Potassium concentration and yields in flowering plants
From the different nutrients that are needed by plants we have known for more than 4 decades that potassium is of critical importance to flowering/fruiting plants. Potassium is one of the most highly limited nutrients in soil due to its high mobility and great increases in yields have been achieved with both potassium fertilization in soil and the use of properly balanced nutrient solutions containing enough potassium in hydroponics. But how important is potassium and what is its ideal concentration in hydroponic nutrient solutions when growing flowering plants? Today we are going to take a look at the scientific literature about potassium and what the optimum levels of potassium for different flowering plants might be in order to maximize yields.
There are many studies in the scientific literature dealing with the effect of potassium on various flowering plants. Earlier evidence from the 1980s pointed to optimum concentrations of potassium being close to the 160-200 ppm range. The book "mineral nutrition" by P.Adams (here) summarizes a lot of the knowledge that was available at the time and shows that for the growing techniques available at the time using greater concentrations of K was probably not going to give a lot of additional benefit.
However newer evidence from experiments carried out within the past 10 years shows that optimum potassium concentration might depend on a significant variety of factors, from which media, other nutrient concentrations and growing system type might play critical roles. For example study on strawberries in 2012 (here) showed optimum concentrations of K to be around 300 ppm for strawberries and the optimum media to be a mixture of peat+sand+perlite (image from this article included above).
Evidence from experiments on tomatoes (link here and image from this article above) also shows that for tomatoes the actual optimum concentration of K might actually be larger under some condition with the optimum in this study in terms of fruit quality and yields being 300 ppm. In this last case the tomatoes were grown using a nutrient film technique (NFT) setup. However there have also been studies under other growing conditions - like this one on reused pumice - which shows that increasing K concentrations to 300ppm can actually have detrimental consequences. In this case tomatoes fed at 200, 290 and 340ppm of K had very similar results when using new substrate but the old substrate heavily underperformed when high K concentrations were used.
Papers published on the effect of different K concentrations in melons (here) and cucumbers (here) also point to optimal concentrations in the 200-300 ppm range and for the optimum N:K ratio to be between 1:2 and 1:3 for these plants. This is probably the reason why you will often find suggested nutritional guidelines for flowering plants - like those below taken from here - mostly suggesting K concentrations in the 250-350ppm range. However you will often find that they directly contradict research papers, like this guideline suggesting K of 150 ppm for strawberries while we saw in a recent paper that 300ppm might be better. This is most probably due to differences in the sources used which might have used different growing systems or plant varieties which responded to other conditions better.
All in all the subject of K concentration in hydroponics is no simple one. Using low K will limit your yields tremendously but increasing your K very high can also harm your plants, especially depending on the type of media you are using. In general aiming for a K concentration between 200-250 ppm is safest but in many cases increases to the 300-400ppm range can bring significant increases in plant yields. A careful study of the available literature and the actual growing conditions that the plants will be subjected to will be key in determining what the best K concentration to use will be. Alternatively carrying out adequately designed experiments under your precise growing environment will help you carry out an evidence-based decision about what K concentration to use.
Potassium may help to prevent heart disease
Spinach, carrots, oranges, and bananas are just some fruits and vegetables that are rich in potassium. According to a new study, we may want to consider increasing our intake of such foods; they could help to protect us against heart disease.
Researchers have found that mice with low dietary potassium are more likely to experience vascular calcification, which is characteristic of atherosclerosis. This is major risk factor for heart disease.
Increasing dietary potassium, however, was found to reduce vascular calcification in the rodents, suggesting that a diet rich in potassium could help to prevent heart disease.
The research team - led by Yabing Chen, Ph.D., a professor of pathology at the University of Alabama at Birmingham (UAB) - recently reported their findings in JCI Insight.
Heart disease is the leading cause of death for both men and women in the United States, killing around 610,000 people in the country every year.
Atherosclerosis is a key risk factor for heart disease. In atherosclerosis, deposits of fat, cholesterol, calcium, and other substances accumulate in the arteries, forming what is referred to as "plaque." Plaque hardens over time, restricting blood flow to the heart.
The new research from Prof. Chen and colleagues suggests that potassium supplementation could be one way to help combat atherosclerosis and reduce the risk of heart disease.
Effect of Potassium and Calcium Ions on Heart Function
In the discussion of membrane potentials in Chapter 5, it was pointed out that potassium ions have a marked effect on membrane potentials, and in Chapter 6 it was noted that calcium ions play an especially important role in activating the muscle contractile process. Therefore, it is to be expected that the concentration of each of these two ions in the extracellular fluids should also have important effects on cardiac pumping.
Effect of Potassium Ions. Excess potassium in the extracellular fluids causes the heart to become dilated and flaccid and also slows the heart rate. Large quantities also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle. Elevation of potassium concentration to only 8 to 12 mEq/L—two to three times the normal value—can cause such weakness of the heart and abnormal rhythm that this can cause death.
These effects result partially from the fact that a high potassium concentration in the extracellular fluids decreases the resting membrane potential in the cardiac muscle fibers, as explained in Chapter 5. As the membrane potential decreases, the intensity of the action potential also decreases, which makes contraction of the heart progressively weaker.
Effect of Calcium Ions. An excess of calcium ions causes effects almost exactly opposite to those of potassium ions, causing the heart to go toward spastic contraction. This is caused by a direct effect of calcium ions to initiate the cardiac contractile process, as explained earlier in the chapter.
Conversely, deficiency of calcium ions causes cardiac flaccidity, similar to the effect of high potassium. Fortunately, however, calcium ion levels in the blood normally are regulated within a very narrow range. Therefore, cardiac effects of abnormal calcium concentrations are seldom of clinical concern.
Growing Conditions Affecting Potassium Availability
Potassium (K) is one of the three primary macronutrients that all plants need to mine from the soil that is necessary for proper growth. On a fertilizer grade analysis, it is the third number listed (in the form of potash) behind phosphorus and nitrogen; yet, potassium is sometimes overlooked as an important plant nutrient until problems arise.
This year, potassium deficiency symptoms appear to be occurring more than in years past. The growing conditions that we have experienced this year have contributed to potassium deficiency. One obvious factor that affects potassium deficiency is low soil K; if there’s little to no K in the soil, plants will show deficiencies. However, K deficiency can appear even in soils that have adequate levels of soil K if certain criteria are met.
Potassium deficiency tends to show up and/or become exaggerated during periods of drought and in compacted fields. Under drought and compacted soil scenarios, plants have smaller, restricted root systems that cannot grow deep enough to access soil K located deeper in the soil profile. Planting into wet fields in the spring can cause soil compaction and glazed sidewalls, leading to K deficiency if dry conditions manifest later in the season. This is exactly what we’ve seen this year in Maryland. Potassium deficiencies do not often appear early in the season, especially in corn. Symptoms typically begin to appear a few weeks after planting, and in corn, begins to appear during jointing.
Potassium deficiency starts out as chlorosis (yellowing) of the leaf margins (edges), progressing down the leaf blade with the midrib staying green (Figure 1). Eventually the tip will begin to die, advancing down the leaf until it is dead. This is opposite of nitrogen deficiency symptoms, which starts as chlorosis in the middle of the leaf and progresses towards the margins. Potassium is mobile in the plant; meaning that plants can sacrifice K in the lower leaves and move it up to the growing point (newly emerging leaves) to support growth. Therefore, symptoms of K deficiency appear first in the lower leaves and progress up the plant as it grows taller.
Figure 1. Potassium deficiency in corn. Notice chlorosis starting on the lower leaves on leaf margins progressing towards the midrib with tip and margin dieback.
Managing a potassium deficiency mid-season can be achieved by topdressing with a K fertilizer if the plants are still small enough. Keep in mind that a granular K fertilizer will need to be dissolved and soaked down into the soil profile, so a mid-season application will only help if it rains or you irrigate following application. Successful management of K should start with your soil test. Test your field to see how much K you have and if you’re low, then fertilize accordingly. Avoid practices that can cause soil compaction, such as working or planting wet ground. If you have compacted soils, consider subsoiling in the fall or planting deep-rooted, diakon-type radishes to shatter the hardpan. Control weeds, as they can compete with your crop for potassium.
Most crops require a large amount of potassium; roughly the same or more than nitrogen. Keep in mind that forage crops, including hay, remove substantial amounts of potassium from the soil (upwards of 200 lbs/acre/yr), so it is necessary to fertilize with K to replace what’s been removed, especially if you plant behind a hay or forage crop.
A well-balanced diet is the key to balanced potassium levels
Working to carry crucial signals to heart, nerve and muscle cells, potassium plays a key role in overall health and function. When the body’s supply of this vital mineral and electrolyte runs too low or too high, one may be at risk for medical issues that include digestive distress, abnormal heart rhythms and potential cardiac arrest.
“Potassium is an electrolyte that helps conduct electricity in the body. It’s crucial to our heart, helping trigger it to squeeze blood through our body,” said Stephen Compston, a licensed and registered dietitian with Renown Health. “Potassium is also crucial for our muscles and skeletal muscles — helping with movement — and the muscles in our digestive system, helping with contraction, which is important for digestion.”
People whose potassium levels are too high or too low typically suffer from medical conditions or are on medications that affect the regulation or absorption of this mineral within the body. For example, people with chronic kidney disease may have too much potassium in their bloodstream.
“Our kidneys work to regulate levels of potassium in our blood, so people who have any disorders or disease related to kidney function need to be careful with their potassium intake,” said Johanna Dibble, a licensed and registered dietitian with Northern Nevada Medical Center. “People who have malabsorption disorders or other states of malnutrition, along with athletes who sweat excessively, are at risk for low potassium levels.”
As far as medications that may affect potassium levels, these include ACE inhibitors and diuretics, among others. If you happen to have a medical condition or take a medication that could affect your potassium level, your doctor can monitor your body’s supply of this mineral through a basic blood test.
“Low potassium levels in the blood, also known as hypokalemia, can be asymptomatic or can cause GI distress, weakness and fatigue, tingling or numbness, heart palpitations, frequent urination, and nausea or vomiting,” Dibble said. “High potassium levels in the blood, also known as hyperkalemia, share the same set of symptoms as hypokalemia. More serious symptoms include a slower heart rate or even sudden cardiac arrest.”
For folks who do not have health problems or prescriptions that could have an impact on potassium levels, maintaining an adequate supply of this electrolyte tends to come down to a well-balanced diet. Experts warn against seeking out a potassium supplement without physician supervision.
“Most healthy adults can get enough potassium through diet alone,” Dibble said. “Do not take potassium supplements unless directed by a doctor, as having too high potassium can be dangerous and even life threatening.”
By filling your plate with a variety of unprocessed foods, chances are good you’ll be boosting your body’s supply of potassium. Prime sources of potassium include potatoes, broccoli, bananas, cantaloupe, avocado, salmon, beans, yogurt and milk.
“As with many things health-related, a diet packed with whole, plant-based foods is best. When you’re eating these foods, you’re not only getting potassium, but also essential fiber, vitamins and minerals,” Compston said. “Most of us don’t get enough potassium in our diet because we aren’t eating enough unprocessed foods.”