Soil samples should be tested in a certified soil lab (e.g. LSU AgCenter Soil Testing and Plant Analysis Lab, Baton Rouge) that uses appropriate soil extraction methods for the state. Soil-test-based fertilizer recommendations in Louisiana are based on Mehlich-3 soil exaction method. Soil samples should be analyzed in the same lab each year to create a historic record.
Soil test results should be interpreted based on the critical soil test
nutrient concentration. The critical concentration is defined as the
soil test nutrient concentration below which crop response to added
fertilizer is expected and above which is unlikely. Critical nutrient
concentration varies with crops, soil types, and states. Therefore, soil
test results should be interpreted with crop, soil type, and state
specific critical nutrient concentrations that are derived from
correlation and calibration research. Usually, soil scientists from
every land-grant university develop their own critical soil test
nutrient concentrations for each crop of that state. So, it would be
better to analyze soil sample in the state soil testing lab.
Fertilizer recommendation should be based on critical soil test
nutrient concentration and fertilization philosophies. There are three
main fertilization philosophies: sufficiency, buildup and maintenance,
and cation saturation ratio. In the sufficiency approach, used by most
land-grant universities including LSU AgCenter, fertilization is only
recommended if the soil test nutrient level is at or below the critical
level and the fertilizer rate is determined based on expected crop yield
increase. This approach is called “fertilize the crop”. In the buildup
and maintenance approach, also known as “fertilize the soil”,
fertilization is almost always recommended unless the soil test level is
very high. The buildup part of this approach is used for soils with
nutrient concentration below the critical level and the fertilizer rate
is determined based on sufficiency rate plus some extra rate to raise
the soil test nutrient concentration above the critical level. The
maintenance part is used for soils with nutrient concentration above the
critical level and the fertilizer rate is determined based on the
expected nutrient removal rate by the crop to maintain soil test
nutrient concentration at the same level. The cation saturation ratio is
not very accurate and economic in recommending fertilizer. In this
approach, fertilizer is recommended based on the cation ratio mainly
calcium (Ca), magnesium (Mg), and potassium (K) on the cation exchange
site. The most used ratio is 65% Ca, 10% Mg, 5% K, and 20% others.
Care should be taken using buildup and maintenance philosophy for K fertilization in coarse-textured soils with low cation exchange capacity (CEC <10) such as loamy sand to silt loam soils. Potassium is highly prone to leach down to the soil profile with excessive rainfall in low CEC soils. So, building up soil test K level in coarse-textured low CEC soils may not be feasible and economic. Please visit LSU AgCenter website for detailed soil-test-based fertilizer recommendations for each crop.
The soil-test-based fertilizer recommendations mainly include phosphorus
(P) and K which can be applied in fall especially for fine-textured
soils with high CEC (>20). For coarse-textured low CEC soils, it is
better to apply all fertilizers in spring or at planting. There is a
misconception about spring application of P (TSP) and K (Potash) that
both fertilizers require long time to dissolve and become available for
plant uptake. Many studies showed that spring application of both
fertilizers is better than fall application in increasing crop yield
especially for soils that are highly prone to nutrient loss via
leaching, runoff, and erosion.
Since both P and K are highly immobile in soils, both fertilizers should
be placed near the root zone (on the top of the bed for furrow
irrigation system) and incorporated with shallow tillage for tilth
fields. Banding of P fertilizer is very effective for both acidic (pH
<5.5) and alkaline (pH >7.5) soils since P availability is greatly
affected by soil pH.
Soil pH is the most important soil quality component that greatly influences soil nutrient availability. Most nutrients are highly available at the soil pH of 6.5. Therefore, soil pH needs to be adjusted to the target pH either by applying lime for low pH (<6.0) soils or by elemental sulfur for high pH (>7.5) soils. Increasing soil pH by liming is a more common practice than decreasing soil pH by elemental sulfur.
The rate of lime depends on the initial and target soil pH and the buffering capacity of the soil (buffer pH, ability of a soil to resist the change of pH). If the soil buffering capacity and the difference between initial and target soil pH are low, lime rate would be low. However, for soils with high buffering capacity (low buffer pH), lime rate would be high even for a small change of soil pH. Clay soils have higher buffering capacity and require greater amount of lime for each unit increase of soil pH than silt loam soils. Note that LSU AgCenter Soil Testing and Plant Analysis Lab does not run buffer pH but indicate the unit change of soil pH with the addition of maximum 3 tons of lime and let the farmers decide how much they would like to spend, assuming higher than 3 tons lime may be too expensive.
The target soil pH should be determined based on the crop to be grown. For example, soybean is more sensitive to low soil pH than corn and cotton. The target soil pH should be set at 6.3 for soybean and 6.0 for corn and cotton. Lime is required if the target soil pH is 0.2 unit more than actual soil pH.
Lime takes at least 6-9 months, depending on liming materials, to react
with the soils and raise soil pH. Therefore, lime should be applied
uniformly and incorporated by tillage in the fall.
The quality of liming materials is very important to raise soil pH. There are two qualities of liming materials: purity and particle size. The purity of a liming material is determined in relation to pure calcium carbonate (CaCO3), calcitic limestone, which is rated as 100% (molecular weight of pure calcium carbonate is 100) and this rating is called calcium carbonate equivalent (CCE). The rate of lime recommended by soil testing labs is based on pure calcitic limestone with 100% CCE. So, the actual lime application rate should be adjusted based on the CCE of the liming materials. For example, if the CCE of the liming material is 80% and the recommendation is 2-ton lime per acre, 2.5-ton lime (2-ton/0.80) per acre should be applied.
Particle size is the fineness factor of liming material and is expressed as the percentage of liming material passes through various sized screens. The higher the percentage of liming material passes through the larger size screen (i.e. smaller hole), the greater the fineness factor would be. Finer particles are more efficient in neutralizing soil acidity (increasing soil pH) by reacting quickly with soils due to greater surface area or soil contact. However, the liming materials should have a good distribution of particle sizes with both smaller and larger particles so that smaller particles can raise the soil pH quickly and larger particles can have a long-term control in neutralizing soil acidity. According to current Louisiana recommendations for ground lime, 90% of liming materials should pass through a 10-mesh sieve, 50% should pass through a 60-mesh sieve, and 20% should pass through a 100-mesh sieve.
Both purity (CCE) and particle size (fineness factor) of the liming material are expressed together as effective CCE (ECCE) or effective neutralizing value (ENV). The higher the ECCE or EVN of the liming material the more efficient it is in increasing soil pH. Like CCE, the actual lime rate also needs to be adjusted with the ENV of the liming material if the recommendations are based on ENV. For example, if the ENV of the liming material is 60%, but the recommended lime rate is based on standard calcium carbonate with 90% ENV, 1.5-ton (0.9/0.6) lime should be applied for every 1-ton of lime recommended. Note that the lime recommendations from LSU AgCenter Soil Testing and Plant Analysis Lab is based on 50% ECCE or ENV.
Cover crop is the most effective and efficient tool in improving soil fertility. Cover crop adds organic materials to the soils that improve soil physical, chemical, and biological properties such as increase soil organic matter content, CEC, water and nutrient holding capacities, water infiltration, and water use efficiency, improve soil structure, bulk density, pH, electrical conductivity, microbial biomass, and nutrient cycling, and reduce soil compaction. Cover crop ensures year-round ground cover that reduces soil runoff and erosion potentials, decreases weed pressure and herbicide needs, scavenges nutrients left over from the previous crop, and prevents leaching loss of nutrients into ground water.
Cover crop species should be selected based on soil’s need. For example, cover crop that produces deep tap root (such as tillage radish) should be selected for compacted or no-till soils. Cereal rye can be a good choice for soils that have low organic matter content or tremendous weed pressure. Legume cover crops (such as crimson clover, hairy vetch, etc.) can be a good source of nitrogen fertilizer for the subsequent corn or cotton crop, which would reduce fertilizer need and cost.
Cover crop should be planted immediately after summer crop harvested and terminated at least 15 days prior to planting the next summer crop. Find the cover crop decision tool on the website.