If the supply of all nutrients is adequate, the crop
will remove characteristic amounts of nutrients from the soil. The removal of
nutrients from soil cannot continue indefinitely without depleting the
productivity of the soil; nutrients must be replaced. If the supply of any
major plant nutrients is inadequate, growth and subsequent yields will be
reduced. With particularly severe nutrient deficiencies, visible deficiency
symptoms will appear.
Tissue Tests
One procedure for determining nutrient deficiency is tissue
testing. This procedure allows the determination of nutrient needs before
visual symptoms appear (that is, before the plant has undergone nutrient
stress). 
Problems with tissue testing are: 1) nutrient stress may occur before the fertilizer can be applied, 2) it is difficult to determine how much fertilizer to apply, and 3) can be effected by the weather. Tissue tests do, however, give an estimate of plant nutrient needs at the time of the test.
Visual Deficiency Symptoms are useful to aid in identifying when the plant is deficient in a nutrient. They are often difficult to interpret because many symptoms look similar or may look like disease or insect damage. When we see the deficiency symptom it is often too late to add additional fertilizer to aid the plants future growth.
Soil Tests
Soil testing is based on the concept that a crop's response to fertilizer
will be related to the amounts of available nutrients in the soil.
Good soil testing requires 3 components:
1) good representative sample;
2) adequate laboratory tests that
determine the amount of nutrients the plant can remove from the soil;
3) considerable experimental work to
correlate the soil test results with fertilizer recommendations and actual crop
yields.
Soil samples can be taken at any time of the year when moisture conditions and temperatures permit. It is not necessary to wait until planting time. For agronomic crops, lawns, and gardens, they can be collected in the fall before the frost is in the ground. For greenhouse soils, it is better to sample after sterilization. Fall sampling allows for adequate planning time before spring planting. For pH, phosphorus, potassium, sulfur, zinc, and soluble salts, fields and gardens should be tested only once every 3 years.
Testing for soil organic matter needs to be done only once every 5 years, since these levels change so slowly. Soil sample containers and information sheets are available from county extension agents, fertilizer dealers, and the University of Minnesota. The University of Minnesota operates a Soil Testing Laboratory in the Department of Soil, Water, and Climate in 125 Crops Research Building on the St. Paul Campus. U.M. Soil Testing Laboratory Web Page
A soil tube or auger is a convenient way to collect a large number of subsamples. However, a shovel or garden trowel will also work. A clean pail is also needed for use in mixing subsamples.
One of the most important steps in collecting the
samples is to insure a uniform sample. With the help of a soils map, divide
each field into uniform areas. The soil in each area should have the same
color, texture, cropping history, fertilizer, lime, and manure treatments.
One sample should not represent more than 20 acres of level uniform field or 5 acres of hilly or rolling land.
When taking the sample in cultivated fields, scrape off the surface litter and sample to plow depth. For lawns, pastures and meadows, sample only to 3 inches; for home gardens, sample to 6 inches. Mix the soil in the pail thoroughly. This is the composite sample.
If the area where you sampled was large and the total amount of soil collected was more than a pint, you will need to collect a subsample of one pint to send to the laboratory. Be sure your sample is thoroughly mixed before you subsample. Lastly, fill out the information on the form indicating past practices and future crops to be grown, since this information is needed to make fertilizer recommendations.
Calculating Fertilizer Rates from Soil Test Recommendations
Lawn and Gardens
All fertilizer recommendations are based on the amount (lbs) of nutrient (N, P2O5, K2O) to apply per given area. Lawn and turf recommendations are given in pounds per 1000 sq. ft. and garden, tree, and shrub recommendations in pounds per 100 sq. ft. From this recommendation it is necessary to select an appropriate fertilizer grade and then determine how much of this fertilizer to apply to the garden area.
Most garden fertilizers are complete fertilizers. They are convenient to use, but it may be difficult to find one that exactly matches the ratio required in a fertilizer recommendation. For complete fertilizers, select one with the closest ratio of N-P2O5-K2O to that recommended. It is most important to accurately match the N requirement when calculating the fertilizer rate and then compromise somewhat if necessary for P and K. However, do not apply additional P to lawns if it is not needed since it may become a pollutant. The Twin Cities will soon have a ban on the use of P in lawn fertilizers.
NOTE: 2 cups (1 pint) of dry fertilizer weighs approximately 1 pound.
Sample Problem to calculate fertilizer rates: A soil test recommendation calls for 0.1 lb. N/100 sq. ft., 0 lb. P2O5/100 sq. ft. and 0.1 lb. K2O/100 sq. ft. The area is 40 feet by 10 feet.
Step 1 Measure the area to be fertilized in square feet.
Formula for calculating size of area to be fertilized=feet long X feet
wide=square feet.
Example: 40 feet long X 10 feet wide=400 square feet
Step 2 Select the fertilizer(s) to use based on the soil test by
matching the ratio of nutrients recommended to the fertilizer grades available.
Example: The N-P-K nutrient ratio based on the soil test should be 1-0-1.
Ideally, a fertilizer such as a 10-0-10, 20-0-20 or 25-0-25 should be selected.
At the local garden store, fertilizer bags marked 20-10-10, 27-3-3 and 25-3-12
were available. The one marked 25-3-12 best matched the ratio of 1-0-1
recommended by the soil test.
Step 3 determine the amount of fertilizer to apply by: dividing the
recommended amount of the nutrient by the percentage of the nutrient (on a
decimal basis) in the fertilizer or lb. nutrient recommended/sq. ft. ÷ %
nutrient in fertilize=lb. fertilizer/sq. ft.
Example: (use nitrogen percentage to determine the rate to apply) 0.1 lb.
nutrient recommended / 100 sq. ft.÷ 25% nutrient in fertilizer (0.25)=0.4
lb. fertilizer / 100 sq. ft Or apply 0.4 lb. of 25-3-12 fertilizer per 100 sq.
ft.
Step 4 Adjust the amount of fertilizer to apply for the lawn or garden
area. lb. fertilizer/ sq. ft. X sq. ft. garden or lawn=lb. fertilizer
garden or lawn
Example: 0.4 lb. fertilizer /100 sq. ft. X 400 sq. ft. garden or
lawn=1.6 lb. 25-3-12 fertilizer garden or lawn or about 3 cups of a 25-3-12
fertilizer per 400 sq. ft. garden or lawn
For more information about your soil test results go to Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants
Agronomic Crops a corn example
You will need to have the extension bulletin N recommendations for Corn to complete fertilizer recommendations for corn.
Sample Problem: Farmer Brown is converting a soybean field into a corn field. The price/value ration is 0.10. The soil is highly productive. The SOM level is 3.2%, soil texture is silt loam, the yield goal is 175 bushels/acre, soil pH is 6.5, P soil test=7 ppm(Bray), K soil test=85 ppm.
Solution: from ( N recommendations for Corn ) The amount of N needed from Table 1 is 90-120 lbs. N- Note: it might be useful to do a nitrate test to determine a more specific N rate.
& 40 lbs P205- Banded - (Table 8- 7 ppm P for 175 bu. corn, row applied );
& 50 lbs K20(Table 9- 85 ppm row applied, 175 bu);
Lawns vs. Corn Fields
Paul Harvey of radio fame once made the statement "City folks put on 10 times the amount of fertilizer as farmers". He was trying to make a point that other people are causing nutrient pollution besides farmers. I wonder if he is right. Turf recommendations can be found at: Turf Grass N Recommendations and here is a guide for N recommendations for Corn. Thus, turf should have about 4 lbs. of N / 1000 ft²/year if the clippings are not removed and we add water between rains. Corn yield of 120 bu/acre that is following corn would need 140 lbs. of N / acre. If we convert the corn N recommendation to lbs./acre we would divide 140 by 43.56 (1 acre=43560 ft²) or 140÷43.56=3.2 lbs./1000 ft²=140 lbs. N / acre. Many growers are using more than 140 lbs. of N per acre on corn and many lawns receive less than 4 lbs/1000 ft². So the amount of N a lawn or golf course gets is about the same as a corn field. Paul Harvey may have "the rest of the story" but he did not have this story right!
Precision Agriculture 
New uses of soil testing information have resulted in the development of Precision Agriculture. Definitions for precision agriculture include:
"...using the best available technologies to tailor soil and crop management to fit the specific conditions found within an agricultural field or tract" ( after Chris Johannsen, 1996)
"...information gathering, management planning, and field operations that improve the understanding and management of soil and landscape resources so the cropping inputs of management practices are utilized more efficiently than with conventional 'one-fits-all' strategies" (after Kitchen et al., 1996)
"The strength of remote sensing lies in the opportunity to learn more about crop growth variability while the crop is still growing. Benefits can be realized by combining this information with grid sampled soil and yield maps in developing an integrated crop production program [...]" (Schepers et al., 1996)
Other terms that are currently being included in precision agriculture include: Precision farming, Variable Rate Technology (VRT), Variable Rate Management, Prescription Farming, Site Specific Crop Management, Farming by Soil, Grid Soil Sampling Agriculture, Grid Farming, Farming by the Inch, Farming by the Foot, GPS Agriculture
The U of M Center for Precision Agriculture Web page is located at Precision Agriculture Center
For information about the Minnesota River Project go to Minnesota River Project
The Soil Analytical Lab can be found at Soil Analytical Laboratory
Private soil testing laboratories are also available, and their addresses can be obtained from the county extension office or your fertilizer dealer. For information on other soil testing laboratories check out New York .
© Terence H. Cooper & Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.
