What’s  all those number on fertilizer for?

 Saturday June 07, 2003



Ever wonder what  fertilizer numbers mean?   Should I use  5-10-10  or  12-12-12  on my tomatoes and corn?   Perhaps this explanation will help you.  Those numbers tell you HOW MUCH  of  certain  desirable  chemicals are in the fertilizer.   Here in Southeastern Ohio we badly need the last 2 nutrients in fertilizer.  



The first number is always NITROGEN.   Nitrogen is what gives our lawns that nice dark green color.   Nitrogen is necessary for plant growth.  Corn grows very tall, and therefore needs 2 applications of nitrogen; one once it is knee-high.  However, too much of a good thing is not good.  Too much nitrogen can kill our lawns !   Somewhat higher nitrogen levels are fine with leafy crops, like lettuce.   But plants that have fruit (eg. tomatoes) are actually hindered by excess nitrogen.  Tomato plants that get too much nitrogen grow very big, but have few tomatoes hanging on the vines.



The second number is for PHOSPHORUS.  Phosphorus gives the plants health and fruit.  Proper amounts assist the plant in disease resistance.  Also it promotes seed germination and fruit (or flowers) production.  



The third number mention in fertilizer is POTASSIUM.  Potassium assists in good root development.  It is also called  by the name POTASH.





What is pH?   This is a measure of the soils acid  or  alkalinity (opposite of acid).  Just like we measure electric by voltage, soil can be measured by its  pH number.   This number is between 1 and 14, with 7 being neutral (not acid or alkaline).   The further you get from the number 7 the stronger the pH gets.  Acid is measured from 1 upto 7.  Alkalinity is measured  beyond  7  to 14.  


Why do gardeners care about this pH number?  It controls the plants ability to digest nutrients already in the soil.   For example I could have the best chemical balance in my garden's soil, but have a pH of  9  that keeps the plants from digesting any nutrients. 


How do I change pH in my garden?  Lime makes the soil more alkaline.  Sulfur makes soil more acidic.  I suggest you adjust your garden's pH in the Fall, after you put the garden "to bed" for the winter.  Do not mix lime with nitrogen fertilizer.  The two chemically react and basically neutralize each other.


How do I know what my garden needs?  Two suggestions.  One is to buy a soil test kit and do it yourself.  They are easy to use, can be used many times  and sell for around $20.  The other suggestion is to have your County Extension Agent test it.  He usually charges about $15  for one test.  I test my own, but once let the extension agent do a professional test to see how accurate my own results were (they were very close). 


Most gardens like a pH around  6.7  (slightly acidic).


Various plants prefer various pH levels.  Here in Southeastern Ohio raspberries grow like crazy along old railroad roadbeds.  Why?  They like the acidic pH  levels found in the coal cinders once used between ties (modern tracks use limestone = alkaline = few berries).  Rhododendron bushes like an acidic pH around 5.0 .   Cabbage likes 6.7 ,  lettuce 6.5 , sweet corn 6.5 , grass 6.0 to 6.5 , tomatoes 6.5 , and peppers like 6.5 .   Don't go crazy trying to adjust your soil's  pH perfectly; just be close to these numbers.   So if your rhododendron bush doesn't flower, test the soil's pH. 


 How "picky" should I be with these pH numbers?  In Sudbury's soil testing kit they say that a pH above 7.3 will prevent plants from taking in phosphorus.

Soil with a   pH above 9  isn't  going to grow anything but tough weeds.