Getting the right nutrient levels in the orchard starts with answering the basic questions: How much of each element does a tree need, and when does it need it?

For Lailiang Cheng at Cornell University, answering the question was a matter of exerting total control and then measuring.

In a series of experiments, he set out to answer one question for a very specific situation: How much nitrogen and other nutrients are needed for high-density Gala apples on M.26 rootstock in their sixth leaf to produce a high yield of good size fruit?

He grew the trees in sand so he could totally control the nutrient supply, and he adjusted the crop load to 8.2 fruit per square centimeter of trunk cross-­sectional area (about 104 fruit per tree). He carefully measured dry ­matter accumulation in the shoots, leaves, roots, and fruit, and also measured fruit size, firmness, and soluble solids as indicators of fruit quality.

Here’s what he found:

—Measured from bud break to the end of the season, each tree accumulated about 10 pounds of dry matter.

—Trees put 72.2 percent of that dry matter into fruit, 17.3 percent into shoots and leaves, and 10.5 percent into the trunk and roots. Fruit yield was 41.3 pounds per tree, about 1,100 bushels per acre.

—Highest nitrogen demand occurred from bloom to end of shoot growth, with lower but steady demand on to harvest.

—Other nutrients had relatively constant demand from bloom to harvest.

—Fruit need more phosphorus, potassium, boron, and iron than do leaves.

—Requirements for each nutrient, on a pounds-per-acre basis, were as follows:

• Nitrogen: 50 pounds
• Phosphorus: 8.5 pounds
• Potassium: 90 pounds
• Calcium: 36 pounds
• Magnesium: 11 pounds
• Sulfur: 4.0 pounds
• Boron: 0.23 pounds
• Zinc: 0.15 pounds
• Copper: 0.12 pounds
• Manganese: 0.46 pounds
• Iron: 0.37 pounds

Assuming the ratio between net accumulation of each nutrient in the whole tree from bud break to fruit harvest and its amount in fruit remains constant across the range of fruit yield encountered in commercial apple ­production, one can readily calculate the total tree requirement for each nutrient at any expected fruit yield, he said (see “Tree nutrient ­requirements in relation to yield” for example).

It should be pointed out, however, that these are tree nutrient requirements, not necessarily the amounts of fertilizer growers need to apply, Cheng said. When developing fertilization programs in apple orchards, soil nutrient availability and tree nutrient status must be taken into ­consideration along with tree nutrient requirements.

The total amount of each nutrient contained in the fruit at harvest is also proportional to fruit yield (see “Fruit nutrient removal at harvest.”)

Since the amounts of nutrients contained in the fruit are permanently removed at harvest from the orchard where the fruit are grown, these are the least amounts of nutrients growers need to return to the orchard, Cheng said.