This section describes the factors that are used to make cropnutrient recommendations from a soil analysis.

Introduction
Definitions of Soil-Test RatingsUsed for P and K
Descriptions of Soil Groupson Which P and K Soil-Test Ratings are Based
Extractable Nutrients inPounds Per Acre
Table VersusFormulas
LimeRecommendations
Lowering Soil pH
MagnesiumRecommendations
Soil Organic Matter
FertilizerRecommendation
Yield Potential (YieldGoal)
Interpreting
Report of Soil Tests (samplecopy)
Report on Soil Tests (sample copy)
Tables

Table 1. Crop Classes and Computer Code Numbers Used in Recommendations

Table 2. Soil-Test Ratings and Pounds Per Acre of Soil-Test P and K for All Soils and Crops

Table 3. Soil-Test Ratings and Pounds Per Acre of Soil-Test Mg and Ca for Soils and Crops.

Table 4. Lime Recommendation Codes.

Table 5. Acidifying Effect of Some Common Fertilizers and Soil Amendments

Table 6. Pounds of Elemental Sulfur Per 100 Sq. Feet Needed to Lower Soil pH of a Silt Loam Soil

Table 7. Magnesium Recommendation Codes.

Table 8. Most Frequently Recommended Fertilizer Grades and Ratios.

This publication presents the fertilizer recommendations made bythe Auburn University Soil Testing Laboratory. The information isorganized for the computer program which is used to makerecommendations on samples analyzed by this laboratory. Crops aredivided into more than 50 classes, about one-half of which are fieldand forage crops and one-half are horticultural or special crops. Thecrops are listed in Table 1 with a summary ofinformation used in classifying crops based on fertilityrequirements, and in making recommendations for each crop. Detailedinstructions and recommendations are presented in individualtables for each crop. These tables contain the followinginformation:

1. Crop code number and a list of the crops included in this code number.
2. K requirement level number. Crops are divided into three classes based on their K requirements. These classes are (1) peanuts, (2) soybeans and corn and other grasses; and (3) cotton, forage legumes, gardens, lawns, shrubs, and other special crops. They are presented in Table 2 along with the pounds per acre of soil test P and K, used to rate the different soil groups from Very Low to Extremely High.
3. N rate. Each crop is assigned a standard, annual N rate based upon research conducted throughout Alabama. However, comments given with each crop may modify this rate based upon potential yield, soil, time of application, cropping system, etc.
4. Lime recommendation code number. Crops vary in the amount of acidity they can tolerate and still make top yields. They are divided into five classes based on the pH ranges in which they produce best. These classes are presented in Table 4. and in the tables which provide fertilizer recommendations for each crop.
5. Mg recommendation code number. Crops are divided into three classes based on their Mg requirements. These Mg recommendations are from Table 7.

Specific comments that go with individual recommendations or cropsare listed by number with the fertilizer recommendations. Commentsused with each crop also are presented with fertilizerrecommendations for each crop.

Definitions of Soil-Test Ratings Used for P and K

Results of chemical tests are used to rate the fertility level ofsoils for each nutrient element tested. The ratings range from verylow to extremely high. They are influenced by the soil and thenutrient requirements of the crop to be grown. The ratings for P andK are based on the relative yield that may be expected without addingthe nutrient and when all other elements are in adequate supply.Fertilizer recommendations are based on these ratings.

Very Low (VL)

Soil will yield less than 50% of its potential. Large applications for soil building purposes are usually recommended. Some of the fertilizer should be placed in the drill for row crops.

Low (L)

Soil will yield 50 to 75% of its potential. Some fertilizer should be placed in the drill for row crops.

Medium (M)

Soil will yield 75 to 100% of its potential. Continued annual applications should be made in this range. On some sandy soils that will not retain much K, it may not be economical to attempt to build beyond this medium level.

High (H)

Supply of the nutrient is adequate for the crop, and none is recommended for field and forage crops. Where this recommendation is followed, it is suggested that the area be sampled each year.

Very High (VH)

Supply of the nutrient is more than double the amount considered adequate. Application of P or K to soils of this rating is wasteful.

Extremely High (EH)

Supply of teh nutrient is more than four times the amount considered adequate. The level is excessive and further additions may be detrimental to the crop and may contribute to pollution of ground and surface waters.

Pounds per acre of soil test P, K, Ca, and Mg on which soil-testratings are based for the different crops and soil groups arepresented in Tables 2 and 3.

Availability of nutrients in soils to plants is affected by theamount in the soil, cation exchange capacity (CEC) of the soil, andother factors. The amount in the soil is determined by the parentmaterial from which the soil is formed, the amount that has beenadded in fertilizers, liming materials or organic wastes, the amountof organic matter, and the amount that has been removed by cropping,erosion, and leaching. The CEC is a measure of the capacity of thesoil to retain nutrients against removal by crops, erosion, orleaching. It is affected by organic matter content and the amount andtype of clay in the soil. Therefore, knowing the CEC of individualsoil samples is important before making fertilizer recommendations.This is determined in the Soil Testing Laboratory and serves as thebasis for classifying soils into four groups.

Soil Group 1.

Sandy soils with cation exchange capacities less than 4.6 cmolc kg^-1 of soil. Examples of soils in this group are Dothan, Orangeburg, Alaga, Ruston, and Troup.

Soil Group 2.

Loamy and clayey soils with cation exchange capacities of 4.6 to 9.0 cmolc kg^-1 of soil. Examples of soils in this group are Madison, Lucedale, Allen, Hartsells, Cecil, Pacolet, and Savannah.

Soil Group 3.

Clayey soils, from areas other than the Black Belt, with cation exchange capacities of more than 9 cmolc kg^-1 of soil. Colbert, Decatur, Dewey, Talbott, Boswell, and Iredell are examples of soils from this group.

Soil Group 4.

Acid and calcareous clayey soils of the Black Belt with cation exchange capacities greater than 9.0 cmolc kg^-1 of soil. These soils require different laboratory procedures for measuring soil fertility. Examples of soils in this group are Sumter, Oktibbeha, Leeper, and Vaiden.

The group in which a soil is classified may affect the fertilityratings and therefore the P and K recommendations. Growers sometimesdo not understand why samples from individual fields change groupsbetween samplings. When a soil is near the borderline between groups,(e.g. 4.6 cmolc kg^-1) it may fall into one soil group thisyear and the other group the following year. Liming the soil may alsocause it to be shifted from Group 1 to Group 2 or from Group 2 toGroup 3. Although CEC is determined on all samples, it is notrecorded on soil test reports. Growers wishing more information aboutthese relationships on their samples should contact the Soil TestingLaboratory.

All results are expressed in pounds per acre of an extractablenutrient. This method makes the assumption that an acre-furrow-slice(one acre of soil to a depth of 8 inches) weighs 2 million pounds.Thus, pounds per acre is equal to pounds per 2 million pounds (pp2m)or mg per 2 kg.

Fertilizer recommendations are given in table format or byformulas. The formulas with each recommendation table may allow forsmaller incremental increases or decreases in fertilizer rates ascompared to those rates presented in the tables. For example, if yoursoil test report shows 50 lb/acre of potassium (K) and you wish touse the formula to calculate your K fertilizer needs; use thefollowing steps:

1. determine CEC range by looking at the soil group on your report;
2. look up the appropriate equation;
3. substitute the pounds per acre of K for X and then solve for Y.

Example calculation:

1. corn
2. 50 lb/acre soil test potassium
3. CEC 0-4.6
4. Equation Y=80-0.99X
1. Y=80-0.99(50)
   Y=80-49.5
   Y=30.5 lb K₂O per acre
5. round off to nearest 10 pounds

The formula will allow for lower fertilizer rates when soil testlevels are approaching a higher fertility rating.

Practically all Alabama soils, except for the calcareous soils ofthe Black Belt, are slightly to strongly acid and may need to belimed for most crops. This acid condition results from the low levelof lime in the original soils, the high rate of leaching fromexcessive rainfall, the use of acid forming fertilizers, cropremoval, and other factors. About one-half of the samples received inthe Soil Testing Laboratory need lime. This ratio has not changedmuch in recent years. Growers should use soil-test recommendations tomaintain soil pH between 5.8 and 6.5 for most crops. Soil testing isthe only practical basis for determining when and how much limeshould be applied. Soil pH is a critical factor in determiningresponse of crops to fertilizers and maintaining a favorable soilenvironment for profitable production. Soils should be sampled every2 or 3 years to ensure that production is not limited by soilacidity.

Lime recommendations are based on two separate tests made in theSoil Testing Laboratory on each sample. These are 1) thedetermination of pH, which indicates the active acidity of the soil,and 2) the lime requirement test, which determines the amount of limerequired to raise the pH to a desired range. The amount of limerequired varies among soils at the same pH because of differences inorganic matter content, and the kind and amount of clay in the soils.Soils that are high in organic matter and clay content require morelime to raise the pH to a specific range than do sandy soils that arelow in organic matter. For example, a sandy soil at pH 5.0 mayrequire only 1 ton of lime to raise the pH to 6.5, while a clay soilat the same pH may require 4 tons of lime. Crops also have differentrequirements for soil pH, and therefore lime recommendations varywith the crop to be grown. Lime is recommended to correct the pH ofthe top 8 inches of soil. Growers who plow deeper than 8 inchesshould increase the rates accordingly. Lime should be thoroughlymixed with the soil because the primary reason for applying lime inmost cases is to adjust the soil pH rather than to supply plantnutrients such as Ca and Mg. Lime should be applied and mixed withthe soil as soon after sampling as possible. It will begin to reactwith the soil immediately after application but the full effect maynot be evident for several months. Fineness and purity of lime areimportant in determining the rate of reaction. Lime recommendationsare based on the minimum quality ground agricultural limestone asdefined by the Alabama Department of Agriculture and Industries: 90%CaCO₃ equivalent:

• 90% passes a 10 mesh sieve
• 50% passes a 60 mesh sieve

The pH requirements on which lime recommendations for differentcrops are based are presented in Table4. 

Most plants grow best where the soil is slightly acid in the rangeof pH 6.0 to 7.0. However, a few plants such as azaleas, gardenias,and blueberries grow best at lower pH values. In rare cases, it maybe desirable to lower the pH by adding an acidifying agent. This canbe done successfully on soils that do not contain large amounts offree lime. Calcareous Black Belt soils cannot be practicallyacidified because much of the soil contains lime (CaCO₃).In other cases, the pH can be lowered simply by using fertilizerscontaining ammonium-N (Table 5). Ammoniumsulfate and sulfur coated urea are two of the best choices foracidifying soils. 

The most practical way to supply Mg is through the use ofdolomitic limestone. Where Mg is low and lime is recommended, acomment will be printed on the report stating that "both soil acidityand low Mg can be corrected by applying dolomitic lime." Some cropshave a higher requirement than others for Mg and it may be suppliedby other sources if lime is not recommended. For some crops, a lowrate of dolomitic lime can be recommended at higher pH values tosupply the Mg. For certain crops such as potatoes, blueberries, andpines a soil pH below 5.5 is desirable and if Mg is low, a fertilizercontaining Mg is recommended. The Mg recommendation codes are givenin Table 7. 

Soil organic matter affects the CEC, nitrogen supplying capacity,and other fertility factors. The organic matter content of mostAlabama soils is low (less than 3%) and does not vary widely amongsoils. Therefore, organic matter analysis is not needed in makingfertilizer and lime recommendations, and routine soil samples are notanalyzed for organic matter. Organic matter content and cationexchange capacity are considered in determining the rates of someherbicides required for satisfactory weed control. The CEC isindicated by the soil group in which samples are classified. Thisshould give some indication of the amount of herbicide that should beused. The Soil Testing Laboratory offers a service to determineorganic matter content of soil samples on special request. A specialcharge is made for this service. Those desiring organic matteranalysis should request it on the information sheets submitted withsoil samples. 

Fertilizers are recommended in pounds per acre of N,P₂O₅, and K₂O. Other recommendationsare printed as comments on the report. The recommendations are basedon regression equations for crops and soil groups. Recommendedfertilizer rates will be in 10 pound increments with a minimumrecommendation of 40 pounds per acre of any nutrient.

Specific fertilizer grades (e.g. 13-13-13, 8-24-24, 15-0-15, etc.)are not recommended because of the availability of so many gradesthat could be used. Most fertilizer dealers will custom blendspecific grades to meet the needs of the customer when ordering morethan 1 ton of fertilizer.

For home gardens and specialty crops, the choice of grades is morelimited. In these cases, specific grades are mentioned in comments.The use of a grade such as 13-13-13 as an example, does not indicatethat purchasers should insist on this specific grade, but that anyequivalent ratio may be substituted. If 10-10-10 is used instead of13-13-13, the amount used should be increased about 30 percent andthe result will be the same. If concentrated superphosphate, whichcontains 46 percent P₂O₅ is used instead ofsuperphosphate which contains 20 percent P₂O₅the amount used should be reduced by about 55 percent. The same istrue for nitrogen sources and other materials. 

Over fertilizing based upon arbitrary "yield goals" has been shownto result in economic losses to the grower and severe water qualityproblems in some part of the U.S. Auburn University's soil testcalibrations and recommendation are based on maximum economicalyields from actual experiments conducted on experiment stations andfarms throughout Alabama under both irrigated (where possible) andnon-irrigated conditions. Phosphorous and potassium rates are rarelyrelated to yield potential but may be adjusted based upon anticipatedcrop removal (e.g. forage crops). Nitrogen rates for grain crops(e.g. corn) and cotton may need to be adjusted up or down based uponyield potential. Conditions for adjustments are usually given incomments. However, sources of fertilizer and timing of applicationmay have as much impact on crop yields as the total annual rateused. 

* Group 4 soils are clayey soils from Black Belt counties and are extracted with the Mississippi extract. All others are extracted with Mehlich1.

** Level 3 = cotton, legumes, gardens, lawns, and shrubs; Level 2 = soybeans, corn, and other grasses; Level 1 = peanuts & pine trees.

* - Tomatoes, pimientos, fruits, and nuts. 

* - Code 1 crops are moved to Code 5 when grown on fine-texturedsoils of Soil Group 3 or 4. 

¹ - If aluminum sulfate is used, multiply by 6.