In the previous post of this series we discussed how some nutrients can be in insoluble forms which are unavailable to the plant since nutrients have to be dissolved in water in order to be absorbed by the roots. This post will discuss other factors which will make nutrients unavailable to the plant.
While we won't get into the details of what pH is, simply put it's the measure of the acidity or alkalinity of a solution. 7 being neutral, and measurements below 7 being acidic and over 7 are alkaline.
pH is the most widely known reason for having a nutrient lockout, most growers are aware of it and work to remedy the problem. Simply put, while a nutrient might be in solution that doesn't mean that the roots are able to absorb it. Each nutrient has a very specific range at which it can be absorbed and unfortunately there is no perfect pH which is optimum for all nutrients. That being said 6.2 is the best compromise for all making nutrients available to the plant.
Be aware that this is not necessarily the case for all species of plants as some have evolved in more extreme environments. For example blueberries are native to bogs and so are accustomed to pH ranges of 4.5 to 5.5. Before adjusting pH be aware of the optimum pH for the specific plant which you are growing.
The growing medium has the biggest effect on pH levels. Water culture is the most susceptible to fluctuations in pH but it is also the easiest to correct, typically by adding a small amount of a Phosphoric Acid based pH Down for alkaline conditions of a Potassium Hydroxide based pH Up for acidic conditions (other acids or bases will work but these are recommended since they have the added benefit of adding additional nutrients at the same time as pH correction).
Most peat based growing media have been optimized for ideal pH levels, peat is naturally acidic but lime is added to compensate for this. When adding a fertilizer solution to this type of medium it should be adjusted to optimum levels before feeding the plants. Coco based medium generally has pH levels pretty close to ideal levels without having to adjust.
For those growing in soil testing pH is essential to optimize crop production and minimize wasting fertilizer. There are several do it yourself methods online and for the hobby grower this should suffice, but in the case of a high value crop a soil test from a professional lab is strongly recommended. Should the test come back too alkaline it can be lowered with sphagnum peat moss or by fertilizing with Ammonium Sulfate or Monopotassium Phosphate. In the case of acidic soil lime is recommended. Keep in mind that soil has a strong buffering capacity and so it can take a lot of effort to optimize the pH.
For serious growers it is strongly recommended that they invest in a good quality properly calibrated pH meter and regularity monitor levels, especially after an adjustment.
*Did You Know?*
Soil with less that a pH of 5.0 will solublize Aluminum in the soil which is very toxic to plants, damaging the roots preventing the uptake of water and nutrient.
This is the third and least know reason why nutrients can become unavailable to the plant. The theory behind it is simply that many plant nutrients have chemical and physical properties that are similar to other elements. When these other elements are in high concentrations the nutrients have a harder time absorbing due to competition. This antagonism can either be from other essential nutrients like between Magnesium and Calcium or by an element not usable by the plant like Sodium competing with Potassium.