Unless you’re fertilizing nut trees the right way, you may be squandering dollars while threatening air and water quality, says agronomist and soil scientist Robert Mikkelsen, Western North America director of the International Plant Nutrition Institute.
He lists four such Fertilizer Rights — or 4 Rs.
“Using the 4 Rs as a mental checklist can go a long way in making sure you’re using crop nutrients in a way that is best for your particular operation and the environment, rather than simply following traditional fertilizer practices, which may not be the best approach for your situation,” he says.
1. Right Source — Identifying the best choices requires knowing the chemistry of the various fertilizers and how they react in your soil and climate, Mikkelsen says.
For example, some forms of nitrogen are more susceptible to volatilization or leaching than others. Similarly, the salt index of the various potassium sources can also vary. Adding more salt to the soil via potassium may not be wise.
“Look at the whole fertilizer package,” he suggests. “Choose the one that is best for your soils, plants, water regime and budget.”
2. Right Rate — Match tree nutrient use with nutrient applications.
The first step is to determine how much of a nutrient you’re removing from the orchard when you harvest the nuts. In general, every thousand pounds of harvested nuts depletes your orchard soils of 60 pounds of nitrogen, 7 pounds of phosphorus and about 50 pounds of potassium.
At one time, many soils in California had ample supplies of potassium, eliminating the need to add more. But, that’s changing as years of farming have drawn those levels down below what trees and other crops need.
In figuring the right application rate of a given nutrient, it’s important to account for any amounts used to grow new roots or wood or, in the case of nitrogen, lost due to leaching or denitrification. And, you may have to adjust fertilizer rates to account for nutrients added to the soil in other ways.
For example, your irrigation water may contain nitrogen in the form of nitrates from geological sources, previous fertilization practices, or from livestock operations or water treatment plants.
“Depending on your location, your irrigation water may be adding anywhere from 10 pounds to 30 pounds of nitrogen per acre,” Mikkelsen says. “Organic matter in the soil will also be contributing nitrogen.”
A soil analysis, including deep soil testing for nitrates, can measure specific levels of the nutrients in your fields. This tool can be especially valuable prior to planting a new orchard.
“It’s your one chance to get fix any soil problems, including plowing in nutrients, and ripping hard pans to mix nutrients down deep, and get your soil in really good shape before the growing trees prevent that,” Mikkelsen says.
It’s also important to provide enough fertilizer to satisfy peak nutrient needs of the trees in a timely way throughout the growing season.
“There are certain times of the year, especially during April and May, when the rapidly growing nuts have a large demand for nutrients. If you don’t meet that demand, the nuts will be smaller or may abort.”
3. Right Time — Wet weather can prevent a grower from applying fertilizers when they will do the most good. The idea is to apply fertilizers in the root zone when the trees need them, but not so early that the nutrients are lost through leaching or volatilization before the trees can take them up.
Because they are much less prone to such losses, timing is not as critical for phosphorus and potassium applications as it is for nitrogen.
Leaching of nitrogen down through the soil profile can threaten groundwater quality. Denitrification, in which nitrates are converted to nitrous oxide gas, is becoming an increasing environmental concern, Mikkelsen points out.
Nitrous oxide, a greenhouse gas, is almost 300 times more potent threat to the atmosphere than carbon dioxide, another greenhouse gas.
4. Right Place — Efficient fertilizer use also requires placing nutrients where the trees can get them when the nutrients are needed.
Phosphorus typically moves less than an inch a year through the soil. Potassium may move several inches, depending on type of soil. That means placing these two nutrients close to actively-growing roots, Mikkelsen notes.
“The majority of these active roots will be in the wetted zones of the soil. So, if you’re applying these fertilizers through a microsprinkler or drip system, the roots will be able to use them. But, if you apply phosphorus or potassium anywhere the soil is dry, they won’t benefit the trees. Nitrogen, on the other hand, moves wherever water takes it, making it difficult to manage.”
Over time, acidity in wetted zones can increase to the point where it starts to limit root growth.
“So, you need to use the form of fertilizer that will move the way you want it, while keeping the wetted zones in as good shape as possible,” he says. “Periodic soil testing will help with this” he says.