Chemical analysis of soils, or soil testing, is a means to determine the nutrient supplying power of the soil .
The sample should be a true representation of the area sampled, as the laboratory results will reflect only the nutrient status of the sample which is received .
To obtain such a sample, the following items should be taken into consideration .
Several different tools, such as an auger, soil sampling tube, or spade may be used . Sample tubes or augers should be composed of either stainless steel or be chrome plated . If using a pail to collect the soil, it should be plastic to avoid contamination from trace elements ( i .e ., zinc ) .
Mix the various cores or slices together in a clean plastic container and take sub samples to be put into the sample bag . A sub sample should be 1 to 1 ½ cups of soil, which is taken from a well-mixed composite from 10 to 20 random locations in the field . It is advisable to air dry extremely wet samples before they are bagged . Identify the sample bags with name, sample number, and field number which correspond with identification on the appropriate sample information sheet .
Area to be sampled generally should not be more than 40 acres . Smaller acreage may be sampled when the soil is not uniform throughout the field . Soils that differ in soil type, appearance, crop growth or past treatment should be sampled separately provided the area can be treated in that manner . Avoid small areas that are dead furrows, end rows, and which are poorly drained. Stay away from barns, roads, lanes and fence rows.
The required depth of sampling is influenced by many factors which are discussed in this section.
1. Tillage Method
- Conventional………………………………………………………..plow depth
- Reduced Tillage……………………………………3/4 of tillage depth if nutritional problems 0-4” and 4-8”
- Continuous Ridging………………………………………….. 0-6” in ridge 0-4” in valley
- No Till…………………………………………………..0-8”, to check pH, 0- 2”
- Deep Placement……………………………… plow depth and below
- Band Placement……………………………………………………plow depth
In general, samples are taken at depth where the main root system exists.
- Established Lawns and Turf Sample; depth of 3 to 4 inches, which is the actual rooting depth. The sample should not include roots and accumulated organic material from the surface.
- Orchards ;The greatest root activity occurs at a depth of 8 to 12 inches. The sampling depth in orchard soils, therefore, should be up to 12 to 14 inches, taken at the edge of the drip line. Take one core sample from each 15 to 16 trees selected at random in the orchard. Mix the cores to obtain a composite sample which should be from an area no larger than 20 acres.
- Flower Beds; One sample per 100 sq. ft. consisting of a composite of three cores taken up to 6 inches depth.
- Vegetable Garden; Sample up to 6-inch depth at various locations and prepare a composite sample.
- Shrubs and Small Trees; Take samples at the edge of the limb spread to a depth of 8 to 10 inches.
3. Sampling for Nematodes
The best time to sample for most nematodes is during the summer months as crop growth can indicate the presence of nematodes by having a stunted appearance. Take the samples, one per every 5 acres, to a depth of 8 inches in the row from 20 to 25 locations. Mix the samples as soon as possible and put a composite sample of 1 to 2 pints into a soil bag. Do not let the soil dry out or get hot. The best method for nematode identification sampling is to collect root tips and feeder root samples. Remember that nematodes can be present in large numbers without any visual symptoms showing on the plant roots.
4. Sampling for Nitrate, Ammonia Nitrogen, and Soluble Salts
Rapid changes in nitrate and ammonia levels can occur after taking a soil sample, if the sample is stored under moist, warm conditions. It is advisable to dry the sample at 40° to 50° C (100° to 110° F) prior to shipping, unless the sample is refrigerated.
Because nitrate nitrogen leaches easily, deeper sampling is required to effectively determine the total available nitrogen in the soil. Sample to a 2-3 foot depth with samples taken at 7-inch to 1-foot increments to form possible composite samples. Sampling for soluble salts should be in accordance with instructions for nitrate sampling. Soil should be air dried before shipping or storage for any length of time.
5. Subsoil Sampling
Subsurface or subsoil sampling is frequently of value, and samples can be collected to explain unexpected crop growth patterns resulting from either chemical or physical characteristics of subsoil layers.
Such sampling is also of importance in areas where deep-rooted crops are grown, which obtain the majority of their nutrient requirements at such depths.
To estimate the available soil nitrogen for crop use, the determination of nitrate nitrogen levels in the soil profile is made.
Separate samples from plow depth and subsurface can be taken if sodium or salinity problems are anticipated.
SOIL SAMPLING TIMING
Generally, soil tests should be taken on all fields at least once every 2 to 4 years, but soils on which vegetables or other high cash crops are grown may need to be tested annually.
It really does not make much difference whether one is sampling cotton, corn, wheat, or soybean fields, the ideal time to sample is right after harvest. At that time of year, fields are generally very accessible and good representative soil samples are easy to obtain. More time is also available for the evaluation of the soil test data and setting up a good soil fertilization program.
Due to the variation in nutrient availability that may be associated with time of sampling, it is suggested that any given area be sampled about the same time each year.
However, samples taken for diagnostic purposes (fertilization response, poor crop growth, evaluation of soil conditions) are best obtained while the problem areas are delineated by crop or other visual differences.
SOIL REACTION (pH)
Soil reaction is important as it affects nutrient availability, solubility of toxic substances like aluminum, the rates of microbial activities and reactions, soil structure and tilth, and pesticide performances.
Soil pH is expressed as a numerical figure and can range from 0 to 14. A value of 7.0 is neutral, a value below 7.0 is acid, and above 7.0 is alkaline.
The pH value reflects the relative number of hydrogen ions (H+) in the soil solution. The more hydrogen ions present, compared to the hydroxyl ions (OH-), the more acidic the solution will be and the lower the pH value. A decrease in hydrogen ions and increase in hydroxyl ions will result in more alkaline or basic conditions.
The ratio between hydrogen ions and hydroxyl ions changes tenfold for each unit change in pH. Therefore, a soil with a pH of 5.0 is ten times as acidic as a soil with a pH of 6.0.
Soils are becoming more acid as a result of the removal of the cations calcium, magnesium, potassium, and sodium through leaching or by growing crops. As the cations are removed from the soil particles, they are replaced with acid-forming hydrogen and aluminum. Most common nitrogen fertilizers also contribute to soil acidity, since their reactions increase the concentration of hydrogen ions in the soil solution.
Many agricultural soils are in the pH range 5.5 to 8.0. The growth of crops on these soils is influenced by the favorable effects of near-neutral reaction on nitrification, symbiotic nitrogen fixation and the availability of plant nutrients. The optimum pH range for most crops is 6.0 to 7.5 and for leguminous and other alkaline preferring crops 6.5 to 8.0. A desirable pH range for organic soils is 5.0 to 5.5.
Hydrogen ions in the soil solution are increased when the salts increase. This results in a more acid condition or lower pH. The salts may be a result of fertilizer residues, irrigation water, natural conditions, or microbial decomposition of organic matter.