Articles by Bob Denman


The Question

Posted on 5/14/2015 to How to articles.....
Girl Watering Garden - Red Pig Tools

“Can I use this 5-5-5 fertilizer for everything?”

“My roses don’t look too good.  What should I feed them?”

“Which fertilizer is the best for citrus?"

“Last year I had very few tomatoes.  What should I put in my soil to have good ones this year?” 

“I have clay soil.  What should I feed my plants?”

“Is there something I can spray on everything to make stuff grow better?”

The question comes in endless variations, but is, nonetheless, the same question.  It is asked every day at my store and at 50,000 nurseries and garden centers coast to coast.

I’m willing to bet that, more often than not, people get the wrong answer “We recommend Garden Fairy’s Megamaxibloomglop.  Just sprinkle it on, one cup to each plant, and stand back so the sudden burst of growth doesn't knock you down.”

Or, “You need rose food for the roses, citrus food for your orange tree, vegetable blend for your victory garden and this little box of wonder for your herbs.  And be sure to feed your azaleas and camellias with this acid blend.  That’ll be $195.47, please.”

There is, in truth, only one correct, honest answer to the question.  

“I don’t know.”

Before telling someone what to add to soil to improve it, you have to know what their soil has, and lacks, in the way of nutrients.

There are dozens of diagnostic questions to ask before making a fertilizer recommendation.  Many of them have to do with soil type and structure, plant type, watering habits, and the symptoms of poor plant health that gave rise to the initial question.  And, often, the real problem isn't a lack of nutrition but something else altogether.  Inappropriate plant choice or siting, for example, or poor watering habits.  In those cases, the recommended remedy may not involve soil amendment or fertilization at all.

However, if poor plant nutrition is the suspected problem, then an analysis of the soil’s available primary nutrient levels should precede any recommendation.  Without such an analysis, the guru making the recommendation has as much chance of being correct as he does of hitting a high-flying bat on a moonless night with a BB gun.

Which brings us to a tool, an analytical tool, one every gardener should have and use—a soil test kit.

There are several such kits available.  All of them enable you to determine the relative acidity or alkalinity (Ph) of your soil and the available levels of nitrogen (N), phosphorus (P) and potassium (K).

These test kits are easy to use.  We’re talking junior high school chemistry here, nothing more complex.  In every case, you take a little soil from your garden, put it into a glass container along with some distilled (Ph neutral) water, and shake.  Then depending on the type of kit you bought, you follow one of two procedures.  With one type, you dip four especially coated test strips, one each for Ph, N, P and K, into the slurry you've made.  Each strip turns a color.  You then compare the color to a chart furnished with the kit.

The color match gives you the level of available nutrient, expressed as excess, sufficient, adequate, deficient or depleted.

The other type of kit requires you to extract a small amount of the water you have mixed with your soil.  This is done with a pipette that comes with the kit.  You place extracted water samples in each of four color-coded plastic vials.  You then add chemicals contained in four color coded capsules to the corresponding vials, blue capsule into blue vial, green into green, and so on.  Then you shake each vial, let it settle for a few minutes and compare the color of the liquid to colors printed on the vial.

Once you know what your Ph, N, P, and K are you can choose the appropriate fertilizer.  You may need nitrogen only, in which case blood meal may be the right choice.  Or you may be deficient in potassium and need to dose with wood ash or sul-po-mag or greensand—your choice varying with the degree of deficiency.

That 5-5-5 may be just fine or you may be better off with a 4-5-2.

 For example, if you have a deficiency of nitrogen but an excess of phosphorus—not uncommon in many Orange County soils—using 15-15-15 fertilizer may solve your nitrogen problem but it will exacerbate your excess of phosphorus and amplify the imbalance of your nutrients.  Don’t forget that imbalance of nutrients can be just as deleterious to plants as excesses or deficiencies.

There is a final reason why knowing the Ph and N, P, K levels in your soil is so important.  If they’re all at acceptable levels and in reasonable balance, there’s a good chance your problem may be a deficiency of some trace element such as iron, boron, copper, magnesium, calcium or one of umpty other “ums” necessary in minute amounts for optimum plant health.  Fortunately, you don’t have to do a lot of testing to identify which ones you’re missing.  You simply add some rock dust that has about 80 or so constituent elements.  But that’s another column.


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Date 12/5/2017
Christine Arthur
When I was reading about your tools, I read that you are writing a book about tools. Has it been published?

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