Nutrient Comparisons Between Fallow and Crop Fields

Katrina Olsen



I tested the pH, nitrogen, phosphorous, and potassium levels in a fallow (uncultivated) field, a soybean crop field and an alfalfa crop field.  I wanted to see what affect the two different crops had on the soil nutrients as compared to the fallow field.  The nutrient levels were affected in both cultivated fields.

Keywords: alfalfa, fallow, nutrients, soil, soybean


Soil nutrients are major contributors to the overall health of plants.  The absence or presence of nutrients in certain amounts can determine whether or not a plant will successfully grow. The pH of soil influences the nutrients’ affinity for the soil, determining how much stays in the soil and how much is absorbed by the plant.  Generally, the nutrients needed by plants are most available in the pH range of 5.5-8.0 (Bickelhaupt 1998).  At the acidic or alkaline extremes, the nutrients become locked in by the soil’s chemical bonds and can’t be taken up by the plant’s root system. 

Different plants have different requirements for optimal growth, but all need nitrogen, phosphorous, and potassium.  Nitrogen is essential for the utilization of phosphorous and potassium, faster crop maturity, the size of fruits and almost all biochemical activities in plant life.  In legumes such as soybeans and alfalfa, nitrogen is converted from its unusable atmospheric form to one that can be taken in by the plant. When a legume crop is plowed, it can leave an abundance of nitrogen in the field and this can be “credited” when determining how much fertilizer to apply for the next crop (Bundy 1998).

Phosphorous encourages growth and activity of plant cells, as well as taste.  It also helps with disease resistance and strong root development.  Depletion of phosphorous can be a problem in most crop fields, since its useable form is present only in small amounts in soil. Potassium is another vital nutrient in plant development.  It is involved in amino acid production, chlorophyll and starch formation, disease resistance, flavor and size of the plant.  Potassium is also not readily available in all forms.  The soil test only tests the amount available to plants directly. 

Generally, soybeans have a lower demand for phosphorous and potassium than alfalfa.  The optimum pH for soybeans to retain these nutrients is 5.6-6.3, slightly more acidic than alfalfa’s 6.8 (Kelling 1999).  The nutrient supplying power is another factor in the properties of soil. For the test area in Sheboygan County, the phosphorous is considered to have a medium level, while the potassium is at medium to low.  The depth is considered optimum at 20-76 centimeters.  Since alfalfa reaches into this subsoil layer, it is at a prime depth for reaching these nutrients.

The purpose of this experiment was to determine what effect growing soybean and alfalfa had on the soil as compared to a fallow field.  I hypothesized the following:

·        The pH levels would be highest in the fallow field and similar in the soybean and alfalfa fields

·        The nitrogen levels would be highest in the alfalfa field and lowest in the fallow field

·        The potassium and phosphorous would be highest in the fallow field and lowest in the soybean field




Materials and Methods

I took 10 samples per field at a depth of 20 cm using a steel soil corer.  The samples were taken on October 23, 2001 from three different fields in a 40 acre area in Sheboygan County, with approximate coordinates of 87°.95 longitude and 43°.68 latitude.  Field one was fallow (for at least three decades), field two had been soybeans, and field three had been alfalfa. Each sample was placed individually in a labeled plastic bag.  The samples were spread out on a pan to air-dry overnight and tested with a Lamotte Soil Test Kit.  I used the pH, potassium, nitrogen, and phosphorous test kits to test each sample.


There were differences in the nutrients and pH of the soil in the three fields.  The pH (Fig.1) was highest (Mean=9.5) in the fallow field and lowest in the alfalfa field (Mean=6.3).  The nitrogen levels (Fig.2) were highest in the alfalfa and lowest in the fallow field.  The potassium (Fig.3) levels were highest in the fallow and lowest in the soybean field.  The phosphorous (Fig.4) was highest in the alfalfa and lowest in the fallow field.




Fig.1.  pH test results on fallow, soybean, and alfalfa soil.




Fig.2.  Comparative nitrogen levels on fallow, soybean, and alfalfa soil.


Fig.3.  Comparative potassium levels on fallow, soybean, and alfalfa soil.



Fig.4.  Comparative phosphorous levels on fallow, soybean, and alfalfa soil.




The differences in the nitrogen levels were as I had predicted.  The alfalfa and soybean fields were higher in nitrogen because a legume crop tends to leave more nitrogen in the soil then what it started with (Bundy 1998).  I had expected the pH levels of the soybean and alfalfa plots to be the same, but the alfalfa soil was less alkaline than the soybean soil. This could be from the higher amount of nitrogen present in the soil.  The potassium levels were as I had hypothesized.  The fallow field has less of a potassium requirement than the soybean and alfalfa field, so it was higher.  The phosphorus levels were lowest in the fallow field, which was not what I had expected.  Reasons for this could have been the particular plants and trees growing there naturally used more phosphorous than the crops did, or leaching affected the amount left in the soil (Kelling 1999). 

In a repeat of this experiment, I would increase the amount of soil samples taken for a better composite view of the field.  I would also wait a period of three to five days after precipitation and dry the soil more completely.


Literature Cited

Bickelhaupt, D.  1998.  Soil pH: What it Means.  State University of New York, New York.


Bundy, L.G.  1998.  Using legumes as a nitrogen source.  Extension Publications, Madison, 4 pp.


Kelling, K.A.  1999.  Optimum soil test levels for Wisconsin.