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Mr. Smarty Plants - Problem With Vegetable Garden Soil

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Sunday - June 09, 2013

From: Liberty Hill, TX
Region: Southwest
Topic: General Botany, Non-Natives, Compost and Mulch, Soils
Title: Problem With Vegetable Garden Soil
Answered by: Mike Tomme

QUESTION:

We live in Liberty Hill on 25 acres and we are working to restore native grasses and plants. We are ardent supporters of the Wildflower center. I say this because my question is not "typical" of what I follow. We have spent an inordinate amount of money putting in raised beds this year for veggies. The gourmet soil mix came from respected organic gurus. We planted in March-plants came up then atrophied. We replanted and now we have pale, emaciated "bonsai" plants. Soil tests show no nitrogen. We added manure, blood meal. The supplier does not believe us. Question: what could possibly make gourmet soil so toxic? Not one plant has grown.

ANSWER:

Normally, Mr. Smarty Plants doesn’t answer questions about vegetable gardening, but since you are an ardent supporter of the Wildflower Center, I’ll make an exception. Besides, I think I know the answer.

First let’s talk about your soil test. It is highly unlikely that your soil has “no nitrogen.” Was this a soil test from the AgriLife Extension Service or a home test kit?  If a home test kit, I highly recommend  you get  a “real” test done by AgriLife Extension.  I strongly suspect you have a nutrient imbalance and it is important that you find out where you stand with your soil.

However, I suspect that your problem lies with phosphorus rather than nitrogen. Commercial garden soil mixes typically contain a lot of compost which is high in phosphorus. The AgriLife Extension Service has a publication entitled Phosphorus: Too Much and Plants May Suffer. This is the opening paragraph of that publication:

“The buildup of phosphorus in lawns, gardens, pastures and croplands can cause plants to grow poorly and even die. Excessive soil phosphorus reduces the plant’s ability to take up required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of those nutrients in the soil.”

The report goes on to talk about corrective measures. I recommend you read the whole thing, but I’ll summarize the main steps:

1. Avoid future applications of phosphorus by eliminating organic composts and manures.

2. If nitrogen is required, use low phosphorus sources like blood meal.

3. Apply iron and zinc by foliar application.

The report goes on to discuss how long it will take to clear up the problem. Unfortunately, it will take years (3 to 5 years is their estimate).

I have personal experience with this problem. I built raised beds and blended my own soil, using lots of compost (no one ever told me you can use too much). My plants experienced problems almost exactly like you describe. I have been doing foliar applications of a solution of iron and zinc (available at many garden centers) and have been adding nitrogen to the soil (I use ammonium sulfate because my dog digs it up if I use blood meal). So far, it is working. My plants are much larger and more vigorous than before I started. But, if I get lazy and don’t do the foliar application on time, they start to droop and turn yellow. 

 

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