Citrus and Cold, Waterlogged Winter Soil management



Freezing Temperatures in young citrus block.

Winter can be a challenging time for growing citrus. While these conditions can vary from year to year, it pays to be prepared for the worst…just in case.

During extreme temperatures, plants are put under a high stress load and often maturing a crop at the same time. This creates an added level of severity to the plant stress. There are many options for dealing with freezing conditions. Today, let’s concentrate on managing “waterlogged”, cold soils.

As a soil and plant nutritionist, I focus on good, balanced soil and plant nutrition to best equip plants to tend to themselves even during stress events. It takes time to adjust soil nutrient levels when they are imbalanced and time is a limiting factor, when Mother Nature comes calling with freezing temperatures. Soil nutrient levels, especially the Base Saturation cations (K⁺, Ca⁺⁺, Mg⁺⁺, Na⁺ and H⁺), should be based upon the soil texture (sand, silt or clay) and adding “more” than what is needed, is not better. This is especially true with calcium and magnesium. Calcium creates pore space and affects the amount of air in the soil. Having surplus amounts of calcium can have a negative impact and actually compete against other needed cations, like potassium and magnesium, by displacing them. Magnesium is equally important, as it affects the soil’s moisture level and like calcium, too much magnesium can create tight-soil conditions that limit water percolation and/or competition with other needed cations. Together, these two need to make up 80% of the total Base Saturation of cations and directly affect your soil-water efficiency and percolation rate. A correct Ca:Mg ratio (i.e. 68% Ca:12% Mg) is important to nutrient efficacy all year round…but especially in times of water saturation and cold soil.

Irrigation water is used to manage cold temperatures and provide affordable protection of a few degrees; it can be the difference between protected and freeze damaged fruit. When this must be done for an extended period of time, the water is applied faster than the plant and/or soil can move it through the soil profile and field saturation occurs. Water saturation creates multiple issues, while cold/freezing air temperatures are present already (stressor); the rootzone is deoxygenated due to irrigation water (stressor), plants are often nutrient deficient due to cold soil temperatures (stressor) and if trees are sprayed with a canopy protectant, respiration is dramatically reduced (stressor). This is a bad mix of stressors, but it happens more often than you might expect.

 

Soil Microbiology - The Missing Link...

Microbiology is foundational to soil health, efficient water usage and nutrient availability/uptake. Applying specific strains of microbiology in the fall, helps prepare the plant and rootzone for wintertime stress events. There are many species and subspecies available, so getting the right type(s) is important.

 

During winter we know certain things to be true:

1.      Phosphate is energy and necessary for plant vitality and to manage stress.

2.      All nutrients, with the exception of nitrogen, move into the plant in phosphate form.

3.      All nutrients, with the exception of nitrogen, are unable to move from the soil/rootzone into the plant until the soil temperature reaches 65 °F.

4.      Nitrogen (nitrate) is a mass-flow mover and susceptible to leaching with heavy amounts of water.

5.      “Waterlogged” soil is depleted of oxygen. Roots cannot grow without oxygen and can actually start to die in 24-48 hours depending on the severity of the conditions.

6.      “Waterlogged” soils stimulate anaerobic microbial activity that feed on the organic matter/humus in the soil and produce methane gas (waste-product), which is highly toxic to roots. This condition will not improve until the rootzone gets air re-introduced into it (oxygenation).

Now, with that in mind, you can see why good, balanced nutrition and nutrient availability is so important. Microbiological activity makes this a reality. Let’s look at how certain groups of microbiology help during cold, wet, winter conditions.

To keep things simple, let’s focus on just two groups that are beneficial in keeping the rootzone active and aerated. Lactobacillus and Yellow soil yeast. Both are proficient at producing high levels of true organic acids (O.A.), right in the rootzone; some of these are citric, acetic, lactic, malic, gluconic, etc. These types of O.A.s are more efficient than humic or fulvic acids as they act much akin to plant enzymes. They are very proficient at “cleaving” fixed nutrients from soil particles and also in keeping them available for later plant use…even during winter. These Organic Acids are not temperature dependent and are exceptional “plant-ready” chelating agents for moving nutrients into plants that would otherwise not be usable until the soil warms. In fact, adding these types of microbes and the O.A.s they produce to your soil will also lessen nitrogen leaching, by conversion into “plant-ready”, non-leachable forms, like proteins and amino acids. In these forms, the nitrogen is stored within the root system for later use, when needed.

In addition to the nutrient chelating plant benefit, the microbiology also increases the oxygen (O₂) capacity of the soil through the production of carbon dioxide (CO₂). A byproduct of active microbes is CO₂ production. Therefore, a benefit of higher microbial population and activity is an increased CO₂ level. As CO₂ is released, it ascends rapidly to the soil surface and creates tiny pores in the soil. It is much the same, as when you pour a carbonated drink into a glass and the CO₂ gases (fizzes) off rapidly. Now imagine the gassing/fizzing action in your soda glass, but from within the rootzone rising up to the soil surface. “Gas exchange” occurs when CO₂ reaches the soil surface and releases to the atmosphere. This rapid rising creates a vacuum within the soil pores and, as a result, oxygen is pulled back down into the soil (oxygenation) via the pores to maintain equilibrium.

Another benefit from Lactobacillus is in the production of bacteriocins. Bacteriocins are specialized natural antibiotics and antibiotic-like compounds. They are vital to maintain soil, root and plant health within the rootzone. They are both gram^- and gram⁺ compounds that directly antagonize methanogenesis anaerobes and soil borne pathogens.

In essence, by adding these specific strains of microbiology you are altering the microbial population to your benefit.

Fall soil inoculation will build microbial population and diversity, in time, to combat the cold, wet soil issues of winter for a better soil and crop response.

Adding microbiology is not a substitute for balanced soil nutrition, but it is an important tool in making your existing soil nutrients more efficient and available for plant use.