A well-planned fertilizer program for your Thuja Green Giant hedge can make the difference between simply good growth, and spectacular, dense and healthy growth, giving you a solid hedge or screen several years earlier than it would otherwise happen. In this mini-series, we have been looking at fertilizers, not just saying ‘this one is good’, but giving the basics. If you understand plant nutrition, then selecting a fertilizer from the array available becomes a thought-out activity, not a hit-or-miss process. You can tailor your choice to what you know your hedge needs, depending on how it looks, and at what stage it is growing. You can make smart price choices, and understand the value of certain ingredients. As well, you have the pleasure of knowing more about your plants, and realizing that good gardening is something that can be learned, without the required ‘green thumb’.
In the first part of this series, posted last week, we looked at the ‘Big Three’, the major plant nutrients – Nitrogen, Phosphorus, and Potassium – that figure in the three numbers of the Fertilizer Ratio on every bag of fertilizer. We looked at what they do, and their roles in the good health of your plants. To summarize that, we can say the Nitrogen is the ‘growing’ nutrient, that causes shoots and leaves to develop; Phosphorus is the ‘rooting’ nutrient, that helps your plants develop strong, extensive root systems; and Potassium is the ‘protecting’ nutrient, that strengthens cells, and makes them more resistant to cold, insects and diseases.
Now let’s look at some of the important minor nutrients, which, like vitamins for us, are only needed in small quantities, but which are just as important as those Big Three.
The Minor Plant Nutrients
There are several nutrients that are used by plants in moderate quantities, although a lot less than N, P, K (these are the scientific symbols for the Big Three). These are Calcium, Magnesium, and Sulphur. Only in very acidic soils, with pH values below 5.5, can there be a lack of calcium and correcting that is not a matter of fertilizer, so we will put that one aside. Sulphur is present everywhere, and is almost never an issue for plants. That just leaves Magnesium.
Magnesium
We all know about the ‘wonder chemical’ called chlorophyll in plants that turns sunlight into sugar. It is what makes leaves green, and it is what feeds everything on the planet, directly or indirectly. Inside the heart of this big molecule is a single atom of magnesium. Without enough of that metal, no matter how fast the plant tries to grow, it will not be able to. Plants can rob older leaves to feed the more important younger ones if magnesium is in short supply, and Thuja will do that, leaving the older parts of the stems yellow instead of green, while the growing tips still look healthy.
This is not very common, mainly because most good fertilizers include magnesium in them. Look for the letters ‘Mg’ to find out if the fertilizer you are looking at has some, which will usually be listed as a percentage. It doesn’t have to be very much, and sometimes it isn’t even needed, but it’s good to see some in there.
The Micro-nutrients
These nutrients are also called ‘trace elements’, and both names tell us that they are only needed in minute quantities. These are sometimes called ‘vitamins for plants’, because they are just as important to plants as the big nutrients, but only tiny amounts are used. There are several, but only a couple are of importance. Iron, boron, manganese, zinc, copper, molybdenum, nickel and chlorine are the micro-nutrients that plants need, bringing the total needed to just 14 elements. Others are sometimes listed, but at this point those are not strictly essential, as they can be replaced with one or other of the 14. From a practical point of view most of this list can be ignored, since some, like chlorine, are so widespread that it is almost impossible to even demonstrate in hydroculture that they are essential. Nickel is needed only in very, very, minute quantities, and it need special equipment to even detect such small amounts. In most parts of the country, boron, manganese, zinc and copper are common in soil, and no supplements are needed. That just leaves iron, which we will look at in a moment.
Because it is hard to decide how much of these nutrients are needed, in modern fertilizers they are almost always simply included in small amounts. You will usually see them listed somewhere on the bag, given in ‘ppm’, which stands for ‘parts per million’. This is a commonly-used way of expressing very small amounts of something. Only the cheapest fertilizers will have no micro-nutrients, and the good news is that the materials in them will probably be a little impure, so they will be ‘contaminated’ with enough of these elements to provide your plants with what they might need.
Iron
This element is the only micro-nutrient that is regularly needed by plants as a supplement. We said that magnesium is in the chlorophyll molecule, well iron is in the enzymes that make chlorophyll, so no iron means no enzymes, which in turn means no chlorophyll. However the difference is very easy to see, because iron deficiency shows up on young, new shoots, which turn pale yellow. Like the other micro-nutrients, iron will often be in fertilizer you buy. Look for the letters ‘Fe’, which is the chemical symbol for iron. To get maximum growth from your Green Giant Hedge, iron is an important additive, since it will give your hedge that rich, lush green color that makes such a perfect backdrop for your garden. Nobody wants a pale hedge, and making sure you are adding iron will prevent that. This is by far the most common micro-nutrient deficiency seen, especially on soils that are alkaline, or if you have recently put down a lot of lime, hoping to improve your soil.
Enough for Now. . .
We seem to be on a roll here, so next week we will look at using organic fertilizers on your Thuja Green Giant hedge.