Soil and Plant Nutrition and its Affect on Giant Ragweed Growth and Proliferation

In the farming world there are several weed species that are more
prevalent and challenging than others. Giant Ragweed (Ambrosia
trifida) is one of those challenging weed species. In some
areas it has adapted to germinate all summer long, and it is
becoming increasingly resistant to many herbicides. In the
certified organic commodity crop world it presents unique
challenges because of deep germination, rapid spring growth in
cool weather, and in some places the ability to germinate all
season long.   

 

It has often been conjectured that different weeds are triggered
to grow during an excess of nutrients (to hold or bind them) or
to accumulate nutrients that are deficient in the soil. For many
weed species farmers have been able to verify anecdotally that
this theory may be generally true. However, experience related to
giant ragweed has not always fit the general information found in
resources that try to match weeds with soil conditions.

 

This study seeks to understand if there are specific soil
conditions that encourage giant ragweed growth, either from
excess or deficient nutrients, and to determine if in season
fertility treatments could assist in giant ragweed control. 

 

I will pick 3 fields with giant ragweed pressure, each with
different soil types. At the beginning and end of the season Soil
Tests will be taken in the chosen weedy spot and in a nearby area
of the field without weeds growing. 

Throughout the season Sap Tests will be taken of the cash crop
growing in a "clean" part of the field, cash crop growing in the
weedy patch, and the giant ragweed itself. Sap tests are
considered more accurate than tissue or soil tests for showing
which nutrients the plants are actively taking up and using. Sap
and Soil tests will be compared between fields to spot any
patterns.

 

Working with a consultant, I will identify any nutrients that are
accumulating differently within the ragweed or appear deficient
or different in the cash crop within the weedy area. Any
deficiencies or nutrients out of balance will be addressed by
foliar applications of fertilizer. 

Sap tests and foliar applications will take place approximately
every 4-5 weeks to monitor crop progress. 

 

Following is the pattern of testing and foliar applications:

1. Soil test in May

 

2. v2-3 Sap test (Late May-Early June.) 

    -cash crop in weedy area

    -cash crop outside of weedy area

    -giant ragweed

2a. Foliar feed half of the weedy area based on test results

 

3. Approximately 4 weeks later (late June-early July)

sap test:

    -cash crop in clean area

    -cash crop, no foliar applied

    -cash crop with foliar applied

    -ragweed with foliar applied

   -ragweed, no foliar applied

3a. Foliar feed based on test results within the fertilized area.

 

4. Approx 4 weeks later (Late July-early Aug)

Repeat step 3.

 

5. Approx 4 weeks later (Late Aug- early Sept)

One final sap test to determine any lingering imbalances. 

 

6. Soil Test (at a similar time as the final sap test)

    -Clean cash crop area 

    -Weedy area without treatment

    -Weedy area with foliar treatment

 

7. Measure and document final ragweed biomass, hand check cash
crop yield to determine any difference in weed impact.

 

Objective: To discover if giant ragweed grows in certain soil
conditions related to available soil nutrients, and to discover
if in-season applications of specific fertility treatments can
reduce giant ragweed vigor or prominence. Ragweed biomass and
visual health will be cataloged at the end of the season to
document differences in ragweed vigor after using targeted
fertility treatments. Cash crop yield checks will be done to
compare weedy cash crop without treatment to weedy cash crop with
treatment.  

 

If giant ragweed is triggered to grow in soil conditions that
have excess or are lacking in certain nutrients it should show up
strongly on the sap test. Example: if giant ragweed sap tests
read high in manganese, but the nearby cash crop reads low, a
foliar application of manganese would be applied that would help
the cash crop, and potentially overload the ragweed, or tell it
that the deficiency that caused it to grow has been
mitigated. 

Foliar applications that address these conditions within the
plant may be able to stunt the ragweed growth or potentially even
undermine the plant health, because the conditions that were
ideal for the weed will have changed internally.

 

This research will look at multiple fields with different soil
types (clay, loam, sand), to determine if there might be a common
nutrient imbalance that triggers giant ragweed to flourish. 

 

Sustainable practices: foliar feeding to address short term,
in-season imbalances, as opposed to dry fertilizer that might be
more prone to leaching or runoff.  All fields will be
certified organic and no herbicides will be used.