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News from the Edible Demo Garden

June 2021: Fertilizing Your Soil

May found our team tucking summer harvest seedlings into our beds and anxiously monitoring them to make sure they have everything they need to thrive.  Fertile soil is a must and the primary nutrients needed are N, (Nitrogen) P (Phosphorous), and K (Potassium).  All fertilizers sold are labeled with the content of these three nutrients as a percentage of the total nutrients supplied. 


Nitrogen -  Of the three primary nutrients, California soil is most likely to be deficient in nitrogen.  This deficiency is increased with the rainy season when nitrogen can be leached from the soil.  Nitrogen is very important for photosynthesis; the most prominent symptom of nitrogen deficiency is chlorosis or yellowing of older leaves.

Phosphorus.  California soils usually contain phosphorous naturally, although highly-weathered soils can be deficient.  The symptoms of a phosphorous deficiency include stunted growth, purplish tint on leaves, and/or poor fruit or seed development. 

Potassium.  Additional potassium is usually not needed in California soils.  However, if deficient, plants can experience slow growth, browning with dieback of leaf edges and tips, weak stalks, and small fruit.    


NPK Testing

Prior to planting, a quick test of the NPK in your soil can alert you to the need for fertilizer.  At most nurseries, you will find simple testing kits for NPK that also test pH.  You will feel like a chemist as you shake water and tablets in test tubes and look at color charts to determine nutrient levels in your soil.  This process can be intimidating for some.  Many gardeners just watch their plants carefully and based on plant symptoms, they get a sense for what is needed.  So what choices do you have when you determine a nutrient is needed?


Fertilizer Types

Inorganic:  These fertilizers are composed of minerals and synthetic chemicals.  They are fast acting and low in cost.  But they contain salts and can leach and burn crops if applied incorrectly.  They do not have the ability to improve the soil.

Organic:  These are derived from plant or animal products.  They are bulkier and more costly than inorganic fertilizers and can have odors.  They are also slower to act. But besides adding nutrients, they work more naturally, improving the soil by decreasing density, which increases water filtration and the soil’s ability to hold nutrients. 

Manures:  These contain animal excrement, plant remains, or both.  They are a good organic fertilizer and they improve soil structure.  Many gardeners raise chickens and they get the benefit of chicken manure, the most concentrated NPK source of any type of manure. 

Fertilizer Use on a Certified Organic Farm

Because we are located on a certified organic farm, the Edibles Demonstration Garden (EDG) can only use organic fertilizers that in most cases are labeled OMRI Certified.  OMRI stands for the Organic Materials Review Institute and is a nonprofit organization that maintains lists of products that are FDA approved for use on certified organic farms. 

The EDG most often uses compost to improve our soil.  But compost alone does not provide the amount of NPK needed for heavy summer feeders like tomatoes.  We also use worm castings from our worm bins.  The approximate NPK percentages for worm castings are 5-5-3, which is much higher than compost.    

In the past few months, we have used fish emulsion (4-1-1) as a nitrogen boost for our artichoke plants and worm compost ‘tea’ for our seedlings that had yellowing older leaves.  We added blood meal (12-0-0) to our raspberries and a balanced fertilizer (7.5-5-7.5) to our fruit trees.  Artichokes, raspberries, and fruit trees are perennials.  Edible perennials often have specific fertilizer requirements especially in the spring so pay attention to their fertilizer needs also. 

As you tend to your garden this summer, look for symptoms of nutrient deficiencies.  Try to choose fertilizers that mimic the way nature provides nutrition for plants. The nutrient boost may take longer for you to see the benefits, but natural fertilizers pay off in the longer term.