General gardening advice

14 Mar

Plants and microbes 2
Understanding the plant – soil fertility cycle

The best growers utilize both minerals and biology in their quest for excellence. These two major pathways for getting nutrients into the plant are not mutually exclusive – in fact they can support each other.

In the mineral pathway, nutrients enter the plant roots directly, without intervention of the soil micro-organisms. There are several ways this occurs, described in more detail in this post. In the biological pathway, microbes use soil minerals to build their bodies, but also receive food from the plant roots directly. When the microbes die or excrete, the resulting compounds can be absorbed directly by the plants. The plants feed the soil microbes who in turn feed the plant in a symbiotic relationship. Balanced and available soil minerals support this relationship.

There are three ways plants can absorb minerals directly: by mass flow, by diffusion or by root interception. Mass flow is the movement of dissolved nutrients into a plant as the plant absorbs water for transpiration. The process is responsible for most transport of nitrates, sulfates, calcium and magnesium. It requires that the nutrients be dissolved in water and enter the plants in ionic form, suspended in solution.

Diffusion is the movement of nutrients to the root surface in response to a concentration gradient. A high concentration in the soil solution and a low concentration at the root cause the nutrients to move to the root surface, where they can be taken up. This is important for the transport of phosphorus and potassium. If there is too much salinity in the soil, this process comes to a halt. That is why we pay close attention to the sodium levels in the soil test and even better, the conductivity. In general we want the conductivity to be under 2 mho/cm, but we don’t want conductivity to be zero – some energetic communication is essential.

Finally, root interception occurs when a root contacts soil colloids which contain nutrients. The root then absorbs the nutrients. It is an important mode of transport for calcium and magnesium, but in general is a minor pathway for nutrient transfer.

In the biological pathway, the soil life and the plant exchange nutrients in a symbiotic relationship.  During plant growth, the roots transport sugars into the soil.  These sugars provide food for soil microbes, who in turn multiply quickly in response to the food.  The microbes extract minerals from the surrounding soil in order to build their bodies.  When they poop or die, the microbe’s bodies are broken down into compounds that the plants can absorb.  Microbe byproducts feed mineral nutrients to the plant via mass flow, diffusion, or root interception. Or, in the case of VAM, the mycorrhizae conduct water and minerals directly into the plant roots.

Capitalizing on the biological plant-soil fertility cycle has some distinct advantages.  If you do not irrigate your crop, even a few weeks without rain at the wrong time can spell disaster.  On the other hand, microbes can continue to work even in very dry conditions.  

Almost all of the OMRI organic minerals OrganiCalc recommends are microbe friendly, encouraging microbes rather than inhibiting them. However, agricultural sulfur and copper sulfate deserve scrutiny.

Copper sulfate is a powerful fungicide, but the Biomin (4%) copper we recommend has no fungicidal properties.

OMRI 90% Ag Sulfur (Tiger Sulfur, for instance) is not without consequences. Often Ag Sulfur is the only way to remove excess cations, and restore balance to the soil. So, it has its uses, but we do have an application limit of 100 lbs/ac (which is not much) on this material because it does impact soil life.

Ag Sulfur is slow acting, usually taking over a year to do its work. Specialized microbes slowly convert the sulfur into sulfuric acid, which is toxic to most other microbes. Once in sulfuric acid form, the sulfur is free to form sulfates with excess minerals, which in turn can be leached by sufficient clean water. This effectively removes excess cations, bringing the soil into balance, and lowering the pH. There is a long history documenting improved plant health in soils with a pH between 6.9 and 6.0. With the addition of up to 100 lbs/ac Ag sulfur we slowly move mildly alkaline soils toward a lower pH.

Sometimes people confuse this process with another important function of sulfur. Sulfur is a required plant nutrient (sulfur works with nitrogen), and optimum quantities are 50 to 80 lbs/ac (depending on CEC). This sort of sulfur is best supplied by gypsum (calcium sulfate), or other sulfates, which do not harm microbes at all. Gypsum supplies about 1/5th the sulfur of Ag sulfur.

When we feed the soil, we feed the microbes who in turn feed the plant.  When we feed the plant, we indirectly feed the microbes who in turn feed the plant.  

If managed properly, certain species of plants can form a symbiotic relationships with vesicular arbuscular mycorrhizae (VAM) who scavenge both phosphorus and moisture from the surrounding soil and return it directly to the roots.   In order for the plant to form this relationship it is critical that it not have ready access to soluble phosphorus early in its growth.   Fertilizing with soluble phosphorus at the seedling stage will inhibit drought tolerance later in the season.  Being mindful of this fact, we initiated a conversation with Dr. Christine Jones, who assured us rock phosphate has no ill effect on VAM. Many plants are capable of forming a relationship with VAM, but the cole crops (cabbage, broccoli, etc.) are not.  

In 1997 a USDA research team led by Dr. Sara Wright discovered glomalin, which has revolutionized our understanding of how fertile soils are made. Her work has provided the foundation for others, including Dr. Christine Jones and John Kempf, who are engaged in building soil humus while growing crops and/or pasture. Her breakthrough is well documented in this very interesting article:

How does one go about feeding the soil?  A healthy soil has balanced minerals but does not contain traces of herbicides, fungicides, pesticides or chemical fertilizers, all of which are harmful to soil life.  To feed the soil, balance the minerals and avoid chemicals.  A healthy soil also contains organic matter, either from additions of compost or humates, or from cover crops and other living plants.  

How does one go about feeding the plant?  Besides feeding the soil, foliar feeding the right nutrients at the right time keeps plants growing rapidly. If plants are very, very healthy they will transfer their excess photosynthetic energy into lipids, which are stored in the soil for later use. These fatty compounds are too complex to be broken down by bacteria (in a process called mineralization), but are food for fungi. Fungal decomposition is an entirely different process, resulting in humification, which builds organic matter in the soil. This is an important topic, best suited to another post…

Click to view article on a single page

02 Nov

Getting minerals to the right place at the right time is the subject of this article: How Minerals Move in the Soil

Click to view article on a single page

12 May
Floating row cover tunnels for summer production

floating row cover tunnels in spring-1
We live where it is windy in the afternoon, with intense sun, cool at night, foggy until late some mornings and did I mention the rampaging deer? Our native plants love it — oak trees grow like weeds here. But some of the veggies are tender and they prefer to live a more moderate lifestyle.

Enter the floating row cover tunnel. These are small hoop houses for the garden, but instead of covering them with plastic, Erica covers them with floating row cover, brand name Agribon to be exact.

Cukes under tunnel-2To construct them she buys 10 foot long sections of plastic electrical conduit — the cheapest kind. Assembly of the tunnel is better as a two person job the first time. We have soft soil so we can just insert the ends of the conduit into the ground. If the soil was hard we might need to pound in a piece of rebar to slide the conduit over. Once one side is inserted, bend the conduit in a hoop and insert it on the other side of the bed. Hoops are placed about every 4 feet along the bed. A straight piece of conduit tied in place along the top will help to stiffen the structure and hold the floating row cover fabric.

Erica buys the 10 foot wide rolls of floating row cover fabric, Agribon AG-19 (, which is expensive but worth it to us for the increase in yield. The only fabric available now is spun bonded and it does not last more than two seasons. There was a time when woven row cover fabric was available and it was much more durable, but alas, those days are gone. She cuts off a length of fabric long enough to cover the tunnel and the ends. The fabric is held on by giant metal clips from the office store, the largest size they make. These are much cheaper than the clips from the garden store.

Tomatoes in tunnel-2Tomatoes bursting out of tunnelSince the local deer have grown so fond of tomato vines she always grows her tomatoes under tunnels. The tunnels are large enough to hold the plants even when caged. In extreme deer years, the row cover needs to stay on throughout the season. As soon as a tomato vine pokes through a hole in the fabric, it gets pruned back.

Cucumbers love it under the tunnels and we now get bumper crops in a small area. Since the tunnels provide some frost protection, they are also good for early zucchini until the plant outgrows it.

Click to view article on a single page