MODULE 6: DEMONSTRATION 1
THE EFFECT OF NITROGEN FERTILIZER AND MANAGEMENT ON NODULATION AND
GROWTH OF LEGUMES
PURPOSE
n Show how mineral nitrogen in soil influences nodulation of legumes.
n Demonstrate that nitrogen in the soil eliminate the differences in plant growth due to rhizobia.
n Demonstrate the effect that management has on nitrogen requirements of legumes and BNF.
CONCEPTS OF THE DEMONSTRATION
Modules 4 and 6 discuss the role of mineral nitrogen on legume BNF. Legumes form an effective symbiosis with rhizobia when the plants need for nitrogen is not met by the nitrogen in the soil. When there is abundant nitrogen in the soil from mineralized organic matter (Module 1) or from fertilizer, there may be no nodulation on legumes, even if they are inoculated with effective rhizobia. Legumes prefer to use available mineral nitrogen rather than form nodules and fix atmospheric nitrogen with rhizobia.
Whether legumes form nodules and fix nitrogen is dependent on the balance between the supply of mineral nitrogen in the soil and the nitrogen requirement of the legume. When there is less mineral nitrogen in the soil available than the plant requires, and there are effective rhizobia in the soil, the legume crop will fix nitrogen. If management affects the nitrogen requirement of the legume, nodulation and BNF will also be affected. Poor management reduces the nitrogen required by the plant for growth and therefore reduces BNF. It is important for extension agents and farmers to realize that good management will increase BNF.
FARMER RECOMMENDATIONS FROM RESULTS OF THIS DEMONSTRATION
Farmers should avoid applying fertilizer nitrogen to legume crops since it may reduce the benefit they obtain from BNF.
Although fertilizer nitrogen produces healthy legume crops, the farmer may lose money by applying fertilizer nitrogen rather than inoculating with effective rhizobia.
The benefit that farmers obtain from BNF increases when management can increase growth and yield of the legume crop.
CONDUCTING THE DEMONSTRATION
Soybean is usually good for this demonstration. Soybean rhizobia are not widely distributed in the tropics which will make the differences in strain effectiveness and nitrogen effects easy to demonstrate. BNF by all legumes is affected by management and mineral nitrogen, and the observations that are made in this demonstration are applicable to other legumes.
Treatments
The test legume used in this demonstration requires soil that is free of rhizobia. Refer to Module 7 Demonstration 2 and Methods in LegumeRhizobiumTechnology for information on conducting a pot trial with soil.
Strain Selection. Select strains of rhizobia that differ in effectiveness (see
Module 4). NifTAL can supply effective strains for many species of legumes.
Management Treatments. Select a soil that is known to have low fertility or pH problems. Consult local soil scientists and extension agents about management strategies to increase yield in soils common to your region.
Follow the guidelines for maximum management treatments in Module 7 Demonstration 2 or use local recommendations for the soil you are using for the demonstration.
In this Demonstration, as in the field trial (Module 7 Demonstration 1), Maximal Management refers to management to maximize growth of the legume in the pot. Farmer management refers to no inputs other than the inoculation and nitrogen treatments used in this demonstration. Nitrogen refers to the addition of fertilizer nitrogen in amounts that will reduce BNF
Table D6/11. Treatments For Soybean
USDA 110 / High / +
USDA 110 / High / ─
USDA 110 / Low / ─
Uninoculated / High / +
Uninoculated / High / ─
Uninoculated / Low / ─
Harvesting the Demonstration
Observations on plant growth and leaf color should be made and recorded.
Plants should be cut at the soil level and the weight recorded if
experimental results are of interest.
The root systems should be removed from the soil carefully so nodules
are not detached. Observations should be made on the size and interior
color of the nodules. Nodules should be removed, counted and weighed.
Record the data:
Treatment / Leaf color / Shoot weight / Color / Number / Weight
USDA 110
High + N
USDA 110
High - N
USDA 110
Low - N
Uninoculated
High + N
Uninoculated
High - N
Uninoculated
Low - N