Plant Nutrition : Macronutrients 

While we wait for the snow to melt and our gardens to dry out, let's review what plants need and   what fertilizers provide.   Also, if you haven't had your garden soil recently tested and think your vegetables are not up to snuff, we'll identify some of the major "aha" nutritional deficiencies you can look for....   You'll just have to try to remember all this once the growing season starts...

----------------------------------------------------------------------------------------------------------------------

Every bear in the woods probably knows that plants use the three elements, Hydrogen, Oxygen and Carbon. Plants obtain these three  from  air and water.  But there are 14 other elements that are vital for plant nutrition. Of these, six are identified as "macronutrients" with the remaining 8 elements characterized as "micronutrients" or "trace elements."  Plants actually use other elements as well, but these 14 are needed by all plants. We'll cover micronutrients later  in  a separate post. 

Macronutrients

 Lack of Ca. Source: Cornell Extension

Calcium (Ca)  - Calcium is generally available in soils and is taken into plants through evapotranspiration (water siphoning from the roots through the leaves to the atmosphere). It is the key ingredient  in plant  cell walls. During times of drought (and/or a gardener's under watering) or high humidity, plants do not move enough water through their tissues to supply cell needs resulting  in calcium deficiency.  This is important to note because calcium may not necessarily be deficient in the soil. In either case, this deficiency dramatically manifests itself in tomatoes and peppers, for example,  as blossom end rot. An over abundance of calcium is more subtle to detect because  too much calcium can interfere with the take up of other elements whose deficiencies may exhibit markedly different symptoms.

Chlorosis.  Source: Cornell Extension

Magnesium (Mg)  - Also typically available in soils, especially in clays.  It is incorporated into chlorophyll and plant enzymes.  Its deficiency appears initially as raised areas of  light yellow in mature leaves.  Later, these areas become more pronounce and eventually turn into dead leaf areas (chlorosis). 

Nitrogen (N)   - Nitrogen is utilized by plants in several forms,  but let's skip  over the complex chemistry of the Nitrogen Cycle and just say  that  plants  absorb  these  compounds  incorporating  much of it into   their leaves and the enzymes essential for photosynthesis.  A deficiency will first appear as a paling of mature leaves, then evolve to  yellowing before ending in chlorosis (dead leaves).  Although  nitrogen is essential for plant growth, too much will also cause chlorosis in leaves.  

[n.b. Fertilizers meant for house plants  are typically loaded with nitrogen; and if you use them on vegetables, you will get lots of vegetative growth and less fruit.]

Excess Phosphorus.  Source: U Maryland Extension

Phosphorus (P)   - Phosphorus is used in cell membranes and serves as the primary carrier of energy throughout plant cells.  It is incorporated into DNA and RNA as well as  regulating enzyme activity. It generally occurs naturally in soils, but its availability to plants depends upon soil pH.  A deficiency shows up as dark green or  purplish tints in tomato leaves. Eventually, flowers and fruit do not develop.

 

Potassium deficiency.  Source: Yara.us 

Potassium (K)   - Plants need potassium almost as much as they   need nitrogen, and   potassium also abets nitrogen take up. It maintains various metabolic activities by regulating water within the plant. Potassium deficiency may first appear as yellowing of older leaves before they turn brown. (This may also be called leaf scorch or leaf burn, but other conditions such as exposure to salts, insecticides or too much fertilizer might also cause it.)   The taste and appearance of fruit can also be affected.  

Sulfur deficiency.  Source: haifa-group.com

Sulfur (S)  - Sulfur is utilized in amino acids, proteins, vitamins  and other compounds, e.g. sulfur gives onions and mustard their distinct flavors.  Although sulfur also occurs naturally in some soils, its concentration is relatively low compared with other nutrients and  tends to be  especially low  in sandy soils. Much of the sulphur that is found in soils derives  from the decomposition of  organic materials, e.g. compost.  [This is another  good reason for you to start composting.] A very small amount of  sulphur can also  come from the atmosphere.  Its deficiency appears as yellowing of younger leaves and woody looking stems.

Nutrients and Fertilizers

From what I have observed of neighbors and friends who  try  container gardening, nutrient deficiencies  can show up  relatively quickly because the gardeners do not replenish  the soils from year to year, fail to fertilize during the growing season and/or do not maintain adequate moisture levels. For gardeners, like me, who dig up their front or backyards, deficiencies may take many growing seasons to appear because we are accessing  a greater reservoir of nutrients. Maintaining a consistent moisture level (apart from timely cultivating) is probably our major challenge!

Soils typically have all of the above macronutrients to some extent because these elements  originate from mineralogical sources. However, large percentages of these elements  frequently  are chemically bonded with other compounds in the soil  and rendered  unavailable to plants.  Over time  the usable forms of these  elements   decline as  crops are harvested, remaining plant stock  removed (i.e. diseased plants pulled up and stuffed into lawn bags), leached or washed away by rains etc. Adding compost, natural or synthetic fertilizers replenishes these otherwise lost nutrients.

When backyard gardeners talk about fertilizers, they  are mainly referring to  commercial balanced fertilizers that supply N-P-K (Nitrogen, Phosphorus and Potassium), three key nutrients of the macronutrient group. The concentration of these elements is typically cited as percentages of package weight, e.g. 5-10-5, 10-10-10. The amounts of other macronutrients may or may not be mentioned, and micronutrients hardly ever. The N-P-K combo get the spotlight because they are significant in foliar, root and fruit development; but, as hopefully discernible  from the above text, they are not the whole story.

Fresh & Mature Compost.  Source: permaculturenews.org

Commercially produced fertilizers offer the advantage that they tell you the amounts of some or all of the macronutrients you are getting as opposed to what you sprinkle on your garden from your compost pile. Nutrient specific fertilizers also are available to address  specific soil needs, e.g. sodium nitrate or superphosphate, that might have been identified from a soil test.  Commercial fertilizers derived from organic sources, e.g. those containing bone meal, fish meal,   vegetative compost, manure etc.,  will include more nutrients and also be sterile. The best summary about  the types fertilizers available, I have found, is this one from the Weekend Garden Net.   Scroll past the self-test and look are the chart.

Putting your compost on your garden and mulching does a lot to maintain nutrient levels, especially micronutrients; and if you do not observe any plant problems, keep doing it. Just be aware that our backyard compost piles typically show a N-P-K ratio of around 1-.5-1.  Although you might come to think that plants need a 7 course fertilizing banquet, in reality they make do with very little.  For example, if you want to apply a liquid fertilizer to your vegetable garden, Cornell suggests the following ratio of a dry fertilizer (15-15-15) to water: 1 1/4 ounces of dry fertilizer to 2 1/2 gallons of water. This concentration provides a nutrient drink of only a couple of hundred parts per million!   If you were going to use this concoction for seedlings, cut it down to about 1/8 of a teaspoon. [And, REMEMBER: it is always better to use less fertilizer than too much and do not put either dry or liquid fertilizers directly on plants.] To get better guidance about using fertilizers in your garden, see the Monroe County Extension's publication,  Soil Preparation and Fertilizers for the Home Flower and Vegetable Garden.

"If the grass on the other side of the fence appears greener... it must be all the fertilizer they are using." -  Kevin Rodowicz, MD