Soil Science/Agronomy/Horticulture 326

59 Pages·2003·9.48 MB·English

by  Phillip Barak

Laboratory Manual Soil Science/Agronomy/Horticulture 326

CONTENTS
Introduction 1
Exercise
1 Plant Response to N, P, and K 2
2 Nitrogen Requirement of Different Plant Species 7
3 Plant Response to Nutrient Sources and Soil Placement 12
4 Soil pH, pH Buffering Capacity, and Organic Matter Content 18
5 Soil Potassium Buffer Power 23
6 Mineralization of Organic Nitrogen 28
7 Tissue Testing 31
8 Total P and K concentrations in Plant Tissue 37
9 Total N in Plant Tissue 40
10 Determination of Available P and K in Soil 43
11 Determination of Soil pH, Lime Requirement and Soluble Salts 47
12 Development of Nutrient Deficiency Symptoms in Plants 53
Growing in Solution Culture
i
Laboratory Manual Soil Science/Agronomy/Horticulture 326
INTRODUCTION
During the semester, you will complete 11 of the 12 greenhouse and laboratory exercises contained
in this manual. The final exercise will be conducted as a demonstration in the greenhouse. In most
instances, you will be assigned to work jointly with another student in your laboratory section. This
provides the opportunity to exchange ideas and discuss results as you observe. However, every
student is expected to turn in individual laboratory reports and data sheets.
The 11 exercises are organized into four units with the following objectives:
Unit I: To demonstrate plant responses to soil applications of essential nutrients under
greenhouse conditions. You will study the response of various crops to applications of
nitrogen, phosphorus and potassium and compare crop responses to different sources
and methods of application of these nutrients. Exercises 1, 2 and 3 make up this unit.
Unit II: To become familiar with analytical methods for determining some of the soil
properties and processes that affect plant growth. Exercises 4, 5 and 6 make up this
unit.
Unit III: To examine plant analysis as a means of identifying nutrient disorders, verifying
the adequacy of soil fertilization, and gaining a more detailed understanding of how
plants respond to soil treatments. Exercises 7, 8 and 9 make up this unit .
Unit IV: To introduce soil analysis as a tool for assessing the fertility status of soils and for
serving as a basis for fertilizer and lime recommendation. Exercises 10 and 11 make
up this unit.
For each laboratory exercise, results obtained by each student or student pair are tabulated and
distributed to the whole class so that all of the students can see how their results fit into the “big
picture”. For several of the exercises, each student will do additional analysis of the group data. The
educational value of these exercises depends on the reliability of the greenhouse and laboratory
results of each student. Sloppy work by just a few students can destroy much of the learning value
of many of the exercises.
Grading
Your performance in the laboratory accounts for 30% of your course grade. The following factors
are taken into consideration in determining your laboratory grade:
1. Careful attention to detail in setting up and carrying out each exercise.
2. Turning in data sheets and assigned reports for all exercises on time.
3. Errors in data entry and calculations.
4. Neatness of your work area in the laboratory and in the greenhouse.
5. Regular watering of pots for each greenhouse exercise.
Subject matter covered in the laboratory will be included as a separate laboratory examination at the
time the final lecture examination is given.
1
Laboratory Manual Soil Science/Agronomy/Horticulture 326
EXERCISE 1
PLANT RESPONSE TO N, P, AND K

Nitrogen, phosphorus, and potassium are justifiably classified as primary nutrients, not only because
plants require them in relatively large quantities, but also because these are the three nutrients that
most commonly limit plant growth and crop production. Thus, unless a soil has been heavily
fertilized in recent times, it is generally possible to observe responses to N, P, and K under the very
intensive cropping that occurs in the greenhouse. This is less likely for secondary and micronutrients.
There are distinct advantages, but also distinct limitations, for studying plant response to nutrient
applications made under greenhouse conditions:
Table 1-1: Advantages and Limitations of Using the Greenhouse
for Studying Plant Nutrient Responses.
Advantages Limitations
1. Through careful control of the environment, 1. The environment is artificial. Plants may
conditions can be such that only the factor react differently in the field because of
being studied is growth-limiting. differences in factors such as temperature,
humidity or radiation level.
2. Root volume is restricted to the soil in the 2. The ratio of crop dry matter to soil volume
pot in which the plants are growing. Aside from explored by the roots is much higher in
gaseous nutrients absorbed through the greenhouse pots than in the field. Because of
stomata (e.g., S as SO ) or falling on the leaves this, nutrient deficiencies occur at a higher level
2
via atmospheric deposition (dust), all of the of available nutrients than in the field. The ratio
nutrients taken up by the plant must come from of transpiring surface to water storage in the
the soil in the pot. In the field, plants take up soil is also much higher in the greenhouse, so
nutrients from an unknown volume of surface frequent watering is required.
soil and also from the subsoil below the
fertilized zone.
3. Large numbers of treatments can be tested 3. Results obtained in the greenhouse cannot
at relatively low cost and low expenditure of be applied directly to field conditions. For
time. For the same cost in money and time, example, a soil testing method for a specific
greenhouse experiments enable one to study a nutrient may correlate well with uptake of that
much wider range of soils and soil nutrient in the greenhouse, but the critical level
amendments. for yield response must be determined from
field calibration studies.
Because of these differences between field and greenhouse conditions, greenhouse studies are
restricted largely to studies such as the relative plant availability of various forms of a nutrient or
to screening experiments that serve to develop technologies such as soil testing methods.
The full advantage of greenhouse studies cannot be realized unless special precautions are
taken to minimize experimental error. The soil must be mixed thoroughly so as to be
homogeneous, nutrients must be applied as uniformly as possible, and efforts must be made to
obtain a uniform stand of the test crop. But all of these precautions are a waste of time unless
soil moisture is controlled properly. Most potted plants grow in artificial potting medium – often a
2
Laboratory Manual Soil Science/Agronomy/Horticulture 326
Exercise 1 (Continued)
mixture containing coarse ingredients such as sand, peat, compost, vermiculite, perlite, etc. –
intended to promote rapid drainage of excess water. Leaching of nutrients is extreme under
these conditions so this practice is not suitable for studying plant response to nutrient addition.
Growing potted plants in soil (other than sand) is generally so tricky due to problems of over-
watering that it is usually not recommended to the public. We will grow potted plants in soil by
carefully controlling soil water content over a narrow range that is favorable to the plant and not
conducive to leaching. The proper soil moisture is that corresponding to the amount of water the
pot of soil can retain against drainage due to the force of gravity. This is commonly referred to as
the field moisture capacity (FMC) of the soil.
The FMC of the soil used in greenhouse studies generally cannot be guessed accurately. It must
be measured. The apparatus commonly employed is shown in Figure 1.
To measure the FMC, you need a beaker that will hold a depth of soil equal to the depth of soil in
the greenhouse pots that will be used, a glass tube, and glass wool or cotton batting. The
purpose of the glass wool or cotton batting and the glass tube is to allow air to escape when the
soil surface is flooded with water. The beaker is initially filled to approximately 1/3 the depth that
the soil will have in the pot in the greenhouse and gently tapped on a table top or in the palm of
your hand two to three times to pack the soil. The process is repeated twice more so that the
final soil depth is approximately equal to that in the greenhouse pot. Water is then added quickly
to the soil surface to get complete coverage of the soil surface. The amount of water added
should not wet the soil to more than about 1/3 of its total depth in about 5 minutes. The beaker is
then covered with plastic sheeting to prevent water evaporation.
After 24 to 48 hours, the soil is examined. If it is wet to its entire depth, too much water has been
added and the whole procedure needs to be repeated. If wet to approximately 80% of its depth,
a soil sample weighing 20 to 50 grams is removed from the middle portion of the wetted zone.
The sample is weighed to +/- 0.01 g and dried for 24 hours at 105 oC. The dried soil weight is
then determined and the soil’s FMC is calculated.
Calculation of FMC

Soil moisture is always expressed on a dry-weight basis. The formula used is:
% H O = (weight H O) x 100%
2 2
(weight dry soil)
= (weight wet soil) (cid:33) (weight dry soil) x 100%

(weight dry soil)
(weight wet soil) = % H O x (weight dry soil) + (weight dry soil)
2
100%
These formulas are used to calculate the soil weights needed in steps 2 and 8 of this exercise.
3
Laboratory Manual Soil Science/Agronomy/Horticulture 326
Exercise 1 (Continued)
Figure 1-1: Apparatus for estimating field moisture capacity of soils
for greenhouse studies.
4
Laboratory Manual Soil Science/Agronomy/Horticulture 326
Exercise 1 (Continued)
Materials
The materials needed are: soil passed through a 2-mm sieve, air-dried, and mixed; pots; pot liners;
balances; plastic sheets; nutrient sources (nutrient treatments are shown in a separate hand-out),
seed; labels; marking pens; deionized water .
Procedure Remarks
1. Select three pots and line each with a 1. Individual pots should weigh within 10 g of
plastic bag. each other.
2. Compute the weight of air-dry soil that is 2. The air-dry soil contains _____ % water
equivalent to 1500 g of oven-dry soil. This (get this value from the instructor).
weight is ________ g.
3. Weigh the amount of air-dry soil calculated 3. Weigh the soil to +/(cid:33) 10 g.
in step (2) into each pot.
4. Spread the soil from the first pot onto the 4. The added nutrients should be uniformly
sheet of plastic provided and add the nutrients distributed throughout the pot.
for the treatment assigned to you. Thoroughly
mix the treated soil and return the soil to the 5-8. Pot label
first pot.
Required Information Example
5. Label the container with your name(s), lab
Name(s) ______________ Jane D. & John Q.
section, and amount of the variable nutrient
Replicate (A, B or C)_ ___ Pot A
(N, P, or K) in mg/kg.

Lab Section ___________ Section 301

Nutrient added_____mg/kg 150 mg K/kg
6. Repeat steps 4 and 5 for the remaining pots.

The total weight is comprised of:
7. Randomly label the pots A, B, or C.
Pot, plastic bag, and label ________ g
Oven dry soil ________ g
8. Compute the weight of the container when
the soil is adjusted to its field moisture Water at FMC ________ g
percentage (FMC) of %. Include this
total weight on your pot label. Total weight ________ g
9. Remove about a cup of soil from the sur-
9. Save the soil for step 11.
face of pot A and level the remaining soil.
10. Add approximately 3/4 of the water that will
10. If all of the water were added to the soil
be needed to bring the soil to FMC. (A specific
surface (step 15), the soil at the surface could
volume of water is not required at this point as
become disturbed and some of the seeds
long as FMC is not exceeded.)
uncovered.
5
Laboratory Manual Soil Science/Agronomy/Horticulture 326
Exercise 1 (Continued)
Procedure Remarks
11. Place ____ seeds of corn on the soil and 11. Wait until all of the water has infiltrated
cover with the soil just removed. the soil. The instructor will tell you how many
seeds to plant.
12. Place the pot on the balance and add 12. Hereafter, all watering will be done by
deionized water to adjust the container to the weight using deionized water. Tap water
total weight computed in step (8). Use a contains Ca, Mg, Fe, N and other unknowns.
graduated beaker.
13. Fold the plastic liner over the soil surface 13. As soon as plants emerge, uncover the
to minimize water loss by evaporation until the plastic bag from the soil surface and fold down
corn germinates and emerges. over the outside of the pot.
14. Thin to 4 seedlings per pot (or other number 14. Shake the soil from roots of the seedlings
as directed by your lab instructor). removed back into the pot; discard plants
removed. The remaining 4 plants should be
spaced uniformly in the pot.
15. Water the pots to the computed weight 15. Water loss by evapotranspiration will be
three times weekly during the first two weeks low the first two weeks until there is significant
and daily thereafter. leaf surface area. Notice changes in water use
between cloudy days and bright, sunny days.

Continue watering until harvest time! Harvest after completing Exercise (7).
16. At the designated time, cut the plants at 16. Save all plant parts including desiccated
the soil surface and place in the paper bag leaves that may have fallen off during harvest.
provided. Label the bag and place in crop drier. Label the bag with the same information as on
Dry at 55 oC. the pot label.
17. After the plants have dried to constant 17. For research, the samples would be kept
weight, record the dry weight of the plants plus in the drier until weighed to avoid absorption of
bag to +/(cid:33) 0.01 g. moisture from the atmosphere.
18. Grind the samples and put the ground 18. Grind to pass a 20-mm screen. Samples
tissue into labeled plastic bags. You will will be taken for Ex. (8) in step (20) and for Ex.
analyze the tissue for N, P and K (Ex. 8 & 9). (9) in step (21).
19. Weigh each empty paper bag and calcu- 19. Subtract the weight of each empty bag
late the dry weight of the tissue. from the weight of tissue + bag. Fill out the
Data Sheet and hand it in.
20. Weigh 150 to 200 mg of each ground 20. Record the weights on the Data Sheet for
sample into a 50 ml beaker for P & K analysis Exercise (8). These samples will be ashed
in Exercise (8). prior to analyzing for P & K.
21. Weigh 100 to 150 mg of ground plant tissue 21. Record the weight to +/(cid:33) 1 mg. The
and transfer quantitatively to a dry digestion digestion tube should be dry so that the tissue
tube for nitrogen analysis in Exercise (9). will not adhere to the neck.
6
Laboratory Manual Soil Science/Agronomy/Horticulture 326
EXERCISE 2
NITROGEN REQUIREMENT OF DIFFERENT PLANT SPECIES
Nitrogen was established as an essential element in plant nutrition in the 19th century. Plant
response to nitrogen is manifested in the production of vigorous plant growth with dark green leaf
color. Nitrogen is an important constituent of the chlorophyll molecule as well as amino acids,
proteins, nucleotides, nucleic acids, amines, and amides.
The plow layer of most soils contains nitrogen mainly in the organic form, ranging from 0.08 to 0.4%
(1,600 to 8,000 lbs per acre plow layer). Over the growing season, only 2 to 3% of this organic
nitrogen is made available to crops under Wisconsin climatic conditions. Soils low in organic matter
will supply very little nitrogen. Continuous cropping without replacement of nitrogen reduces a soil's
ability to supply nitrogen; thus, the need for nitrogen fertilizers to supplement natural supplies.
Nitrogen is taken up by plants as nitrate (NO -) or ammonium (NH +) ions. Most plants can utilize
3 4
both forms of nitrogen in their growth processes. An imbalance of nitrogen or an excess of this
nutrient in relation to P, K, and S prolongs the growing period and delays maturity. Too much
nitrogen produces succulent plants, which makes them more susceptible to disease. Some plants
show weakening of stems causing lodging.
Nitrogen requirements vary among plant species. In this exercise, you will determine the optimum
nitrogen rate for biomass production under greenhouse conditions by different plant species. In the
field, the nitrogen concentration varies with stage of maturity and portion of the plant sampled. The
nitrogen concentration of most plant parts decreases as the plant matures. When nitrogen is the
yield-limiting factor, chlorophyll in the lower leaves breaks down and nitrogen is translocated to the
upper leaves. Thus, deficiency symptoms for this element show up first on the older leaves. The
range in nitrogen concentration in the leaves of several crops is shown in the accompanying table.
Table 2-1. Nitrogen Concentration in the Leaves of Various Crops
Crop Nitrogen range
(%, dry wt. basis)
Vegetable Crops (Geraldson and Tyler, 1990)
Celery 2.5 - 4.0
Kale and collards 4.0 - 5.0
Lettuce 2.5 - 4.0
Onion 1.5 - 2.5
Pea 3.1 - 3.6
Pepper 3.0 - 4.5
Potato 3.0 - 5.0
Spinach 4.0 - 6.0
Sweet corn 2.6 - 3.5
Sweet potato 3.2 - 4.2
Tomato 2.5 - 6.0
Turnip 3.5 - 4.5
Watermelon 2.0 - 3.0
Cotton (Sabbe and Zelinski, 1990) 3.0 - 4.3
Peanut 2.7 - 3.8
Soybean (Small and Ohlrogge, 1973) 4.3 - 5.5
Sugar cane blades (Bowen, 1990) 1.5 - 2.7
7