Growing Organic Blackberries in the Desert Requires an Emphasis on Soil Microbiology

Who says you can’t grow blackberries in the desert? 

South of the Organ Mountains, near Vado, New Mexico, soils are characterized by heavy calcium carbonate (aka caliche) deposits on or near the blackberries-in-alkaline-soil-04152015surface.  This caliche becomes sticky when wet, binding together to form an impervious, concrete-like layer that water cannot easily drain through. The natural soil pH is between 8 and 8.5, and the native vegetation is dominated by tarbush, creosote bush, and other desert shrubs. This alkaline soil can dominate desert environments, but it is not the kind of soil experienced growers recommend for blackberries.

Most gardening guides indicate that blackberries require well drained soils with a pH of 6.5.  Such acidic pH is rare in the Chihuahuan desert of Southern New Mexico.  High pH can be problematic for growers, because at a high (alkaline) pH, soil nutrients become unavailable to most plant species.  This is one reason why heat-loving plants that are popular in the deep south, like magnolia trees, may become disease prone and chlorotic (yellow) when growing in the southwest.  The same can be expected for blackberries.  However, by managing soils for microbial biodiversity and abundance, blackberries, magnolias, and other, more acid loving plants may thrive, even in the desert. Ironically, successful establishment of robust healthy plants is the best way to sustainably reduce soil pH.  This is because plants themselves release organic acids continuously.  So do the microbes that make nutrients available to the plants.  For this reason, the trick for growing blackberries in the desert is to enrich the soil with a lot of high quality organic matter, grow other plants on the site, and allow time for soil microbial communities to develop.  Questions many of us have include:What kind of organic matter is “good quality?”
How much organic matter is enough?
How much is this going to cost me?

One grower decided to test these limits by utilizing the cheapest, most readily available sources of organic matter to prepare the soil for blackberries, and monitor soil development over time to assess progress.   The end goal?  Blackberries that require only additions of water and of materials available on site.  Here we summarize progress over three years.  The photo on the left is of a blackberry plant growing only meters away from where the upper photo was taken.  The soil for the blackberries was prepared by covering the existing soil with 4 inches of composted mulch (obtained for free from an urban yard waste composting facility) and tilling it into the top 8 inches of soil.  The reader should note that tilling in this situation is not always necessary, and may not always be beneficial.  Simply covering bare soil with organic mulch, and digging only where plants are to be inserted is much less disruptive to those soil microbial communities that are going to help your plants survive in the desert.

For the blackberries shown here, a microbial density and diversity analyses (MDD analysis) was run prior to planting to assess the quality of 1) the soil and 2) the composted mulch that was added to increase soil organic matter. Both assays indicated an absence of beneficial fungi, nematodes, and other soil microbes that support plant growth.  The absence of microbial biodiversity in the soil was attributed to the arid conditions and the absence of soil organic matter, including humic acids.  In the composted mulch, the absence of biodiversity was attributed to the composting process used (many composting facilities utilize raw materials and composting conditions that do not support adequate growth of beneficial fungi) and to compost storage procedures.  This grower chose not to add the recommended humic acids or commercial sources of mycorrhizal fungi, and was content to wait for naturally occurring fungi to establish themselves.  Two drought tolerant blackberry varieties, Arapaho and Navaho were planted in May of 2011.

  •  Navaho spelled with an “h” looks like a typo to readers in the Southwest, but it is not.  The plant cultivar was developed in Arkansas, and released with the spelling: Nava”h”o.  One might presume that either the plant breeder did not consult with Navajo before naming the cultivar, or s/he decided to spell the word with an “h” so that people from eastern states would not be inclined to pronounce it “Nav-a-joe”.

Lavender, asparagus, and grapes were planted within 5 meters of the ten original blackberry canes. These perennial plant species are somewhat more tolerant of alkaline soils than blackberries, and their growth adds viable organic matter that supports beneficial fungi.  The fungi then help to condition the soil for blackberries. After planting, the soil was covered with about two inches of leaf and twig clippings taken from nearby native shrubs.  These clippings act as a mulch to protect the soil and conserve moisture.  They also contain microbes that help plants adapt to harsh desert environments. Each year, new clippings from nearby native and cultivated plants are added to the soil. Native astragalus, a low-growing legume, appeared on its own within months after planting.  Not every weed is bad.  The astragalus, which grows close to the ground, does not compete with blackberries for sunlight.  It was allowed to remain because legumes add nitrogen to the soil.Within weeks of planting the blackberries, chlorosis (yellow leaves with green veins) was visible in both varieties.  This was anticipated, based on the absence of beneficial microbes in the compost. However, the appearance of new leaves suggested plants were establishing.  At this stage, the grower had the option of intervening with commercial supplements to achieve more rapid growth.  However, the decision was made to simply give the plants and soil time to build their own productive microbial communities.  Bi-annual soil biology monitoring (MDD analysis) was used in conjunction with plant growth monitoring to assess progress.  The analysis revealed a slow increase in the number of fungal spores detected, and by the end of the third year, beneficial nematodes and earthworms were observed for the first time. Chlorosis continued to be problematic, particularly in late summer, as fruits were ripening.  Nutrient deficiencies impacted fruit development, but plant growth was vigorous.  So many new canes had appeared by year three that many had to be removed due to over-crowding.

This vigorous growth is encouraging, since biomass helps to restore the soil.  The grower chose to reject conventional wisdom about allowing debris to build up, and clip pruned canes into one inch segments, returning them to the soil to add nutrients.  The nutrients the canes could return to the soil were deemed more important than the potential for healthy cane clippings to harbor disease. Today, the blackberry plants shown are in their fourth year, and diverse microbial populations are evident in the soil.  Soil microbial density and diversity detected via MDD analysis has increased each year, though total microbial populations still have not reached optimum levels.    To date, no agrochemicals have been added to the system.  All trimmed branches and debris from surrounding plant materials are consistently returned to the soil. Heavy budding is present and a rewarding summer harvest is expected.     

This grower’s decision to monitor the natural changes in microbial populations that occur over time has illustrated three important points:   

  1. Diverse soil microbial populations are increasing slowly and naturally under the current management scheme.
    Since soil microbial populations and organic matter has not yet reached adequate levels, the crop may continue to experience nutritional deficiencies that impact yields.
  2. The combined observations of vigorous plant growth, periodic nutrient deficiencies, and poorly developed soil microbial communities suggests that nutrients to support plant growth are present, but are not uniformly bioavailable.  This hypothesis could be confirmed with a soil chemistry test.   If nutrients are present but not bioavailable, deficiencies will be eliminated as soil microbial community function is restored.
  3. The MDD analysis utilized here has provided a useful tool for guiding this grower’s decision making.   Trends show increasing microbial diversity and abundance, and suggest plant productivity and soil health will continue to improve with time.  If the grower is interested in harvesting blackberries this summer, he should add commercially available humic acids in combination with nitrogen rich, biochemically complex organic compounds to accelerate the growth of healthy microbial communities.  Bioactive compost (that contains optimum microbial diversity) offers an alternate option.

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