Soil Fertility Considerations for Growing Organic Tree Crops

Even though trees grow with air all around them, sufficient air in the soil is still a critical factor (photo by Caleb Adams, Nichols Farms.)

There is a rule all growers should seriously consider: “You can’t properly manage what you don’t correctly measure.” When it comes down to growing tree crops organically, there are so many soil types and so many different trees used for different purposes that most growers are led to believe that there can be no basic program to use for growing trees. So, perhaps the first question that needs to be answered is can a basic fertility program for growing trees in general even be properly established, let alone correctly measured?

Every organic grower would likely agree with the concept that soil biology, the plant roots and all that supports them is the foundation for growing organically. What type of soil environment works best in such cases?

To answer that, consider what the most important needs for life are where trees are concerned.


Basic Tree Needs

What is needed for life itself? Four needs to consider are proper shelter, food, water and air. But to supply air, water, “food” and even proper shelter for the roots of trees, what is necessary?

Of these needs what should be considered as the order of least importance for trees? Physical shelter would likely be the least important requirement for trees growing in their proper habitat. Food is necessary, but not as essential as water in terms of which one trees can go without for the longest time. But of these four needs, air is the need that takes on the most importance for life itself. Air provides the oxygen we need and the carbon dioxide that plants need. When we lose access to fresh air, life will cease in a very short time.
And even though trees grow with air all around them, sufficient air in the soil is still a critical factor. Without proper soil aeration, the microbes and other soil organisms cannot function as they should to provide nutrients from the soil to the trees. So, the point here is that without the proper amount of air in the soil, trees do not function as well as they should.

Textbooks on soil fertility report that based on physical structure, the ideal soil contains 45% minerals, 5% organic matter, 25% water and 25% air. And when soils measure up to those numbers, anyone who recognizes the best soils will agree that such is the case. But most soils throughout the world do not have those most favorable conditions.

In fact, most sandy soils, just left as they are, have too much air space, which adversely affects water holding capacity. Such soils need more water and less air to be most effective. On the other hand, clay soils tend to hold too much water and not enough air to be most effective for growing trees or any other types of plants. These are not ideal soils with a good balance of air and water. And in such cases, there are only two real choices: grow trees and let them do the best they can, or change the air to water relationships based on the measured needs of each soil.

Soils closer to the ideal physical structure are also those that are closest to ideal in nutrient availability (courtesy Rex Dufour, NCAT.)

There is only one way to effectively change the amount of pore space in each soil. That method is to establish a more desirable physical structure by the use of the proper nutrients that affect the porosity (air to water relationship) of each soil.

There are four principal elements that exert the most influence upon the relationship of air to water in soils where trees or other plants typically grow. These four elements are calcium, magnesium, potassium and sodium.

Of the four, calcium is the key element for increasing soil porosity. Calcium causes the clay particles to clump up or flocculate, which increases the pore space in the soil that determines the general balance between air and water. Magnesium, potassium and sodium do the opposite job in the soil. All three will disperse the clay particles in the soil and cause a reduction in pore space.

For sandy soils, maximize magnesium and potassium to excellent levels, which will reduce air space and increase water holding capacity. For clays, we need to maximize allowable calcium which increases pore space and maximizes aeration needed for the biology to function best there.

In effect, this means using soil chemistry to correctly feed the soil what it needs which naturally regulates the space needed for air and water in each soil. Therefore, the use of soil chemistry to supply the right amount of missing nutrients that help feed the plants is the correct way to achieve the ideal physical structure, which then provides the best balance of air and water in each soil. What this does is to effectively build the most conducive environment for soil organisms which are the key to nutrient uptake by trees and all other plants.


Ideal Soils

The ideal soil is described in terms of its physical condition. The physical condition determines how well the biology of the soil can do what is needed for the plants. The solid portion of an ideal soil has roughly 45% minerals and 5% humus. The other half of an ideal soil is pore space. Half of that pore space should ideally be filled by water and the other half with air. However, that physical condition is influenced by the chemistry of the soil, which can be measured using specifically designed soil tests to do so.

There are always certain parameters that must be met to have the most ideal physical structure, which then enables any soil to do its best. That structure is determined by the nutrient makeup of each soil. When the soil nutrients are properly provided, the physical structure will be closest to ideal. Or in other words, the soils closer to the ideal physical structure are also those that are closest to ideal in nutrient availability. Soils cannot have one without the other. When accurately measured, the physical structure backs up the ideal nutrient level, and the ideal nutrient level of the four elements that principally affect soil structure will only be there in the right proportions when the physical structure is also correct.

The question posed at the beginning of this article was can a fertility program for trees in general even be properly established? The answer is provided by studying the nutrient makeup of what is defined as the ideal soil.

Since trees are grown on all types of soils, from very light sands to very heavy clays, and for so many different purposes, how can anyone possibly establish what provides the best fertility for trees? In such cases, perhaps too many people try to put the cart before the horse. For fertility considerations, first concentrate on general principles of fertility that all trees need, then on any additional specifics for different varieties and purposes.

Test where the very best trees of each type grow, where their growth is just average and where the worst problems with growth occurs. See what the best has in terms of nutrient levels that the others do not. Then begin to correct and build the soil to reflect the levels that grow the best trees. If the correct type of materials is used to build fertility levels in each soil, then as the soil test numbers get closer to those where the best trees grow, they begin to do better. In other words, the closer those soils begin to conform to what is needed, the closer the trees will perform as the better ones do.

There is good reason as to why trees do well on some soils and not on others. Two clients who grow English walnuts, one from central California and one from northern California, provide a good example to help illustrate this point. Both clients wanted to grow more walnuts, but had stopped planting because the soils that were left were classified as unsuitable for growing walnuts. Both were familiar enough with the fertility program so that when it came to advice on the proper approach, their confidence was sufficient to follow through. So, when it was shown that manganese was the most critical deficiency on those soils when compared to soils that grew profitable walnuts, though each had very different soils, each applied a sufficient amount of manganese along with the normally needed fertility. The soils previously considered as unsuitable grew excellent walnuts, with top yield performance from the trees.

A basic fertility program for trees should be considered as follows. First provide adequate amounts of N, P, K and S for producing the crop. Next, measure and correct the needed levels of calcium and magnesium. Then, build the essential micronutrients to reach the minimum requirements. Once these needs have been satisfied, then consider the special needs for growing trees that set them apart from other crops.

This topic will be addressed in Growing Organic Tree Crops – Part 2 in the next issue.

Neal Kinsey is owner and president of Kinsey Agricultural Services, a consulting firm that specializes in restoring and maintaining balanced soil fertility for attaining excellent yields while growing highly nutritious food and feed crops on the land. Call 573-683-3880 or see for more information.