Exercise and Diet for your Soil

My last garden before I moved to Colorado was a backyard plot in a quaint English village.  Founded by the Vikings, the village was dominated by a Norman church with buildings dating back to the 16th century.  My garden topsoil was deep and fertile.  Cultivated for more than a 100 years, it had been a victory garden through both World Wars.  With two feet of fertile loam, it never crossed my mind to do more than compost, dig and plant. 

Rocky Mountain gardening was a shock.  I had to start from scratch -- create topsoil, amend, enrich and nurture to make anything grow.  The essential thing I learned was that soil is alive, more like a colony of bees than a pile of ground up rock.  As a living organic system, soil cycles and recycles nutrients and energy. It maintains a constantly changing equilibrium between all of its organic and inorganic ingredients.  The key to soil fertility is to foster and maintain that amazing living ecology.

In last month’s column I suggested some simple methods to get a feel for your soil and its needs.  Hopefully you have a better sense of your soil’s composition: humus, minerals, water, air and soil life.  The next step is to develop an individualized health program consisting of exercise and diet.  You exercise your soil through different methods to cultivation.  You feed your soil using the necessary minerals, amendments and inputs to support or restore its optimum health. 

As with all health programs, moderation is key.  More recent ideas on organic cultivation tend toward minimal disturbance of the soil.  Constant churning of the topsoil breaks up delicate fungal networks; the living soil web has to rearrange itself after each disturbance.  Often an initial deep cultivation is required to get rid of weeds and loosen compacted soil.  After that, shallow tilling, crop rotation, organic mulches, green manures and layering with compost may be all that is required.

In general, nothing should be added to the soil that could be toxic or over-balancing.  Too much chemical nitrogen (N) and phosphate (P) will severely damage soil life.  Even adding quantities of natural materials such as wood ash can cause detrimental effects.  A good question is: what is the least I need to do to achieve my desired result. 

Below are some suggestions on how to work with different types of soil.  Remember, your own soil may not fall conveniently into either sand or clay, so adapt the advise according to your specific situation.

Sand
If you are familiar with sand, as I am, you know it is weak and hungry. Table 1 shows it to be deficient in humus, water, minerals and soil life.  Sand has to bulk up; it needs strengthening and feeding.  Too much exercise in the way of digging or working the soil allows the water and nutrients to simply pass through (I dig only the top 5” of soil).  Without humus to hold the soil life and water, plant roots are starved.

When there is enough added humus, the soil life has a place to live and thrive.  Then is a good time to add microbial life using compost tea and other microbial inoculants.  The ecology of microbes, single-celled organisms and worms produce organic glue to bind the sandy particles together and retain moisture.  Sand is hungry so it needs added minerals to provide major and minor nutrients.  Gradually as sand turns to sandy loam, the soil ecology becomes more resilient and less needy. 
  
Table 1 -- Soil Composition 

Clay
Unlike sand, clay is challenging, stodgy and heavy.  Table 1 shows it to be deficient in humus, air and soil life but with plenty of minerals.  These, however, are locked up in tight chemical bonds.  Clay needs a lot of exercise to release its nutrients; the traditional cultivation method is deep digging and amending with plenty of humus.  However, the secret to releasing plant-available minerals is always soil life.  Certain microbes digest clay.  Roots, fungi, and worms create pathways for the air and water to penetrate.  Instead of manually or mechanically loosening the soil, use the power of nature to help you. 

Layering the surface with compost and organic matter and planting deep rooting green manures breaks down the clay without breaking your back.  Adding life in the form of compost tea, fungal and microbial products will speed up the process.  Eventually, with patience and effort you will have fertile clay-loam soil that naturally provides lots of plant nutrients. 

Soil pH and NPK
Unlike most gardening columns you may have noticed I seldom mention soil pH or NPK.  With a more biological approach, concerns about soil chemistry become less pressing.  I very occasionally test pH levels with those urine/saliva test strips available from a pharmacy.  Last year it saved me from amending potatoes with very alkali manure that was likely to encourage potato scab.  In general, I ignore pH and instead focus on supporting soil life. 

If soil ecology is healthy, the pH tends to stay in balance and can even adjust itself to the needs of different plants.  Roots attract and feed certain species of microbe that are specially adapted to that plant.  In return, these microbes help create a special microenvironment around that plant’s roots.

This biological process also works for plant nutrients.  Over-focusing on nitrogen (N), phosphates (P) and potassium (K) is misleading.  Plants need from 26 to over 100 different elements (depending on who you read) and they prefer them in a organically available forms.  Research shows it is more effective to add minerals to your compost pile or compost tea that directly to the soil.  There they are digested before being fed to your plants.

Amending you Soil
Table 2 offers general information about the use of organic soil additives, inoculants and amendments.  These are all available online and from reputable seed or gardening catalogs or even local animal feed stores.  Use your own soil tests to determine what would work best for you.  You will be surprised at the amazing results possible with even the least investment of the right amendments.  The secret is knowing your soil and giving it just the right exercise and diet it needs.     

Table 2 – Soil Amendments

Resources
ATTRA - National Sustainable Agriculture Information Service: http://attra.ncat.org/attra-pub/altsoilamend.html
Peaceful Valley Farm and Garden Supply: Fertilizer Solutions Chart --
http://www.groworganic.com/library_219.html