Part 1, Section 2: Soil Fertility Management
Soil Fertility Management
The plant nutrients taken up by crops during the growing season may come from many sources, including soil reserves, added fertilizer or manure, and crop residues. Nutrients such as nitrogen, phosphorus, and potassium are required in large quantities, while others are required in very small quantities. Nutrients such as sulfur, calcium, and magnesium are required in intermediate quantities. Soil supplies for nutrients required in small quantities (micronutrients) need not be large, but the nutrients must be available to the plant.
All the nutrients listed in Table 1.2-2 are essential for plant growth. The roles and deficiency symptoms of the primary nutrients are discussed below. The symptoms are rarely clear cut, so it is important to use both soil and plant analyses when trying to diagnose a suspected nutritional problem.
|Primary nutrients||Secondary nutrients||Micronutrients|
|Nitrogen (N)||Sulfur (S)||Iron (Fe)|
|Phosphorus (P)||Magnesium (Mg)||Manganese (Mn)|
|Potassium (K)||Calcium (Ca)||Boron (B)|
Nitrogen (N) is a critical component of proteins, which control the metabolic processes required for plant growth. It also is an integral part of the chlorophyll molecule and thus plays a key role in photosynthesis. An adequate supply of nitrogen is associated with vigorous vegetative growth and a plant’s dark green color. Nitrogen deficiency is characterized by reduced plant growth and a pale green or yellow color. This yellowing generally begins at the tip of the leaf and goes down the middle of the leaf. If the deficiency is severe, the affected area eventually turns brown and dies. Since nitrogen is mobile in the plant, older leaves show the first symptoms of nitrogen deficiency. Nitrogen behavior is extremely dynamic in the soil and very dependent on the weather. Much of the soil N is in the organic matter and must be mineralized into soluble forms by microbes before it can be taken up by plants. Because of this dynamic behavior, soil testing for N ahead of the growing season in humid areas like Pennsylvania is not reliable. In-season testing can be used to adjust N management during the season (see the N recommendations section later in this chapter and in the corn chapter). Also, because of its behavior, N is very susceptible to loss both to the atmosphere and to water, and can be an environmental threat.
Phosphorus (P) is a critical component of nucleic acids, so it plays a vital role in plant reproduction, of which grain production is an important result. Considered essential to seed formation, this mineral is often found in large quantities in seed and fruit. Phosphorus is essential for the biological energy transfer processes that are vital to life and growth. Adequate phosphorus is characterized by improved crop quality, greater straw strength, increased root growth, and earlier crop maturity. Phosphorus deficiency is indicated by reduced plant growth, delayed maturity, and small fruit set. These symptoms may be accompanied by a purple coloring, particularly in young plants. Like nitrogen, phosphorus is mobile in the plant; therefore, any deficiency symptoms show up first on older leaves. Most soil P occurs in relatively insoluble minerals and organic matter in the soil. The availability of these forms is very sensitive to soil pH. Phosphorus availability can be reliably estimated with soil tests. Phosphorus can be lost from the soil. Loss is primarily through erosion of soil particles and organic matter containing P and loss of soluble P in runoff from soils with very high P levels at the surface. Phosphorus lost to water can be an environmental threat.
Potassium (K) is not an integral part of any major plant component, but it does play a key role in a vast array of physiological processes vital for plant growth, from protein synthesis to maintenance of plant water balance. Potassium deficiency is characterized by reduced plant growth and a yellowing and/or burning of leaf edges. Since potassium is mobile in the plant, the symptoms appear on the older leaves first. Another indication of potassium deficiency is reduced straw or stalk strength, which results in lodging problems, reduced disease resistance, and reduced winter hardiness of perennial or winter annual crops. The secondary nutrients, calcium, magnesium, and sulfur play a variety of roles in plants. Excess K in the soil can lead to excess K in plants and can in some cases result in animal nutrition/health problems. Potassium in the soil is mainly in the form of insoluble minerals and as the K+ cation on the soil CEC. Soil tests can estimate available K usually by measuring the K+ cation on the soil CEC.
The secondary nutrients calcium, magnesium, and sulfur play a variety of roles in plants.
Calcium (Ca) is an integral part of plant cell walls. Calcium deficiency is rare among agronomic crops under Pennsylvania conditions. When a soil is properly limed to maintain an optimum pH level, calcium is usually adequate for agronomic crops. Calcium is mainly found in soil minerals and as the Ca2+ cation on the soil CEC. Soil tests can estimate available Ca usually by measuring the Ca2+ cation on the soil CEC.
Magnesium (Mg), a key component of chlorophyll, plays a critical role in photosynthesis. Magnesium deficiency is characterized by white stripes between the leaf veins. Magnesium is best supplied by a limestone that contains this nutrient. Magnesium is mainly found in soil minerals and as the Mg2+ cation on the soil CEC. Soil tests can estimate available Mg usually by measuring the Mg2+ cation on the soil CEC.
Sulfur (S) is a common component of proteins and vitamins. Sulfur-deficient plants have a general yellowing and are very spindly. Symptoms of sulfur deficiency are similar to those of mild nitrogen deficiency, except that they appear sooner in new growth than in old growth, since sulfur is not mobile in the plant. Under Pennsylvania conditions, sulfur deficiency is not common. Rainfall supplies significant amounts of sulfur, and it is recycled efficiently through the manure applications that much of the cropland in the state receives. However, as efforts continue to reduce acid rain, sulfur deficiencies may become more common. Similar to nitrogen, most of the S in the soil is found in the organic matter. Also like N, soil tests for S are not reliable in humid environments like Pennsylvania. Plant analysis is a better tool for monitoring S in the crop.
Micronutrients are key players in many of the processes important for plant growth. Few micronutrient problems exist in Pennsylvania, because (1) the heavier loamy texture of our soils helps to maintain adequate levels of micronutrients (sandy textured soils, by contrast, often show micronutrient deficiencies); (2) the slightly acidic nature of our soils helps to maintain micronutrient solubility; and (3) Pennsylvania agriculture is largely animal based, so much of our cropland gets periodic applications of manure, a good source of micronutrients.
Of the micronutrients, boron (B) and zinc (Zn) are the two that occasionally are deficient in the state. Boron deficiency is most likely in alfalfa and can be overcome by periodically applying boron when topdressing alfalfa. Zinc deficiency is sometimes observed on corn, particularly when phosphorus levels become excessive from overapplication of phosphorus fertilizer. Symptoms on corn appear as a rough stripping on either side of the midrib of the corn leaf. Zinc deficiency can be corrected with a periodic broadcast application of zinc. Routine application of zinc is not recommended.
The lack of general response to micronutrients has made it difficult to calibrate a micronutrient soil test for use in Pennsylvania. In addition, it is not possible to rely on calibrations developed for other soil, climatic, and cultural systems. The best tool for evaluating a plant’s micronutrient status is plant tissue analysis.