With the price of fertilizer rising, orchardists are trying to use nitrogen in the most efficient and economical ways possible, says Lee Gale, horticulturist with Northwest Wholesale, Inc., in Wenatchee, Washington.
However, they can’t afford to cut rates any lower than they are now because of the impact it would have on production, he said. Most growers have already changed the way they use nitrogen and are applying less nitrogen than they did five years ago.
"We have to get nitrogen on," he said. "We can’t cut the rate of nitrogen. The only thing we can do is try to be a little more economical in our choice of nitrogen. Things like custom blends of fertilizers are probably something that won’t happen any more. Growers are going to be focusing on just the nitrogen applications."
The cheaper options are urea (applied both as ground and foliar applications), calcium nitrate, and ammonium sulfate. However, with the ammonium sulfate, which acidifies the soil, it’s important to watch the soil pH level to make sure problems with low pH don’t outweigh the benefits of the nitrogen, he said. "There are tradeoffs you have to make."
Growers are also trying to get the most out of the nitrogen they put on by applying it at the times of year when it will have the greatest impact. Gale recommends a split application. One application would be made in late summer, which would be after harvest for cherries and Gala apples, and another would be made in April or May after the ground warms up. It’s important to apply it during the periods when the roots are active, he stressed. The spring application is designed to stimulate vegetative growth, and the late summer application to promote fruit size.
Richard Leitz, a crop consultant who specializes in nutrition, said only about 30 to 50 pounds of nitrogen need to be applied in the fall for good tree vigor. He believes that foliar urea applications are more efficient than soil applications, though both are necessary.
Leitz, who grows tree fruits and grapes at Mabton, Washington, said urea or ammonium sulfate are the cheapest nitrogen fertilizers available, but he thinks organic growers may have the advantage nowadays because they’ve been used to having to find alternative materials. Now, conventional growers are also looking for compost-type materials, and a lot of fish byproducts are being used.
"I think the high cost of fertilizer is going to push us into a better fertility system,’ he said.
Growers will be making sure that the fertilizers they apply are working for them, he added. "They’re going to evaluate their decisions. They’re going to want to make sure when they put on $1 of fertilizer, they get $2 back. You’re going to want to know that what you’re doing is actually working for you."
For Granny Smith or Pink Lady apples, where green fruit is acceptable or desirable, growers might consider growing some of their own nitrogen in the form of a legume cover crop, he suggested. Dutch clover, for example, is a shallow-rooted plant that is not at all competitive with the tree. It can supply as much as 60 to 70 pounds of nitrogen a year and does not need to be mowed, because in mature orchards, the tree roots extend beneath the alleys.
A legume cover crop raises the fertility of the soil. On a sandy soil, if there are 10 to 20 pounds of residual nitrogen, the clover can raise it to 30 pounds in a few years.
However, one of the problems is that certain insecticides should not be applied when there are blooming plants in the orchard where bees might be foraging.
When trying to grow large fruit, it’s important to avoid growing big apples that are calcium deficient, Leitz said. To grow high quality fruit with good storability takes a good, balanced nutrient program that includes nitrogen, phosphorus, calcium, and micronutrients. "If you don’t have it all, it doesn’t work," he said.
Good calcium uptake in apple trees also depends on strong root growth, Leitz said. In the 2007 season, low fruit calcium was widespread in Washington State, and he attributes this to poor root growth in the spring. The weather was warm in March, but turned cold in April. Tree roots that had started to grow, suddenly shut down. There’s a window of opportunity to get minerals into the tree, and if trees don’t develop a healthy root system during the period from bloom to when the fruit are 25 millimeters in diameter, it can be difficult to make up afterwards.
"The only time you get calcium uptake is when you have actively growing roots," he said.
Roots need carbohydrates from photosynthates from the upper part of the tree in order to grow. If they’re not getting them, a phosphorus fertilizer can stimulate root growth and enhance calcium uptake, Leitz said. A 8-26-0 fertilizer that contains organic solids will dissolve gypsum and calcium carbonate in the soil and make it available to the tree.
Leitz recommends having an annual soil analysis done at the same laboratory each year so that the results are comparable. The ratios of the nutrients can indicate if there’s a problem. The amount of phosphorus in the soil is dependent on the pH level. Years ago, monoammonium phosphate (MAP) was considered a miracle cure for trees that were deficient, but Leitz said an orthophosphate fertilizer is far more efficient. He guesses that an application of just five pounds of orthophosphate will give a similar growth response as 30 to 40 pounds of MAP.
Once the orchard has good base fertility with the right nitrogen and phosphorus levels, the orchardist can then address magnesium and the micronutrients zinc, manganese, iron, copper, and boron. Unless the leaves have adequate magnesium and potassium, the photosynthates in the leaves do not translocate to other parts of the tree, as they should. Applications of biological products, such as humic acids, will take the nutrient status up a notch, Leitz said. "I’m not looking to replace anything. I’m going to the next level."