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Examples of How to Adjust to Your Soil Test Results

By Neal Kinsey

Reading your soil test and knowing what to do are two different things. Here are some examples of how to apply results to your fields based on what your soil test is indicating. It will give you confidence that you are on the right track to increasing your overall soil health.

Except in the case of wheat and other small grains, if the soil test indicates that you need ten pounds of copper sulfate but that amount represents a cost that is too high, you could consider putting on two pounds for the next five years instead of ten pounds all at once. You can build copper levels this way because that nutrient is so stable in the soil. Almost every soil analyzed is deficient in copper. The few exceptions are where there is natu­rally occurring copper in the soil or where manure has been used, or along rivers where soil has been moved in by constant flooding, and where alluvial deposits keep the soil built up. The other place where we don’t have copper deficiencies is where large amounts of pesticides containing copper have been used in the past, and where turkey litter is commonly used.

When crops on soils with adequate fertility that test low in copper fail to respond to copper applications, a molybdenum test should be considered. Both need to be present in adequate amounts, since either one can influence the amount a crop can take up of the other one.

A farmer from Iowa I had worked with for eight years called me one day. He said he had a problem with some of his corn going down. He wanted me to look at the soil test. He had 80 acres split into four 20s, and one had a 1.5 copper, his lowest level. I told him that his copper was 1.5 ppm and he was going to have the weakest stalks there. It was true. He said, “Well, since that is true, what about the next field?” We went through all the data until he hit 2 ppm. In each case he lost corn from lodging, but the lower the copper level below 2 ppm, the worse the corn was lodged.

Copper gives resilience to a plant. The other key to flavor after sulfur is copper. Copper doesn’t move in the soil. If you see a copper deficiency at all, it is like sulfur. It shows the deficiency in the young­est growth first. Adequate nitrogen means you get better copper uptake. Too much nitrogen means you actually decrease the avail­ability of copper to the plant. Wheat is a crop that responds well to copper. If the other needed nutrients are adequate, but you are below 2 ppm copper and you put enough copper on your wheat to get above 2 ppm, it will produce an extra five bushels (300 pounds/acre) of wheat.

I was asked to make a consulting trip to Germany in 1985. The client who asked me there had arranged for speaking engagements to several groups of farmers. He told me to tell them just what I had done on his farms. In Germany, they use the taller varieties of wheat, but they also use growth regulators to keep plants short. When the subject turned to copper, I mentioned that if the soil had below 2 ppm and copper was put on, it would increase yields by five bushels per acre. I also told them that every soil I had checked in Germany was below that level. One man raised his hand, stood up and spoke for two or three minutes. The interpreter leaned over and said, “This is the man who is in charge of fertility for the university in this area. He is telling the farmers why they don’t need to really worry about the copper that you said should be put on.” When he had finished, a doctor of fertility research rose and reported a ten-year study (unknown to me) just completed which had shown exactly the same results I had mentioned on wheat production.

Copper sulfate, 22.5 or 23 percent, is water-soluble and can be applied to soil or as a foliar. A word of caution here. When cop­per is mixed in solution with other elements it may show as compatible in the mix, but use it up. Do not let it set in your spray tank, pumps or hoses overnight. It can harden to the point that the entire spraying apparatus has to be scrapped. I won’t bother with other copper sources if I can get copper sulfate because I know it works, and I know how much can be applied safely. A ten-pound application of copper sulfate—after 12 months time—will raise the copper level by 0.6 ppm on the spe­cific test we use. A copper level is the easiest to build and main­tain, and zinc is next in the pecking order. Both have staying power in the soil.

Zinc aids in the absorption of moisture. Along with potassium, think of zinc in critical moisture situations. It also helps transform carbohydrates. It regulates plant sugar use. Zinc plays a role in enzyme system functioning, as well as with the growth regulators normally present in the plant and protein synthesis.

Consider zinc needs, especially in sensitive crops such as corn and grain sorghum, also soybeans and dry beans. As far as zinc levels are concerned, the minimum is 6 ppm. Below that, enough should be applied the first year to get up above the 6 ppm figure. There has never been a soil—in my experience at least—that required more than 30 pounds of zinc sulfate to completely take care of the worst zinc deficiency, provided limestone didn’t have to be applied at the same time. If lime is applied, the lime won’t drive out the zinc, but it will affect zinc availability. If you need zinc and lime and put on the lime but don’t put on the zinc, expect your zinc level to get worse. Zinc probably gives a response more often than any other micronutrient when it is applied for crop production. I see many soils that need zinc. All can be taken care of, generally with excellent response. Only a few crops do not respond well to zinc when only slightly deficient, one of them being wheat. Again, for zinc, 6 ppm is the minimum. Excellent means at least 10 ppm, and an excess is 35+ ppm.

A classic sign of zinc deficiency in corn is the whitish stripe in the leaf color, which looks much like a magnesium deficiency. In the field, often I can’t tell whether it is a zinc deficiency or a magnesium deficiency. The difference between zinc and magne­sium deficiencies should be that zinc will be white and magne­sium will be white on top with a purplish color on the bottom.

High phosphorous, high calcium or high potassium levels can induce zinc deficiencies as does the overuse of nitrogen. Also, as the pH goes up from 6, when a soil has good zinc levels, availability begins to decrease. It can go as high as pH 7 before it decreases good zinc availability to the point that it becomes a problem. Heavy cuts, such as when the field has been graded or the topsoil has been taken away, or when eroded soil allows sub­soils to appear, strong zinc deficiencies become evident.

Zinc is not easy to leach away. It is held well on clay and humus. Once zinc levels are improved, it is relatively easy to keep them up. When zinc sulfate is applied on the tests we have, an exact relationship between the amount of zinc applied and the increase shown on the soil test can be expected. Using 36% zinc sulfate at an application of 10 pounds per acre will exhibit a 3.6-pound increase of zinc on the test. The correlation is classic. Putting on ten pounds of 36% zinc sulfate means putting on 3.6 pounds of zinc per acre. A soil test — when the zinc is finished breaking down — should show an increase of 3.6 pounds of zinc. That translates into 1.8 ppm, meaning every 10 pounds of zinc sulfate applied will raise the zinc level by 1.8 ppm. There is one other thing to be remembered about pure zinc sulfate. Put on ten pounds of zinc sulfate today, then come back next year and pull a soil test on the same day. That zinc is only going to be halfway to its final level. You are only going to see a 0.9 ppm increase in the soil. When you put on sufficient zinc it will not reach the desired level for two years, but will supply enough zinc for the crops grown for both years.

Some firms say they have a 36% zinc product, it being a 36% oxysulfate. It is cheaper at the counter, but it does not always build zinc levels. There are some zinc oxide products that have been pulverized and then prilled, which have also proved effec­tive for increasing the levels in the soil. Apply ten pounds, the same as with 36% zinc sulfate, and see if next year the deficiency is still there. Pure zinc sulfate is the sure choice.

With the exception of boron, on most soils the technology exists in order to build the levels of trace elements to a point that it is not an annual expense, but basically an initial expenditure. Then afterwards, it is a matter of testing and fertilizing as need­ed over the years to keep the levels up. Many farmers, ranchers and growers initially resist the addition of trace elements to increase fertility levels, objecting to the added expense. But when you consider the years spent taking from the soil without adding the traces back, replenishing should be expected. And supplying those micronutrients have helped increase corn yields by 20-30 bushels, and wheat from 5-25 bushels per acre. Micronutrients, when applied in the right form, to build up the levels in the soil will help quality and crop yields accordingly.

Some farmers who have livestock have always felt they shouldn’t have to be concerned with trace elements because their manure or compost would take care of it. I work with many farmers who use manures and compost, and this is rarely the case. Think about it. When a soil is deficient in copper and adequate copper is not being supplemented, how can enough be in the manure? Manure is generally low in sulfur, boron and copper—the nutrients most often lacking in our soils used for growing the crops. Keep in mind, when manure is applied, you can influence the soil nutrient level to only the extent of what is there in the first place. Nevertheless, manure is certainly helpful and in certain soils even sufficient to keep the trace elements that are present in a soil most available for plant use, while at the same time helping to recycle those that are picked up in the feed. And therefore, as the use of manures in an area declines, the need for trace elements will increase.

Most soils we analyze just do not have an adequate supply of trace elements to assure that the crop will do its best. So keep in mind that just because there are enough of the major elements for the crop, does not assure that trace element levels will also be adequate.

Under the present economic circumstances, every farmer needs to have the confidence that he is on a solid footing, and doing all he can to supply his crops the fertility needed from start to finish. The misconceptions and misunderstandings about soil fertility make this even harder to accomplish. The more farmers or those involved in a soil fertility program understand the reasons behind micronutrient recommenda­tions, the more confidence there will be in those recommenda­tions and the decisions made on how to use them. The lack of any nutrient, whether needed in major, secondary or trace amounts, hurts the soil and all that must live from it.

Source: Hands-On Agronomy

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