1. Why is soil testing so important? Give two reasons.
2. Why is a composite soil sample more representative of an area than a single core?
3. The recommended sampling depth of a lawn is 8-10 cm, while that of a dug garden or plowed field is 15-30 cm. Why the difference?
4. What is the "minimum soil level? and why is it important?
5. For each of the major plant nutrients (N, P, K, Ca, Mg), what is the most common, readily-available form stored in the soil? Of all of these, which is most likely to leach into groundwater?
6. Limestone and fertilizers containing P, K, Ca, Mg, etc. are usually applied in single applications, before or at the time of planting. Nitrogen fertilizers are often applied in split applications; some applied at planting and the rest as a side-dress or top-dress application. Why the difference?
7. What is the importance of the previous crop history for fertilizer recommendations?
8. A fertilizer recommendation (for 100 ft2) calls for 1.5 lbs of 10-10-10. Alternatives include 5-10-10 and 5-10-5. Which of the three would be the cheapest?
| 10-10-10 | @ | $4.60 per 25 lb |
| 5-10-10 | @ | $6.50 per 40 lb |
| 5-10-5 | @ | $4.00 per 25 lb |
9. A fertilizer recommendation (for 100 ft2) calls for 1.5 lbs of 10-10-10. The only fertilizer in your local shop with a similar ratio is 14-14-14. How much 14-14-14 should be applied per 100 ft2?
Study Questions Answers
1. It is important for determining the fertility status and for monitoring levels of chemical compounds in the soil.
2. A single core may not be representative of the field, due to any number of irregularities. A composite sample of 15 to 20 cores will be much more representative of the field - irregularities in the individual cores will be minimized.
3. Soil amendments for a lawn will usually be applied on the surface rather than digging up the lawn. The greatest amount of grass roots are in the upper part of the soil, too.
4. The "minimum soil level" is the level required to provide a healthy ecosystem.
5.
| Nutrient | Common Form |
| N | NO3- |
| P | HPO42- |
| K | K+ |
| Ca | Ca2+ |
| Mg | Mg2+ |
Of these, nitrate (NO3-) is most likely to leach into the groundwater.
6. Most nitrogen in soil is nitrate (NO3-). Because it carries a negative charge it is not adsorbed on the soil colloids, so is susceptible to leaching during heavy rains or irrigation. If conditions are right it can also be lost into the atmosphere as N2 (nitrogen gas), N2O (nitrous oxide), NO (nitric oxide), or NH3 (ammonia).
7. Some crops add nutrients that will be available to the current (future) crops.
8. For each alternative multiphy the cost per pound by the recommended rate.
($4.60/25 lb) * 1.5 lb = $0.28 per 100 ft2
($6.50/40 lb) * 3.5 lb = $0.57 per 100 ft2
($4.00/25 lb) * 3.5 lb = $0.56 per 100 ft2
9. 14-14-14 is 14/10 stronger than 10-10-10. Multiply the recommended rate of 10-10-10 by 10/14:
1.5 lb 10-10-10 * (10/14) = 1.1 lb 14-14-14.
Exam Questions from Labs
Example Exam questions from Lab
1. Which of the following would violate standard soil sampling guidelines?
a. take cores from at least 3 spots in the field to obtain a representative composite sample.
b. one composite sample should not represent more than 20 acres
c. take separate composite samples from problem areas
d. mix cores, allow to air dry, and remove roots and stones
e. take a shallow composite sample for no-till areas to monitor surface acidity.
2. Limestone and fertilizers containing P, K, Ca, and Mg are usually applied in a single application at the time of planting while nitrogen fertilizers are frequently applied in split applications. This is done because
a. nitrogen is more mobile in the soil than the other materials.
b. all the materials other than nitrogen are cations (positively charged).
c. all the materials other than nitrogen need time (weeks) to react with the soil.
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3. A fertilizer recommendation calls for 8 pounds of P2O5 per 1000 ft2. Which of the following would be the cheapest source of phosphate on a basis of the cost per pound of phosphate?
| a. 7-10-0.5 @ $100 per ton | 100/(2000 * 0.10) = 0.5000 |
| b. 5-10-5 @ $4.5 per 50 lbs | 4.5/(50 * 0.10) = 0.9000 |
| c. 0-0-60 @ $2.50 per 25 lbs | 2.50/(25 * 60) = 0.0017 |
| d. 33-0-0- @ $3.95 per 80 lbs | 3.95/(80 * 0.33) = 0.1500 |
| e. 0-20-2 @ $180 per ton | 180/(2000 * 0.20) - 0.4500 |
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