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Soil: TESTING & MANAGEMENT

The Soil Tests and The Nutrients Involved

Brian Jervis, Tulsa County Horticulturist

The Soil Test

Soil tests will help you develop and maintain a more productive soil by providing information about the fertility status of your soil. Information from a soil test will help you select the proper liming and fertilization program to obtain optimal growth of lawn, garden and ornamental plants. BUT, a soil test will not be a magical cure-all. Think of it as a blood test. When you are sick and go to the doctor, the doctor does not always draw blood right away. He or she will try to rule out any obvious possibilities that your illness can be. We need to look at plants the same way. Most soils are fairly stable and will hold their nutrients with some exceptions. This article will explain how to take a soil sample correctly and also will give you examples of some of the nutrient deficiencies of plants. Again you need to assess your problem and overall goals and then make a decision on weather or not to take a soil sample.

A Representative Sample

One of the most important steps in soil testing is collecting the sample. Soil test results can be no better than the sample submitted to the laboratory for analysis. A soil sample weighing about one pound is used to represent thousands of pounds of soil in the landscape or garden. Therefore, it is extremely important that soil samples be properly and carefully taken.

The area included in the sample should have been uniformly fertilized and limed in the past. But DO NOT take a sample if the area has been fertilized or limed in the past two months. This is very important in getting a representative sample, even a small piece of fertilizer will throw off your sample and you will be wasting your money. When collecting the sample, avoid small areas where the soil conditions are obviously different from those in the rest of the area -- for example, wet spots, areas where wood piles have been burned, old building sites, fence rows, fertilizer bands, eroded areas and areas immediately adjacent to roads. If a field contains more than one soil type, collect separate samples from each soil area. Sample problem areas within a field separately. Again your soil sample weighing about one pound is used to represent thousands of pounds of soil in the landscape or garden. For the sample to be representative of the area tested, follow these steps for sampling:

  • Remove any surface litter such as turf thatch or mulch.
  • For lawns, sample to a depth of 4 inches. For gardens, ornamentals and fruit trees sample to a depth of 6 inches.
  • Use a trowel or sampling tube to collect soil samples.
  • Place the cores in a clean plastic bucket or other non-metal container and thoroughly mix the soil. Note: Do not use a galvanized bucket for collecting samples, especially if the soil will be analyzed for zinc or other micronutrients. Ensure that buckets and sampling tools are clean and free of fertilizer and limestone residues. Even a small amount of fertilizer transferred from the sampling tools to the soil can seriously contaminate the sample and produce misleading results.
  • Bring the soil sample to the the Tulsa County Extension Office at 4116 E. 15th Street at gate 6 of the Fair Grounds for processing.

Turn-around time varies with the season but is usually 14 days or less. The standard test includes soil reaction pH, Nitrogen (N), Phosphorus (P), Potassium (K). The secondary and micronutrients can also be tested if specified. (See Nutrients Below)

Cost

The cost of your soil sample will vary depending on which type you choose. The Routine Sample is adequate for most home lawn and/or garden and the cost is $10. This test will include pH, Nitrate-Nitrogen, available P, and available K. For a complete list of prices go to the Soil, Water & Forage Analytical Laboratory

When and How Often Should Soils Be Tested?

Soil can be tested any time during the year; however, be sure to sample well in advance of planting or spring green-up. This is particularly important in areas where lime is likely to be needed. Lime reacts fairly slowly and should be mixed with the soil several weeks before planting. Generally, fall is the most desirable time to sample soil, because landscapes and gardens are usually dry and easily accessible. Soil should be dry enough to till when sampling. If wet samples are collected, they should be air dried before being placed in the soil sample bag. Once medium or high fertility levels are established, lawn and ornamental areas only need to be sampled every two to three years. Vegetable gardens should be sampled every one to two years.

The Nutrients

Plants require a total of 16 elements in differing quantities to grow and mature properly. Three of the 16, carbon, hydrogen and oxygen, make up about 95% of the plant and are supplied by air and water. The remaining 13 must be supplied to the plant by the soil solution. These nutrients are grouped according to plant usage -- primary, secondary, and micronutrients. The primary group, which is the group that is used most by the plant has three nutrients, Nitrogen (N), Phosphorous (P), and Potassium (K). All commercial fertilizer is sold must have the exact amount of N, P, & K listedon the bag. When there is a nutrient deficiency present it is more-than-likely going to be one of these three. The secondary group includes Calcium (Ca), Magnesium (Mg), and Sulfur (S). The third grouping is the micronutrients or trace elements, those used in minute amounts. This group includes Zinc (Zn), Iron (Fe), Manganese (Mn), Copper (Cu), Boron (B), Chlorine (Cl), and Molybdenum (Mo). Click on the fertilizer bag to the left to see the guaranteed analysis for that bag of fertilizer.

The pH Factor

Soil reaction (pH) is a measure of the acidity/alkalinity of the soil. On the pH scale 7.0 is neutral, less than 7.0 is acidic and above 7.0 is alkaline. The major influence of pH is on nutrient availability. Most plant nutrients, particularly phosphorous, are most available in mildly acid soils, i.e. those with a pH between 6.0 and 7.0. Agricultural soils tend to become more acidic overtime. Excess acidity is corrected through application of lime to the soil. Click here to see the pH Primer"

Deficiency Symptoms

  • Nitrogen
    Overall decreased vigor and light green foliage color. Symptoms begin in older foliage. All converts to nitrate which can leach from the soil. Ammonium held to soil. Plants use mostly nitrate, some ammonium.
    Nitrogen Deficiency
    Phosphorous Deficiency

  • Phosphorus
    Leaves dark green to blue-green with petioles and lower leaf surfaces becoming reddish-purple, eventually yellow. Easily tied up by soil and made unavailable to plants. Availability reduced at high or low pH and soil temperature below 50? F. Form for plant use varies with soil pH.
    Potassium Deficiency
  • Potassium
    Marginal yellowing of older leaves, then scorching and leaf drop. Mimics drought stress.
  • Calcium
    Death of shoot tips, yellowing necrosis of younger leaves, leaf distortion, not common. Major component of the cell wall. Deficiency related to blossom end rot of tomato and others. Can be deficient in acid soils, but easily corrected by liming.
  • Magnesium
    Chlorosis and necrosis in interveinal areas. This element is a component of chlorophyll. Deficiency may show on sandy acid soils. Corrected by adding dolomite limestone.
  • Sulfur
    Light-green or yellowing starting with the newer leaves progressing downward. Sulfur is often a contaminant in fertilizer.
  • Boron
    Leaves appear scorched and rosetted, stem die back often occurs.
  • Chlorine
    Required by the plant but seldom deficient, symptoms, which include marginal and tip scorch, occur in toxic levels.
  • Copper
    Death of young shoots and leaf scorching.
    Iron Deficiency
  • Iron
    Deficiencies are apparent when pH is high in the soils. Interveinal chlorosis while the veins stay green.
  • Manganese
    Interveinal chlorosis of young leaves beginning at the leaf margins and progressing toward the midrib.
  • Molybdenum
    Seldom deficient, marginal then interveinal chlorosis in middle and older leaves that may be cupped shape. Essential for N fixation by legumes.
    Zinc Deficiency
  • Zinc
    Leaves are yellow to bronze, narrow and rosetted due to short internodes. Terminal growth areas affected first. Deficiency can be caused by excess phosphorous.