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When is the best time to fertilize fruit trees? In this interview, Tree Fruit Production Extension Specialist Robert Crassweller, Ph.D. from Penn State College of Agricultural Sciences, explains how the nutritional needs of fruit trees change throughout the growing season. The interview is lightly edited for clarity. To listen to the entire, hour-long interview, download the podcast below.
Robert Crassweller: That's a good question. NPK of course stands for nitrogen (N), phosphorus (P) and potassium (K). And that's why you see NPK values on the labels of fertilizers. Those are what we call macronutrients. When I went to school, there were 16 essential nutrients that are needed. And one of the ways we learned how to remember them was a little memory trick where we had a saying, it went C. Hopkins Café Mighty Good Clean Mob Comes In (or C. HOPKNS CaFe Mg Cl MoB CuMnZn)
C. HOPKNS CaFe Mg Cl MoB CuMnZn represents 16 of the key essential nutrients that plants need. It stands for: Carbon, Hydrogen, Oxygen, Phosphorus, Potassium (K), Nitrogen, Sulfur, Calcium, Iron (Fe), Magnesium (Mg), Chlorine (Cl), Molybdenum , Boron, Copper (Cu), Manganese (Mn) Zinc (Zn).Tree Fruit Production Extension Specialist Robert Crassweller, Ph.D. from Penn State College of Agricultural Sciences
Robert Crassweller: The easiest way to find out if your tree is malnourished is to through plant analysis. And that's where you take some of the leaves, and you dry them off, and you send them off to a laboratory. They'll analyze these chemically and they'll give you a percentage of which nutrients your plants lack.
Robert Crassweller: In some cases the tree can tell you if you know what to look for, but usually when you see those symbols or to see those symptoms, you're really in trouble. For example, if you're low on nitrogen, your tree won’t be getting as much shoot growth. Or the plants are a little bit yellowish and not really lush green. And by the same token, if your plants are really lush green and have a lot of growth, then they have too much nitrogen. So you can have too much of elements as well as not having enough.
Robert Crassweller: We do. The way fruit trees come out of dormancy is that as temperatures start to warm up, then they start to have a metabolic reaction. They start to use the reserves that they put down last fall which they stored over the winter in their trunks, roots, branches and stems.
So early in the spring, most of the growth is coming from those reserves that are in the tree. And so we want to fertilize about four to six weeks before bloom because it’s going to take that much time for those nutrients to move down into the soil and get reabsorbed by the tree once the soil temperature warms up. And they use these reserves to start off vegetative growth.
Robert Crassweller: This time of year, they probably need nitrogen, phosphorus, potassium, maybe calcium. W may need to adjust the soil pH. They may need some magnesium if they are magnesium deficient. But basically they need what we call the “macronutrients”. Those are the nutrients that are needed in the greatest supply.
Robert Crassweller: If you're going to use compost or organic matter type of fertilizer, then that also needs to go on early. And that's because it now needs to be absorbed down into the soil. It needs to be transformed and then it'll be available for the roots to take up into the tree.
Robert Crassweller: The initial growth up until about bloom, is using reserves that are in the tree. Once you get into bloom and thereafter, your tree is now using all the nutrients that you applied in your late winter, early spring fertilizer. The roots are taking the nutrients up and they're distributing into the tree. And depending on what nutrient it is and where the biggest amount of growth is, that's where these nutrients will be delivered to.
With the stone fruit, like peaches and cherries, the first thing the tissue opens up are the flowers. In pome fruit, like apples and pears, what happens is actually the shoot starts growing and then the flowers come out. And so there's that little bit of lag where the flowers are set. So really in the spring, for fruit to set on stone fruit trees, they really need to make sure they had enough reserve from the previous year to support the initial fruit set of those stone fruit. Whereas apples, they've got to support shoot growth too, and then afterwards they have to set flower. So, pome fruit may need that double dose of nutrients.
Robert Crassweller: Calcium relates to a problem that some apple trees get called “Bitter pit”. That happens when there is a calcium deficiency. The easiest way is to use calcium sprays starting shortly after bloom and about every 10 to 15 days application. We used calcium chloride. But there are other calcium products you can use.
Robert Crassweller: The critical point when growing fruit trees is we want to have fruit to set, right? So there are definitely nutrients that are needed during that time. Initially fruit set and growth is primarily by cell division. So all the cells are dividing. Later in growing season it's by cell expansion. And so if you have a lot of cells set and divide early, right after bloom, then you have more cells that can expand later on and so your fruit size is larger.
So you need to have good nitrogen levels. The other thing that is important, really critical during pollination, is boron levels. And so we need to have good levels of boron. And a lot of commercial growers will actually make foliar applications of boron fertilizer, either in the previous fall previous or shortly after flowering.
Some growers will throw on a foliar spray of nitrogen. But most of the time you've got enough if you did it right in the spring. Now the boron, this is from the standpoint of homeowners, this is going to sound crazy, but a tablespoon or two per gallon of 20 Mule Team Borax. This product will give you your boron and you can dilute that and spray it on your trees if they have a boron deficiency.
You would use one to two tablespoons of 20 Mule Team Borax and a gallon of water. And spray it on during bloom.
Robert Crassweller: You can do a leaf tissue test. And it really depends on the age of the tree. Normally what we would say is you apply, for apples, apply 0.02 actual pounds of nitrogen per year of the tree’s age.
So we're looking for 12 to 18 inches of shoot growth on apple trees on an annual basis. On stone fruit like peaches, we're looking to say 18 to 24 inches. Pears are somewhat less than apples. Cherries are in between peaches and say apples. So you want anywhere from 12 inches and up to 24 inches of shoot growth depending on what type of fruit crop you're growing.
Robert Crassweller: Normally the range we like to say is somewhere between 6 to 6.5. If you go out in certain areas on the west coast, it may be a little bit higher like six to seven.
Robert Crassweller: Okay, we've gone through the season, we're harvesting the fruit, and all of a sudden you look up, "Well, there's a little bit of cork and bitter pit on our fruit or doesn't look right." A lot of time commercial growers will apply some of the micronutrients in the fall.
Those micronutrients are only required in very small amounts. Those include boron, manganese, zinc, and iron. So a lot of times, in the fall once the fruit is off but before the leaves drop, commercial growers will come in with a micronutrient spray.
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