Soil Lab Write Up

Abstract

Around San Diego, there are a diverse amount of soil types. The environments they’re in, how the soil is tended, and numerous other factors determine soil quality. Measuring pH level and the content of the three primary soil nutrients (Nitrogen, Phosphorous, and Potassium)is one way to determine soil quality and its suitability for growing different plants.  Most soils had a low amount of nitrate, a medium amount of phosphorus, and a high amount of potassium; with mostly acidic and neutral pH levels. These trends were surprising to find considering the variety of places the soil samples were taken from.

Introduction

Most of life on earth depends on soil. It is easy to understate the necessity of testing its textural, compositional, and chemical characteristics, but it is invaluable to land ecosystems due to the fact that soil is where autotrophic land organisms (the foundation of the food chain) thrive. Our objective is to measure soil pH and nutrient content, as they are important factors in determining the quality of this complex mixture of inorganic and organic materials, microorganisms, water, and air. We will determine whether soil is acidic, neutral, or basic, and how much (if any) primary macronutrients—nitrate, phosphorous, and potassium (commonly abbreviated NPK)—it contains.

Chemical tests were performed in order to approximate the nitrogen, phosphorous, and potassium content in various soil samples collected from around San Diego by students. The tests were conducted by adding chemical testing tablets to soil “supernatant”—water poured over the soil—and results were obtained by comparing the resulting color of the solution with color charts to approximate a reading of Low, Medium, or High.

I expect the soil to have a more acidic pH since rainwater tends to be more acidic and the nitrate content should be less since most of the soil samples are from non-fertilized and unmaintained locations. Potassium and phosphorous content might be around medium since there should be decomposing and dead organic matter in most of the soil areas.

Results & Analysis

There were four measurements taken from the soil and despite the variety of samples they all followed similar trends despite the expected outliers. Look for the trends of low Nitrate content, medium Phosphorous content, and high Potassium content, and predominately acidic to neutral pH levels.

It seems that the soil samples had a lot of organic matter in order to achieve higher levels of Phosphorous and Potassium, but not as much nitrogen fixation was occuring or the plants were using up nitrates faster than they could be replenished. Although desert soil, which is native to San Diego, often has low nutrient levels and is basic, the soil samples were more acidic and neutral which might be due to the fact that they are watered by hose and rainfall has increased in the San Diego biosphere over the last century or more.

Acidic < 7                                           Neutral = 7                                          Basic >7

Reflection

This investigation explored a necessary part of our existence I had never before considered. I was happy to find there were trends in the data despite the fact that our soil came from many places around San Diego. Also, it goes to show how complex and important soil really is. I think that the investigation went great and that people had fun doing the lab and were neither frustrated nor stumped. I didn’t see many sources for error in a text like this unless the error was a mistake on the part of the person (i.e. not following instructions…). As a result, I definitely enjoyed this lab and the knowledge it had to offer. The only thing I can say, and it is a generality for many bigger assignments in Environmental Science, is that I think an assignment description and due date should always be posted on the class blog.

Conclusion

In determining the quality of various soils from around San Diego, there was a collective realization of the amazing yet underappreciated role soil plays in the circle of life. We are dependent upon it, but it is something many take for granted. Once we performed chemical tests, I was certainly surprised at the trends we found. I expected results to be erratic, but for the most part our soil had Low levels of Nitrogen, Medium levels of Phosphorous, and High levels of Potassium, with a pH hovering around neutral or just slightly acidic.

  

Short Answer Questions

Some investigations will include short answer questions that will be answered here. When answering the questions, rewrite the question first, and then provide the answer. All questions must be numbered and answers must be typed.

1.      The proportions of what three particles are used to determine soil texture? Which of these particles has the   smallest surface area? Which has the largest?

The portions of sand, silt, and clay are used to determine soil texture. Clay has the smallest surface area, whereas sand has the largest surface area.

2.      Using the soil texture triangle (Next Page), to determine the type of soils with the following particle sizes:

a.       20% silt, 10% clay, 70% sand

Sandy loam contains 20% silt, 10% clay, and 70% sand.

b.      30% sand, 10% clay, 60% silt

Silt loam contains 30% sand, 10% clay, and 60% silt.

c.       10% silt, 50% sand, 40% clay

Sandy clay contains 10% silt, 50% sand, and 40% clay.

d.      30% clay, 30% sand, 40% silt

Clay loam contains 30% clay, 30% sand, and 40% silt.

e.       40% sand, 10% silt, 50% clay

Clay contains 40% sand, 10% silt, and 50% clay.

f.       Determine the soil type for your sample based on your % volumes.

3.      Looking at the Soil Texture Triangle (Next Page), which soil type has the greatest:

a.       water retention ability?

From looking at the Soil Texture Triangle, Clay has the greatest water retention ability.

b.      water percolation rate?

From looking at the Soil Texture Triangle, Sand has the greatest water percolation rate.

4.      What role does humus play in soil fertility?

Humus makes for fertile soil due to the fact that it is organic matter that has decomposed until it cannot break down any longer. Thus, the nutrients from this material are readily available to plants in need of them. Plant fertility is dependent upon a number of variables, but without organic matter in the soil, not only do plants not have a source of the nutrients they need to grow and reproduce, but the other living things within soil cannot thrive without humus either.

5.      Why is pH such an important aspect of soil fertility?

Soil fertility refers to the amount of nutrients and the overall ‘health’ of the soil. Therefore, the less nutrients there are, the less fertile the soil is. Certain nutrients are more readily available in certain pH levels than others. For instance, microorganisms synthesize nitrogen and sulfur better at a slightly acidic pH, creating an abundance of nitrates and sulfates for plant use. If the pH is too acidic, less nutrients will be available, which means soil fertility is negatively impacted.

6.      What are some natural sources of the nitrogen, potassium and phosphorous found in soil?

We intake more Nitrogen than Oxygen every time we breathe in since earth’s atmosphere is rich in N. As a result, most nitrates in the soil originally come from the air. Potassium comes from organic materials, especially decomposed organisms, which are rich in the K that the soil absorbs. Similarly, phosphorous also comes from decomposed organisms, like bone meal. From these examples, it becomes evident how important humus is once again.

7.      How are the three primary plant nutrients used by living organisms?

Plants use the three primary plant nutrients in photosynthesis, growth, and the formation of various cell and system parts. Living organisms use Nitrogen to produce complex organic molecules; Phosphorous is essential for nucleic acids and cell energy (ATP), and plays a role in all the nervous and circulatory systems; and Potassium is vital to neurological/nervous system function.

8.      By what process is atmospheric molecular nitrogen (N2) converted into a form that plants can readily absorb through their roots? What form of nitrogen is this?

Atmospheric molecular nitrogen (N2) is converted into ammonia (NH2) through a process called Nitrogen Fixation.

9.      What are some possible sources of error in this experiment?

Some possible sources of error in the experiment are that the soil was left uncovered and it is possible that organic/inorganic material was added or lost. Even pH could be affected by moisture in the room. Also, results (which are determined by visual color comparison) could be tainted by how well the soil sample settled.

10.  Evaluate the fertility of the soil used in this lab activity based upon your results.

11.  What types of vegetation does soil of the type and pH you sampled best support?