Leaf Nutrient Sampling Is Critical

Why do I encourage leaf analysis more often than soil analysis for tree crops? Leaf analysis is the most accurate method of determining the nutrient status of fruit and nut trees. You can find out what is actually getting into the tree in acceptable as well as excess or deficient concentrations.

While soil analysis is useful for pH determinations, gypsum requirements, lime requirements and changes in certain elements through soil layers – it does not tell you what the tree is utilizing. Some types of nutrients may be present in the soil in sufficient quantities but can’t be utilized by plants because of insolubility, competing nutrients, weakened root system, drought or other causes.

The best time to collect leaves for leaf analysis is in the month of July. Collect a representative sample of 50 to 100 leaves chosen from throughout a block of trees with similar symptoms. Fully expanded, mature leaves from non-fruiting spurs are recommended for apples, apricots and cherries. For walnuts, collect 25 to 50 leaves from the terminal leaflets of compound leaves on spurs or the middle of moderately growing shoots.

Pick the leaves at random from trees across the block to be analyzed. Collect separate samples from each block, soil type or symptom. Some researchers like to sample from individual marked trees each year to reduce sampling variability but a composite sample is adequate for most situations.

The most common deficiencies on the central coast of California are nitrogen, zinc, potassium and iron. Consult with your regional farm advisor for advice on the soils in your area.

Deficiency Symptoms

Nitrogen deficiency results in lack of growth and light green leaves. Zinc deficiency symptoms are mottled, light green leaves, small leaves (little leaf) and tufts of leaves at shoot tips (rosetting). On walnuts, wavy leaf margins may be another indicator of zinc deficiency. Potassium deficiency symptoms are leaves curled upward (boat-shaped) with grey or silver edges. As for other minor nutrients, occasionally manganese and magnesium may be deficient.

Iron cannot be accurately measured by leaf analysis. For iron, I rely on visual symptoms of chlorosis – green veins with yellow leaf blades. More severe cases will have the leaves completely bleached out followed by dieback.

Excesses may include chloride, sodium and boron – all of which may cause tip or marginal leaf burn. Boron excess in particular has some good diagnostic leaf symptoms. On walnuts, the area between the veins is yellow or burned. Stone fruits have enlarged nodes and gumming.

Nitrogen may also be in excess, which can increase bitter pit in apples and result in fruit quality problems, brown rot and delayed harvest in stone fruits. Manganese can be in excess in apple leaves when associated with measles and poor growth in acidic soil conditions.