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Nutrient availability can be impacted by soil chemical and physical properties, including parent material and naturally occurring minerals; amount of organic matter; depth to bedrock, sand, or gravel; and permeability, water holding capacity, and drainage. In addition, environmental conditions and crop characteristics have an important impact on nutrient availability. It is not unusual for crops in fields or portions of fields to show nutrient deficiencies during periods of the growing season, even where an adequate nutrient management plan is followed. The fact that nutrients are applied does not necessarily mean they are available. Plants obtain most of their nutrients and water from the soil through their root system. Any factor that restricts root growth and activity has the potential to restrict nutrient availability. This is not because nutrients are not plant available in the soil, but because the ability of the crop to take up those nutrients is restricted. Understanding how these factors can cause nutrient deficiency in crops is important to avoiding excessive concern about the need for additional fertilization when a sound nutrient program is already in place.
Soil compaction can limit or completely restrict root pene- tration and effectively reduce the volume of soil, including nutrients and water, which can be accessed by the plant. To limit soil compaction, avoid entering fields that are too wet, and minimize the weight per axle by decreasing load weight and/or increasing tire surface area in contact with the soil. Planting when soils are wet can create a com- pacted wall next to the seed that will prevent the seedling from developing an adequate root system. Tilling wet soils will result in clods that become hard and dry out quickly on the surface, preventing roots from accessing resources inside the clod.
Soil water content is critical not only to supply the water needs of the crop but also to dissolve nutrients and make them available to the plant. Excess water in the soil, however, depletes oxygen (O2) and builds up carbon dioxide (CO2) levels. While O2 is needed by roots to grow and take up nutrients, high CO2 levels are toxic.
Temperature is important in regulating the speed of soil chemical processes that make nutrients available. Under cool soil temperatures, chemical reactions and root activity decrease, rendering nutrients less available to the crop. Portions of the plant nutrients are taken up as roots extract soil water to replenish water lost through the leaves. Cool air temperatures can lower evapotranspiration and reduce the convective flow of water and nutrients from the soil to the root.
Light intensity is low on cloudy days. Low light intensity reduces photosynthetic rates and nutrient uptake by the crop. Since low light intensity sometimes occurs when soils are waterlogged or temperatures are cool, cloud cover can exacerbate the capacity of the crop to take nutrients.
Diseases and pests can have an important impact on crop-nutrient uptake by competing for nutrients, affecting physiological capacity (such as reduction in photosynthesis rates), and diminishing root parameters through root pruning or tissue death.
Soil tests are not perfect, so a soil test value should be considered not a single value, but rather a value within a range. There are multiple reasons why soil tests are not perfect: a soil test represents a measurement at one point in time, while a crop takes nutrients through an extended period, and typically under very different soil water and temperature conditions than at the time of sampling; the information generated typically comes from a sample from the plow layer, but the crop roots extract nutrients below that layer; laboratory precision is typically within 5% to 10% of the true value. Despite these imperfections, soil testing is the most important guide to profitable applica- tion of phosphorus, potassium, and lime because it provides a framework for determining the fertility status of a field. In contrast, plant tissue analysis is typically more reliable than soil testing for secondary macronutrients and micronutrients. Since crop yield response to application of these nutrients has been very limited in Illinois, there is not a large enough database to correlate and calibrate soil-test procedures. Ratings in Table 8.1 can provide a perspective on the reliability, usefulness, and cost effectiveness of soil tests as a basis for planning a soil fertility and liming program for Illinois field crops.
Traditionally, soil testing has been used to decide how much lime and fertilizer to apply to a field. With increased emphasis on precision agriculture, economics, and the environment, soil tests are also a logical tool to determine areas where adequate or excessive fertilization has taken place. In addition, they are used to monitor the impact of past fertility practices on changes in a field’s nutrient status. Of course a soil test report can only be as accurate as the sample sent for analysis. In fact, the spatial variability of available nutrients in a field makes soil sampling the most common and greatest source of error in a soil test. To collect samples that provide a true measurement of the fertility of an area, one must determine the sampling distribution; collect samples to the proper depth; collect samples from precisely the same areas of the field that were sampled in the past; and collect samples at the proper time.
Chemical elements exist in solution as cations (positively charged ions) or anions (nega- tively charged ions). In the soil solution, the plant nutrients hydrogen (H), Ca, Mg, K, ammonium (NH4), Fe, Mn, Zn, and Cu exist as cations. The same is true for non-plant nutrients such as sodium (Na), barium (Ba), and metals of environmental concern, including mercury (Hg), cadmium (Cd), chromium (Cr), and others. Cation exchange capac- ity (CEC) is a measure of the amount of attraction for the soil with these chemical elements.
In soil, a high CEC is desirable, but not necessary, for high crop yields, as it is not a direct determining factor for yield. CEC facilitates retention of positively charged chemical elements from leaching, yet it gives nutrients to a growing plant root by an exchange of H. Cation exchange capacity in soil arises from negatively charged electrostatic charges in minerals and organic matter. The CEC of organic residues is low but increases as the residues convert to humus, which requires from 5 years to centuries. Thus, farming practices that reduce soil ero- sion and maintain soil humus favor the maintenance of CEC. It is influenced very little by fertilization, slightly decreased with soil acidification, and slightly increased with liming.
Plant analyses can be useful in diagnosing nutrient problems, identifying hidden hunger, and determining whether current fertility programs are adequate. Critical tissue-nutrient level (below which deficiency occurs) is the concentration needed for a crop to complete its life cycle. These concentrations are largely independent of soil or growing conditions, so the values typically apply across environments and provide a more reliable measurement for micronutrients and secondary nutrients than do soil tests.
Micronutrients
Boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) are the seven essential micronutrients (also known as minor or trace elements). Although these nutrients are required only in small (micro) amounts, if any of them is deficient, it can result in severe yield reduction. Deficiencies of these nutrients are not common, making it challenging to study and to correlate and calibrate soil tests. Micronutrient tests thus have very low reliability and usefulness.
Soil compaction can limit or completely restrict root pene- tration and effectively reduce the volume of soil, including nutrients and water, which can be accessed by the plant. To limit soil compaction, avoid entering fields that are too wet, and minimize the weight per axle by decreasing load weight and/or increasing tire surface area in contact with the soil. Planting when soils are wet can create a com- pacted wall next to the seed that will prevent the seedling from developing an adequate root system. Tilling wet soils will result in clods that become hard and dry out quickly on the surface, preventing roots from accessing resources inside the clod.
Soil water content is critical not only to supply the water needs of the crop but also to dissolve nutrients and make them available to the plant. Excess water in the soil, however, depletes oxygen (O2) and builds up carbon dioxide (CO2) levels. While O2 is needed by roots to grow and take up nutrients, high CO2 levels are toxic.
Temperature is important in regulating the speed of soil chemical processes that make nutrients available. Under cool soil temperatures, chemical reactions and root activity decrease, rendering nutrients less available to the crop. Portions of the plant nutrients are taken up as roots extract soil water to replenish water lost through the leaves. Cool air temperatures can lower evapotranspiration and reduce the convective flow of water and nutrients from the soil to the root.
Light intensity is low on cloudy days. Low light intensity reduces photosynthetic rates and nutrient uptake by the crop. Since low light intensity sometimes occurs when soils are waterlogged or temperatures are cool, cloud cover can exacerbate the capacity of the crop to take nutrients.
Diseases and pests can have an important impact on crop-nutrient uptake by competing for nutrients, affecting physiological capacity (such as reduction in photosynthesis rates), and diminishing root parameters through root pruning or tissue death.
Soil tests are not perfect, so a soil test value should be considered not a single value, but rather a value within a range. There are multiple reasons why soil tests are not perfect: a soil test represents a measurement at one point in time, while a crop takes nutrients through an extended period, and typically under very different soil water and temperature conditions than at the time of sampling; the information generated typically comes from a sample from the plow layer, but the crop roots extract nutrients below that layer; laboratory precision is typically within 5% to 10% of the true value. Despite these imperfections, soil testing is the most important guide to profitable applica- tion of phosphorus, potassium, and lime because it provides a framework for determining the fertility status of a field. In contrast, plant tissue analysis is typically more reliable than soil testing for secondary macronutrients and micronutrients. Since crop yield response to application of these nutrients has been very limited in Illinois, there is not a large enough database to correlate and calibrate soil-test procedures. Ratings in Table 8.1 can provide a perspective on the reliability, usefulness, and cost effectiveness of soil tests as a basis for planning a soil fertility and liming program for Illinois field crops.
Traditionally, soil testing has been used to decide how much lime and fertilizer to apply to a field. With increased emphasis on precision agriculture, economics, and the environment, soil tests are also a logical tool to determine areas where adequate or excessive fertilization has taken place. In addition, they are used to monitor the impact of past fertility practices on changes in a field’s nutrient status. Of course a soil test report can only be as accurate as the sample sent for analysis. In fact, the spatial variability of available nutrients in a field makes soil sampling the most common and greatest source of error in a soil test. To collect samples that provide a true measurement of the fertility of an area, one must determine the sampling distribution; collect samples to the proper depth; collect samples from precisely the same areas of the field that were sampled in the past; and collect samples at the proper time.
Chemical elements exist in solution as cations (positively charged ions) or anions (nega- tively charged ions). In the soil solution, the plant nutrients hydrogen (H), Ca, Mg, K, ammonium (NH4), Fe, Mn, Zn, and Cu exist as cations. The same is true for non-plant nutrients such as sodium (Na), barium (Ba), and metals of environmental concern, including mercury (Hg), cadmium (Cd), chromium (Cr), and others. Cation exchange capac- ity (CEC) is a measure of the amount of attraction for the soil with these chemical elements.
In soil, a high CEC is desirable, but not necessary, for high crop yields, as it is not a direct determining factor for yield. CEC facilitates retention of positively charged chemical elements from leaching, yet it gives nutrients to a growing plant root by an exchange of H. Cation exchange capacity in soil arises from negatively charged electrostatic charges in minerals and organic matter. The CEC of organic residues is low but increases as the residues convert to humus, which requires from 5 years to centuries. Thus, farming practices that reduce soil ero- sion and maintain soil humus favor the maintenance of CEC. It is influenced very little by fertilization, slightly decreased with soil acidification, and slightly increased with liming.
Plant analyses can be useful in diagnosing nutrient problems, identifying hidden hunger, and determining whether current fertility programs are adequate. Critical tissue-nutrient level (below which deficiency occurs) is the concentration needed for a crop to complete its life cycle. These concentrations are largely independent of soil or growing conditions, so the values typically apply across environments and provide a more reliable measurement for micronutrients and secondary nutrients than do soil tests.
Micronutrients
Boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) are the seven essential micronutrients (also known as minor or trace elements). Although these nutrients are required only in small (micro) amounts, if any of them is deficient, it can result in severe yield reduction. Deficiencies of these nutrients are not common, making it challenging to study and to correlate and calibrate soil tests. Micronutrient tests thus have very low reliability and usefulness.
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