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biowheel

Watering

biowheel
11 years ago

It's my understanding that you can't apply water with fertilizer on a plant that has dry soil because the roots can burn. I guess the theory is that the plant is so thirsty it will take in the ferts directly and it can hurt things. Well - my question then is how do I do it? I mean - I guess I can use plain water first and then maybe come back 1/2 hour later and apply the ferts but won't that just run right through the pot? My container plant is a tuberous begonia and it looks like it's showing symptoms of fertilizer burn. Ideally I'd like to just water it every time with a weak fert solution but only if the soil is wet? Should I just keep the soil wet? I don't want to kill it anymore. :-(

Comments (10)

  • tapla (mid-Michigan, USDA z5b-6a)
    11 years ago
    last modified: 9 years ago

    Adding solutes to a solution increases its osmotic pressure. Water can only move into plant cells when the osmotic pressure inside the cells is higher than in the solution outside the cells. The moisture state of a soil also is a determining factor in the osmotic pressure of the soil solution. Fast draining soils that hold less water tend toward having already higher osmotic pressures, so when you add fertilizer solution to dry or nearly dry soils, the osmotic pressure of the soil solution can end up being markedly higher than that of the fertigation solution, and MAY be approximately the same as or greater than the osmotic pressure of cellular solutions. When this occurs, water can't enter cells, or can actually be removed from cells. The end result of this condition is a collapsing cell and cell plasma being torn from cell walls. The scientific term is plasmolysis, but we commonly refer to it as fertilizer burn.

    If you're using continual low doses of fertilizer, and are fertigating when the soil is still damp and plants aren't drought-stressed, there won't be a problem doing what you proposed if your soil allows it. WET, isn't a term I would consider to be an ideal state of a soil. Damp or barely moist is ideal. If you're using a soil like Miracle-Gro, there's a high probability you won't be successful fertigating each time you water. That approach requires that you flush soils regularly, and most peat/compost-based soils remain soggy for too long after a thorough watering to ensure good root health/function.

    Al

  • dsws
    11 years ago
    last modified: 9 years ago

    From what I've read, fertilizer burn is caused by having too high a concentration of dissolved stuff in the soil water.

    The way plants take in water is by actively taking in dissolved stuff (especially potassium) and letting the water move from where there's less dissolved stuff to where there's more. So if the soil water has lots of fertilizer dissolved in it, eventually the plant can't take in any water even if the soil is wet to the point of being almost waterlogged.

    Just use a weak enough solution of fertilizer. I don't have enough experience to say how weak is weak enough, but I imagine that if it's weaker than the weakest option on the directions that should be ok.

    Also, people tend to recommend intentionally letting some fertilizer run through and go to waste. It's not possible to be 100% efficient, anyway. You give the plant a mix of nutrients, and it takes what it needs. If you have a good fertilizer for your plant, it's a pretty good match. But there's no such thing as perfect. There will always be some difference between what you offer and what the plant takes. If that difference stays in the soil, the ratio gradually gets further from what the plant is using. If it gets rinsed out, the ratio stays where it was, hopefully close to what the plant uses.

  • dsws
    11 years ago
    last modified: 9 years ago

    I wonder about those time stamps. Didn't seem as though it took me 16 minutes to type that.

    Anyway, yeah: what Al said.

  • tapla (mid-Michigan, USDA z5b-6a)
    11 years ago
    last modified: 9 years ago

    To expand just a little on dsws's point, when using fertilizers that more closely mimic the plants' actual usage of nutrients, you can get away with watering in sips longer than when using a badly skewed ratio. For instance, if you use FP 9-3-6 or MG 24-8-16, watering in sips isn't nearly the issue it would be if using say Bloom Booster 15-30-15. 15-30-15 supplies at least 12X as much P as the average amount plants can/will use (in relation to N usage), so you can see how quickly repeat applications can cause trouble on multiple fronts, beginning with the first through all subsequent applications if watering in sips. Fertilizers like 9-3-6 and 24-8-16 (3:1:2 ratios) come closest to mimicking plants' average usage, and because of that it takes much longer to end up with a badly skewed ratio in the soil solution, even if you DO water in sips.

    Another way of looking at it is, it's much more important to flush the soil regularly when using fertilizers with ratios not close to the ratio actually used by plants (like 1:1:1 ratios or 'bloom booster' high-P formulations), than it is when using ratios like 3:1:2 or 2:1:2.

    Al

  • PRO
    The Ficus Wrangler
    11 years ago
    last modified: 9 years ago

    Hi Al - Some of my plantscaping colleagues feel that using a 3-1-2 fertilizer is suited more toward encouraging growth, whereas the 1-1-1 is better suited to maintaining plants at a slower rate of growth and in low-light conditions. My thought is that by using dilution to limit the amount of nitrogen (as well as other elements, but nitrogen being the major factor affecting foliage growth), there will be fewer problems related to salt buildup when using the 3-1-2 as opposed to 1-1-1. Do I have that right? Can you reference any studies on fertilizer formulation?

  • biowheel
    Original Author
    11 years ago
    last modified: 9 years ago

    Thank you all so much for the replies and the wealth of information. Please don't look down at me but I'm still a bit confused and might just require simpler terms. If I'm approaching a dry plant (my particular plant needs it 'feet' dry a bit before rewatering) then can I use ferts? The answer sounds like yes but that I need to dilute it. I usually use standard MG at about 1/4 strength but looking at the last reply that may be too much. Maybe I need a lot bigger dilution. And - it probably depends on the type plant. I think I just might moisten the soil a bit with straight water and then come back a short time later and hit it with some fert and water but at lower strength then recommended. With the numbers everyone is throwing around I'm surprised MG doesn't tone down their recommendation amounts.

    Thanks again. I love plants.

  • PRO
    The Ficus Wrangler
    11 years ago
    last modified: 9 years ago

    Bio,
    I must apologize, I'm afraid I kind of went off on a tangent when I read Al's post. I'll try to address your particular questions a bit more directly. First of all, we need to define what you mean by "dry." If the soil feels "a bit dry," as you state at one point, that is an ok time to water and fertilize. If the soil feels dry to the "scatchy" point, and/or the plant is wilted, that is the time you DON'T want to fertilize, you need to rehydrate your plant first. You might have to soak pot and all in a basin to thoroughly wet the soil. But from what you've written, it doesn't sound like you're letting the plant actually go dry;
    dry is when, if you pinch some soil from at least 1/2 way down into the pot, the soil doesn't stick together at all, and has only the barest trace of moisture.

    As for the question of watering with a dilute solution at every watering as opposed to fertilizing at intervals, once a month, every other month, every 3 or 4 months, or whatever, this is a subject of debate, and there are many adherents of each method. I personally prefer to fertilize plants that are indoors all the time only 3 or 4 times a year, and plants that are usually outside in pots, more like every month.

    If you think your begonia is showing signs of fertilizer burn, though, your best course would be to not fertilize for awhile, don't you think? One thing you might want to consider is that plants don't need "feeding" the same way a puppy or a goldfish need feeding. In other words, plants actually make their own food from light, air, and water. Fertilizer just adds a few extra minerals that are used in the construction of enzymes, hormones, and other processes.
    If you are fertilizing regularly, like at every watering, are you making sure that the water is flushing through the soil mass? This is the best way to guard against over-fertilization, other than to not fertilize so much.

    So the idea is that you actually WANT the fertilizer to run through the soil. And if you have a begonia, you really don't want to keep it wet, moist, or even damp, all the time. I hope that clears up a few questions for you. If not, don't be shy about asking again. Sometimes it takes a while for everyone to get onto the same page.

  • tapla (mid-Michigan, USDA z5b-6a)
    11 years ago
    last modified: 9 years ago

    Some of my plantscaping colleagues feel that using a 3-1-2 fertilizer is suited more toward encouraging growth, whereas the 1-1-1 is better suited to maintaining plants at a slower rate of growth and in low-light conditions.

    I think you're kidding yourselves - here's why. The chart below is commonly accepted as the average usage of all plants as compared to N usage:

    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 (16) 1/6
    K 45-80 (62) 3/5
    S 6-9 (8) 1/12
    Mg 5-15 (10) 1/10
    Ca 5-15 (10) 1/10
    Fe 0.7
    Mn 0.4
    B(oron) 0.2
    Zn 0.06
    Cu 0.03
    Cl 0.03
    Mo(lybdenum) 0.003

    The basic chart is taken from a university level text on horticulture, and it appears widely on the internet. The numbers in parenthesis and the fractions following the macros and secondary macros are mine, but they are only using math for comparison to make usage easier to understand.

    First, plants grown in low light use less N, but they don't use less N in relation to the other nutrients - the ratio of usage stays the same. Plants grown in low light can indeed be slowed down vegetatively by curtailing the amount of N that they are given, but plantscapers can't afford to ACTUALLY work with a N deficiency. You guys are ALWAYS supplying an adequate amount of N. If you weren't, if you were actually LIMITING the amount of N you provided to where it was a mild deficiency, it would show up in the form of older and interior leaves being shed as the plant scavenges the N it needs to fuel new growth. IOW, the plant will sacrifice itself in order to fuel growth, and you guys try to avoid that at all costs.

    The ideal you're chasing is admirable, and it SOUNDS good, but the route you're taking is less so. IF you choose a 1:1:1 ratio fertilizer, and use it to supply the plant with adequate amounts of N, you are supplying too much P and K. It can't be any other way. These extra measures of elements unnecessarily add to the EC/TDS of the soil solution (makes it more difficult for the plant to take up water and the nutrients dissolved in water), AND because they skew the ratio of nutrients in the soil solution, wreak havoc with the plant's ability to absorb other nutrients in appropriate measure (antagonistic deficiencies)

    for every 1 part of N supplied, you should supply (on average) .17 parts of P. Anything beyond (above) that number is an excess, and tends to make it more difficult for the plant to absorb other nutrients - particularly Fe, K, Ca, Cu, and Z. From this, you can see it's impossible to supply enough N with 1:1:1 ratio fertilizers w/o providing too much P & K. Why would you want to do that? More important to keeping plants compact is the type of N you deliver, rather than how much, but I haven't seen that mentioned. N in nitrate form, as opposed to ureic or ammoniacal forms, are what you need to keep plants compact & full - especially since we know with certainty that you really are not limiting the amount of N your plants are getting. N isn't limited by the fertilizer ratio, it's limited by the hand of the grower.

    The problems using 1:1:1 fertilizers become more acute as soil density and the resultant increase in water retention increases to the point where the grower is forced to water in sips. This increases the skewed ratio of nutrients in the soil solution dramatically, as each subsequent fertilizer application throws the ratio of available nutrients further out of whack.

    If you really do want to limit the amount of N supplied to a plant, you need to be skillful enough to be able to read the plant, to be sure you actually are limiting N. The best way to do that would be with a fertilizer ratio of 2-2.5:1:3, by actually decreasing the amount of N to what is an appropriate ratio for the other nutrients.

    I'm not saying you can't raise healthy plants using 1:1:1 ratios, or that you can't limit N by using them, but it's more difficult for a number of reasons, and unwanted excesses or deficiencies are absolutely assured.

    Bio - you're pretty safe when applying 1/4 the recommended dose to any plant that isn't drought stressed, but it's always a good idea to eliminate all doubt by watering and waiting an hour or so before you fertilize if you suspect the soil might be too dry. Watering twice also helps ensure that all, or a larger fraction of the soil mass will be dampened.

    Al

  • dsws
    11 years ago
    last modified: 9 years ago

    Some plants drop leaves as part of their normal growth. I'm guessing that would mean they use a somewhat different ratio of nutrients from the soil, with N and P being recovered from senescent leaves at a higher percentage than K.

  • tapla (mid-Michigan, USDA z5b-6a)
    11 years ago
    last modified: 9 years ago

    Plants do use slightly different ratios of nutrients, and the best program would be one specifically designed for the particular plant .... but that requires that we have the wherewithal to determine what is in the plant and in the soil, in the way of nutrients. Few hobby growers have that ability. The next best thing is to cover our bases as closely as possible by playing to the averages.

    N, P, and K are very mobile in the plant, so a deficiency of any of these nutrients would cause the plant to absorb them from other plant parts in order to fuel new growth .... which may not really be growth at all - it could be just elongation w/o an increase in plant mass.

    Al