You should not be applying fertilizer without a good idea of your fertilizer needs. There are two types of tests to answer this question, soil tests and tissue tests. Soil tests reveal the nutrient levels are in the soil. Tissue tests give an accurate picture of the nutritional status of the plants. Because perennial fruit crops have large root systems and recycle nutrients from year to year, tissue testing is a better way to determine 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. The extra information in a tissue test is well worth the cost.

Too much nitrogen fertilizer results in excess vegetative growth and reduces the reproductive growth that 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?

Tissue tests should be done on a regular cycle every two to five years. Tissue testing is normally gone in late July and early August, later in the growing season when nutrient levels in the leaves have stabilized. 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. If you are using foliar nutrient sprays, you need to either leave some trees unsprayed or wash the spray residue off the leaves after sampling. Even minerals in your water can influence the test so you should use distilled or bottled water.

Heavy crops can concentrate nitrogen in the leaves and potassium in the fruit, so it is important to know what the crop load is when the sample is taken. 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 a 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.

For tree fruit and blueberries collect 100 leaves from the middle of this years growth. Collect these leaves from as many different plants as possible within the sample area. Wash the leaves by swirling them in soapy water then rinse and blot dry. Place the dry leaves in a paper bag and let them dry at room temperature. Samples can be submitted to any MSUE county office.

Grapes are a little different. In grapes, we usually wait until the beginning of veraison, in early to mid August when the berries change color. The tissue sampled is the petiole or leaf stem and not the entire leaf. The sample is taken from the most recently matured leave on the shoot. It is not necessary to wash grape petiole samples.

"Plant Tissue Tests for Determining Fertilizer Needs of Michigan Fruit Crops", E-2482, E-2482 PDF File, describes the procedures for collecting and submitting these tests for Michigan fruit crops. MSU has two other fruit fertilization bulletins, E-852, "Fertilizing Fruit Crops", and E-2011, "Managing the Nutrition of Highbush Blueberries" E-2011 PDF File. 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 grower's library.