How is soil quality affected by soil management?
By Aurora, Eleanor Roosevelt High School, Greenbelt, MD
Soil is one of our most precious natural resources, as it integrates all parts of the ecosystem. It provides a medium for plant growth so that we can have food, clothing, and other materials. Soil filters water, decomposes waste, stores heat, and exchanges gases. Soil is alive- it is the home to billions of micro- and macroscopic organisms. It is a material used for construction, medicine, and art. It produces a snapshot of the geologic, climatic, biological, and human history at the place that they are found. Unfortunately, there is a limited amount of soil that can actually be used for growing food, and all of the other uses that we require it for. When improperly managed, soil can become eroded, polluted, or destroyed. It can also cause damage to other parts of the ecosystem.
Humans have used soils since we have been on the Earth. In the 1930's, technology caught up with us. After the war, we had leftover chemical bombs, weapons, and oils which we could not use. So, companies took theses bombs, weapons, and oils and processed them into chemical fertilizers* and pesticides* which were then used in our soils to eradicate the pest population and to help to fertilize the soils. One of the most popular chemical compounds used at the time was anhydrous ammonia, a petroleum-based pesticide which was fatally toxic if it made contact with the skin, eyes, and lungs. It also was slowly disintegrated, so the unused portions of it were found in streams and rivers, and was later discovered to produce air pollution and made a large contribution to the widening of the hole in the ozone layer. As time went on, the chemical fertilization and pest control phase grew. By the early 1950's, all of the fruits and vegetables grown in our country were treated with man made chemicals such as treflan, parathylon, DDT, and 424D- all toxic to the human system (Adams, 1986). By this time, chemical farming had become very, very popular and so had gained the name "conventional agriculture*." There are a few benefits to conventional agriculture, such as that it allows farmers to grow the same crop in the same place every year, which is convenient for farmers because crop rotation* requires uprooting all plants and starting new in different place (until the soil becomes degraded), and that it treats soil with a few elements (i.e.: nitrogen, potassium, phosphorus) which may have been depleted by farming (Brady and Weil, 1996). It had been proven to cause soil degradation*, which would make it easier to be effected by erosion*. Some methods in conventional agriculture have led to the removal of 15-20 tons of top soil* per acre annually, and now scientists estimate that, if we continue with conventional agriculture, in 40-50 years, there will be no more top soil left in the world (Miranda, 1995). Another problem also arose from conventional agriculture- although the bugs and pests that were specifically targeted were killed, the chemical compounds that were used often attracted other, unanticipated bugs and pests, and caused some of the targeted pests to adapt and become tolerant of the pesticides (Miranda, 1995).
While more than half of the world is using conventional agriculture to grow crops, there is a new, more radical approach to farming- organic farming*. Although organic farming is a fairly recent idea in our society, it had been used before by our ancestors. The principle behind organic farming is to grow crops completely without using man-made chemicals, and to help to nourish the earth while also gaining something from it (namely, strong healthy crops). Organic farming is very beneficial, both to the soil and to us. Using organic farming, the soil maintains its structure, consistence,* and diversity* of life within the soil (both micro- and macroscopic life), which is considered very important in a healthy soil. It also allows farmers to grow many crops in the same area, which provides diversity for the soil and also is a natural way to keep out pests. By organically treating the soil with compost* and with animal manure, the soil stays healthy and also is enriched so that any tillage* of the soil will not harm it in the long run. Also, the organic material supplied to soil through organic farming serves as a sponge to water, and helps to store nutrients (Federation of Ohio Naturalists, 1995).
The idea of chemical vs. organic farming is the main reason for this project. By looking at conventionally-treated, organically-treated, and forested soil of the same type, the question of whether different management practices affect the soil will be addressed. This question will be answered by comparing a) the chemical composition of all three soils (i.e.: pH, nitrate nitrogen, potassium, phosphorus, etc.), b) the consistence, c) the texture, d) the soil structure*, e) the color, f) the macro invertebrate activity, and g) the bulk density. The soils that will be tested are in the Mattapex soil series. Both the soil that is cultivated and the soil that is farmed with compost have been farmed from 1985-1992 with alfalfa, and from 1993-1995 with no-till corn. The cultivated soil, starting in 1996, has been farmed with corn and has been treated with commercial fertilizers and herbicides. The composted soil, also starting in 1996, has been farmed with corn and treated with chicken manure and compost- a total of 4 tons per acre per month.
Soil Vocabulary and Definitions
Color Wheel- a color description of the different possible colors of soil- the soils are titled with letters that represent the actual color of the soil, and by numbers which represent the intensity of the color of the soil
Compost- organic residues that have been piled, moistened, and allowed to undergo biological decomposition
Consistence- a description of how easily a soil breaks apart when pressed- can be friable, which means that it breaks apart easily, or firm, which means that it takes a lot of pressure for the soil to break apart.
Conventional Agriculture- the popular term for tillage practices and application of chemicals
Crop Rotation- a planned sequence of crops growing in a regularly recurring succession on the same area of land
Diversity- the presence of many different organisms in a soil
Erosion- the wearing away of the land surface by water, wind, ice, or other geological agents
Fertility- the quality of a soil that enables it to provide essential chemical elements in quantities and proportions for the growth of specified plants
Fertilizers- any organic or inorganic material of natural or synthetic origin added to a soil to supply certain elements essential to the growth of plants
Munsell Soil Color Book- a handbook which displays the possible colors of soil and titles each one according to their place on the Color Wheel
Organic Farming- farming without the use of man made chemicals and using only organic matter which helps to enrich the earth at the same time as it helps in producing crops
Organic Matter- any dead matter within the soil of a natural origin (e.g.: compost)
Pesticides- any material added to the soil in order to eradicate existing pests and insects and to prevent the recurrence of any new pests or insects
pH-the negative log of the hydrogen ion concentration which is an indicator of how acid or basic the soil is. The lower the number, the more acidic. The scale runs from 0-14, with 7 being neutral.
Soil Degradation- a decrease in the productive capability of the soil (e.g.: erosion or compaction)
Soil Quality- the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation
Soil Series- the finest level of classification consisting of soils that are similar in all major profile characteristics
Soil Structure- the combination or arrangement of primary soil particles into secondary particles, units, or peds ( the shape of the soil units). Some types of structure are granular (which looks like cookie crumbs), blocky (which looks like small blocks), prismatic (which is prism shaped). Grades of structure are weak, moderate, or strong depending on how easy it is to distinguish the type of structure.
Soil Texture- the way the soil feels, based on the amount of sand, silt, and clay present in the soil
Tillage- the mechanical manipulation of soil for any purpose (e.g.: crop production)
Top Soil- the layer of soil on the surface of a soil- typically considered to be the most fertile section of a soil
How is soil quality affected by soil management? Is there a difference in soil quality between the same soil type which has been forested, farmed conventionally, and farmed using compost? If so, what?
The quality of soil will be affected by the use of conventional farming. Soil properties which control soil quality such as soil structure, organic matter content, earthworm activity, and other properties will be different from the unmanaged soil or the soil treated organically. Fertility may be higher in the conventionally treated soil than in the other types of soils because of the addition of fertilizers.
- unmanaged soil such as a forest (control group)
- agricultural field to which pesticides and fertilizers have been added (conventional farming)
- agricultural field to which compost has been added
- LaMotte Soil Testing kit (N, P, K)
- PG County Soil Survey Report
- liter-sized plastic bags for each site and marker
- newspaper for drying soil on
- 250 mL. can for sampling (3 for each site)
- nail and hammer for poking a hole in the bottom of each can
- experts from the Beltsville Agricultural Research Center (BARC) and NASA
- scale for weighing soils
- Munsell soil color book
- pH pen and buffers
- trowel for gathering samples
- 100 mL beakers
- 100 mL graduated cylinder
- distilled water
The procedure for the completion of this project is as follows:
Step 1: Find 3 soils of the same soil type of which one has been conventionally farmed, farmed with compost but with the same farming history, and one that has been forested.
Step 2: Collect 3 surface samples from each site, each filling a 250 mL can
Step 3: Carefully observe soil properties (such as structure, consistence, earthworm activity, and color) in the field. Structure, consistence, and earthworm activity is tested by simple observation and by feeling the soil's texture and consistency. Color is tested by comparing a sample of moist soil to the colors displayed in the Munsell Soil Color Book (see "Introduction to Science Project, pt. 2"). Step
4: Take samples back home, and dry them, sieve them, and test them for the following chemical properties: pH, nitrate nitrogen (N), phosphorus (P), potassium (K). pH will be tested using a pH pen, calibrated with buffers. The nitrogen, phosphorus, and potassium will be tested using a LaMotte soil testing kit. The LaMotte soil testing kit has materials to extract solution from the soils containing the elements to be tested. Reagents for each of these are then used to change the color of the extracted solution to compare with color charts that tell the relative amount of each nutrient in the soil. pH will be tested for because acidic soils can be harmful to plants, and controls the presence of other nutrients in the soil. Therefore, the soils can be partially proven harmful or helpful to plant growth by testing pH. Nitrate nitrogen, phosphorus, and potassium are nutrients that are important for plant growth, and may be affected by different soil properties.
pH Test Results
The results of the pH tests in the three differently managed soils were varied. In the composted soil, the pH was at three different levels- in sample one, it was 7.1, in sample two, it was 6.4, and in sample three, it was 6.8. The levels of pH in the chemically cultivated soil were are different levels as well, although not quite as disparately- in sample one, it was 6.6, in sample two, it was 6.7, and in sample three, it was again, 6.6. In the forested soil, the pH levels were again varied- in sample one, it was 6.6, in sample two, it was 6.8, and in sample three, it was 5.5.
Click the thumbnail to see a graph of the average pH results:
Chemical Test Results (NPK Tests)
The soils were tested for three different chemicals in this series of tests. The results for the levels of nitrate nitrogen in both the forested and the composted soils were all the same- there was no nitrogen in the soils. In the cultivated soil, there was a larger amount of nitrogen in the soil, but still not a very large amount. The results for the levels of phosphorus in the soils were again, all very similar. In both the composted and the forested soils, there was a small amount of phosphorus present. In the chemically cultivated soil, there was also, for the most part, a small amount of phosphorus present, but in sample two of the cultivated soil there was a spike in the amount of phosphorus and the level reached a medium amount. The results for the levels of potassium in the soils was also quite similar between the different soils. In the composted soil, there was mainly a medium amount of potassium present, but, in sample two, the amount spiked to a high level. In the forested soil, there was a medium amount of potassium for all of the samples. In the cultivated soil, there was also a medium amount of potassium but, like the composted soil, the levels spiked to a high amount in sample one.
Bulk Density Test Results
The bulk density was different for each different soil. The average bulk density of the composted soil was 1.04g/cm3. The average bulk density of the chemically cultivated soil was 1.17 g/cm3. The average bulk density of the forested soil was .90 g/cm3.
Field Test Results
There were several tests involved in the field testing process- structure, color, consistence, texture, crusting, macro invertebrate activity, and roots. The structure of the composted soil was strong granular, as was the structure of the forested soil. The structure of the chemically cultivated soil was medium blocky. The color of the forested soil was the darkest- 10YR3/2 as determined by the Munsell Soil Color Book. The color of the composted soil was next darkest- it was 10YR3/4 as determined by the Munsell Soil Color Book. The cultivated soils were the lightest in color- 10YR4/4 as determined by the Munsell Soil Color Book. All three soils had a friable consistence and a silt loam texture. Only the cultivated soil had crusting present on its surface. Only the forested soil had macro invertebrates (earthworms) and roots present in its contents.
The overall conclusion of this project was that the hypothesis, that the quality of soil would be different in the conventionally managed, composted, and forested soils was correct. It was most evident in looking at the physical properties of each of the soils. The first sign of deterioration in physical properties was in the conventionally cultivated soil with the presence of crusting. When a soil "crusts," it means that the soil is losing its structure and is forming a crust on the surface. This is a signal that the soil will lose its ability to let water move into it, which may lead to soil erosion. There was no crusting on the composted or the forested soils.
The second sign of physical deterioration in the conventionally cultivated soil is in the color of the soil. Darkness in color of a soil usually represents the presence of organic matter, which is good for the soil as it allows the soil to absorb water, improves soil structure, and adds more natural nutrients to the soil which allow for plant growth. In comparison to the composted and the forested soils, the conventionally cultivated soil is the lightest in color- a 10YR4/4, as determined by the Munsell Soil Color Book. The notation "YR." stands for yellow-red on the Color Wheel. The number 10 in front of the YR. stands for the hue of the soil color or its position on the color wheel. The number before the slash is called the "value" which has a scale of 0-10, with 0 being the darkest and 10 being the lightest. The number after the slash is called "chroma" and it represents the saturation of the color. This, in relation to the conventionally cultivated soil, means that the soil is a light yellow-red color, and both the fact that the soil is so light and that it is yellow-red indicates that it probably has a lower amount of organic matter which is not good for soil quality.
The third sign of physical deterioration in the conventionally cultivated soil is in the bulk density. When a soil is dense, it means that it is compacted and has little to no pore space or room for the accumulation of air and water. The conventionally cultivated soil has the greatest bulk density when compared to the composted and the forested soil, which means that it is more compacted, so that less air and water can not move through it.
The second part of the hypothesis to this project was that although the quality of soil would be different in each of the soils, the conventionally cultivated soil would be more fertile due to the addition of fertilizers.. This also was proven in the experiment. The conventionally cultivated soil was the only one of the three soils to contain nitrate nitrogen, which, due to its negatively charged properties, is not usually attracted to soil (soil is also negatively charged). Because the soil doesn't hold it, it can be easily leached out the bottom of the soil with water, taken up by plants, or changed to different forms of nitrogen. This indicates the probable addition of nitrate nitrogen through fertilizers which allows the soil to have a higher fertility. Another chemical, phosphorus, is also usually present in the soil as a negatively charged element, and behaves in a similar way as nitrate nitrogen. Phosphorus was found in a sizable amount in the conventionally cultivated soil when compared to the composted and the forested soils. The addition of phosphorus probably is also added in the conventional soil. The only chemical component that was present in all three soils was potassium. This is probably because potassium is positively charged, and when is added to the negatively charged soil, is attracted to it.
If this experiment were to be done again, there are several things about it that could be changed. First of all, the soil sites could be observed over a relatively lengthy period of time (i.e.: years). By observing the soil sites over time, changes in soil chemistry and physical properties could be noted, and this would add to the final data. Another thing that could be done to improve on this project is to test the soil for more chemicals (i.e.: sulfur, humus content, etc.). This also would allow for more diversity in the results and would help to make the hypothesis more easily proven or disproven. A final thing that could be done to improve on this project is to dig the samples at different depths, as opposed to all at the surface. By gathering samples at different levels, it would show whether or not the results gathered at one level change as the soil deepens, or if the results stay the same.
Organic Gardening; Vol. 43, No. 4; Apr. 1996; p31-39; "Get Your Soil Tested"
Adams, John Anthony; Dirt; Texas A&M University Press; College Station, Texas; 1986
Federation of Ontario Naturalists; Don't Treat Soil Like Dirt; Soil and Water Conservation Society; Ontario, Canada; 1995
Brady, Nyle C. and Weil, Ray R.; The Nature and Properties of Soils; Prentice Hall; Upper Saddle River, New Jersey; 1996
United States Department of Agriculture; Prince Georges County Maryland Soil Survey Report; US Government Printing Office; USA; 1966
I would like to thank Dr. Elissa Levine (Soil Scientist) for all of her assistance on understanding the terms and concepts required in this project, and for being patient when I couldn't understand what I was talking about. I would also like to thank Dr. Laura Lengnick for assisting me in locating soil samples and in collecting them. Thanks also go to Dr. Patricia D. Millner for helping me locate an appropriate