How Plant Growth is Limited (container forum version)
In a recent post, I suffered criticism after I tried to explain why light could not make up for or 'trump' the negative affects of other factors that potentially limit plant growth. Liebig's Law of the Minimum is a universally accepted concept that defines how the growth of plants is limited. Originally the law was viewed by Justus Von Liebig, a German chemist who is often referred to as 'the father of the fertilizer industry', as a fitting way to define the fact that plant growth is not limited by the total of the available resources, but rather, by the single resource in shortest supply.
Though Liebig's focus at the time was on nutrition, his concept was later expanded to include other limiting factors as they were discovered. Not only are each of the elements commonly regarded as essential to plant growth recognized as having the potential to individually limit growth, but the law has also been expanded to recognize the limiting effects of cultural conditions like light, temperature, levels of soil moisture and aeration, insects, disease, and others.
Liebig used a barrel with staves of varied heights, like you see in the picture, to illustrate how his concept worked. Imagine the barrel also had a stave for light, soil moisture/aeration, temperature ..... for each and every potential limiting factor, insects and diseases included. The picture above is illustrating that in this case, N is the limiting factor. The plant is not growing as well as it could be because it is N deficient. When we add more N, and N is no longer the nutrient or potentially limiting factor in shortest supply, something else takes its place as the limiting factor. Even if the supply of N was increased to the point where it was in perfect supply, the least available nutrient or cultural condition would STILL be the limiting factor. We raise the stave representing N, but then another stave representing another resource becomes limiting.
You can see that if light levels are made perfect, it wouldn't compensate for the effects of a N deficiency or a soggy soil. If it could, we would be able to grow our plants in peat porridge with no supplemental fertilization at 32* F in a wind tunnel .... as long as it was a bright wind tunnel .... or we focused on perfecting light levels. The same is true of soils. The most perfect soil we are able to build will not make up for or 'trump' the effects of a nutritional deficiency or poor light.
Our goal then, is to try our best to make sure ALL the cultural conditions are optimum - making ALL the staves taller, as it were. It doesn't do us any good to make all but one stave taller, because it is that pesky short stave that is going to limit growth - EVERY SINGLE TIME! Surprisingly, it is not as difficult as it sounds.
Light and temperature are actually very easy. The onus of learning your plants' preferences for these cultural conditions is on you, but they are very easy to learn and easy to correct, so that issue needs no more attention. Insects and diseases might be a little tougher, but IPM practices are derived from common sense. Identify the pest/disease and use the least noxious remedy possible to reduce the problem to something below your tolerance threshold.
Modern fertilizers make it easy to supply nutrients at near optimum levels and in a ratio to each other that is favorable. Tucked into Liebig's Law is the fact that too much is as bad as not enough, so there is incentive for us not to cater to the idea that because a little is good, more is better. As we look at the barrel example, we can see that increasing the N supply so the N stave is taller than the P or K staves is not going to help. So, using fertilizers with a favorable ratio and applying them wisely is actually something we can all manage.
Because this is the Container Gardening Forum, the most frequent source of trouble and the issues that arise with the most frequency are soil related. Soil moisture and aeration are staves as critical as any other in the barrel. Just as a perfect soil cannot 'trump' the effects of other short staves, optimizing other conditions cannot offset or 'trump' the effects of a poor soil. The necessity of making sure your plants are adequately supplied with water is an obvious given. The effects of excessive water retention and inadequate aeration are widely discussed on the forum. You can learn how to avoid these issues entirely or almost entirely by reading about How Water Behaves in Container Media by clicking this highlighted text; or you can read some tips about
How to Deal With Water-retentive Soils by clicking on this highlighted text.
Keep learning. The more you know about how your plants grow, what cultural conditions they prefer, and the effects varying cultural conditions will have on your plants, the better equipped you are to deal with them, keeping all the staves tall and minimizing limiting effects.