PLANT DISEASE AND PH: AN IMPORTANT CONNECTION
by Adrian White
20 December, 2021 by
PLANT DISEASE AND PH: AN IMPORTANT CONNECTION
Lucija Johum

PH: it’s that one aspect of growing you know is important, but that you maybe don’t yet fully understand.

Learning how to tweak pH in your growing medium is easy enough to get the hang of. The science behind it is not so difficult to grasp, either. But do you know how levels of pH – ranging from more acidic to more alkaline – truly affect your plants?

If you find your crop suffering from chronic disease (particularly when planted in a certain spot, soil, or other medium), you may want to check on the acidity in those areas. While acidity (or alkalinity) furthers the health of certain plants, it can also be very detrimental to others, especially at the wrong levels.

For the best of gardeners and growers, knowing something’s up with pH doesn’t usually come from obsessive soil testing (though if you’re up for it – doesn’t hurt!). More realistically, it comes from knowing your plants really well: they’ll tell you early on if pH isn’t right. But how can you know?

THE PH AND PLANT NUTRIENT CONNECTION

Here’s a good question: if you close your eyes, can you envision what pH looks like in your head? With words like “soil acidity,” do you imagine that very acidic soils might be toxic or astringent, while basic (alkaline) ones are more powdery or ammonia-like?

This is a funny image that I myself have thought of when trying to imagine soil acidity. But then I learned the true science of it: acidity has to do with the level of available elemental nutrients in the soil that your crops need. 

As depicted in the Jeff Lowenfels horticultural classic, Teaming With Nutrients, different levels of various elements make a medium acidic, while a different set make it alkaline.

In fact, the term “pH” stands for “potential of hydrogen.” The more hydrogen around, the more acidic the pH (0-7); the less hydrogen, the more alkaline (7-14). But, it also involves the presence of other minerals like magnesium, calcium, potassium, and more.

Almost always, certain levels of acidity are revealing of what levels of other elements are there. This is where the connection between pH and disease establishes: plants are choosy about the mineral amounts their roots are exposed to. If things get off kilter, disease can develop.

And as every clever grower knows, you can easily diagnose nutrient excesses and deficiencies just by looking at plants! Plus, nutrient imbalances are one of the leading causes of disease in crops. To put it simply: if you know your nutrients and your plants, then you know your soil pH, sometimes more than you think you do. 

ACIDIC, ALKALINE, AND NEUTRAL MINERALS

What elements in your soil signify acidity or alkalinity? What nutrient deficiencies in your crop can you pay attention to, in order to know if your growing medium is too acidic or too alkaline? When you get down to it, there’s quite a few.

  • Boron – Most present at slightly acidic levels.

  • Calcium – Most present at slightly alkaline levels.

  • Copper – Most available in slightly acidic levels.

  • Iron – Most present in very acidic to slightly acidic levels.

  • Magnesium – Most available in slightly alkaline levels.

  • Manganese – Most available at slightly acidic levels.

  • Nitrogen – Most available at neutral levels.

  • Phosphorus – Most available at neutral levels.

  • Potassium – Available at all ranges, including highly alkalinity, least present in highly acidic soils.

  • Zinc – Most available in slightly acidic levels.

WHAT DO THESE NUTRIENTS TELL US?

The answer is easy: if you diagnose a nutrient deficiency in your plant, then it can tell you what’s lacking in your soil. That, in turn, can tell you a bit about the pH of your soils, and especially if what you grow has more than one nutrient deficiency.

Keep in mind, however: certain plants have preferences towards more acidity or alkalinity. In some cases, a lack of certain nutrients may not manifest if that is what the plant prefers.

BORON DEFICIENCY

Your plants will have trouble flowering or fruiting, and may have damaged growth tips. May mean soil is a bit too alkaline (though not excessively), especially if they are also copper, iron, manganese, and zinc deficient.

CALCIUM DEFICIENCY

Malformed new shoots, roots, or leaves at the growing tip. May mean soil is too acidic, especially if your plants are also magnesium and potassium deficient; though is not present the higher alkalinity gets either, especially if nitrogen, iron, copper, and zinc is lacking.

COPPER DEFICIENCY

Yellowing of the leaves in some, curling of leaves in others, or too much top growth. May mean soil is too alkaline, especially if they show signs of being iron, manganese, and zinc deficient, too.

IRON DEFICIENCY

New leaves get yellowed or whitened, while old growth maintains green color, though may lose color at edges. Means soil might be much too alkaline, especially if boron and zinc are not present.

MAGNESIUM DEFICIENCY

Older leaves lose color, though veins continue to be green – over time, this happens to young leaves too. Means soil may be too acidic, especially if potassium and calcium deficient too; though is not present the higher alkalinity gets, especially if nitrogen, iron, copper, and zinc is lacking.

MANGANESE DEFICIENCY

All leaves start to yellow but show green veins. Tends to mean that soil is too alkaline, especially if copper and zinc are low also.

NITROGEN DEFICIENCY

Plants will start to yellow all over, though most notably in their new and young growth. Lack of nitrogen can mean extreme acidity, especially if they show signs of all nutrient deficiencies except iron. Or, it may indicate extreme alkalinity, especially if calcium, iron, magnesium, manganese, copper, and zinc deficiencies are noticed. 

More than anything, lack of nitrogen means that soils need to be neutralized with the addition of more organic matter (like compost, compost tea, blood meal, etc.).

PHOSPHORUS DEFICIENCY

In young plants, the deficiency shows itself as stunted growth. In mature ones, however, root growth and blooming are affected. A phosphorus deficiency means that soils are most likely too acidic, and especially if all other nutrient deficiencies manifest save an iron deficiency.

POTASSIUM DEFICIENCY

If your plants need more potassium, the old growth will show wilting and spotty leaves due to cellular death. If many other nutrient deficiencies manifest, except an iron deficiency, this could mean that your soils are getting too acidic.

ZINC DEFICIENCY

Symptoms include yellowing, along with slowed and even squat growth. May mean soil is too alkaline, especially if they show signs of being copper, iron, manganese, and zinc deficient, too.

NUTRIENT EXCESSES (TOXICITY)

Nutrient deficiencies can point to certain acidity levels in your soil. Likewise, nutrient excesses can also be very telling about pH, too. As described by the American Phytopathological Society, some nutrient excesses can also be called “toxicities,” and be equally harmful and conducive to disease in plants. 

Again: different plant varieties have different acidity preferences. Just because a plant isn’t showing toxicity relating to certain nutrients doesn’t immediately indicate a certain pH, or that a nutrient isn’t present. Be observant, and test acidity to make sure!

BORON TOXICITY

Too much boron in soils can create yellowish lesions or tipburn in some crops, though not all. Excess of this mineral can indicate very high alkalinity, though not always. If potassium levels are also high, it would be wise to check pH, because high alkalinity is possible.

IRON TOXICITY

Too much iron is a common problem in greenhouses when chemical fertilizers are used too much, and no soil biology remediation helps replace the soil life over time. Necrosis (tissue death) and chlorisis (yellowing) begins to appear on the leaves of some, usually on the margins. Too much iron almost always means that soil is much too acidic, something most plants don’t tolerate.

POTASSIUM EXCESS

Potassium excess is not considered a “toxicity” per-se, but too much of it can interfere with the uptake of other important minerals – particularly magnesium. Look for magnesium deficiency symptoms in your plant – if magnesium is present aplenty in your soils, this most certainly points to high alkalinity.

NUTRIENT IMBALANCES = PLANT DISEASE

Fortunately, not all plant diseases are caused by nutrient deficiencies. Still, any sort of nutrient deficiency is bound to have a negative impact on your plant. For this reason, testing pH is important!

Certain common categories of disease tend to be connected to nutrient deficiencies. No doubt, this can also mean that pH is sometimes involved, or even a culprit in the problem. My Agriculture Information Bank lists common disease and deficiency connections:

  • Hollow stems/soft rot – Signals boron deficiency, which can point to slightly too alkaline soils.

  • Red leaf spot diseases – Caused by deficient nitrogen, which can point to either high acidity or alkalinity – make sure to soil test and check for other nutrient deficiencies.

  • Rust leaf diseases – Points to potassium deficiency, which may mean soils are too acidic. Make sure first, however, that you are not dealing with a fungal disease.

  • Dwarfisms – Lack of phosphorus can stunt healthy plant growth. Most likely points to high acidity, though it can also relate to slightly high alkalinity.

  • Blossom end rot – Calcium deficiency is often responsible for failed flowering. Check for high acidity, but high alkalinity may also be a culprit.

SICK PLANTS? GET TO KNOW PH!

If you have crops that are getting sick often – or all the time – getting in touch with your soil’s pH is one of your logical next steps, and one that growers should consider on a regular basis. By watching for soil deficiencies and analyzing certain plant diseases, you can get an idea of what soil acidity you’re dealing with – and without testing soil nearly as much.

Still, routine testing is wise, especially if you apply chemical fertilizers, pesticides, or herbicides on your crops often. Organic growers, on the other hand, don’t have to worry as much about pH being a part of crop disease issues, since natural methods keep soil organisms alive, which maintain pH at a neutral level naturally.

PLANT DISEASE AND PH: AN IMPORTANT CONNECTION
Lucija Johum 20 December, 2021
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