One of the most common reasons people replace their nutrient solution is because controlling its pH to stay within its range keeps them running in circles. As a solution ages and nutrients are removed, the ability for the solution to buffer against future pH shifts becomes less. Hint:
As a rule of thumb, when running in a bio-system, it has been my experience that the closer you become to that 100% add-back mark that the ph will also begin to lean towards the ph of the original water source, here is were you need to have a maintains plain to do one of two things:
- Flush the entire system, and replace with fresh solution. This will not be a problem if outfitted with drain valves. A fresh solution has a pH behavior that's generally predictable, it will fluctuate but will do so within the acceptable range, thus requiring no adjustments or maintenance other than add backs. An older solution finds the pH wanting to run out of range (usually in the direction of the source water pH), this runaway pH drift constantly needs attention. At this point pH can start to become more trouble to maintain than the trouble it takes to replace the solution and return to the lower maintenance of a balanced and well buffered fresh solution. The problem here is that by the time a grower realizes he's been going in circles chasing a pH ghost, the solution can have already passed its useful life in other respects.
- Or, have a plain ready that would replenishes the aged solution? Or not, I have found that it pays to use mathematics rather than guess work when it comes to the useful life of your solution.
as a yardstick by which to gauge a solutions' useful life can be tricky. That TDS drops 25%, or 250ppm, isn't of itself an indicator for possible nutrient deficiencies, or that plant yields will suffer because of it. The assumption often made here is that the starting solution was at or near the nominal threshold of the plants' ability to sustain healthy growth, thus concluding the reduced TDS to be well below the threshold, and possibly deficient in one or more elements. Since it's relative to the starting TDS of a solution, if the starting solution was originally mixed 25% stronger than the nominal threshold, then when the solution TDS had dropped 25% it would be at the threshold instead of below it. Plant nutrient requirements are not something that can be nailed down to the ppm, for that reason thresholds for many crops are given as a range of recommended minimum and maximum elemental ppm values (not to be confused with TDS ppm values). For example, a flowering recommendation might be given as N 40-100 ppm, P 70-100, K 100-200, Mg 30-60. To know your crop's limits is to be able to use it to your advantage. As you can see from the above example, a grower has a good deal of latitude in how he can configure his nutrient solution mix. A safety margin for TDS measurements can be built-in to the original mix by mixing the solution nearer to the high end of a crops' recommended range, doing so will also provide more buffering power thus extending the solution's life to a degree where it relates to pH stability. In other words, TDS can have an affect on pH changes, but pH has no effect on TDS changes, so TDS also plays a role in controlling pH.
A common rule-of-thumb estimate of water usage in a greenhouse is about 1 liter/sq ft/day for vine crops such as tomatoes. It has been my experience in my bio-bucket system, that in-between maximum/minimum of water/solution uptake, (this is not a static time frame,)
for a mature indoor garden under strong artificial HID lighting is about (1qt, US Gallons) per plant.
Water uptake based management determines the useful life to end at a point where the original volume has been completely replaced by plain water add backs. For example, in my bio-bucket system, which has a total of 205 gallons of water in it, when the 205 gallons has had 205 gallons of water added back to it. This is sometimes also referred to as the 100% add back point. As you add back plain water, simply make note of the quantity and replace the solution when the total quantity of all add backs equals to the total capacity. For example, I have a 205 gallon bio-bucket system, 36 buckets/plants are using per plant or bucket 1 quart per day,
thatís 36 quarts per day and that equals out to 9 gallons a day.
Tow ways that I have grown in the Bio-Buckets
- To do a grow without a reservoir change-out, requires mathematical skills and a great deal of knowledge of hydroponics solutions, but can be done if you calculate your reservoir correctly.
- This other way will probably render a more piece of mind for the beginner in the bio-buckets, water uptake based management determines the useful life to end at a point where the original volume has been completely replaced by plain water add backs. For example, when a 25 gallon reservoir has had 25 gallons of water added back to it. This is sometimes also referred to as the 100% add back point. As you add back plain water, simply make note of the quantity and replace the solution when the total quantity of all add backs equals the reservoir capacity. It should go without saying that I have tried both of these methods and there are very little deferentís between them.
In case you haven't noticed, the determining factors behind a reservoir's useful life can all be traced back to the rate of water uptake, which is directly tied to the current demands of the crop. These demands will constantly increase as plants slowly fill their allotted space, often taking sixty or more days and spanning multiple growth stages before peak water uptake is eventually seen by the reservoir for the first time. As more water is being used by the plants, more nutrients are being removed from the nutrient solution, this naturally affects the nutrient balance in the remaining solution. In essence, the nutrient balance is also being controlled by the rate of water uptake. Simply put, a fuller garden space uses more nutrients because it uses more water. So what we have here is a direct relationship between solution volume maintenance (add backs) and pH/TDS maintenance. When that relationship is recognized, and this strategy enhanced to take advantage of it, additional gains in labor can be realized.
Reservoir Sizing, to buffer ph and nutrient uptake
An indoor home grower wanting a starting point for determining his reservoir size to go the entire grow start/finish, I have used this method with great success, hereís how I did it by approximately calculated 3 US Quart(s) or (2.839 liters) of reservoir water volume for each square foot of mature crop/bud canopy space.
This is not to say, the entire veg canopy space of your grow, only crop/bud space!! This is how I calculate my overall canopy space, with each Bio-Bucket calculate one square foot, so you would go the weith plus linth of you grow buckets, which in my case each Bio-System is two buckets wide by eithteen buckets long that comes out to be 18sq feet times two is 36sq feet, this is a rule-of-thumb what I am about to say next, I calculate 3 US Quarts per-square foot and that comes to 108 quarts now dived that total number by four and you should get 27gl and that should be the size of your reservoir. So my reservoir size is 27 gallons, this gives each sq-foot of mature canopy crop/bud space, three quarts per sq-foot. This water volume to space ratio has been found to produce both low maintenance and solution life expectancies that can easily coincide with growth stage nutrient formula changes. Waste not, want not:-)