Value of Leaf Analysis vs Soil Tests
Fertilizing orchards and managing nutritional needs is completely different from agronomic and vegetable crops. The latter rely predominantly on soil test results. Soil tests are of limited value for perennial fruit crops because of the difficulty in obtaining a representative sample over a wide and varying rooting zone. There is not a satisfactory test for nitrogen, which is one of the most critical nutrients for growth.
Soil tests are primarily useful in pre-plant situations or to monitor soil pH and the need for lime application once the trees are established. Perennial fruit crops recycle many of their nutrients within the tree. In the fall as trees go into dormancy they can store nutrients in the wood and roots of the trees. These nutrients are then re-mobilized in the spring and support the growth of spurs, shoots, flowers and early fruit growth. Nutrient management plays a more critical role in new high density plantings because the trees come into production earlier, have higher per acre yields and smaller root systems.
Plant tissue analysis is used to directly measure the amount of nutrients in the trees, and for established perennial crops, is usually a better indicator of nutrient status than a soil test. Recommendations are based on the levels of essential nutrients in the leaves at a specific time. In the case of tree fruits the sampling period is mid-July to mid-August. Leaf analysis can also be a tool to help diagnose problems outside of this period by collecting two samples--one of good healthy leaves and a separate one from trees that exhibit abnormal growth or coloration. The advantage of foliar nutrient analysis is that it can alert you to nutrient levels that are approaching critical levels before they are visible and allow for corrective action before yield or fruit quality are reduced. The other important aspect of foliar analysis is that it alerts you to the possibility of over fertilization and potential interactions affecting uptake of other mineral nutrients.
Lailiang Cheng, Cornell University, conducted a study of six year old Gala/M.26 in which he destructively sampled trees at significant growth phases of the tree--budbreak, bloom, end of spur leaf growth, end of shoot growth, rapid fruit expansion, fruit harvest and after leaf fall. He separated the trees into their various components--fruit, leaves, spurs, shoot leaves, one year-old stem, branches, central leader, above ground root shank, below ground root shank and roots. Dry weights were determined of each type of tissue and each type was analyzed for the major nutrients. Gala/M.26 trees at a density of 1,125 trees per acre used about 50 lbs of nitrogen, 8.5 lbs of potassium, 90 lbs of calcium, 36 lbs of magnesium, 11 lbs of sulfur, 4 lbs of boron and less than 1 lb of each of the minor elements of zinc, copper, manganese and iron.
Nutrient Loss due to Harvest of Fruit
Even with the recycling of nutrient elements the harvest of fruit represents a loss of minerals, so how much is lost? Palmer and Dryden (2006) conducted a study in New Zealand in which they collected yields from mature commercial orchards over a 2 year period and then a representative sample of the fruit was analyzed for mineral content. Based on the yields they could determine the nutrient amount per acre that was removed (Table 1). Potassium removal was the highest, 74.0 lb/A, followed by nitrogen, 27.7 lb/A, while the removal of phosphorus, calcium and magnesium was less than seven pound for each. At the very least these amounts would represent what should be replenished. As you can see in the table the amount of mineral nutrients removed can vary both by cultivar and yield.
Table 1. Average mineral nutrient removal from mature orchards in New Zealand.
Adapted from Palmer, J. & G. Dryden. 2006. NZ Jour. Crop & Hort. Sci. 34:27-32
Under high density systems, factors such as rootstock, soil physics and chemistry, water availability, temperature and other factors influence mineral uptake and quality. Rootstock influenced calcium levels with more dwarfing stocks having higher levels of leaf Ca (Fallahi et al., 2001a). More vigorous rootstock had higher Mg levels. Leaf N on B.9 was significantly lower in one year and slightly lower the next year; while trees on Ottawa 3 had significantly higher leaf K. Crop load will also affect some nutrients. Fallahi et al. (2001b) found that the optimum nitrogen content for Fuji in the "on year" was approximately 2.30% while in the "off year" it was 2.05%. I have observed over a number of years that potassium levels in leaf samples submitted to Penn State tends to be lower in dry years.
When and How to Collect Leaf Samples
In order to monitor nutrient levels in orchards it is critical that a regular pattern of foliar analysis be utilized. We recommend that you divide your orchard into thirds and collect a sample every third year. As much as possible divide your orchard into blocks that are uniform in age, soil type, cultivar and rootstock. As previously indicated leaf samples are best collected from mid-July to mid-August. Nutrient levels are constantly changing in the trees but during this period they are the most stable and after years of research the standards are based upon desirable levels during this period.
Collect 50 to 60 leaves from across the orchard block. The most accurate sample is one from a single cultivar on a single rootstock. Mixing cultivars and rootstocks in a single sample does not give an accurate reading of the nutrient status of the trees. Collect fully expanded leaves that are midway on the current season's growth (See Figure). Select shoots that are shoulder to eye level from the outside of the trees. To avoid contamination from fungicides, collect the leaves as far as possible past your last cover spray. Place the leaves in a small paper bag.
Penn State provides a leaf analysis service through its Agricultural Analytical Service Lab at University Park. Plant analysis submission forms are available at lab's website. Be sure to fill out the information on the form as this will help us provide the best recommendation. Mail the leaves, information form and a check for $24.00 payable, to Penn State. The standard analysis includes the macronutrients N, P, K, Ca, Mg plus the micronutrients Mn, Fe, Cu, B, Al, Zn.
There are also commercial firms in Pennsylvania that provide leaf analysis services and these are listed below:
- Agri Analysis Inc, 280 Newport Rd, Leola, PA 17540; 1-800-464-6019
- Growmark FS, 3150 Stoney Point Rd, East Berlin, PA 17136; 717-259-9573
- Cheng, L. and R. Raba. 2009a. Accumulation of macro- and micronutrients and nitrogen demand-supply relationship of Gala/Malling 26 apple trees grown in sand culture. J. Amer. Soc. Hort. Sci. 134:3-13.
- Cheng, L. and R. Raba. 2009b. Nutrient requirements of Gala/M.26 apple trees for high yield and quality. New York Fruit Quarterly 17(4):5-10.
- Fallahi, E., I. Chun, G. Neilsen & W. M. Colt. 2001a. Effects of three rootstock on photosynthesis, leaf mineral nutrition, and vegetative growth of BC-2 Fuji apple trees. J. Plant Nutrition. 24:827-834.
- Fallahi, E., W. M. Colt, B. Hallahi. 2001b. Optimum ranges of leaf nitrogen for yield, fruit quality and photosynthesis in BC-2 Fuji apple. J. Amer. Pom. Soc. 55:68-75.