At a very practical level, if you are using a balanced low urea commercial fertilizer at a reasonable rate (say 50 –100 ppm nitrogen), your likely only serious mineral nutrition concerns are calcium and magnesium levels (which are related to also to your water analysis), and the pH of your irrigation mix. If you are using a pure water source then to a certain extent your resolution of the calcium and magnesium levels is somewhat easier, you simply have to supply them at the level you want. Several commercial fertilizers are available that provide one or both of them, but many have neither. Your choices are to select one of the fertilizers that contain sufficient levels, or to feed calcium nitrate once every 2-3 weeks and Epsom Salts (magnesium sulfate) at another feeding every 2 – 3 weeks. Even some of the fertilizers that have these two elements in them contain them at sub optimal levels for use in pure water, so a periodic feed of calcium nitrate and Epsom Salts may still be indicated even with them. If you are working with water with higher concentrations of these elements you may need to choose a fertilizer that does not contain them to maintain as much balance as possible in your program. As discussed earlier, if you have extremely high water salinity, you may have to fertilize at well below the sub optimal levels to avoid salt build up problems for your plants roots.

Water/ fertilizer pH is another commonly overlooked area, and an area where I think many folks could drastically improve their plants health. Various elements are available at different levels depending on the pH of the solution they are dissolved in, with some becoming virtually unavailable if the pH goes too far out of range. We use a base line pH in the 6.6 range for our irrigation mix, and use calcium supplements on the mix of strongly calcicolous plants and those associated with ultrabasic rock. This is explained in more detail in Use of Calcium Based Supplements for Paphs. Be aware that most municipal water supplies have artificially raised pH’s to reduce leaching of toxic substances into the water. Also, purified water is extremely susceptible to wide pH swings when fertilizer is added, having little buffering capacity (our RO with Peter’s Cal-Mag at 100 ppm N drops to about pH 4.3, with Jerry’s Grow Foliage formula at 1 teaspoon per gallon it gives us pH 6.4). Rain water has both the lack of buffering and the possibility of very acid pH to start with. Fertilizers differ in their alkalinity or acidity depending on composition. You may be able to match a fertilizer to your condition to arrive at an acceptable pH, for example an acidic fertilizer used with pH 8.0 municipal water may end up at a range in the high 6’s. It is important to know the pH of your final irrigation mix, and make adjustments if necessary to get it into an appropriate range. We use potassium hydroxide to raise our pH, phosphoric acid is commonly used to lower pH. Vinegar is commonly mentioned as useful for lowering pH, however we do not have experience with it. It is also possible to use a limited amount of potassium silicate to raise your pH, follow label directions to avoid using too high a level. Do not try to raise the pH levels of fertilizer concentrates that contain calcium or magnesium and phosphorus, you may cause precipitates to form. The pH adjustment of these fertilizers must be made in the diluted form.

The charts below summarize additional information regarding mineral nutrition. The relative numbers of atoms in plant tissue is a generalized approximation based on molybdenum being 1, to give a rough’ idea of the relative needs of the plants for each of these elements, note that it is based on a number of atoms basis, not weight basis. It illustrates the large differences in the plant’s needs for macro versus micronutrients, and also helps illustrate the higher needs for calcium and magnesium, which are often overlooked, and are present in reasonable quantities in only about 1/3 of the commercial fertilizers I’ve surveyed.

The elements tranlocatability in the plant is also important in disgnosing deficiencies. If the element is translocatable, that is the plant can remove it from tissue in one area and transport it for use in another, then symptoms of deficiency typically appear in the older tissue as the plant mobilizes the element from the more expendable older tissue to the newer growing areas. Conversely, if it is not translocatable, then the deficiency will show more in the new growing areas of the plant. Another diagnostic indicator to pay attention to is the location of the symptoms, are they marginal, interveinal or generalized on the leaf? Again, I’d like to point out that the most likely cause of a mineral deficiency (unless you are not fertilizing at all) is the loss of the roots of the plant, leaving it unable to take up the needed nutrient levels.