Nutrient Requirements of Fruit Production

By Tami Schlies


Bob Purvis observed some nutrient deficiencies in fruit trees during his visit here in September and was kind enough to share with all of us some of his knowledge on these issues.  I have also investigated some other common deficiency symptoms and expanded on his advice in this article.


We all know that plants need 3 major nutrients to grow well – Nitrogen (N), Phosphorus (P), and Potassium (K).  Bob also brought up Boron (B) and Calcium (Ca) as essential to good fruit production.  In particular, B, Ca, and K are essential in fruit quality and storage, even though trees with minor deficiencies in these elements may produce just fine.


According to Bob, each year an apple tree needs about 1/10 pound of nitrogen per year of age for optimum growth and production.  To figure out how much of whichever fertilizer you use, look at the label – the first number is how many pounds of nitrogen in every hundred pounds of fertilizer are in it.  So if you use a fertilizer with a ratio of 10-0-0 you would need to apply 1 pound to be adding 1/10 of a pound of nitrogen.  Sandy soils will need fertilizer applied in smaller amounts more often, while clay soils may require less fertilizer and need it less often.


Younger trees should put on more growth than older, bearing trees.  New apple trees should grow at least 18 inches a year and mature ones should put on 6 to 12 inches.  If your tree is putting on little or no shoot growth, has yellow-green foliage that appears first in older leaves, or perhaps has very small fruit size even if you fruit thinned, you may need to add nitrogen.


Apples are extremely good at withdrawing phosphorus from soil, so they are not commonly deficient.  However, our cooler soils have been known to need higher levels of P for other crops.  Trees low in P may exhibit purple leaf veins, petioles, and young shoots early on in the season and then green up later, sometimes becoming very dark green in the expanding leaves.


Apples use a lot of potassium due to heavy fruit load.  If there is a deficiency, it will be seen in older leaves that scorch along the edges and then curl upwards, especially on stone fruits.  The plants will also flower poorly and have low fruit set.  Lack of K may also show up as poor color development on red fruited apple cultivars.  Pears in particular may need extra amounts of K to set fruit.  Potassium also helps promote frost resistance in blooms and is necessary in the maturation process of woody stems for winter hardiness.


Magnesium (Mg) is another nutrient needed in regular amounts for fruit production.  Mg deficiency can be caused by excessive watering or using to much high potassium fertilizer.  It is more common in highly acidic soils.  Yellowing will appear between the leaf margins, leaving behind red or yellow pigments, usually in older leaves first.  Epsom salts are a quick fix at a rate of 8oz in 2.5 gallons of water used as a foliar spray (you can add a few drops of dish soap to act as a wetting agent.)  This will need to be done every couple of weeks, or you can apply epsom salts to the soil at 1.5 oz per square foot.


Unlike the major nutrients, Boron and Calcium deficiencies are seldom seen in the leaves of fruit trees.  Usually it is not until fruit set that the lack shows itself.  Boron deficiency can lead to terminal dieback and certain bark conditions, but mostly it is noted in a tree that flowers well and yet has poor fruit set.  Apples that do set may have cracking early on that is not due to over watering.  The cracks may heal and leave black wounds on the fruit.  The flesh may also be corky in areas.  Pear trees in particular are sensitive to boron deficiency.  Alaskan soils sometimes exhibit B deficiency first in crops of brassicas with a disorder called whiptail, where the leaves grow deformed.  It can also cause beets to have black spots and carrots to split open.  There is a very fine line between boron deficiency and boron toxicity, so care should be taken in application.  Bob suggests a foliar spray made of 1T of borax per 1 gal of water, soaking the tree during the growing season one time each year.  Do not apply boron if you do not have good reason to suspect a deficiency.  It is probably best to get a soil analysis first.


Calcium is often provided in the lime we use to bring our acid soils into a better pH range.  But some fruits need more than that application provides.  Ca provides disease resistance, makes apples sweeter, and makes strawberries firmer.  Tomatoes are notorious for blossom end rot, a disorder related to a lack of calcium either because of infrequent watering or low Ca in the soil.  Apples that are low in calcium may develop bitter pit, which shows up as sunken areas on the bottom of the fruit that turn dark and develop corkiness in the flesh beneath.  Constant water is the best treatment for keeping calcium levels constant.  There are also several products on the market that can provide a quick boost of calcium for your plants if you have had these symptoms on your fruit in the past.