Mid-summer leaf samples for nutrient analysis

Now is the time to collect leaf samples for nutrient analysis, which is the best way to assess the nutritional health of perennial fruit plantings.

As shoots grow and leaves age, nutrient concentrations change. Mid-summer is the standard time to sample because levels of most nutrients are relatively stable and are most easy to interpret by comparing them to known values. Sampling tissues at other times can also be useful to diagnose specific problems. In this case, samples from affected and healthy plants are needed for comparison since desired values at non-standard sampling times are less well-defined.

Many growers rely too heavily on soil testing to guide fertilization practices. Although soil tests provide a useful measure of pH, soil phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) levels are often misleading because they do not closely reflect levels in perennial fruit plants. This may be due to several factors, but the bottom line is that basing fertilizer choices on soil nutrient levels only is inadequate. Most importantly, there is no effective means of monitoring soil nitrogen (N) availability to perennial crops, so soil tests are of no value in guiding fertilization decisions for this key nutrient.

Leaf analyses can be used to diagnose nutritional problems and to identify developing problems before growth or yield is affected. Sample young plantings every one to two years and established plantings every two to three years. The whole farm can be sampled in the same years, or portions sampled more frequently.

Define sampling units. Divide the farm into sampling units or areas that have uniform soil types, management history and variety. Farms with variable soils or history will require more sampling units to provide an accurate picture of the nutritional health. If the farm is very uniform, with large blocks of the same age and varieties, units can be as large as 15 acres.

Sampling. Sample leaves in late July to early August. Collect at least 50 leaves from different plants throughout the sampling unit. Select healthy leaves from the middle of this year’s shoots. If the leaves are dusty, rinse briefly in tap water and lay leaves out on a table top until they are dry to the touch. For vineyards, only the petioles, or leaf stems, are collected.

Submitting samples. Package leaves in clearly labeled paper bags and send them to a reputable laboratory.

Diagnosing nutritional problems. If you wish to diagnose a suspected nutritional problem, collect one sample from plants beginning to develop symptoms of the problem and a second from nearby healthy plants. These samples can be collected at anytime during the season.

The total cost tissue analysis (sampling labor, postage, laboratory fees) can be as high as $40. However, if the sample represents 10 acres, per acre cost is $4. This small input can readily be covered if results show that fertilizer rates can be reduced. Test costs are incidental if fruit yields or quality are improved due to fertilization changes.

Some Midwest labs offering tissue analysis (no endorsement is implied)

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