The objective of agriculture is to modify the conditions under which the crop is growing in order to increase productivity. This is accomplished by a number of methods, including selection of improved varieties, management of water, control of pests and the application of fertilizers.
Applications of only the macronutrients (nitrogen, phosphorus, potassium and sulfur) do not provide all the minerals required by the crop for efficient production. It is necessary to provide a balance of all the nutrients in order to maximize production and quality.
Elements in plants play two important roles. First are the elements that make up the structure of the plant. These include carbon, hydrogen, oxygen, nitrogen, sulfur and calcium. Second are the reactive elements. These enter into the biochemical reactions within the plant such as respiration and photosynthesis. The reactive elements include magnesium, zinc, iron, manganese, copper and boron.
Building good nutrient status into your tree is a year round program. Flower buds for the next season’s crop begin to form shortly after bloom and complete their development shortly before flowers appear the following spring. During the early spring, new leaves are developing and new shoot growth begins. Full bloom occurs, young fruits set and development begins. Fruit growth continues through summer until harvest. Trees then begin to shut down, drop their leaves and go into dormancy in the Fall. This cycle known as the phenology cycle is important to understand. It’s role in nutrient uptake, nutrient movement within the tree, relationship of nutrients in metabolism, nutrient balance and reserve levels all come into play.
Spring is the most stressful time of the year for any fruit tree. Your orchard is drawing from reserves as it progresses through bloom. Soil temperatures are cold and limiting on nutrient availability. Fruit size is partially determined by the number of cells each fruit contains. Cell division occurs immediately after bloom (from 14 to 28 days) when the stress is at a peak. Healthy well fed, trees are better able to handle and size large crops, survive winter injury and pull buds through spring frosts. Next years’ flower buds are determined this season. Keeping a tree well fed is a year round program. Properly timed foliar applications of nutrients can help relief the stress!
Liebig's Law of the Minimum states that growth is controlled not by the total of resources available, but by the scarcest resource. Increasing the amount of plentiful nutrients did not increase growth. The plants did not need the plentifully available nutrients, it needed the scarcer nutrient in relation to “need”. Only by increasing the limiting factors was plant growth improved. This is easily pictured by the barrel with a “short stave”. You can fill it only as high as the shortest stave.
I often get asked, “when should I be adding foliar nutrients and what minerals should I use?” The following is a seasonal use guide for nutrition. Remember, foliar feeding by definition implies the presence of active, growing leaves! The absorption of minerals through twigs and bark is minimal at best.
I have an excellent set of spray programs for organic growers. They're set up like the Washington state spray guide, moving from one crop stage to the next. They list the diseases and pests present at each crop stage along with suggested control measures. The guides cover apricot, peach, apple, pear and cherry. Let me know if you want a copy.
SEASON PHYSIOLOGICAL ACTIVITY CRITICAL ELEMENTS
Spring Bud-break & Initial growth N & P
Early Season Flowering, Foliage growth, N, P, Zn, B & Other Micros
Pollination, Fruit set
Mid Season Fruit & Foliage growth Ca, K, Mg & P
Late Season Sizing & Maturation Ca, P & K
Postharvest Nutrient Storage N, B & Zn
I looked back through a pile of notes from meetings over the years. The following is a random collection of thoughts relating to the theme of nutrition:
Proper soil nutrition should be the foundation upon which foliar nutrition programs are developed. Foliars are a boost during any key stress period, they can’t replace ground applications of macro nutrients.
When are foliar applications of nutrients beneficial? The first reason is to correct a mineral deficiency. The second is to maintain optimal levels during periods of stress.
When it comes to foliar nutrition, remember two points: 1) Invigorate early, vigor has to do with cells dividing and cell numbers has to do with potential fruit size. 2) Color and store late.
Use surfactants when applying foliar nutrients.
Nutrient uptake from the soil begins when soil moisture and soil temperature are at proper levels. New roots begin to develop in early spring and are very efficient at absorbing nutrients and water versus older roots. Nutrient uptake throughout the entire season may not be adequate enough to satisfy the needs of the above ground portions of the tree. Tree development in the early spring depends upon stored reserve levels of nutrients from the previous year before root uptake begins. (This is the reason I’m such a believer in the fall shutdown urea sprays and bloom time boosts.)
Low soil temperatures impact the uptake of N from the soil. At a soil temperature of 46˚F, uptake of N occurs very slowly. By contrast, uptake of N in 70˚F soil temperatures is rapid. From experience, 70˚F soil temperatures at CSU OM occur late May to early June. This fact may result in insufficient N levels being available to the trees in the critical post bloom period. Early spring applied N primarily goes to the new and expanding shoots and leaves.
Drought stress limits any nutrition response. Water management is of first importance.
Complex subject, but essentially you should have stopped feeding any nitrogen by the end of July on all fruiting trees. This is a general rule, but must vary by variety.
Newly planted trees use high phosphorus. Research shows that one shot is as good as multiple applications. Also, feed nitrogen early and hard to maximize growth, taper out N in late July to allow trees to harden off and set fruit buds.
Foliar feeding with a full complement of nutrients is essential for newly planted trees. Feed weekly to every ten days once growth starts.
Nutrient absorption; greatest amount occurs between spray application and the droplet drying. Within 2 – 4 hours the majority of the element is inside the leaf. Within 48 hours it’s all done
Most nutrients are in the top 2-3” of soil, the majority in the top 6”.
Cover crops won’t contribute N to the tree until you destroy the cover crop by disking it under or mowing and discharging the clippings under the tree.
● The average day/night temperatures equals the average soil temp in root zone.
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