Importance of Soil Testing -
Why and How
Terrence S. Marshall, County Agent, 1890
East Baton Rouge Parish Southern University Cooperative Extension Program
Why Soil Test?
- Determine best rates and types of fertilizer to use
- Determine whether lime is needed and how much to use
- Determine causes of poor growth
- Takes guesswork out of fertilization - can save money
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What do the numbers on a bag of fertilizer
mean?
1st Number - % of nitrogen in bag
2nd Number - % of phosphorus in bag
3rd Number - % of potassium in bag
What do nutrients do?
- Nitrogen is responsible for rapid growth and green
color of plants. Nitrogen deficiency is usually expressed as a yellow plant.
- Phosphorus is responsible for root growth and fruit
set. Phosphorus deficiency is often expressed as a purple or reddish plant.
- Potassium is responsible for disease resistance and
water movement in plants. Potassium deficiency usually shows as dead areas on the edges of
the leaves.
Micronutrients
- Micronutrients are boron, zinc, copper, iron, chlorine, molybdenum,
and manganese.
- Usually, micronutrient deficiencies are found in high pH soils.
- Micronutrieints are usually not a problem in well fertilized crops in
the South.
What is pH?
- pH is how much acid or alkalinity a soil contains.
- Alkaline soils are sometimes called "sweet". Acid soils are
sometimes called "sour." Most soils in the South are acidic.
- pH scale ranges from 1.0 - 14.0
- pH of 7.0 is neutral (nether acid nor alkaline).
- Most plants prefer a pH of 6.0 - 7.0
Effect of pH on Availability of Plant
Nutrients
How to Correct Acid Soils
- Acid soils require lime. Most crops need lime when the pH drops below
5.5.
- Never apply lime without a soil test.
- Lime is best applied in the fall so it can fully react with the soil
before spring. However, lime will still do some good if applied in the spring.
- If soil test magnesium levels are low, use dolomite lime. Otherwise,
any type of agricultural lime is OK.
Proper Soil Sampling
- It is important to take a good sample for the lab to analyze.
- Divide large fields into blocks no larger than 10 acres across each.
Sample soils that are obviously different separately
- Sample problem areas separately.
- From each field, collect soil from at least 10 places to make 1
sample. Mix thoroughly. Sample 6 inches deep.
Mailing address for author: Cooperative Extension Program, Southern University, 805
St. Louis Street, Baton Rouge, LA 70802-6457
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