Fertilizer Placement Affects Weed Germination and Crop Growth

Introduction

There are three methods used to apply controlled release fertilizers (CRFs) to container crops: topdressing, incorporating, and dibbling.  Currently, topdressing and incorporating fertilizers are the most common methods.  Dibbling fell out of favor with fertilizer manufacturers and growers due to concerns of plant injury with roots being so close to the critical mass of fertilizer.  

Dibbling use to be a common and recommended practice.  Sierra Chemical (now Scotts) use to provide growers with a technical bulletin called "Dibble Application of Osmocote or High-N CRFs (Bulletin #30)".  When a few cases of injury occurred because of dibbling fertilizers, manufacturers and subsequently growers stopped dibbling fertilizers completely.  This occurred despite the fact that it was a very successful fertilization method that generally resulted in better crop growth with lower fertilizer rates and less nitrogen leaching compared to topdressing and incorporating fertilizers.

Work by Sven Svenson demonstrated that incorporating fertilizers resulted in fewer liverwort infestations compared to topdressing.  He determined that liverwort thrive in high N and P environments, and by incorporating fertilizers you remove most (though not all) of the N and P from the container surface.  Perhaps dibbling fertilizers would even further reduce available N and P and further reduce liverwort growth.

A Few Mistakes That Prove A Point

First, my technician wanted to find a better way to collect weed seed (we collect weed seed regularly to use in experiments, I know that sounds weird).  She used a method of floating seed as a way to separate the seed from other debris.  To compare her method to mine, she placed some seed in containers to test seed viability.  She filled up one gallon containers with 100% Douglas fir bark amended with micronutrients, then overseeded the bittercress.  I told her to topdress with Osmocote fertilizer, but she didn't listen.  Within 1 week all the seed germinated........ but 2.5 months later, the weeds were still at the cotyledon stage.  The photo to the right is a picture of the bittercress seed taken 2.5 months after germination.  Without fertilizer (N, P, and K), the weeds appeared to have germinated just fine, but failed to grow beyond the cotyledon stage.  Note that there was sufficient micronutrients available to the seedlings, they were only lacking N, P, and K.



A second mistake occurred.  I decided to put together a demonstration for our September field day (2002) that evaluated all the common preemergence herbicides used in container production.  We potted up 3 species, topdressed them with 12 g of Osmocote 18-6-12, overseeded them with either bittercress, oxalis, or annual bluegrass, and then applied to each container one of 14 herbicides.  By 45 days after herbicide application, no herbicide provided control of any of the weed species.  Most herbicides should provide control for at least 80 days, but at 45 days all had failed, including our most commonly used products at the maximum recommended rate (Rout, OH2, Snapshot, Regal O-O, Ronstar, and others).  


The Experiment

We decided to test these ideas with a controlled experiment.  In all, we conducted 5 experiments  which all had the same general results.  Below is a description of one of the experiments performed by our summer intern, Kathy Von Arx.

We used the follow experimental setup:
We applied the following herbicide and fertilizer treatments, each to 10 containers:

Snapshot rate (lb/acre)
Fertilizer placement
0
topdressed
0
incorporated
0
dibbled
75
topdressed
75
incorporated
75
dibbled
150
topdressed
150
incorporated
150
dibbled

We used a maximum of 150 lbs of Snapshot because the label recommends this rate for oxalis control (the species we overseeded with).  We also used 1/2 of that rate and no herbicide to see if fertilizer placement along with low or no herbicide could be used to provide adequate oxalis control.

The Results

Weed growth was dramatically improved by dibbling fertilizer (Chart 1).  Even at the recommended herbicide rate (150 lb/acre Snapshot), weed control was improved by dibbling the fertilizer instead of topdressing.  Most surprising was the high level of weed control obtained when no herbicide was used.  With no herbicide, topdressing and incorporating fertilizers resulted in about 60% weed control (not commercially acceptable) while dibbling fertilizers resulted in over 95% weed control.






Growth of lavender was also affected by fertilizer placement (Chart 2).  Regardless of herbicide rate used, dibbling fertilizer resulted in larger plants compared to topdressing or incorporating fertilizers.  The authors noticed at the beginning of the experiment that plants with dibbled and topdressed fertilizers began growing more quickly than those with incorporated.  This was likely the result of an available supply of nutrients in containers that were topdressed or dibbled,  and a more diluted supply of nutrients in containers where fertilizers were incorporated.




Despite the result of larger plants with dibbling, roots of plants that were dibbled were smaller than those that were incorporated and slightly smaller than those that were topdressed (Chart 3).  Root evaluations were done by subjectively rating the % of the rootball-container interface that were covered by roots.  This may not be the best measure of root growth, but it does serve as a good indication if  a production practice influences root growth.



While root ratings were lower for dibbled plants where,  it was noted that all plants had root systems that would allow them to be marketable.  

roots

Summary


If you are considering dibbling fertilizers to improve your weed management program, consider the following suggestions:


With proper irrigation management (next year's research) and proper fertilization management, weed control can be drastically improved without changing your current herbicide program.



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