More Is Loss

Learn to identify and overcome nutrient excesses in your crops

Most soil- and plant-health management techniques use analytical methods that compare the elements found in the sample to sufficient or desired levels for a particular crop. The levels of most nutrients are typically shown to be at or near desired ranges, or slightly below and in need of replenishment. Typical crop programs aim to apply the 4 R’s of fertilization (right rate, right source, right placement and right timing) in order to satisfy the needs found in analysis reports through the growing season, with the goal of reaching certain numbers of units of each element.

However, on any given soil report, there are usually one or more elements that may be significantly higher than desired — and not just by a little, but by 300 to 500 percent above maximum. I have seen both soil and plant analysis methods that have had well over 500 percent of the desired maximum amount of magnesium, potassium, iron, manganese, boron, nitrate and others — to name just a few. 

This dilemma is often brushed aside as something that doesn’t affect crop outcomes or that doesn’t need to be considered when developing a crop plan. However, after careful examination, it is my opinion that this issue may be having a much larger impact on crop performance than commonly accepted. 

Agronomic systems typically consider element-to-element ratios in the soil to optimize plant growth, such as carbon:nitrogen, base saturation percentage, sulfur:phosphorus and others. But these element-to-element ratios can be disrupted very quickly if one or more elements is in extreme overabundance and the plant does not have access to selective root uptake from actively cycling soil microbes. 

The Costs of Excesses

I have personally noticed that many crops with one or more of these nutrient excesses can also express plant physiological symptoms, such as flower abortion, fruit drop, premature senescence of a plant organ, and a host of other unwanted symptoms. There have also been observed instances where an excess of one nutrient was applied, inducing a subsequent depression of another needed element. This is often referred to antagonism, and it presents significant challenges to crop input management.

A quick literature review of the unwanted influences of plant nutrient excesses reveals many physiological symptoms associated with a nutrient excess of any given element. Excesses can:

1. Decrease overall productivity
2. Limit fruit growth expansion
3. Require increased amounts of other nutrients
4. Induce oxidative stress
5. Disrupt nutrient uptake
6. Limit leaf expansion
7. Limit root development
8. Reduce root branching
9. Create undersized root systems
10. Limit cell wall strength
11. Increase susceptibility to frost stress
12. Cause leaky cellular walls
13. Limit reproductive capacity
14. Result in loss of seed viability

Taking this concept one step further, in perennial cropping systems, these nutrient excesses seem to persist and/or increase and to compound physiological symptoms that are manifested in the crop as systemic issues, requiring constant management attention. A few of the management issues we have observed related to nutrient excesses are increasing annual expression of visible leaf discolorations, necrosis, tip burn, persistent fruit drop, ineffective pollination, root decline, trunk decay and more. 

What seems to be happening is that when plants have too much of a particular type of food, they begin a cleaning process of removing all overfed, sick cells. This starts with the creation of special proteins that only form under certain stressful conditions, such as heat, drought and flooding, and that fortify the weak cells. Once this process is finished, the plant selectively gathers these proteins around the dead cells, dissolves them to the best of its ability, and restores the normalized growing state. This process is repeated until the plant reaches restored plant growth or senescence of the organ. The purpose of this is to prevent sick cells from reinfection and to help the plant shed unhealthy cells.

Correcting Excesses

Overfertilization is costly enough to the pocketbook — let alone if it actually reduces crop yield and quality. It is one thing to consider soil excesses that are naturally occurring, and another to consider soil excesses that are induced by our own actions. But both of these problems can be managed with insight, application and management.

In order to manage these types of imbalances, one needs to perform the below actions until plants achieve the desired growth potential:

1. Identify excesses
2. Implement corrective action
3. Refine and repeat steps 1 and 2 

It is necessary to utilize precision forms of mineral analysis in order to properly identify excesses. This includes newer forms of soil analysis, such as water-based extractions; field lysimeters; real-time ion monitoring; and other modernized ways of assessing mineral availability without involving acids. The refinement of leaf sap analysis techniques over the past 20 years has also given growers a tool to identify nutrient excesses effectively and quickly inside the plants themselves. Lastly, low detection limits in mineral-based water analysis can also play a key role in identifying the source of a mineral excess, as water sources are often contaminated.

Once a nutrient excess has been located, the baseline corrective action usually follows a three-step process:

• Plant detoxification
• Mineral balancing
• Biological augmentation

Each of these typically requires using a variety of forms of carbon as crop inputs. These can come from sources such as humates, fulvic acid, biochar, organic acids, plant residue, cover crops or other forms of soluble carbon. Soluble carbon is a vital plant nutrient that is often in short supply for growers due to the underuse of cover crops, overuse of soluble fertilizers, lack of animal rotation, over-tillage, reliance on chemicals or other means of mismanagement. The depletion of all soil carbon pools is damaging, but the active pool that plants use to buffer stress seems to be a major source of crop limitation.

It is well known that plants exude carbon-based molecules from their roots in order to obtain minerals from the soil, but what many people forget is that soluble carbon fertilizers are also taken up by plants as food to build their plant organs — in other words, plants don’t only access carbon through CO₂ intake, but also through their roots. What’s more is that higher levels of gas exchange in soils (respiration) seem to correspond to higher levels of plant health. Extremely healthy natural ecosystems are prime examples of this, literally breathing life from soil into the air and back through the plants.

Set a goal to rebuild gaseous exchange between the soil and the air that is moderated by the plant, instead of attempting to reach a specific number of units of N per season. This sort of fertilization can produce similar yields, but with much higher quality because the plant is not up-taking some elements at the expense of others. This is because the units of nitrogen (for example) needed often corresponds not to what the plant actually needs, but to what the plant needs in order to overcome the desired ratio of other excessive elements, such as sulfur, magnesium, iron, aluminum, chloride, etc.

Instead of relying on a constant stream of soluble N, P and K, which in the long term can have unbalancing effects on the soil and lock out needed micronutrients, diverse input programs can be implemented, rebuilding the soil and plant on many physiological levels. If we are applying the same crop inputs year after year into our cropping system, we may need to consider the cumulative effects that these inputs can have on the element-to-element ratios.

Utilizing the soil active carbon method known as POXC can also be a helpful tool. Developed by top-level scientists and frequently promoted by the NRCS and the ASA, POXC identifies the pool of soil carbon that can be utilized by a plant during a normal cropping year. This is helpful when used simultaneously with a soluble nitrogen test for assessing soil C:N ratios. This can help growers to limit unnecessary N feeding and instead allow the proliferation of soil microbes.

Once you have a basic assessment of how well your plant-soil environment is managing active carbon, you can then form a plan to restore that carbon pool to its needed state. A simple three-step process can overcome simple soil depletion, kickstart microbial cycling and mineral intake from soil parent materials, and improve plant health through allowing the plant to build higher-level defense compounds that require large amounts of carbon from the soil.

1. Rebuild biology via inoculants and/or soluble carbon
2. Rebuild soil health by buffering soil excesses with minerals and carbon
3. Rebuild plant health by supplementing needed minerals in a foliar spray

In using these simple techniques, we have seen dramatic increases in plant health and have observed visible changes — typically in under 14 days. This sort of outcome is driven by technical analytics, effective implementation of these analytics, and an understanding of antagonistic relationships between elements. Previously, these could only be learned through careful study of books on mineral interactions in plants and soil, combined with years of experience and observation. 

While soil analysis methods have revealed how to grow monocrops on scale, leaf sap analysis reveals how to grow resilient crops on scale with consistent outcomes across a myriad of agronomic variables. Using precision leaf sap analysis to accelerate the learning process for growers and consultants helps tremendously to understand how to use these sorts of application techniques consistently. Learning how the plant is managing nutrients opens many new data points for how a plant-soil system is functioning. 

When used in conjunction, the analysis of soil carbon, soil minerals, and plant uptake can pave the way for lowering fertilization needs while increasing plant health and improving crop outcomes.

David Knaus is the founder and CEO of Apical Crop Science, an agriculture laboratory and crop advising service that provides plant and soil analysis, regenerative crop inputs and ASA certified crop advising services focused on regenerative and organic crops.