Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209701
07/10/97
Extension Bulletin E-486, Revised August 1994
By M.L. Vitosh, D.D. Warncke and R.E. Lucas Department of Crop and Soil Sciences Michigan State University Extension
General Information
Plant nutrients in fertilizers are classified as major nutrients and micronutrients. The most important major nutrients are nitrogen (N), phosphorus (P) and potassium (K). Plants require these nutrients in relatively large amounts, and these are the nutrients most likely to be deficient for plant growth. The other major nutrients, also called secondary nutrients, are calcium (Ca), magnesium (Mg) and sulfur (S). They are also required in relatively large amounts but are less likely to be deficient. Micronutrients are essential for plant growth, but plants require relatively small amounts of them. They include boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). These elements may also be referred to as minor or trace elements, but "micronutrients" is preferred.
Extent of Secondary and Micronutrient Deficiencies
Secondary and micronutrient deficiencies in Michigan are not widespread, but a deficiency of any of these elements carcase plant abnormalities, reduced growth or reduced yield. Increased need for these nutrients in crop production has drawn attention for the following reasons:
* More information on crop responses and availability of nutrients in various soil types has been accumulated.
* Today's higher crop yields require larger amounts of these nutrients.
* Long-time cropping has removed measurable amounts of these nutrients.
* Today widespread use of animal manures has decreased and use of high-analysis fertilizers low in nutrient impurities has increased.
* High potassium levels in the soil and greater use of nitrogen fertilizer can cause a magnesium imbalance in some plants that creates the potential for hypomagnesia (grass tetany) problems in livestock.
* High phosphorus levels in the soil may induce zinc deficiency in plants.
* Air pollution abatement practices have reduced sulfur emissions from industry and decreased the amount of sulfur added to the soil by precipitation.
* More attention is being given to crop quality and nutritional value of today's crops.
Identifying Problem Areas
The information used to help identify secondary and micronutrient problems are soil tests, plant analysis, soil-type, crop species and plant symptoms. A good way to confirm a suspected nutrient deficiency is to obtain plant and soil samples from adjacent areas with normal and abnormal plant growth and have them tested.
The MSU Soil Testing Laboratory will test soil samples for pH, available phosphorus, exchangeable (available)potassium, calcium, magnesium and extractable (available)zinc, manganese and copper. Some laboratories may also offer soil tests for sulfur, boron and iron.
Many soil testing laboratories also offer plant tissue analysis. (Vis. 1) shows the acceptable or sufficiency nutrient concentrations required for production of several important Michigan crops. The values reported for vegetables are general and should be used only as guidelines. Nutrient values below the sufficiency concentration may indicate a deficiency. Values above the sufficiency concentration maybe excessive and possibly toxic.
When using tissue analysis to diagnose nutrient deficiencies, it is important to sample the specified plant tissue at the proper time. It may also be helpful to note whether deficiency symptoms existed on the plant or tissue sampled and any climatic conditions that might have caused the symptoms to occur.
Secondary and Micronutrient Fertilizers
There are several ways to add secondary and micro- nutrients to nitrogen, phosphorus and potassium (N-P-K) fertilizers. They may be incorporated into granulated fertilizers during the granulation process so that each granule of fertilizer contains an equal amount of all nutrients. They may be blended with N-P-K fertilizers at a bulk blending plant. If the particle size of secondary and micronutrients is greatly different from the size of particles containing the primary nutrients, a sticker may be needed to prevent particle size separation. Separation can lead to segregation of particle sizes and non-uniform application. These nutrients may also be added to liquid or suspension fertilizers. Chelated secondary and micronutrient formulations of these nutrients are generally preferred to non-chelated materials for mixing with liquid fertilizer because a larger amount of the nutrient can be added before precipitation occurs. The amount of secondary or micronutrients required in mixed fertilizers depends on the application rate. (Vis. 2) can be used to determine the appropriate percentage of elements needed in mixed fertilizers based on the amount of fertilizer to be applied and the amount of element required per acre. In this bulletin, all secondary and micronutrient recommendations are given in pounds of element per acre.
Fertilizer Laws
The Michigan Fertilizer Law requires each fertilizer manufacturer who claims secondary and micronutrients are present in fertilizer to guarantee the minimum composition of these nutrients in the fertilizer. Some states have set minimums for claims at levels recommended by the Association of American Plant Food Control Officials (AAPFCO). These levels are lower than those permitted by Michigan regulations. We believe that these low levels often do not supply sufficient quantities to correct a deficiency. For example, one application of 300 pounds of fertilizer containing 0.05 percent manganese (minimum set by AAPFCO) per acre will supply 0.15 pounds of manganese. In deficient soils, field trials have shown a need for at least 5 pounds per acre-a 32- fold difference.
Some manufacturers claim that small amounts of secondary and micronutrients are needed to maintain soil fertility. Such claims may have merit for some micronutrient elements in very sandy soils. However, for most agricultural soils, the problem is not replacing the nutrients at a maintenance level, but maintaining the nutrients in an available form. Most soils have adequate total iron and manganese, but these nutrients are largely in an unavailable form. The availability of copper and zinc is also governed by their adsorption to soil particles. Boron is a highly mobile element that does not accumulate to a great extent in sandy soils. Therefore, it may be difficult to maintain boron in these soils at sufficiently high levels for certain crops. In other soils, boron may be adsorbed and unavailable for crop use.
| Visual title - Visual size | Visual title - Visual size |
|---|---|
| Nutrient sufficiency rages for corn, soybeans, alfalfa, wheat,etc - 57K | Percentage of the element needed in mixed fertilizer, based on - 20K |
