Managing plant nutrient inputs to sustain or optimize plant productivity economically while minimizing environmental consequences is the goal of soil fertility management. Nutrient management plans document available nutrients by source (inorganic or organic), crop production practices, and other management decisions that influence nutrient bioavailability, plant growth and productivity, and environmental stewardship. This publication integrates many years of research about the economic and environmentally responsible use of plant nutrients in Mississippi.

Nutrient management planning (NMP) is a best management practice, or BMP. It incorporates all nutrient inputs, whether organic materials, livestock production by-products, or commercial inorganic fertilizers.

What Is Nutrient Management Planning?

Nutrient management planning principles are the basic, sound fundamentals of good business management.

Nutrient management planning is:

knowing what you have,
knowing what you need,
managing wisely, and
documenting your management.

Nutrient management plans are site-specific and tailored to the available inputs, soils, landscapes, and management objectives of the farm. They should be a blueprint to implement the four Rs: use the right amount of the right fertilizer at the right time in the right place. Following are steps to nutrient management planning:

Get accurate soil information for each field or management unit.
- Develop farm maps with soil series, surface water bodies, and other management concerns present in the landscape.
- Soil sample each field or management unit and process through a reputable soil testing laboratory. Note that some government programs in the state require that testing be done by the Mississippi State University Extension Service Soil Testing Laboratory.
Develop fair, realistic yield potential estimates for each field based on recent production history, agronomic practices, and soil potential.
Determine what plant nutrients may be required to reach the yield potential based on the soil test results. In some cases, you may need to consider nutrient uptake and removal data for common crops. This information is available from various resources, including this publication. It is important to distinguish between growing crop uptake versus nutrient removal in harvested biomass.
Determine plant-available nutrients provided by livestock by-product amendments such as broiler litter that may be used to fertilize the crop. The BMP is to sample the manures that will be used. Table values are available, but the accurate nutrient content of manure is site-, animal-, diet-, and management-specific (see Using Poultry Litter to Fertilize Agronomic Crops on page 23). More information on testing broiler litter is available in MSU Extension Publication 3749 Soil and Litter Testing Basics.
If manures were applied to previous crops, estimate any residual nutrient contributions. Usually, 50–60 percent of the nitrogen in animal manures is available to growing plants the first year following application. Subsequent manure nutrient use by plants without additional applications is usually on a declining scale for three growing seasons. The MSU Extension Service credits carryover from previous inorganic fertilizer applications in only specialized circumstances for some crops.
Environmental assessment tools such as the Mississippi Phosphorus Index (PI) are available to determine the potential risk of off-site phosphorus movement. Risk is evaluated on a field-by-field basis when animal by-products have been used previously and are being considered for the planning cycle. The PI incorporates site-specific soil conditions and applies BMPs in the evaluation process. Soil test phosphorus levels, soil permeability, field slopes, litter application rates, distance to surface water, and other factors are used to determine the probability of nutrient movement in the landscape. If the PI rating is low, NMPs may be based on crop nitrogen needs. If the PI is medium, additional BMPs may need to be used. If the PI shows a high potential risk for phosphorus movement in the landscape, nutrient management should be based on crop phosphorus requirements as determined by the soil test.
Apply rates of commercial fertilizers and/or animal manure to supply nutrients based on the soil test recommendations and the PI risk assessment process. Overapplication does not improve yields and increases the risk of environmental issues.
Keep records of nutrient sources, application dates, rates, methods, and climatic conditions. This simplifies future planning.

Nutrient Management

Know your soils and fields.
Be realistic about yield potential.
Determine nutrient removal.
Find out what is available from this year’s application.
Calculate nutrients available from previous applications.
Assess the environmental risk of nutrient movement.
Use common sense when putting nutrients out.
Keep relevant field records.

This guide is a brief introduction to nutrient management in Mississippi. Practices used on particular farms will vary throughout the state due to soils, weather conditions, and other localized considerations. See MSU Extension Publication 3681 Best Management Practices for Plant Nutrient Management for more information on nutrient BMPs. For crop-specific information, see the MSU Extension Soils page or contact your county MSU Extension office.

The Soils of Mississippi

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Soils form from parent materials, climate, biological factors, and topography interacting over time. The diverse factors present in Mississippi yield a wide variety of soils.

Three general land resource regions are recognized in Mississippi (Figure 1):

An alluvial (water-deposited) flood plain along the current and former channels of the Mississippi River (the Delta).
A loess region, which is a band of soils formed in windblown material adjoining the Delta.
Coastal Plain areas, which make up the rest of the state.

The Mississippi loess and Coastal Plain regions have smaller units based on common soils, geology, climate, water resources, and land use.

Soils of the world are classified into 12 soil orders; seven have been found within Mississippi (Figure 2). About 260 individual soil series (the most specific soil classification) have been mapped in the state.

As human land management transitioned from before European colonization to the modern, mechanized era, the predominant surface activities within regions have evolved based on the underlying soil resource. About 75 percent of Mississippi’s annually seeded crops, such as cotton, corn, and soybeans, are grown in the relatively flat, deep alluvial soils of the 18 Delta and partial-Delta counties conducive to mechanized farming. Animal production and forestry predominate in the shallower soils of the hills in east and south Mississippi.

Federal soil conservation efforts since the 1980s significantly reduced acreages of tilled crops in the hill regions with very erosive soils. Much of this repurposed acreage is planted in pine trees.

Southern Mississippi Valley Alluvium: The Delta

Mississippi Delta soils originate in sediments left by flooding of the various rivers in the region; it is not a traditional river delta. Much of the Delta region is used to produce annual crops; three-fourths of the total cropland is in the northern counties. Water management issues, including flooding, crop irrigation, and internal field drainage, are critical considerations for soil management in the region.

Delta soils are diverse at small scales due to the alluvial (flooding origin) parent material. Particle sizes within sediment deposits are smaller because they are farther from the originating stream (i.e., soils closer to running water have proportionally more large silt and sand particles than soils farther from the stream). Delta soils also reflect the long-term effects of surface water movement; soils formed under standing water differ from soils formed under running water.

Mississippi Delta soils with more clay particles (the smallest basic soil solid) have unique features. When these soils dry, small, round aggregates form at the surface. These resemble the pellets within shotgun shells and, accordingly, are often called “buckshot.” These clay soils have very slow water infiltration rates, making them ideal for aquaculture and rice production.

Southern Mississippi Valley Uplands:
Thin Loess Areas and Brown Loam Hills

When floodwaters receded in what is now the Delta, strong west to east winds blew dry sediment deposited by the floods to adjacent uplands. This wind-deposited material is called loess and is the parent material of soils formed in the hilly region along the eastern edge of the Delta. The depth of loess decreases from west to east across the state as the distance from the originating flooded lands increases.

This area, referred to as the Brown Loam region or Bluff Hills, has some very deep deposits, such as the bluffs outside Yazoo City. Natchez silt loam, a soil series found on about 170,000 acres in this area, is designated as the Mississippi state soil. Diverse agriculture is found in the loess region; however, erosion is a significant resource concern because the soils tend to erode when exposed.

Coastal Plain

Mississippi Coastal Plain soils occur on the edge of a soil region that forms an arc along the United States coast from New Jersey to Texas. These soils form on unconsolidated fluvial (stream or river) or marine sediments deposited at the edges of ancient seas. These soils usually are best suited to pastures and forests, but they can support other crops.

The northern portion of the Coastal Plain is commonly called the Mississippi Sand Clay Hills. The Southern Coastal Plain is the Piney Woods region of the state.

Blackland Prairie

Mississippi has two prairie regions: the Blackland Prairie of northeastern Mississippi in the Tupelo, Aberdeen, and Columbus area, and the Jackson Prairie in south-central Mississippi.

Many soils in these areas are very dark and are prone to developing wide cracks when dry. On the surface, they may look like Midwestern prairies. The Mississippi Blackland soils are formed in soft limestone or chalk parent material. Lighter color horizons underlie the very dark surface layers. Midwestern prairie soils form in glaciated areas predominated by grasslands where centuries of nutrient cycling produced soil with very high soil organic matter and dark soils throughout the profile. The Mississippi soils have lower organic matter levels.

The Mississippi prairie soils support a wide variety of agricultural production.

Gulf Coast Marsh

Zones of the marsh along the Gulf of Mexico are almost treeless, have marsh vegetation, and are uninhabited. This area is part of the estuarine complex that supports marine life. Most soils of the Gulf Coast marsh are very poorly drained, with the water table at or above the surface most of the time. These soils are susceptible to frequent flooding. They formed in alluvial and marine sediments and have organic accumulations.

Eastern Gulf Coast Flatwoods

This is flat to gently sloping land along the Gulf of Mexico that includes the highly developed coastal part of the state. Much of the undeveloped land is owned by paper companies and managed as forests. The military also has significant acreage in the region.

For the Soils on Your Farm

This is only a brief introduction to the soil regions of Mississippi, which contains about 260 different soils with a wide range of properties. Site-specific information about the soils in your area is readily available from the U.S. Department of Agriculture’s Web Soil Survey.

Web Soil Survey is easy to use to identify the soils of any location and their properties. You can download user-defined reports that inform appropriate conservation or nutrient management plans. Local offices of the Natural Resource Conservation Service and the MSU Extension Service also are available to help identify the soils on your farm.

Figure 1. Mississippi land resource regions.

Plant Nutrients

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The soil provides most nutrients needed by plants. Insufficient nutrients potentially limit or end plant growth, resulting in decreased yields and lower profitability. Excess nutrients can also limit plant development and decrease yields, leading to lower profit and enhanced environmental risks. Nutrient management planners should optimize nutrient relationships to decrease undesirable outcomes.

Essential nutrients are chemical elements required by plants to complete their life cycles. Plant scientists currently recognize 18. Three—carbon (C), hydrogen (H), and oxygen (O)—are assimilated by plants via photosynthesis. The other plant nutrients are overwhelmingly obtained from the soil through root uptake.

In native landscapes, nutrients recycle between plants and soils constantly; however, agricultural crops may use more nutrients than natural turnover can provide. Furthermore, harvested crop biomass removes significant amounts of nutrients from the field. Supplemental nutrients may promote optimal crop development and profitability. These may be provided using inorganic (commercial) fertilizers, animal manures, green manures, and legumes.

Essential nutrients are traditionally classified into three categories: macronutrients, secondary nutrients, and micronutrients. The category names reflect more the nutrient supply characteristics (both in the soil and through amendments) than the actual quantity required by the plants. Remember that an insufficient supply of these nutrients leads to poor plant performance or even death.

Using the right amount of the right fertilizer at the right time in the right place is the foundation of nutrient management. See BMPs for Nutrients in Agronomic Crop Production on page 29 and MSU Extension Publication 3681 Best Management Practices for Plant Nutrient Management for more information about economically and environmentally sustainable nutrient management. For more information about fertilizer management in agronomic crops, see Extension Publication 2500 Inorganic Fertilizers for Crop Production and Publication 1466 Fluid Fertilizers.

Macronutrients

Nitrogen

Nitrogen (N) is a component of chlorophyll, which gives the green color to plants and is vital for photosynthesis. It is required for protein production in plants and animals and is a component of the nucleic acids DNA and RNA. Crop nitrogen use efficiency is relatively poor, and significant quantities are often lost via leaching, volatilization, or denitrification. Because of these loss mechanisms, the warm and humid Mississippi environment is not conducive to predictable soil nitrogen carryover from one growing season to the next. Year to year variability may result in some nitrogen carryover in some circumstances.

Highly variable moisture and temperature changes in Mississippi mean the oxidation state of nitrogen transforms continuously. These rapid changes make nitrogen soil testing difficult for most crops, even with a collected sample. Therefore, nitrogen measurements made in the laboratory are not well calibrated with nitrogen available to plants in the rooting zone. More information about nitrogen in Mississippi soils and plant nutrition is available in Extension Publication IS767 Nitrogen in Mississippi Soils.

Legume plants such as soybeans and clovers produce almost all their nitrogen through a symbiotic, or mutually beneficial, relationship with bacteria (Rhizobium species) attached to plant roots. Rhizobium bacteria convert atmospheric nitrogen (N2 gas) into plant-usable forms. Because this relationship provides large quantities of nitrogen, legumes with active nitrogen-fixing bacteria rarely need additional nitrogen fertilization. Additionally, if these crops are fertilized with nitrogen, bacterial nitrogen production will decrease. Bacterial populations do not persist longer than a few years in fields where their host legume crop is not planted. In these instances, legumes must be inoculated with the proper nitrogen-fixing bacteria species before planting.

Phosphorus

Phosphorus (P) is integral to converting solar energy to the chemical energy that plants need to synthesize sugars, starches, and proteins. Plant-available phosphorus is usually high or very high in Mississippi Delta soils, so phosphorus fertilization is minimal in those 18 counties. Phosphorus levels in other regions of the state vary but, in the absence of extenuating circumstances (such as long-term application of high amounts of fertilizers), are usually lower than in the Delta region.

Phosphorus is relatively immobile in the soil matrix but can be transported by erosion of soil solids. Phosphorus builds up over time when the phosphorus from fertilizers and organic materials is greater than the amount removed by harvested crops. Early-season plant deficiencies are possible when plant root growth is slow. This is exacerbated by the immobility of the nutrient in the soil.

Phosphorus fertilization is inefficient as it becomes unavailable for plant use through reactions in the soil with iron, aluminum, and calcium. Plant-available phosphorus is much less than the total quantity present in the soil and can only be estimated through soil tests. Additional information on managing phosphorus is available in Extension Publication IS871 Phosphorus in Mississippi Soils.

Potassium

Plants use potassium (K) in photosynthesis, sugar transport, water and nutrient movement, protein synthesis, and starch formation. Adequate potassium plant nutrition improves disease resistance, water stress tolerance, winter hardiness, plant pest tolerance, and uptake efficiency of other nutrients.

Potassium removal by crops under good growing conditions is high, often equal to nitrogen uptake and several times the uptake of phosphorus. Conversely, where levels of soluble potassium in the soil are high, plants may take up more potassium than needed (“luxury consumption”) that is not reflected by higher yields.

Potassium mobility in soils is related to soil texture: movement is greatest in soils with more sand content. The buildup of potassium in soils is related to soil texture, with the greatest accumulation generally in clay soils, followed by loam and coarse-textured sands.

Secondary Nutrients

Sulfur

Sulfur (S) is a component of some amino acids used in building proteins. Plants require about the same quantity of sulfur as phosphorus. In Mississippi, soil-test sulfur is reported by the Mississippi State University Extension Soil Testing Laboratory for some crops. The amount of sulfur reported is not a direct measurement; it is based on the soil organic matter content found in the sample.

Like nitrogen, sulfur is mobile in soils and can be lost by leaching. Unlike nitrogen, sulfur is immobile within plants, so deficiency symptoms first present on younger tissues. Nitrogen symptoms are presented in older tissues.

Mississippi soils once received sulfur as an extra nutrient in formerly used fertilizers. For many years, about 25 pounds per acre was deposited in rainfall in the state annually. Neither of these is true as of 2021. When sulfur was added via fertilizer and rainfall, low sulfur issues were confined to coarse, sandy-textured soils prone to leaching already-low organic matter levels. More deficiencies have been diagnosed recently.

More information about sulfur in Mississippi soils and plant nutrition is available in Extension Publication 3669 Sulfur Nutrition for Mississippi Crops and Soils.

Calcium

Calcium (Ca) makes up part of the cell wall and stabilizes cell membranes. Calcium deficiencies are usually manifested in growing points of the plant at the fruit, stem, leaf, and root tips. Calcium deficiency is rare in Mississippi soils, but some crops such as peanuts may use more calcium in one season than the soil can supply.

Magnesium

Magnesium (Mg) is the central part of the chlorophyll molecule where photosynthesis occurs. It is also involved in energy metabolism in the plant and is required for protein formation.

Magnesium deficiency is rare in Mississippi soils but has been diagnosed in some unusual circumstances on sandy soils with low cation exchange capacities and high soil-test potassium. This may lead to a deficiency level for grazing animals. More information on calcium and magnesium nutrition can be found in Extension Publication 3727 Calcium and Magnesium in Mississippi Crop Production.

Micronutrients

This is a brief introduction to micronutrients. More information is available in Extension Publication 3726 Micronutrients in Mississippi Soils and Plant Nutrition.

Copper

Plants require copper (Cu) in very small amounts. It is involved in respiration, protein synthesis, seed formation, and chlorophyll production. Copper is immobile in soils, so it accumulates when application rates exceed use. Copper is also held tightly by organic matter.

Despite being an essential nutrient, copper may be toxic to plants in some situations. Applications of copper-containing municipal biosolids to fields are regulated to manage soil accumulation and other potentially toxic elements. See Extension Publication 3663 Biosolid Applications to Mississippi Soils for more information.

Zinc

Zinc (Zn) is involved in starch formation, protein synthesis, root development, growth hormones, and enzyme systems. As with copper, zinc is relatively immobile in soils and tends to accumulate. Zinc deficiencies are most common on sandy, low organic matter soils with high pH and phosphorus levels, especially under cool, wet conditions. Zinc deficiency symptoms are evident on small plants as interveinal light striping or a whitish band beginning at the base of the leaf.

Manganese

Manganese (Mn) is involved in chlorophyll formation, nitrate assimilation, enzyme systems, and iron metabolism. A high soil pH generally causes manganese deficiency, whereas manganese toxicities occur at low soil pH. Liming programs likely are the best way to address manganese toxicity issues.

Boron

Boron (B) is involved in sugar and starch balance and translocation, pollination and seed production, cell division, nitrogen and phosphorus metabolism, and protein formation. Boron is highly mobile in soils and is not readily retained by sandy surface soils. In contrast, boron has limited mobility in plants. It must be added annually for crops sensitive to boron deficiencies. In Mississippi, boron is recommended for all alfalfa production and cotton production in all non-Delta areas. In Delta areas, boron may boost yields on non-irrigated soils in dry weather, particularly if the soil has been recently limed. However, excessive rates of boron fertilization should be avoided.

Molybdenum

Molybdenum (Mo) is involved in protein synthesis, legume nitrogen fixation, enzyme systems, and nitrogen metabolism. Deficiencies of molybdenum generally occur on acidic soils with high iron (Fe) and aluminum oxides. The availability of soil reserves of molybdenum to the plant are largely regulated by soil pH. Mississippi State University Extension recommends applying 0.5–1 ounce of sodium molybdate or equivalent annually per bushel of soybean seeds at soil pH less than 7.0.

Iron

Iron (Fe) is used in chlorophyll and protein formation, enzyme systems, respiration, photosynthesis, and energy transfer. Iron deficiency is believed to be caused by an imbalance of metallic ions, such as copper and manganese, excessive amounts of phosphorus in soils, and a combination of high pH, high lime, cool temperatures, and high carbonate levels in the root zone.

Iron deficiency chlorosis in soybeans is a concern in the Blackland Prairie soils of the state where high pH limits iron solubility and, therefore, plant uptake.

Chlorine

Chlorine (Cl) is involved in photosynthesis, water-use efficiency, crop maturity, disease control, and sugar translocation. While chlorine leaches quite readily in coarse-textured soils, deficiencies are not very common and have not been identified in Mississippi.

Nickel

Plants require nickel (Ni) for proper seed germination. Additionally, it is the metal component in urease, an enzyme that catalyzes the conversion of urea to ammonium. Deficiency symptoms are poor germination and chlorosis. It has not been identified as an issue in Mississippi agronomic crops.

Table 1. Macronutrient removal by selected crops.

Crop	Unit	N	P2O5	K2O
Alfalfa	lb/ton	51	12	49
Bahiagrass	lb/ton	43	12	35
Barley	lb/bu	0.89	0.41	0.28
Barley straw	lb/ton	15	5	30
Bermudagrass, common	lb/ton	25	8	34
Bermudagrass, hybrid	lb/ton	50	12	43
Clover, grass	lb/ton	50	15	60
Clover, crimson	lb/ton	47	9	59
Clover, red	lb/ton	41	10	40
Clover, white	lb/ton	56	15	49
Coastal bermuda	lb/ton	50	12	43
Corn, grain	lb/bu	0.90	0.44	0.27
Corn, silage, 67% water	lb/ton	10	3.1	7.3
Corn, stover	lb/ton	22	8	32
Cotton, lint	lb/bale	32	14	19
Eastern gamagrass	lb/ton	40	4	40
Fescue	lb/ton	27	12	54
Lespedeza, Korean	lb/ton	42	10	21
Lespedeza, striata	lb/ton	40	29	22
Oats, grain	lb/bu	0.77	0.28	0.19
Oats, straw	lb/ton	13	8	40
Orchardgrass	lb/ton	36	13	54
Peanuts, nuts	lb/ton	70	11	17
Peanuts, vines	lb/ton	43	6.6	20.5
Rice	lb/bu	0.57	0.30	0.16
Ryegrass	lb/ton	60	16	50
Sorghum, grain	lb/bu	0.66	0.39	0.27
Sorghum-sudangrass	lb/ton	40	15	58
Soybeans	lb/bu	3.8	0.84	1.30
Sweet potatoes	lb/cwt	0.52	0.23	1.00
Tomatoes	lb/ton	3	1.3	6.2
Watermelons	lb/cwt	0.42	0.12	0.74
Wheat, grain	lb/bu	1.3	0.60	0.34
Wheat, straw	lb/ton	13	3	23

Data derived from International Plant Nutrition Institute Nutrient Database, North Carolina State University Extension Service Publication AG-439, Section 4 of the Mississippi NRCS Field Office Technical Guide, and http://plants.usda.gov/.

Introduction to Soil Testing

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Soil sampling and testing are critical for successful nutrient management. Soil testing assesses the nutrients available for plant uptake. Soil testing services available through Mississippi State University Extension or commercial laboratories should—

accurately determine the nutrient status in the soil,
convey whether a nutrient deficiency or excess is serious,
serve as the basis for fertilization management, and
allow an economic assessment of the options.

Soil testing calibration and correlation were developed for crop response. However, soil test-based nutrient recommendations are a foundation for environmentally sensitive stewardship of nutrients. Soil tests alone are insufficient to provide accurate environmental assessment. Soil fertility interacts with other soil-related properties within production systems, including soil series, slopes, and erosion potential.

Representative samples are crucial for reliable test results and interpretation to optimize production, investment return, and environmental quality. Sound field sampling ensures that the soil collected represents the area of interest and improves the relevance of the test results. The soil itself is highly variable, and this variability can increase with vegetation differences, terrain or slope, drainage, organic matter content, texture, and previous fertilizer applications. Errors in test results are usually caused by poor sampling.

Defining Areas to Soil Sample

Soil sampling should always begin with a field plan or map of the different areas to be tested. Knowledge of soils, field acreage, soil characteristics, crop growth patterns, drainage, and other factors can help you identify areas that can be sampled on a “whole field” basis. More intensive sampling schemes such as geometrical grid patterns or user-defined soil management zones often are used within fields. Information about soil series, texture, drainage, yield maps, or use history can help you define and locate soil management zones. Growers may work with crop consultants, Extension Service personnel, or others to determine the best sampling pattern for their situation.

Soil Sampling

Attention to detail provides robust, effective soil sampling that maximizes the quality of the resulting recommendations. Follow these guidelines:

Use the proper equipment for the situation.
Random-sample the field, grid, or zone.
Account for previous banded fertilizer applications.
Sample the appropriate soil depths for the situation.
Include an adequate number of subsamples.
Properly identify and code each sample.
Consistently sample during the same season from year to year.
Handle the samples appropriately.

Equipment

Soil may be collected in several ways. Use only stainless steel or other non-reactive metal tools to extract the subsamples in the field. Specialized soil test probes are available but not necessary for soil sampling.

Random Sampling

Sample in whole fields using a random walk pattern across the entire area. Mix the subsamples thoroughly, and transfer the mixture to properly labeled boxes or bags.

Intense soil sampling schemes characterize soil fertility at more detailed levels than whole-field sampling. Grids or management zones are commonly used. The grid cell method considers each grid as a separate whole field and should be sampled with a random walk. Grid point methods sample within a relatively small radius of a midpoint in the grid. In areas where you believe nutrient levels are relatively high and variable, grid cell is the better option. Grid point is the better option when in-field nutrient variability is low. Fields with a history of banded phosphorus and/or potassium fertilizer applications should be accounted for in the sampling plan.

Depth of Sampling

Most agronomic fertilizer recommendations assume samples were collected from the top 6 inches of soil. However, broadcast application of immobile phosphorus and potassium fertilizers leads to the stratification of these nutrients near the soil surface. When minimum or no-till is used, a soil sample depth of 4 inches is typically recommended.

Pastures and hay fields may need double sampling if nutrient or pH stratification is suspected: a 2-inch depth for pH and lime requirements and the regular 6-inch depth for other nutrients.

Number of Cores

Getting a sample that properly represents a field requires multiple subsamples to be mixed thoroughly. Variability of nutrients is typically high due to the lack of uniformity of previous years’ fertilizer applications. Surface soil is generally more variable, so more cores should be collected. In general, a composite sample of 20–30 individual borings should be taken to represent a 20-acre area. Take 15–20 cores for 10-acre fields.

Time of Sample Collection

While MSU Extension offers three crop years of recommendations for each sample submitted, it is recommended to collect samples every year in fields that grow multiple crops. This allows you to monitor trends and more accurately manage the fertility program. For less intensive cropping sequences, sampling every 2–3 years is adequate. In either case, for better consistency, fields should be sampled during the same month each time because some nutrient, pH, and lime requirements vary seasonally due to climatic conditions, crop growth, and other factors.

Sample Handling and Record Keeping

Place soil samples into clean plastic buckets and mix well. If testing micronutrients, do not use galvanized metal buckets because samples could be contaminated, particularly with zinc. Break up cores and make sure the mixture is well homogenized before taking a composite subsample for laboratory analysis. Most soil testing laboratories provide small, moisture-resistant containers that hold about a pint of soil. The laboratories prefer these for operational ease, but if none are available, samples may be submitted in plastic bags. Give each sample box or bag a unique name and use the same name on the submission form.

Maintain records of field maps and names; sampling points; timing, cropping, and fertilization history; and other management activities. This information and the soil test reports will allow you to monitor changes in the fertility status of fields and field areas over time.

Soil Test Recommendations

Three growing seasons of phosphorus and potassium recommendations will be generated for the client for each sample submitted to the MSU Extension Soil Testing Laboratory. These recommendations are based on calibration and correlation research by the Mississippi Agricultural and Forestry Experiment Station. The correlations for phosphorus and potassium are summarized in Tables 2 and 3.

The numeric values for each sample are in pounds per acre for phosphorus or potassium; index terms of very low, low, medium, high, or very high are assigned based on the pounds per acre. Potassium values are also evaluated by the soil cation exchange capacity (CEC) as determined from the sample. CEC is a measure of the soil’s ability to store positively charged nutrients such as potassium, calcium, magnesium, and sodium. MSU Extension uses CEC to determine soil texture; sandier textures typically have lower CEC. This adjustment is based on research with many Mississippi soils that found that crop response to potash additions differed based on soil CEC.

Note that the units used for the indices are pounds per acre of phosphorus and potassium, but the fertilizer recommendations are given in pounds of phosphate or potash fertilizer per acre. This is because fertilizers are usually marketed using these conventions. Introduction to Inorganic Fertilizers on page 17 has information on converting between the two systems.

More Information

More information on soil sampling is available in the following publications online or from your local MSU Extension office:

IS346 Soil Testing for the Farmer

IS1294 Soil Testing for the Homeowner

P3749 Soil and Broiler Litter Testing Basics

Table 2. Soil testing indices for phosphorus (phosphate equivalent) used by the MSU Extension Soil Testing Laboratory for all crops.

Phosphorus soil test level (pounds per acre)	Index
0–18	very low
19–36	low
37–72	medium
73–144	high
> 144	very high

Table 3a. Soil test potassium levels (pounds K per acre) and indices using the Mississippi soil test extractant for perennial winter grass pasture (fescue or orchard grass); small grains for pasture; peanuts; perennial summer grass pasture (bahia, dallis, or bermudagrass); rice; or annual legumes with ryegrass.

Index	CEC < 7	CEC 7–14	CEC 14–25	CEC > 25
very low	0–40	0–50	0–60	0–70
low	41–80	51–110	61–130	71–150
medium	81–120	111–160	131–180	151–200
high	121–210	161–280	181–315	201–350
very high	> 210	> 280	> 315	> 350

Table 3b. Soil test potassium levels (pounds K per acre) and indices using the Mississippi soil test extractant for dryland corn for grain, soybeans, oats, wheat, barley, summer pastures (bahia, dallis, or bermudagrass) with annual legumes (white clover, red clover, lespedeza, arrowleaf clover, ball clover, or subterranean clover); temporary summer grass pastures (millet, sorghum, sudangrass, sorghum-sudangrass hybrids, or Johnsongrass); forage legumes; perennial winter grass pasture with clover (white clover, red clover, subterranean clover with fescue or orchardgrass); pasture grass with annual legumes (crimson clover, annual lespedeza, arrowleaf clover, ball clover, or subterranean clover with bermuda, dallis, or bahiagrass); johnsongrass hay; mixed grass hay; annual or sericea lespedeza hay; or sunflowers.

Index	CEC < 7	CEC 7–14	CEC 14–25	CEC > 25
very low	0–50	0–60	0–70	0–80
low	51–110	61–140	71–160	81–180
medium	111–160	141–190	161–210	181–240
high	161–280	191–335	211–370	241–420
very high	> 280	> 335	> 370	> 420

Table 3c. Soil test potassium levels (pounds K per acre) and indices using the Mississippi soil test extractant for alfalfa, cotton, corn or sorghum for silage, sweet potatoes, irrigated corn, or hybrid bermudagrass hay.

Index	CEC < 7	CEC 7–14	CEC 14–25	CEC > 25
very low	0–70	0–90	0–120	0–150
low	7–150	91–190	121–240	151–260
medium	151- 200	191–240	241–290	261–320
high	201–350	241–420	291–510	321–560
very high	> 350	> 420	> 510	> 560

Table 4a. Magnesium calibrations for the Mississippi soil test (CEC less than 5).

Index	Magnesium (pounds per acre)
very low	0–12
low	12.1–24.0
medium	24.1–48.0
high	48.1–96.0
very high	> 96.1

Table 4b. Magnesium calibrations for the Mississippi soil test (CEC greater than 5).

Index	Percent magnesium saturation
very low	< 0.85
low	0.86–1.75
medium	1.76–3.30
high	3.31–6.60
very high	> 6.61

Table 5a. Zinc fertilizer recommendations for corn and rice.

Soil test rating	Soil test K rating	Zinc (pounds per acre)
very high	very high	0
high	high	0
medium	medium	1–2
low	low	2–3
very low	very low	3–4

Table 5b. Zinc fertilizer recommendations for pecans.

Soil test rating	Soil test K rating	Zinc (pounds per acre)
very high	very high	0
high	high	0
medium	medium	10–20
low	low	20–30
very low	very low	30–40

Introduction to Inorganic Fertilizers

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A shortage of any soil-provided nutrient limits plant growth. Nutrients are recycled from plants to soil to meet plant needs under natural conditions. Agricultural crops use more nutrients that may be removed in harvested crops than natural vegetation.

Supplemental nutrients ensure optimal crop growth and profitability. The added nutrients may be fertilizers, animal manures, green manures, and legumes. Annual and perennial crops respond to phosphorus (P) and/or potassium (K) fertilization in many situations. Responses to other nutrients have occurred in limited crops and locations.

Supplemental nitrogen is usually needed for almost all non-leguminous agronomic crops grown in the state. However, plant nutrition is just one of many factors that limit potential crop yields. Others include soil physical problems, low or excessive rainfall, other climate issues, poor stands, inappropriate variety selection, weeds, insects and diseases, and crop genetic potential.

The macronutrients, also known as the primary nutrients, are listed on fertilizer labels as elemental nitrogen (N) and the oxide equivalents of phosphate (P2O5) and potash (K2O) in the order N-P2O5-K2O. This is the fertilizer grade or analysis. Numbers in a fertilizer grade such as 10-20-10 indicate that the fertilizer by weight is 10 percent N, 20 percent P2O5, and 10 percent K2O. The relationships to convert between the oxide and elemental forms are:

Phosphorus: P × 2.29 = P2O5 and P2O5 × 0.44 = P

Potassium: K × 1.21 = K2O and K2O × 0.83 = K

Results from the MSU Extension Soil Testing Laboratory are given in pounds of P or K per acre; however, the fertilizer recommendations from MSU Extension are in pounds of phosphate or potash per acre (see Introduction to Soil Testing on page 12).

Fertilizer Formulations

Various commercial fertilizers are available as solids, liquids, or gases. Each physical form has its own advantages and limitations, which must be considered when selecting the best material for the job.

Granulated fertilizers are solid, homogenous mixtures of fertilizer generally produced from materials such as anhydrous ammonia, phosphoric acid, and potassium chloride. Granulated materials are N-P or N-P-K grades of fertilizer.

The chief advantage of granulated materials is uniform distribution of nutrients. Each fertilizer particle contains all nutrients in the grade. For example, each particle of a 10-20-10 granulated fertilizer theoretically contains 10 percent nitrogen, 20 percent phosphate, and 10 percent potash. Nutrients do not segregate while handling or spreading (ensuring uniform nutrient application), and there is little tendency to cake or dust.

Blended fertilizers are mixtures of materials such as urea and potassium chloride. Granulated compound fertilizer materials can be blended or combined with other mixtures. Be aware that nutrients in a blended mixture may segregate into their individual particles. This directly impacts application rates when spread over a field.

Properly made blends are generally equal in effectiveness to other compound fertilizers and have the advantage of allowing a wide range of fertilizer grades that can match a fertilizer exactly to a soil test recommendation. Blends are often used as starter fertilizers. Urea and diammonium phosphate are not appropriate for use in starter fertilizers because both materials produce free ammonia. This can hinder seed germination and seedling growth when placed near seeds.

Fluid fertilizers are used widely in Mississippi. Fluids can be either straight materials, such as nitrogen solutions (e.g., 32-0-0), or compound fertilizers of various grades. Fluid fertilizers are categorized into two groups: clear solutions and suspensions.

In clear solutions, the nutrients are completely dissolved in water and are easy to handle. In addition, phosphorus in these materials is highly water soluble. Only relatively low analyses are possible, especially when the material contains potassium, and the cost per unit of nutrients is generally higher. When equal amounts of plant food are compared, clear solutions are equal in agronomic effectiveness to other types of fertilizers.

In suspension fertilizers, solubility of the components has been exceeded and clay added to keep the very fine, undissolved fertilizer particles from settling out. They can be handled like fluids and can be formulated at much higher analyses—even as high as dry materials. Suspensions require constant agitation, even in storage, and suspension fertilizer cannot be used as a carrier for certain chemicals. The agronomic effectiveness of suspensions is equal to other types of fertilizer materials when equal amounts of plant food are compared.

Anhydrous ammonia is a gaseous fertilizer that requires special handling and use considerations. It is stored as a compressed liquid that expands during application to a gas that must be injected into the soil to prevent loss to the atmosphere. Special handling methods and safety precautions are required because the material can cause serious chemical burns and asphyxiation.

Fertilizer Properties

Solubility indicates how readily nutrients are dissolved in the soil water and taken up by plants. Solubility usually is not a major consideration, but some materials (e.g., raw rock phosphate) are very insoluble in water.

Particle size of a fertilizer material is important for agronomic and handling reasons. In agronomic applications, particle size is most important for the sparingly soluble materials such as rock phosphate. These materials must be very finely ground to ensure sufficient solubility. For most soluble fertilizers, particle size is not critical for agronomic purposes but is very important in determining ease of handling the materials.

Inorganic Fertilizer Calculations

Proper application rates are critical to optimal economic and environmental management of inorganic fertilizers.

Fertilizer grade or analysis is always referred to on a weight percent basis, not on a volume (i.e., gallon) basis. Therefore, you must know the weight per gallon of the material to determine the actual plant nutrient content. Most fluids weigh between 10 and 12 pounds per gallon.

Example: 10-34-0 weighs 11.4 pounds per gallon, so 1 gallon contains:

11.4 × .10 = 1.14 pounds nitrogen per gallon
11.4 × .34 = 3.88 pounds phosphate per gallon

About 9 gallons of this fluid is equal to 100 pounds of fertilizer. To compare fluid fertilizer prices per ton, divide the weight per gallon into 2,000 for the number of gallons per ton.

For the above example: 2,000 ÷ 11.4 = 175 gallons in each ton. This calculation can be used to compare a liquid priced in dollars per gallon with a solid priced in dollars per ton.

As above, fertilizer labels identify the percent by weight of N, P2O5, and K2O in the material. To determine how much N, P, or K is in a particular fertilizer:

70 pounds of a 7-14-7 fertilizer has 4.9 pounds of N (70 × .07), 9.8 pounds of P2O5 (70 × 0.14), and 4.9 pounds of K2O (70 × 0.07).

40 pounds of a 0-14-28 fertilizer has no N, 5.6 pounds of P2O5 (40 × 0.14), and 11.2 pounds of K2O (40 × 0.28).

To calculate how much fertilizer to apply to provide a specific amount of nutrient to a given area:

Amount of fertilizer = amount of nutrient needed ÷ percent nutrient in the fertilizer

Example 1: How much 34-0-0 is needed to supply 50 pounds of N?

147 pounds (50 ÷ 0.34) of 34-0-0 supplies 50 pounds of N

Example 2: If 20-10-15 was used to apply 45 pounds of N, how much P2O5 and K2O also would be applied?

It requires 225 pounds (45 ÷ 0.20) of 20-10-15 to apply 45 pounds of N. Therefore, this 225-pound application of 20-10-15 also supplies 22.5 pounds of P2O5 (225 × 0.10) and 33.75 pounds of K2O (225 × 0.15).

Calculations for liquid fertilizers are similar

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Nutrient Management Guidelines for Agronomic Crops Grown in Mississippi

Introduction to Nutrient Management