Mineral nutrients are the last big requirement. Though we often say we’re
"feeding" our plants when we fertilize, that’s not quite accurate. When we
fertilize plants we are providing them with the nutrients they need so they can
manufacture their own food by the process of photosynthesis.You can think of mineral nutrients in plants as similar to the
vitamins and minerals we get from supplements. Without the vitamins and minerals our
bodies require, we wouldn’t be able to carry out all the various metabolic functions
necessary for good health.
The nutrients plants require in relatively large
quantities are: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium,
magnesium, and sulfur. These are called the macronutrients.
Plants also need a number of other nutrients, though in much smaller quantities. Plant micronutrients include iron, copper,
zinc, manganese, molybdenum, and boron. (Some species require additional micronutrients,
and scientific research continues on the subject of plant mineral needs.)
Plants get carbon, hydrogen, and oxygen from the
atmosphere. And, as we mentioned in the previous section, some plants can fix atmospheric
nitrogen. Plants must get all other minerals from the soil, or from fertilizers. Some
soils are naturally deficient in certain minerals; others have had their reserves depleted
through continuous farming. In either case, if you want to grow healthy plants in that
soil, you’ll need to supply the missing minerals. Diagnosing mineral deficiencies is
complex, and we’ll cover it in more detail in the soils course. Here are a few
generalizations:
If the entire plant is chlorotic—that
is, it looks pale and weak—it may be suffering from nitrogen deficiency. Chlorosis
can also be caused by an iron deficiency.
Phosphorus is important for good root development, which
is why it is often added at planting time. Deficiency symptoms include poor growth and
sometimes a purple tinge to the foliage.
A potassium deficiency sometimes shows up as yellowing or
browning of leaf margins, especially in older leaves. Another symptom is weakened stems.
