I've been studying fertilizing and plant nutrition as it relates to container culture for some time now. This subject has been discussed frequently, but in rather piecemeal fashion on the Container Gardening forum and forums related. So, prompted by a question about fertilizers in my mail, I decided to collect a few thoughts & share an individual's considered overview. I hope it leads to as much discussion and garners as much interest as the Container Soil post did. ;o)
Fertilizer Program - Containerized Plants
Let me begin with a brief and hopefully not too technical explanation of how plants absorb water from the soil and the nutrients/solutes that are dissolved in that water. Most of us remember from our biology classes that cells have membranes that are semi-permeable. That is, they allow some things to pass through the walls, like water and whatever is dissolved in it, while excluding other materials. Osmosis is a natural phenomenon that creates a balance (isotonicity) in pressure between liquids and solutes inside and outside the cell. Water and ionic solutes will pass in and out of cell walls until an equilibrium is reached and the level of solutes in the water surrounding the cell is the same as the level of solutes in the cell.
This process begins when the finest roots absorb water molecule by molecule at the cellular level from the surface of soil particles and transport it, along with its nutrient load, throughout the plant. I want to keep this simple, so I’ll just say that the best water absorption occurs when the level of solutes in soil water is lowest, and in the presence of good amounts of oxygen (this is where I get to plug a well-aerated and free-draining soil), ;o) but of course, when the level of solutes is very low, the plant is shorted the building materials (nutrients) it needs to manufacture food and keep its metabolism running smoothly, so it begins to exhibit deficiency symptoms.
We already learned that if the dissolved solutes in soil water are low, the plant may be well hydrated, but starving; however, if they are too high, the plant may have a large store of nutrients in the soil, but because of osmotic pressure, the plant may be unable to absorb the water and could die of thirst in a sea of plenty. When this condition occurs, and is severe enough (high concentrations of solutes in soil water), it causes fertilizer burn (plasmolysis), where plasma is torn from cell walls as the water inside the cell exits to maintain solute equilibrium with the water surrounding the cell.
Our job, because you will not find a sufficient supply of nutrients in a container soil, is to provide a solution of dissolved nutrients that affords the plant a supply in the adequate to luxury range, yet still makes it easy for the plant to take up enough water to be well-hydrated and free of drought stress. Electrical conductivity (EC) of the water in the soil is a reliable way to judge the level of solutes and the plant’s ability to take up water. There are meters that measure this conductivity, and for most plants the ideal range of conductivity is from 1.5 - 3.5 mS, with some, like tomatoes, being as high as 4.5 mS. This is more technical than I wanted to be, but I added it in case someone wanted to search "mS" or "EC". Most of us, including me, will have to be satisfied with simply guessing, but understanding how plants take up water and fertilizer and the effect of solute concentrations in soil water is an important piece of the fertilizing puzzle.
Now, some disconcerting news - you have listened to all this talk about nutrient concentrations, but what do we supply, when, and how do we supply them? We have to decide what nutrients are appropriate to add to our supplementation program, but how? Most of us are just hobby growers and cannot do tissue analysis to determine what is lacking. We can be observant and learn the symptoms of various nutrient deficiencies though - and we CAN make some surprising generalizations.
What if I said that the nutritional needs of all plants is basically the same and that one fertilizer could suit almost all the plants we grow in containers - that by increasing/decreasing the dosage as we water, we could even manipulate plants to bloom and fruit more abundantly? It’s really quite logical, so please let me explain.
Tissue analysis of plants will nearly always show NPK to be in the ratio of approximately 10:1.5:7. If we assign N the constant of 100, P and K will range from 13-19 and 45-70 respectively. I’ll try to remember to make a chart showing the relative ratios of all the other 13 essential nutrients that don’t come from the air at the end of what I write.
All we need to do is supply nutrients in approximately the same ratio as plants use them, and in adequate amounts to keep them in the adequate to luxury range at all times. Remember that we can maximize water uptake by keeping the concentrations of solutes low, so a continual supply of a weak solution is best. Nutrients don’t just suddenly appear in large quantities in nature, so the low and continual dose method most closely mimics the nutritional supply Mother Nature offers. If you decide to adopt a "fertilize every time you water" approach, most liquid fertilizers can be applied at ¾ to 1 tsp per gallon for best results. If you decide that’s too much work, try halving the dose recommended & cutting the interval in half. You can work out the math for granular soluble fertilizers and apply at a similar rate.
The system is rather self regulating if fertilizer is applied in low concentrations each time you water, even with houseplants in winter. As the plant’s growth slows, so does its need for both water and nutrients. Larger plants and plants that are growing robustly will need more water and nutrients, so linking nutrient supply to the water supply is a win/win situation all around.
Another advantage to supplying a continual low concentration of fertilizer is it eliminates the tendency of plants to show symptoms of nutrient deficiencies after they have received high doses of fertilizer and then been allowed to return to a more favorable level of soil solute concentrations. Even at perfectly acceptable concentrations of nutrients in the soil, plants previously exposed to high concentrations of fertilizer readily display these symptoms.
You will still need to guard against watering in sips and that habits accompanying tendency to allow solute (salt) accumulation in soils. Remember that as salts accumulate, both water and nutrient uptake is made more difficult and finally impaired or made impossible in severe cases. Your soils should always allow you to water so that at least 10-15% of the total volume of water applied passes through the soil and out the drain hole to be discarded. This flushes the soil and carries accumulating solutes out the drain hole.
I have recently switched to a liquid fertilizer with micronutrients in a 12:4:8 NPK ratio. Note how close this fit’s the average ratio of NPK content in plant tissues, noted above (10:1.5:7). If the P looks a little high at 4, consider that in container soils, P begins to be more tightly held as pH goes from 6.5 to below 6.0, which is on the high side of most container soil’s pH, so the manufacturer probably gave this some careful consideration.
To answer the inevitable questions about specialty fertilizers and "special" plant nutritional requirements, let me repeat that plants need nutrients in roughly the same ratio. Ratio is an entirely a separate consideration from dosage. You’ll need to adjust the dosage to fit the plant and perhaps strike a happy medium in containers that have a diversity of material.
If nutrient availability is unbalanced, if plants are getting more than they need of certain nutrients, but less than they need of others, the nutrient they need the most will be the one that limits growth. Whatever nutrients are available in excess, will be absorbed by the plant to a certain degree, and in some cases, this may lead to toxicity or even symptoms of shortages of other nutrients as toxicity levels block a plant's ability to take up other nutrients. Too much nitrogen will lead to excessive foliage production and less flowering. Too much potassium or phosphorus will not lead to ill effect, but will show up as a deficiency of other nutrients as it blocks uptake.
What about the "Bloom Booster" fertilizers you might ask? To induce more prolific flowering, a reduced N supply will have more and better effect than the high P bloom formulas. When N is reduced, it slows vegetative growth without reducing photosynthesis. Since vegetative growth is limited by a lack of N, and the photosynthetic machinery continues to turn out food, it leaves an expendable surplus for the plant to spend on flowers and fruit.
The fact that different species of plants grow in different types of soil where they are naturally found, does not mean that one needs more of a certain nutrient than the other. It just means that the plants have developed strategies to adapt to certain conditions, like excesses and deficiencies of particular nutrients..
Plants that "love" acid soils, e.g., have simply developed strategies to cope with those soils. Their calcium needs are still the same as any other plant and no different from the nutrient requirements of plants that thrive in alkaline soils. The problem for acid-loving plants is that they are unable to adequately limit their calcium uptake, and will absorb too much of it when available, resulting in cellular pH-values that are too high. Some acid-loving plants also have difficulties absorbing Fe, Mn, Cu, or Zn, which is more tightly held in alkaline soils, another reason why they thrive in low pH (acid) soils.
The point I’m trying to make in the last three or four paragraphs is simply that nearly all the variables in a fertilizer regimen pertain to the plants ability to handle nutrients, not to the actual nutrient needs of the plant.
So, If you select a fertilizer that is close in ratio to the concentration of major elements in plant tissues, you’re going to be in pretty good shape. Whether the fertilizer is furnished in chemical or organic form matters not a whit to the plant. Ions are ions, but there is one consideration. Chemical fertilizers are available for immediate uptake while organic fertilizers must be acted on by passing through the gut of micro-organisms to break them down into usable elemental form. Since microorganism populations are affected by cultural conditions like moisture/air levels in the soil, soil pH, fertility levels, temperature, etc., they tend to follow a boom/bust cycle in container culture, which has an impact on the reliability and timing of delivery of nutrients supplied in organic form.
What am I using? I start with a quart of 12-4-8 liquid Miracle-Gro all purpose plant food. To that, I add 3 Tbsp. of Epsom salts, 2 Tbsp. STEM (Soluble Trace Element Mix), and 1 Tbsp Sprint 138 Fe chelate and agitate until the concentrate is dissolved. I then try to fertilize my plants weakly (pun intended) with a half recommended dose of the concentrate and a little added 5-1-1 fish emulsion. The fish emulsion is for no particular reason except that I have lots of it on hand. This year my display containers performed better than they ever have in years past & they were still all looking amazingly attractive this third week of Oct when I finally decided to dismantle them because of imminent cold weather. I attribute results primarily to a good soil and a healthy nutrient supplementation program.
What would I recommend to someone who asked, for nearly all container plantings? If you can find it, a 12-4-8 liquid blend that contains all the minor elements would a great find and easy to use, but I don‘t think it‘s available. What I’m using does not have all the minors but I supply them with the STEM. You’ll likely find a 24-8-16 product readily available in granular, soluble form with all the minors, which is the same ratio as 12-4-8, so if I had to pick one fertilizer for use on all my plants, it would be that.
The chart I promised:
I gave Nitrogen, because it's the largest nutrient component, the value of 100. Other nutrients are listed as a weight percentage of N. N 100 P 13-19 K 45-80 S 6-9 Mg 5-15 Ca 5-15 Fe 0.7 Mn 0.4 B(oron) 0.2 Zn 0.06 Cu 0.03 Cl 0.03 M(olybden) 0.003
If you're still awake - thanks for reading. It makes me feel like the effort was worth it. ;o) Let me know what you think - please.
Al Fassezke _____________________________________________________________________________
Fertilizer Program for Containerized Plants III, Posted by tapla z5b-6a mid-MI (My Page) on Fri, May 6, 11 at 10:44 Is the later version. There are so many myths circulating arround,but the bottom line is nearly the same. Our experts feed DAP near bloom time, they are reducing the Nitrogen. Most growers arround the world use higher N during growing season, low N during blooming period and more K for winterring. What I do not understand is they recommend different NPK ratio but talk of same PPM of N when feeding. His other postings on container soil and water movement are also very educating and can be very helpful if modified to suit our climate and available materials.