There are many ways in which people determine the longevity, or useful life, of their hydroponic nutrient solutions. These range from the "replace it every week or two to be safe" method, to not replacing it at all except between crops. The latter, meant primarily for commercial operations, requires costly plant tissue and nutrient solution lab analyses to be performed. This becomes more attractive as an operation grows larger, for example, when a point is reached where costs to replace an 8,000 gallon solution start to approach the lab and personnel costs required to maintain it. For the home gardener who hasn't the resources nor the financial incentive to have lab tests performed, other management methods are used. As home gardeners not using lab tests, in this article we will not consider maintaining the elemental makeup of the solution. Attempting to do so without such tests would have no basis in fact and would be based solely on guesswork.
Useful Life
Useful life can mean many things to many people depending on their definition of useful. Two factors can be used to define useful where it relates to nutrient solutions; plant health and economics.
Plant Health
A solution is no longer useful when it has potential to negatively impact growth or the health of plants, and ultimately yield. The nutritive quality of a solution is determined by the gardener at the time he mixes a new solution. Over time, water and nutrients will be used by plants and will slowly change the elemental composition (or balance) of the original mix, leaving some elements in short supply while others become proportionately over-abundant. There are two facets involved with elemental availability. One is the existence of an element, the other is the effect the chemistry of an imbalanced solution has on the availability of that element. An aged solution's imbalance can be such that it either has an insufficient quantity of an element existing in the mix, or that the imbalance has changed other properties of the solution to cause the element to become unavailable to the plants. For example, a solution may have had all but a trace of its nitrogen depleted, or it may still contain adequate nitrogen but it will be unavailable because of the pH shift resulting from the imbalance. Either condition is unfavorable to plant health. The difference being that the former points to a spent solution that has no more useful life and needs to be replaced, and the latter points to a solution which may still be useful but is starting to require more maintenance than desired.
It should go without saying that using the plants themselves as a means of measuring the useful life of a solution is counterproductive. The purpose of nutrient solution management is to avoid any unhealthy solution condition, waiting for plants to show signs of nutrient stress defeats that goal. Instead of using the plants as guinea pigs, we use indicators in the solution that will alert us to approaching potential problems so that we can avoid those problems thus insuring uninterrupted plant health for the life of the solution.
Useful Life
Useful life can mean many things to many people depending on their definition of useful. Two factors can be used to define useful where it relates to nutrient solutions; plant health and economics.
Plant Health
A solution is no longer useful when it has potential to negatively impact growth or the health of plants, and ultimately yield. The nutritive quality of a solution is determined by the gardener at the time he mixes a new solution. Over time, water and nutrients will be used by plants and will slowly change the elemental composition (or balance) of the original mix, leaving some elements in short supply while others become proportionately over-abundant. There are two facets involved with elemental availability. One is the existence of an element, the other is the effect the chemistry of an imbalanced solution has on the availability of that element. An aged solution's imbalance can be such that it either has an insufficient quantity of an element existing in the mix, or that the imbalance has changed other properties of the solution to cause the element to become unavailable to the plants. For example, a solution may have had all but a trace of its nitrogen depleted, or it may still contain adequate nitrogen but it will be unavailable because of the pH shift resulting from the imbalance. Either condition is unfavorable to plant health. The difference being that the former points to a spent solution that has no more useful life and needs to be replaced, and the latter points to a solution which may still be useful but is starting to require more maintenance than desired.
It should go without saying that using the plants themselves as a means of measuring the useful life of a solution is counterproductive. The purpose of nutrient solution management is to avoid any unhealthy solution condition, waiting for plants to show signs of nutrient stress defeats that goal. Instead of using the plants as guinea pigs, we use indicators in the solution that will alert us to approaching potential problems so that we can avoid those problems thus insuring uninterrupted plant health for the life of the solution.