By Dr. Harold Willis
Corn, much more than many other crops, needs high soil fertility, especially at modern high populations. But there are several factors to consider.
Before about 1950, most corn fertilizer came from manure and legumes, with considerable lime being used. Corn crops were in rotation with other crops, so that only 1/3 to 1/2 of the cropland was planted to corn at a time. Since then, there has been the overwhelming tendency to grow higher yields and continuous corn, and to use increasing amounts of synthetic commercial fertilizers and pesticides. These trends have been accompanied by decreasing humus content, harder “deader” soil, and worse erosion, weed, pest, and disease problems. Could there be a connection?
Absolutely! Wrong methods of fertilization and soil management can make the difference between healthy, fertile, “living” soil and “dead,” sterile soil. The two big factors are adequate humus and a high population of beneficial soil organisms. The beneficial organisms can’t live and do their jobs if the soil is saturated with toxic substances. Toxic substances not only come from pesticides (herbicides and insecticides), but also are produced by anaerobic bacteria in tight or waterlogged soil, and from certain harmful fertilizers. Yes, that’s right, there are good and bad fertilizer materials, and right and wrong ways of using even the good ones.
Problems and Causes
Some of the problem-causers include: (1) anhydrous ammonia, which is toxic to nearby roots and soil organisms, causes large pH changes (first high, then low), and destroys humus (by making it soluble and leachable), causing hard soil; (2) urea and diammonium phosphate (DAP), which can release ammonia when they break down, causing some of the same problems as ammonia; (3) chloride-containing fertilizers, especially the commonly used potassium chloride (muriate of potash), which is a high-salt fertilizer and therefore can cause root damage and which kills or inhibits beneficial soil bacteria; (4) high magnesium materials on soil with adequate magnesium, primarily dolomitic limestone, which is slow to become available, and can lead to hardened soil and can release soil nitrogen reserves; and (5) wrong use of animal manures, either over-application or plowing or injecting them too deep, causing toxin production by anaerobic bacteria
Wet or Dry Fertilizer?
Then there is the question of liquid vs. dry fertilizers. Which is best? While it is true that liquids offer some advantages, namely that they can be handled easily and applied rapidly, and can easily be custom mixed, still they have some disadvantages and are not as versatile as dry fertilizers, which can also be custom blended. For one thing, liquids cannot be mixed in very high analysis solutions in blends (generally less than about 30 units of fertilizer) because of crystallization, or “salting out,” of the components (although slurry suspensions are possible). So you are hauling and spraying a lot of water. Another more serious problem is the possibility that dry conditions could follow application and result in the fertilizer salts being concentrated and “burning” the seedlings. Liquids should never be applied on the seeds, for that reason. On the other hand, dry fertilizers will not be activated in dry weather, eliminating possible damage to plants (although dry fertilizers can also damage seedlings if placed too close to the seed). Dry fertilizers can become available to plants just as rapidly as liquids when there is normal soil moisture, since dry fertilizer particles attract moisture and quickly dissolve.
Liquids can be used effectively if care is used in the choice of materials being applied and of application methods. Chloride-containing materials should always be avoided. You should have a sample of any liquid fertilizer tested to be certain it is chloride free (or at least below 2% chloride).
Corn Fertilizer Needs
Does it make much sense to apply a heavy dose of starter fertilizer just before planting? Stop and think, when does the growing plant need the most nutrients, when it is a seedling, or when it is actively growing and maturing? Most of the fertilizer in starters is not needed early in the season, and much of it (mostly nitrate nitrogen) can be lost (from leaching and denitrification) before the plants have a chance to use it. Also, too much nitrogen early in the season promotes lodging and produces a poorer balance of amino acids in the grain.
According to studies of nutrient uptake by corn, it can be seen that the plant’s need for soil nutrients begins at near zero (during germination) and gradually increases throughout the growing season. Therefore, the optimal way to supply nutrients to the crop is to either use slow-release fertilizers (ammonium nitrate or organic matter) or to use split applications throughout the season (by side dressing and/or foliar feeding). Now, if the soil is healthy and “alive” with beneficial organisms and has plenty of humus and no toxins, a proper balance of nutrients will be made available to the plants in increasing amounts through the growing season. This is because the activities of the soil organisms increase with increasing temperature. Considerable nitrogen will be taken from the air by non-symbiotic nitrogen fixing bacteria and algae. Soil microbes will even help feed nutrients to the roots, and some will protect roots from diseases and pests. That’s the way it should work. With less than perfect soil conditions and with high plant populations, supplementary fertilizers will be needed, but they should be the types that are compatible with soil life, not toxic to it.
Corn Fertilizer Requirements
Let’s see briefly what elements a growing corn plant needs and their functions in the plant. The elements needed in largest amounts are called major elements. Corn needs considerable nitrogen (N is its chemical symbol), mainly in the nitrate form, although an application of an ammonium-containing fertilizer about 45 days after planting will more effectively help the plants “switch over” to flowering and grain production activities. Nitrogen is needed by the plant for certain enzyme functions and for protein production, and it is a necessary part of chlorophyll, nucleic acids, vitamins, and several other molecules.
Phosphorus (P) is a very important element for general growth, photosynthesis, and metabolism. It carries energy from one part of the cell to another and supplies energy for the transport of nutrients and food from one part of the plant to another. It is a part of cell membranes, nucleic acids, etc. It is necessary to grow high quality crops and is especially needed by young and actively growing plants.
Potassium (K) is used in enzyme reactions, protein and chlorophyll production, food transport from one part of the plant to another, and in regulating water balance in the plant’s tissues. Potassium improves disease resistance if not too much is applied.
Calcium (Ca) is vitally important for cell division and root hair growth, enzyme activities, and for normal cell walls. Adequate calcium improves plant resistance to diseases and gives better quality crops with regard to animal (or human) nutrition.
Secondary elements are needed in lesser amounts than the major elements. Magnesium (Mg) is needed as part of chlorophyll and for the structure of nucleic acids, cell membranes, and cell structures that produce proteins. It is also required for some enzyme functions.
Sulfur (S) is a necessary part of certain amino acids (methionine, cysteine, cystine) and is important in many enzyme functions. (Elemental sulfur should not be applied to the soil; sulfur should be supplied in the sulfate form.)
The remaining elements are needed in very small amounts and thus are called trace elements or micronutrients. Each one has different functions, but in general most of them are involved in enzyme activities in metabolism and photosynthesis. The trace elements include iron (Fe), zinc (Zn), copper (Cu), boron (B), manganese (Mn), molybdenum (Mo), cobalt (Co), and chlorine (Cl).
Corn Fertilizer Balance
It is very important for crop health and high quality yields that nutrients be available to the plants in the proper amounts and in proper balance. Too much or too little of some elements can cause deficiencies of others, as can too high or low soil pH. For example, adding potassium may cause a magnesium deficiency, while high pH can cause zinc, manganese, iron, and boron deficiencies by tying up these trace elements on soil particles.
Added to this is the problem that some experts tend to recommend amounts of some fertilizers that lead to a balance of elements that produces good yields (quantity), but poor quality.
Corn Fertilizer Testing
To find out what fertilizers need to be added to your soil, it is important to have your soil tested. But the trouble with soil testing is that there are dozens of ways of doing it and interpreting the results, and some of them are very misleading. Most testing labs use methods that test the soil under artificial (dried, finely ground) rather than field conditions, and that determine the total amounts of some nutrients rather than what is readily available to the plant roots at any one time. Also, many labs recommend too high amounts of some fertilizers, just to “be on the safe side.”
The most realistic method of soil testing uses weak extracting fluids that simulate the nutrient extracting power of roots. This soluble testing shows you what soil nutrients your crops could absorb at that time. By testing more than once during the year, you can get a better idea of what is happening in your soil than by testing once every year or two. Unfortunately, few soil testing labs routinely use these methods, although most will run soluble tests if requested.
If soil tests reveal that your soil needs large amounts of certain fertilizers and you want to (or your budget allows you to) go on a soil building program, then the proper fertilizers should be broadcast. On the other hand, in-row pre-plant fertilizer and split applications of side dressing during the growing season are a very economical way to grow a good crop one year at a time.
Such soil building additives as crop residues, animal and green manures, compost, lime, and soft rock phosphate are best added in the fall, although the latter three can be applied in the spring. Manures and compost should always be plowed or disked down several inches, but not deeper (they need to be in the aerobic, or aerated zone). Up to 6–10 tons per acre of fresh animal manures are beneficial for corn, applied in the fall (only 1/2 to 1/3 as much poultry manure should be applied as cattle manure). Ammonium sulfate is a good fall fertilizer because it helps warm up the soil faster in the spring and keep it cool in hot weather.
Especially in cool, wet soils (in the North, and for early planting), an in-row starter fertilizer is valuable. It gets a uniform stand off to a quicker start, helping the corn to stay ahead of weeds and hastening maturity. The best placement for a band of starter fertilizer is about 11/2 to 2 inches below and to one side of the seed. That way the early roots can encounter it easily. “Pop-up” fertilizers (low levels of fertilizer on the seed) run the risk of fertilizer “burn” in dry weather. Adequate levels of nitrogen and phosphorus are especially important early in the season. Since there may not be high release of these elements by soil organisms at cool temperatures, good synthetic sources are ammonium sulfate, ammonium nitrate, and mono-ammonium phosphate, depending on what the soil tests in dicate is best. Soft rock phosphate, a natural product, also supplies phosphate in a form readily used by plants.
OK, so let’s say you prepared a good seedbed, applied your starter, and planted your seed. You now have a good looking stand of young corn plants “off and running.” Can you now take a vacation and come back at harvest time? Not if you want to be farming five years from now.
A growing crop should be monitored closely to see how it is doing and to check for developing problems. Large reductions in yield and quality can occur from things that happen early in the life of the plant. In corn, the period of tasseling and silking is especially critical. Don’t forget the pollen and cob development are first begun way back when the plant is only knee high.
Therefore, no matter how busy you are, it is worthwhile to walk around in your fields at least weekly. Learn to be alert for signs and symptoms of problems. Don’t just look at the leaves and stalk; also check the other half of the plant—the root system—and the soil it is growing in. Carry a shovel and dig up a plant, shake the dirt off the roots and see if you have a healthy set of roots—main roots, feeder roots, and root hairs. Are there brown or dying roots, or roots whose outer layer stripped off when the plant was pulled from the ground? This may indicate possible toxic or anaerobic soil, or salt damage. If the root system isn’t healthy, the rest of the plant won’t be either, and it can’t function effectively to make food and produce good grain. Sick plants are easy prey for pests and diseases.
Since the greatest need for soil nutrients comes in the latter 2/3 of the corn plant’s life, a mid-season soil test should be run to see if adequate nutrients are available, and in the right balance. As we mentioned earlier, soluble testing can give this information. If soil fertility isn’t what it should be, mid-season fertilization can make the difference between a near disaster and a terrific yield. What happens in all too many soils (those without adequate life, humus, and air) in the last part of the growing season is that the soil just runs out of adequate available nutrients. The cob doesn’t fill completely, or the kernels dent excessively, or the grain won’t dry down naturally. Weather stresses (heat, drought, excess rain, etc.) make the problems worse. Generally, pests and diseases take a large toll.
There are two ways of supplying an extra boost, or a “spoon feeding” for a crop while it is growing: side dressing and foliar feeding. They have different purposes and capabilities. Side dressing, either with liquid or dry fertilizers (dry is preferred, see earlier discussion in this chapter), is able to supply moderate amounts of major, secondary, or trace elements, as well as to give the plants a “shot in the arm” of a high energy fertilizer, which will stimulate them to take up more nutrients from the soil (which means the soil must have enough nutrients available or else growth will be slowed). For example, in a Door Co., Wisconsin, corn field which had sufficient calcium, phosphorus, and other nutrients, I have seen beautiful corn with well-filled ears (two per stalk) in mid-August after side dressing with 0-20-0 (superphosphate). Foliar sprays are mainly used to supply secondary or trace elements, but can also be used to “energize” plants.
Side dressing should be done starting when the corn is about knee high, or about 40–45 days after planting, and can be cost-effective even when done two or three or even four times during the middle of the growing season. It can even be done after tasseling if you have a highboy tractor. A convenient way of side dressing is to do it during cultivation by mounting fertilizer boxes on your cultivator. In side dressing, a small amount of fertilizer (perhaps 100 to 200 lbs. per acre) is applied between the rows on the surface (fertilizer should not be allowed to get into the whorl of the corn plant). Even a dry fertilizer on the surface will dissolve (from rain or dew) and become available to roots just below the surface. Side dressing is a very valuable aid to growing top quality corn if done properly. It can reduce wasted fertilizer that would leach away from a too-heavy pre-plant application, supply nitrogen during the time it is most needed, and give plants an extra boost of “energy” when it is vitally needed. However, the soil still should have an adequate supply of the important elements of phosphorus, calcium, and potassium before side dressing is done, for side dressing is best done to activate plants to absorb more nutrients that should already be available in the soil. An application of side dressing containing ammonium nitrogen at 40–45 days after planting will aid the plant to “switch over” to producing tassels and cobs.
On the other hand, foliar feeding is a little trickier to do effectively, and may not be cost effective on a crop such as corn. Ordinary field sprayers do not produce a very fine spray, and thus are wasteful of material. More expensive sprayers atomize or homogenize the spray, so you can sometimes spray a field for less than a dollar per acre of materials, especially if you mix your own spray formula. But that is too complex a subject to cover in this book. However, a good “all-purpose” spray is a mixture of about 3/4 liquid fish and 1/4 seaweed, diluted 2 gal. in 100 gal. water (deionized or soft water is best). The fish and seaweed mixture should be acid (pH 5-6.5); first adding 1 pint to 2 quarts of liquid phosphoric acid to the 100 gal. of water will take care of that. The liquid fish should be strained to keep from plugging your sprayer.
The trace element most often deficient in corn is zinc, especially in cold, wet soils and at high pH. You should never add trace elements just because you think they might be deficient, since they are needed in only very small amounts, and too much can be toxic. A soil test should be done and only the recommended amount should be applied. “Shotgun” mixtures of trace elements can be detrimental.
If you want to try foliar spraying, a good way to see if the spray will help your crop is to spray some on several plants with a hand sprayer or squeeze bottle. After a half hour, test the sugar content of the sprayed plants with a refractometer (see Chapter 6) compared to unsprayed plants. If sugar increases, spray. The most effective time to spray is early in the morning (3–4 a.m. is even better!), since that is when plants take in the most materials through their leaves. Be sure that your sprayer has had no herbicides or insecticides in it.
Source: How to Grow Top Quality Corn