Chemical Analysis of Soil: Two Slopes of a Tobacco Patch

 Objective:

The goal was to investigate the pH and concentrations of phosphorus, nitrogen, and potassium at different points on two different slopes of a tobacco patch. I wanted to see if the concentrations would be the same and whether or not the steepness of the slope made a difference.

The tobacco crop was harvested and the field was left untilled for four weeks. I then collected the topsoil, approximately one-inch deep, and placed it into zip lock bags and they were stored in the refrigerator. The collections were four meters apart and ran parallel with the previously standing tobacco plants. The length of slope one is not as long as slope two so I collected two extra samples (an extra 8 meters). In addition, slope two was slightly steeper than slope one. The next step was to perform the chemical tests. I used a LaMotte Soil Chemistry Test Kit provided by Bellarmine University’s biology department. The four tests involved a series of dilutions and then matching the results with a color chart. The results were recorded and analyzed.

Important Information:

Soil is the product of natural decomposition forces acting upon native rocks, vegetation and animal matter over a long period of time. It is a complex, living mixture of minerals, organic compounds, microorganisms, water, and air. Soil formation begins with a simple organism, such as lichen or a moss, settling on a rock. These organisms release carbonic acid, which begins to dissolve the rock and release minerals. At the same time, the structure of the plant begins to catch wind-blown dust that is also added to the soil mixture. The simple organism also begins to attract fungi, protists, bacteria, and animals, which begin to add organic materials to the soil through their deaths and excretion. As this mat thickens, higher forms of life can start taking advantage of the site. In turn, the rate of soil formation accelerates.

Soil productivity is dependent on certain factors. Phosphorus, Potassium, and Nitrogen are need in the proper form in order for the plants to be able to utilize them. Microorganisms are necessary to convert these elements into the compounds that the plants can use. Also, pH affects the productivity by changing the charge of the soil.

The proper soil pH is extremely important to plants because it directly affects the availability of the plant food nutrients, which the plants need for efficient growth. Soil charge allows it to hold on to nutrients. Soils that are too acidic or alkaline will not favor the solution of compounds and restrict the presence of ions of essential plant nutrients because many of these elements do not dissolve easily in extreme soil conditions. Most plants prefer a soil reaction that is neutral or close to the neutral point. Soils in the pH range of 6.0 to 8.0 are usually satisfactory except for certain acid-loving plants that grow best in soils with a pH value of 4.0 to 5.0.

Nitrogen is a major essential element supplied through the soil. It is a unique element in that it composes 80% of the earth’s atmosphere. However, most plants can not utilize this form of "free" nitrogen. Legumes have the capability of converting atmospheric nitrogen into a form that is used by the plant. Those non-legume plants get useful nitrogen through the decomposition of organic matter and application of commercial nitrogen fertilizers.

Nitrogen is the element that stimulates above ground growth and produces the rich green color that is characteristic of a healthy plant. In addition, it influences the quality of the plant’s fruit and it increases the fruit’s protein content. Also, the presence of nitrogen in the plant stimulates the plant’s utilization of other major elements. However, excessive nitrogen can have adverse effects on crops. Excessive nitrogen can delay crop maturity, increase lodging due to weakened stems, produce extreme vegetative growth at the expense of the fruits, and cause potential health hazards for man and animal due to nitrate accumulation in leafy forage or vegetable crops.

Phosphorus is necessary for the hardy growth of the plant and activity of the cells. This element is abundant in the fruits of plants and seeds and also in the parts of the root, which are involved in the rapid uptake of nutrients and water. Phosphorus plays a major responsibility in plants in the processes requiring a transfer of energy. By stimulating the rapid cell development in the plants, phosphorus is a natural means of increasing the resistance to disease. An excess of phosphorus does not cause any harmful effects and has a balancing effect on the plant.

In many soils, the phosphorus content is low and is often present in forms that are not available for effective plant uptake. In acid soils, particularly, phosphorus may be converted into aluminum and /or iron phosphates, both of which have relatively poorer plant availability. On the other hand, calcium phosphate is more available; therefore, it is desirable to apply phosphates to soils that are properly limed and show slightly acidic reactions. Phosphates applied to properly limed soils are kept in accessible forms.

It is significant that the best agricultural soils are all high in readily available phosphorus, since the abundance of readily available phosphorus favors all the conditions, which go to make a real fertile soil. A difference of 25 pounds of available phosphorus per acre in the lower range is sufficient to exert a marked influence on the crop producing power of the soil. The supply of phosphorus can be quickly depleted by continuous cropping if provisions are not made for the return of phosphorus in the form of commercial fertilizers or farm manure. Importantly, as soon as the content of phosphorus in a soil goes below a certain level, maximum crop yields drop below a profitable level.

Potassium is a positively charged basic metal cation whose total content in most mineral soils is greater than most other major nutrient elements. The average potassium content of the earth’s surface is estimated at 2.3 percent. Most of this content is readily available to plants because it is either bound in primary minerals or is fixed in the interlayers of clay minerals. This element has much to do with the vigor and vitality of the plant, encouraging the development of a healthy root system and it offsets the damaging effects of excessive nitrogen. Potassium also tends to counteract a delay in ripening and thereby exerts a balancing effect on excessive nitrogen levels. It also appears to play a role in the synthesis of starch and the translocation of carbohydrates within the plant. Since clay soils develop from the decomposition of potassium rich primary minerals, soils high in clay content usually have high potassium content. As potassium in the soil solution is diminished by plant uptake, it is replenished by exchangeable potassium from soil colloids.

Being that my research concentrated on the soil from a tobacco patch, I searched for the ranges that best suited the growth of tobacco. From literature, I have found that tobacco plants prefer to live in soil with a pH range of 5.0-6.0 or 6.0-8.0. In addition, tobacco plants need a very high amount of nitrogen, a medium amount of phosphorus, and a very high amount of potassium.

Importance of Soil Testing:

Soil must be measured by chemical means and at frequencies that will insure against unexpected shortages. With the advent of high yielding hybrids and the increasing interest in double cropping systems, there has been an increase in the rate of removal of raw materials. This has placed a much greater demand on the soil and could lead to exhaustion if these reserves are not monitored regularly and replenished properly. A soil test serves as a tool for monitoring and maintaining these vital nutrient elements.

The Crider series was established in Caldwell County, Kentucky in 1957 and was named after a community in that county. The Crider soils are extensive, making up 500,000 acres in Kentucky and occurring in 35 counties in the state. Most areas are used for pasture and crops. The main crops are corn, soybeans, small grain, hay, and tobacco. Crider soils are highly productive and many acres of these soils are prime farmland.

The Crider series consists of very deep, well drained, moderately permeable soils on uplands. Slopes range from on to 20 percent. The average annual precipitation is approximately 48 inches and the average annual temperature is 57 degrees F.

Results:

According to the color charts for the Phosphorus, Nitrogen, and Potassium tests, the results were word values (Very low, Low, etc.). In order to graph the results, I assigned a numerical value to the results for Phosphorus, Nitrogen, and Potassium. The following is the assignment:

I then graphed my results to better understand the correlation of the slopes.

There is an obvious difference between the two slopes. Slope one has a much lower pH range, between 4.0 and 4.5, while slope two has a range of 5.0 to 6.0. Slope one is more acidic and its ranges are below the desired ranges of 5.0-6.0 or 6.0-8.0. However, slope two falls right in the desired ranges. As for the curve of both slopes, there is no trend for the lay of the land. Both slopes increase in pH and then decrease. No conclusions can be drawn for a correlation between slope curve and pH.

As for phosphorus, there are some differences between the two slopes. Slope one starts out with a medium low amount of phosphorus, progresses to low, jumps back up to medium low, settles at low for eight meters and then finally reaches medium high. As for slope two, medium high is the starting value. It progresses to low, jumps up to medium over several collection and finally falls and settles on low. This is not what I expected. I was expecting a gradual increase in the phosphorus amounts as collections proceeded down the slope. Unfortunately, no generalizations can be drawn from my results.

There are some differences in the amounts of nitrogen among the two slopes. Not as I expected, slope one progressed from very high to very low and then jumped to high. It then fell to low and bounced back to high and finally ended on very high. I expected that the starting value would be rather low since the collection was at the top of the slope and the values would eventually increase as I progressed down the slope. For slope two, the values did seem to increasingly progress. The levels eventually rose but fell on the last collection point. The increase in nitrogen levels could be due to the fact that slope two is slightly steeper than slope one.

As for the levels of potassium in the soil, slope one remained at a constant level of medium all the way until the sixth collection point were it decreased to medium low followed by an increase to high and then settling on medium high. Thus, the levels did rise at the bottom of the slope, as was expected. As for slope two, the levels fluctuated between medium and medium low and then rose to medium high but fell to medium. Thus, slope two did not pan out as I expected. Consequently, no generalizations can be deduced about the two slopes and potassium levels.

As for the ideal amount of phosphorus for growing tobacco (medium), slope two seems to fall into that amount better than slope one. The ideal amount of nitrogen is very high and both slopes possess high levels, but slope two has gradual increase in levels so it would be better suited. As for potassium, the ideal level is very high. Slope one and two are lacking in the desired levels of potassium. These levels are affecting the tobacco crop yield. The tobacco grown in slope one has progressively shown decreases in plant growth and survival over the past three years. However, no hard conclusions can be drawn from my results and further testing should be conducted.