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I have been thinking about what I want to say about houseplant soils here, and how I should open. I’m going to talk a little about soils, primarily from the perspective of what is best for the plant - not the planter. ;o) More often than not, the two ideas are mutually exclusive, and the plant suffers loss of vitality for the sake of grower convenience - even if unknowingly. There is absolutely nothing wrong with that. Probably none of us can afford the time it would take to give our plants the very best care possible, and we need to decide on an individual basis how much attention we can pay our plants. I’ll explain later, but hope to leave you with enough insight so that, at at a minimum, you understand there are options and alternatives to using commercially prepared houseplant soils.
Let me start by saying that whenever I say ’plants’ I mean a very high % of house plants and freely allow that there are exceptions to every rule; but, we need to learn some rules before we can recognize the exception. I’m going to offer a few (of what I think are) rules I believe are difficult to challenge, and that I have adopted in my growing practices after a fair amount of study and consideration. I’m going to leave light levels out of this conversation after acknowledging that they are probably just as important as soil to a planting, the difference being, we can recognize and change poor light levels easily if we choose, but poor soils are not so easily remedied.
Rule: Plants need air in the root zone as much as they need light and water. The soils we usually buy in a bag either do not supply enough aeration from the outset, or they do not supply it for a long enough period. Most, or at least many readers are expecting their plants to live in the same soil for several years, when the fact is that most peat based soils substantially collapse within a single growth cycle. That is to say that the peat particles break down into continually smaller pieces. This reduces the number of macropores (large air pockets), causes compaction, and increases the amount of water the soil holds in root zone and increases the length of time it remains there. Use of materials like compost, worm castings, or fine sand have the same effect.
What does this mean to our plants? Well, there is the specter of root rot, but even if we set that aside, there is something more subtle occurring. Whenever roots are deprived of oxygen (O2) they soon begin to die - incrementally. First, and after only a few hours in saturated, anaerobic (lacking O2) conditions, the finest roots that absorb water and nutrients begin to die. Already, the plant would be operating under stress. Gradually, thicker roots die unless the plant uses the water in the root zone or it evaporates and O2 is allowed back into the soil. When adequate aeration is restored, the plant will have been disadvantaged, because fine rootage has died. The plant begins to regenerate the lost roots, but guess what? It has to call on energy reserves it has stored because the roots cannot efficiently take up water and the building blocks from which it makes food (nutrients/fertilizer). This stored photosynthate that goes to root regeneration would have been used to increase biomass - flowers, fruit, foliage, stem thickness. See how subtly aeration affects growth?
Rule: Our number one priority when establishing a planting should be to choose a soil that guarantees adequate aeration for the expected life of that planting. We can easily change every other cultural influence if we choose. Light, temperature, nutrients, moisture levels ….. all can be changed, but we cannot change aeration, so we really need to consider that as a priority.
It is in our choice of soils where we need to bring attention to the fact that, as alluded to above, convenience has costs. I’m not saying that in chiding fashion. I simply want to make the point that when you’re able to go several days to a week without watering, in a high % of cases, the cyclic death and regeneration of roots is taking place. The plant is growing under stress and is weakened to varying degrees, depending on the severity of O2 deprivation in the root zone.
Rule: A fast soil that drains freely will be far superior from a plant vitality perspective than a more convenient soil that stays wet. The cost: You’ll need to decide if you’re willing to water and fertilize more frequently to secure the added vitality.
I could go on for days about soil, but I’m hoping that I’ll be able to discuss HOW we can get to a better place with regard to our soils through answering any questions that might come up, and exploring options. Before I close, I would like to talk for a minute about another bane of poor soils.
Many of us recognize what we consider the main danger of over-watering - root rot, and do our best to prevent it. Most often, we try to prevent it by watering sparingly, in small sips, so the soil is never saturated, but let me explain what happens when we do this.
Plants best take up water and the ions dissolved in it when the ion level is very low. This ion level is measured by either electrical conductivity (EC) or the total amount of dissolved solids (TDS). Problems arise when the TDS/EC level is too low, even when the plant can take up water easily. It easily remains hydrated, but starves for nutrients because there is not a high enough concentration of ions in the soil water. If the level of TDS/EC is too high, the process of osmosis is affected, and the plant cannot efficiently take up either water OR nutrients, so the plant can starve or die of thirst in a sea of plenty. It’s up to us to supply the right mix of all the nutrients in a favorable range of TDS/EC.
I’m sorry if I got a little technical, but I’m getting to a point. When using soils that are not fast enough (free draining) to allow us to water copiously and continually flush the salts that accumulate from fertilizer and irrigation water something unwanted occurs. Salts and dissolved solids from fertilizers and irrigation water will continue to accumulate. This pushes up the level of TDS/EC and makes it increasingly difficult for the plant to take up water and nutrients.
Imagine: A soil that is killing our most efficient roots (the finest are the first to succumb), which stresses the plant and makes it more difficult to take up water due to the lack of those roots, while it insures that the level of TDS/EC will rise, making it difficult or impossible on yet another front for the plant to take up water and nutrients. Is it any wonder that our plants start to struggle so mightily toward winter’s end? Are we really seeing the effects of low humidity or do you think it might be drought stress brought on by either an inappropriate soil and less than favorable watering practices? Probably a little or a lot of both.
Rule: Whenever you consider a plant in trouble, you must consider not only the plant, but the rest of the planting as well - including the soil. The insect infestations, diseases, and stress/strain we so often need help with here on the forum can almost always be traced back to weakening of the organism due to an inappropriate soil (or, as noted, inadequate light - though in an extremely high % of cases, it is indeed the soil).
This only touches on the cause/effect relationship of the soil to the planting. If there are questions, I’ll try to answer them. If there is disagreement on a point or points, I’ll offer the science behind my thinking and you can decide individually if the things I set down make sense.
I would strongly urge anyone who was not long ago bored to tears to follow this link to another thread I offered on the container gardening forum. If you would like to get into the science and physics of how water behaves in container plantings, this will help explain it: http://davesgarden.com/community/forums/t/719569/ It's likely you would come away with additional knowledge of what makes a good houseplant soil.
I hope this starts a lively discussion and provokes plenty of questions, but more importantly, I hope it eventually, as the thread progresses, helps put a few more pieces of the puzzle together for at least a few forum participants. ;o)
Al Fassezke
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