The Aerobic Zone
When considering how to build up soil fertility, the most important zone to consider is the aerobic zone – the top 6 to 7 inches (as deep as a fence post rots). This is the part of the soil that contains the micro-organisms that can only operate properly when adequate oxygen is present.
There are some exceptions to sampling the aerobic zone [roughly considered 6.75 inches because an acre of soil down to this depth will generally weigh about 2 million pounds]. One is when all materials must be placed on top of the soil and will not be worked into the top 2 inches (5 centimeters) or more. Tree crops such as citrus, almonds, apples, pecans, and even timber or trees and landscape plantings are a part of this category. No-till crops, pastures, hay meadows, lawns and turf grass are other important examples.
Take the topsoil sample from only the top 4 inches (10 centimeters) for such cases in order to avoid over-saturation by adding too much of any nutrient to those top few inches. This is necessary and extremely important to consider because too much of one nutrient added to the soil can cause a deficiency of one or more of the other nutrients needed there. For example, too much phosphate will tie up zinc, too much potassium ties up copper, and too much of any one of the various liming materials will tie up at least a portion of just about all the elements we can measure with a soil test, with phosphate usually being the exception. Thus, borderline levels would now become deficiencies for the plants intended to be grown there.
Another exception to the rule of sampling the aerobic zone is when the fertility of the land for establishing new trees or vine crops is being considered. We have some clients who test for certain nutrients to a depth of 48 inches (125 centimeters) to consider what can possibly be done to positively influence fertility levels. In such cases deeper sampling and analysis may be utilized by taking samples in 6 inch (15 centimeter) increments to the depth soil nutrients can be safely applied and mixed.
Think of the aerobic zone as the most important consideration for soil testing. Even more, track this area separately because, given the choice, every plant will feed in the aerobic zone. The soil is the plant’s stomach! The aerobic zone is the area where nutrients are most available because the microbial activity is greatest there. Plants when allowed to grow normally will send the major feeding roots into that zone because it is the area that will provide optimum nutrient uptake.
Depth of Fertilizer Application The depth to which soil sampling should be done after considering the aerobic zone depends on the circumstances and the way you intend to use the information gained. If a ripper or subsoiler will be pulled through at 5 feet deep or more, it may be of interest to see what is down there. But once you know what is lacking, how can you affect fertility levels at that depth?
Essentially at that depth, you have what you have to work with and not much chance of affecting it except to break up any hardpan and allow roots and moisture to penetrate more easily so as to better utilize what is already there. As this happens over the years the levels in the lower depths of the soil will begin to show a change, and the more roots the more the changes become evident.
A major key to effective soil fertilization is the use of sampling to measure what you have, as deeply as the roots may penetrate. But then, continue to utilize testing at the deeper levels to prove or disprove what changes are thought to be possible there.
Fertilizer Mixing, Nutrient Concentration Next, determine how deep you have the ability to work the soil and the efficiency of the equipment used to mix the applied materials with the soil. The key is to achieve a homogeneous mix. The rule of thumb is “adequate mixing of the nutrients applied is only satisfactory to half the depth the equipment is able to break up and work the soil”.
Keep in mind that thorough mixing is of utmost importance. Where the material is applied in clumps, if it is something the plant requires, roots will concentrate there and we again have to consider the “law of the maximum”, P ties up zinc, etc. But also, would the contents of the clump cause toxicity problems for roots that would normally penetrate that area? Better to be safe than sorry!
When the depth to which fertilizer can be thoroughly mixed is determined, figure the needs in 6 – 7 inch increments once you drop below the aerobic zone. For example, if it is determined that implements that can work the soil to a depth of 30 inches will assure adequate mixing to 15 inches, back off to 12 inches and apply the materials calculated for that depth. If 18 – 20 inches, figure to the 18-inch depth, etc.
The question that matters most to this point is how well will the fertilizer, lime, compost, etc. be mixed with the soil. When it comes to soil fertilization, it is a good rule to follow that in the long run too little is always better than too much.
Considerations for Specific Nutrients
For soils needing an increase in fertility, the following comments are of particular importance, keeping in mind the points made above:
Phosphate If a soil can be thoroughly mixed and it would not provide an excess of nitrogen, applying 250 lbs./acre of 11-52-0 for each six inches of depth would not be too much phosphate. Furthermore, the nitrogen will be gone in no more than 8 months, likely far sooner. However, better to thoroughly mix 250 lbs./acre in the top 6 – 7 inches just before planting than do a poor job to a greater depth. For organic growers using 500 lbs./acre of soft rock phosphate, the concept would be the same, just so long as nutrient availability would also not be harmed from the extra calcium that will be added there.
In addition, the deeper you place the phosphate, the less efficient it will tend to be at remaining in an available form over time. But from what we have seen, if it is correctly applied and mixed, it will increase P levels in the form the plant needs, to the depth it is applied, even though availability is not as great the deeper it is placed.
Sulfur will tend to move downward through the soil. Placing it extremely deep is not an efficient use of sulfur unless there is some other nutrient excess that could be affected properly by its use. Many sampled areas would not benefit from sulfur placed in the 12 – 24 inch levels, but others could benefit, due to very high or excessive sodium levels.
Concerning the application of lime and/or gypsum to the soil, if the topsoil and subsoil need the same amounts, consider the complete depth for receiving all that is required. But if the subsoil needs more than the topsoil, the question is, how well would the amount needed deeper down ever be kept from the top portion? For this reason, only apply to that soil what the subsoil can stand when it is equivalent to or less than the topsoil and when it can be completely mixed as required. So long as it can be mixed correctly, needed potassium from 0-0-50 would benefit when recommended and applied to the complete depth required.
Boron should only be applied to the aerobic zone. If there is ever a hardpan, the boron moving from the top could accumulate to toxic levels somewhere in the root zone. The more that is applied at once, the greater the chance that damage can happen. Since boron will leach out anyway, or move downward from the top, it is best to apply it on top and only where shown to be required.
Iron sulfate (in the proper form – white or blue green in color) can benefit the crop if truly needed. However if the aerobic zone is deficient and the immediate subsoil exceeds 40 ppm and the ppm of manganese is adequate, but below iron, deep rooting crops can compensate and will likely receive an adequate amount to supply the need. However, if liming materials are placed in the subsoil and iron is barely adequate or deficient, this will only make the problem worse for future root and top growth. Even more, the extent of the problem will not be determined until the end of the third year after the lime has been mixed in. On specific samples that have the need, applying the recommended lime should not be a problem.
However where calcium is too high, iron availability may be just as much a problem at greater depths. Iron applied here will only benefit to the extent it can be worked in at all depths. Even if the iron were thoroughly mixed and it was twice the amount needed in the top 12 inches it would only serve to benefit in the long run, so long as it can be kept in an available form, or converted to an available form by sulfur acidulation.
Manganese tends to tie up in the soil the deeper it is placed or even naturally as you continue to check it at lower depths in the soil. An application to the aerobic zone is likely the only time manganese will have a lasting benefit for the amount of material required and the subsequent cost.
Copper tends to release slowly and remain available, but good microbial activity in the aerobic zone seems to help provide a more even distribution. Without further information, the expense of building copper in the subsoil may not be cost-effective because of the lack of microorganisms to help. Another consideration is whether it can be thoroughly mixed so as not to cause a problem. In general, no problems have been experienced with topsoil applications of copper sulfate up to 35 lbs./acre when broadcast and well mixed.
Although zinc tends to decrease in availability with depth, the greatest problem would be if clumps or high concentrations would accumulate in specific areas and cause phosphate tie up. It is likely that an even distribution, even at deeper levels, would pay dividends in years to come.
Compost can only be properly evaluated with a representative analysis in order to determine what it can do for individual soils. Depending on the nutrient levels present, and the amounts available for application per acre, some or all of the phosphate and potassium may be supplied. The big question is, can it be mixed, and will sufficient amounts be available, for the lower soil depths.
If there is any doubt, it is recommended that fertilizer materials be applied and worked in to the minimum depth of homogenous mixing as a certainty. We can always continue to treat from the top and make better progress than where excesses occur that we cannot then easily and economically correct.
– Neal Kinsey