HORTICULTURE HORTICULTURE
Optimizing Fertilizers Application for Fruit Crops
by Mark Longstroth
District Extension Horticultural Educator for SW Michigan
Many fruit growers ask me, how much they should fertilize their crops. The way they ask makes me think that they expect a quick, simple answer. There are a lot of things I want to know about your fruit planting before I answer that question. My job makes me an expert in growing fruit but the real expert in your trees is you. You know more about your orchard, vineyard or berry field than anyone. You know the variety, rootstock, soil type, and past performance of your planting, and you also have years of experience growing fruit at your location. My goal with this article is to get you to think about ways to cut your fertilizer applications and use that money elsewhere in your operation.
There are basically two approaches to fertilizing fruit plantings. Do what your neighbor does, or try to adjust your fertilizer rates to the needs of your plants. Perennial fruit crops are quite a bit different from annual crops. Corn and wheat growers can almost predict what their final yield will be from the amount of fertilizer they apply. Yields in annual crops increase as the amount of fertilizer increases. In the beginning, yields increase rapidly but after enough fertilizer is added, the yields do not increase as quickly and then yield flattens to a plateau. The point on the curve where the increase in yield is no longer worth the cost of the fertilizer to get that increased yield is known as the maximum economic yield. It is not the maximum yield, but the point at which more inputs into the system do not make economic sense because they do not justify their cost.
Fruit crops have maximum economic yields too, and applying more fertilizer than the crop can use doesn't make sense. Perennial fruit crops have deep root systems which can bring in nutrients from a large volume of soil. Because the perennials can store nutrients in the fall and reuse them again and again during later growing seasons, perennials do not need as much fertilizer as annuals. An annual plant's whole life is dedicated to setting as much seed as possible and sacrifices the entire plant at the end of the growing season. A perennial plant's goal is to make it to next year and the years beyond and set seed in the years it can. Different goals and different strategies.
Growers seem to worry most about if they are putting on enough fertilizer. They believe that if they put on just another hundred pounds of this or added some foliar nutrient mixture that their yields would increase. I think of this as the "if a little bit is good a lot must be better" syndrome. Please believe me when I tell you that you can put on too much fertilizer and it isn't always just a waste of money when you do. Too much nitrogen fertilizer results in excess vegetative growth and reduces the reproductive growth which we need to maintain large yields of fruit. In apples, excess nitrogen can result in soft fruit with poor color. In grapes, it will delay ripening. A thick vegetative canopy increases disease by slowing drying and increases the cost of disease control. Too much potassium can cause bitter pit in apples by upsetting calcium uptake. Is it really worth it if you are not seeing an increase in yield?
Determining fertilizer needs
You really shouldn't be applying fertilizer if you don't have a good idea of what your
fertilizer needs are. There are two types of tests which can answer this question, soil
tests and tissue tests. Soil tests reveal what the nutrient levels are in the soil and
tissue tests reveal what the nutrient levels are in the plant. I seldom see samples of
both from the same field. When I do, it surprises me how different they can be. Soil tests
are useful tools in determining the gross levels of nitrogen, phosphorus and potassium
(NPK) in the soil, as well as calcium and magnesium. More important is the soil pH, or
acidity, which determines nutrient availability in the soil. Soil tests should be done
every three to five years. Most growers base their fertilizer programs on soil tests. That
is the way field crop growers do it and it works for them. But I don't think fruit growers
should use that approach. Because perennials have a larger root system and recycle
nutrients, tissue testing is a better means of determining plant nutrient needs. Growers
are being penny wise and pound foolish to base their fertilizer programs on soil tests
simply because the soil test is cheaper.
Tissue tests should be done on a regular cycle every two to five years. (Plant Tissue Tests for Determining Fertilizer Needs of Michigan Fruit Crops, E-449, describes the procedures for collecting and submitting these tests for all Michigan fruit crops.)
Tissue tests give a accurate picture of the nutritional status of your plants. Eric Hansen, the fruit nutrition specialist at MSU, last year revised Michigan's two fruit fertilization extension bulletins (E-852, Fertilizing Fruit Crops; and E-2011, Managing the Nutrition of Highbush Blueberries). Cornell also has an excellent publication, Orchard Nutrition Management (1991). These bulletins have color pictures of nutrient deficiencies and discuss the signs of healthy and deficient nutrition in fruit plantings and should be in every fruit growers library.
Tissue tests are so accurate that we need to worry about contamination from minerals in sprays and interactions between the nutrient and crop levels in the plant. I do a lot of grape samples and almost always see extremely high levels of zinc and manganese due to the prebloom use of ODBC fungicides which contain these minerals. Heavy crops can concentrate nitrogen in the leaves and potassium in the fruit, so it is important to know what the crop load. If some nutrient is deficient it may cause high levels of other nutrients to accumulate giving the false impression of good fertility. If growers are using tissue tests to determine if part of their field is deficient in some nutrient it is a good idea to also take a sample of healthy looking plants of the same variety so that the two samples can be compared. These samples should be labeled so that the agent making the recommendation knows what you are interested in. I receive many samples with no comments not even the variety of fruit sampled.
Foliar sprays
The type of fruit, the variety and your cultural practices determines your fertilizer
needs. Growers are often looking for a foliar spray that will enhance their yields. I am
not a great fan of using foliar nutrient sprays to enhance yields. To me it is as if you
were shooting a shotgun in a dark room to let some light in. You might hit a window you
might not. Growers liken it to taking a vitamin pill every morning and claim that they can
see a difference in their fields. My reply is that I don't take vitamins unless I'm sick
and I can probably do the same thing to your field with a spray of urea and iron,
especially if it is in good condition. The real test is whether there is an increase in
yield or fruit quality. I do recommend foliar sprays usually to accomplish a particular
outcome in response to a specific need. Foliar sprays should be thought of as supplements
to your regular soil applications and not replacements for them.
There are good reasons to use foliar sprays to increase nutrient levels at certain times of year, or when soil levels are low and the soil application has not had time to become really effective. You should be applying foliar nutrient sprays to your plantings for a specific reason and not just because your neighbor says it works for him.
Soil types
The type of soil you are farming determines a lot of your fertilizer needs.
Soil tests will give you several important pieces of information. Soil type, cation
exchange capacity, soil pH and the amounts and ratios of the most important nutrients. The
soil type is determined by the amounts of sand, silt, clay and organic matter in the soil.
The cation exchange capacity (CEC) is a measure of the soils ability to hold on to cations
(positively charged ions, such as potassium, calcium and magnesium). The cation exchange
capacity is a good measure of the innate fertility of the soil and is determined largely
by the amount clay and organic matter. Soils with high cation exchange capacities do not
need as much fertilizer as sandy soils with low cation exchange capacities. Soil pH
determines how soluble different nutrients will be. Some materials such as zinc and iron
are less soluble at pHs above 5 and are really scarce if the pH is above 7. Other
nutrients become less soluble as the pH drops below 6. Most tree fruit crops like a soil
pH near 6.5. Blueberries like the soil pH below 5. Most of the fertilizers we use acidify
the soil and we use lime to restore the pH to the levels we want (E-471, Lime for Michigan
Soils, 1993). If the soil has a high cation exchange capacity it will change pH slowly and
will only need liming before the field is planted. Sandy soils with low cation exchange
capacities can change pH quickly and may need to be limed every five to 10 years to
restore the pH.
Nitrogen needs
Nitrogen is the fertilizer most needed for commercial fruit plantings. Sandy soils need
more nitrogen than heavy soils. If you have a cover crop you need to apply more nitrogen
for the cover crop which is competing with the trees for nutrients. If you prune your
trees regularly you do not need as much nitrogen because pruning will stimulate growth.
The amount of growth you had is a good measure of the vigor of your plants and we want
good but not excessive growth. Applying fertilizer for this year's crop doesn't make a lot
of sense because this year's fertilizer application will be used for shoot growth. This
year's nitrogen doesn't show up in the fruit until next year. Additionally, sandy soils
can't hold much nitrate nitrogen and growers should use split applications to provide
smaller amounts of nitrogen to the plant and reduce losses due to leaching.
Potash and phosphorus
Potassium or potash is another frequent fertilizer. Generally this is due to sandy soil
which can not hold a large reserve of potassium. Many growers on sandy sites apply annual
applications of potash in moderate amounts. Too much potash can result in bitter pit in
apples (see, Bitter Pit Management in Apples).
Many grape growers may think they were deficient in potash because many vineyards displayed the distinctive black leaf symptoms of potassium deficiency. This is not necessarily true because last year we carried a heavy crop which would have concentrated potassium in the fruit and reduced leaf levels. The dry soil conditions last year would also have reduced the plants ability to get the potassium it needed from the dry soil.
Both nitrogen and potassium are readily soluble in water and uptake of these nutrients is really driven by the evaporation of water from the leaves which draws soil water to the roots and carries soluble nutrients with it. This means that there is very little uptake unless there are leaves on the tree so most of the uptake takes place in the late spring and summer.
Phosphorus is seldom recommended because it is not very mobile in the soil and fruit crops rarely respond to it. Phosphorus fertilizers stay near the surface and slowly move lower into the soil profile where the roots can get to them.
Finding recommendations
Basic fertilizer recommendations and recommended tissue concentrations can be
found in the MSU and Cornell bulletins which I have mentioned earlier. You as a grower
have the ability to observe your plantings and determine what your fertilizer needs are.
You should be able to use your experience to fine tune fertilizer applications to your
specific needs. Both of these bulletins also include recommendation for fertigation, the
application of fertilizers through your irrigation system.
