline solutions) will create smaller particles over time. The result is that the initial volume of the growing media will be reduced, possibly of detriment to the current crop, as the area for root growth will be shrinking while the demand for root space is increasing with crop maturity.

At the consumer level, coarser growing medias tend to contain higher additions of perlite/pumice, with the aggregate size typically being larger in size. Good blends are often very similar to a rich, finer organic blend in terms of ingredients, with the principle difference of their simply being an addition of coarser aggregate to improve drainage. Due to this addition, there are likely lower nutrient levels contained in the growing media, so a stronger concentration may be supplied via the nutrient solution. The blends may also be coarser due to the nature of their parent material. Coco coir provides an excellent consistency with a good balance of water to air capacity. Materials with higher carbon to nitrogen ratios will provide additional aeration and drainage. However, the carbon (i.e. bark chips) will lessen the potential in the availability of organic nutrients, as soil life will be more focused on decomposing the higher carbon content material than the organic nutrient additions.

Over time, and particularly with re-use, soils that have been fed with more organic based nutrients will have accumulated some organic matter which may be beneficial for plant growth and the microbial populations contained within the soil. To loosen finer soils, or those that have become finer over time, additions of ground oyster shells work well to aerate peat based growing mediums, to help buffer pH, to supply calcium, and to possibly supply growth promotants.

If reusing soils, it is beneficial to amend the growing media between crops to supply organic matter and re-charge microbial population levels.

Aside from the physical characteristics of a soil, the chemical characteristics are of equal importance. Blends should be well composted, this includes the parent material prior to blending and an aging process for the blend itself. If the mix is too “hot” it will be chemically unstable, putting the crop on a roller coaster ride for nutrient availability. Poorly composted or unstable growing medias will also lessen nutrient availability when chemical based nutrients are supplied via fertigation. Even with a pure soilless mix such as coco coir, ensure that the supplier has stated a significant composting period for the product, ideally 18 months or longer. Naturally, the chemical composition of the parent material is of great significance to the chemical properties of the soil. This not only includes the types of ingredients themselves, but also of consideration are the sources of those materials. Not all soil amendments are created equal. For example, has the parent material been exposed to potentially harmful agricultural chemicals which still may be present which they were not metabolized by animal sources (manures) or may be found as residues on agricultural crop materials such as cotton seed meal? In pure growing mediums such as coco coir, ensure that salinity levels are relatively low, and the pH is stable. This is accomplished by the supplier during the conditioning period. Double washed materials tend to contain lower levels of impurities, such as sodium. Materials may be chemically buffered for their cation exchange capacity, typically with calcium and or potassium solutions.

Now, let’s have a look at some popular ingredients and possible additions for a good quality, organic based growing medium. Note that the plants featured in the images in this article were grown in a healthy organic-based potting mix. No additional nutrients were supplied via fertigation. A typical ingredients label on a superior quality growing media may contain:

Earthworm Castings:

Having a high content of good quality earth worm castings is the kindest thing a person can do to an organic based growing media. Not all worm castings are created equal. Sources derived from African Night Crawlers tend to produce richer deposits that contain significantly higher levels of beneficial organic matter versus Red Wriggler worm types. The castings should be dark to black in appearance and have a rich earthy texture and smell. Some materials marketed as worm castings are really more compost based rather than containing significant levels of worm castings (the digested material produced by worms).

Seabird and Bat Guanos:

Originating from very old fossilized deposits, these materials are highly concentrated. Depending on the diet of the animal source (regional variations) different macro nutrient values may be obtained. Bat guanos are an excellent source of soluble phosphates and trace minerals.

Seaweed/Kelp Meals:

Kelp meal (dried and ground kelp) is an excellent and fast acting source of potassium and trace elements. An excellent organic “greening” product, kelp is also one of the best natural sources of an array of growth promoting hormones including cytokinins. It is most beneficial throughout the vegetative and early flowering phases of growth.

Leonardite Ore:

This is the parent material where humic and fulvic acids are derived from. Humic acids help to improve soil structure, microbial life, and water movement in the soil. The lighter fulvic fraction is more closely associated with nutrient absorption and transport within the plant. Even when finely ground, it takes some time for the humic/fulvic fractions to become available. This why it is best to purchase a pre-mixed soilless medium, as some time has elapsed between the blending of the mix and the time the grower plants the crop.

Manures/Compost:

Well composted green and animal manures are an excellent source of healthy organic matter and slow release nutrients, and may introduce a wide variety of beneficial microbial to the mix. Composted chicken and rabbit manure are especially good. Horse manure can be very beneficial too, especially if the animal’s diet has included alfalfa. The importance of good composting cannot be stressed enough if manures are added to the mix for use with indoor applications. Well composted manure is virtually odourless.

Crushed Oyster Shells:

This is an excellent component for buffering pH and supplying a slow release of calcium. Significant quantities of coarser crushed shells also help to aerate the soil, and their decomposition may also release beneficial growth promotants. Overall root health appears to increase with additions of oyster shells.