Surefire Ways of Identifying a Phosphorus Deficiency in the Garden—and How to Correct the Problem

When we’re deep in the throes of the Chinese blessing “May you live in interesting times,” I fully believe there will be no soil testing available at the county extension office.  We’ll be on our own. 

Unfortunately, plants possess poor communication skills.  A common response is to turn yellow and die.  Too much water?  Too little of a nutrient?  High (or low!) pH?  But plants actually do have some other responses to conditions they don’t like.  And if we look closely and really pay attention, it’s possible to identify and rectify some problems.  A phosphorus deficiency is one of the easiest problems to identify.

As a primary macronutrient, phosphorus is required for plant respiration and photosynthesis.  It’s essential to the production of flowers, fruit, and seeds.  Adequate phosphorus helps roots grow better in general and plants grow better in cold temperatures.  It also helps plants use water efficiently. 

The following factors all affect phosphorus availability in the soil:

• Aeration.  Poorly aerated soil, due either to compaction or poor drainage—reduces the amount of oxygen available to the plant roots.  This, in turn, can reduce phosphorus uptake by up to 50%.
• Cation exchange capacity.  The amount of phosphorus in the soil is a function of the amount of clay and organic matter in the soil.  The more there is, the more phosphorus there is. Soil with a high cation exchange capacity holds more organic matter, thus facilitating the movement of phosphorus into plant roots.
• Compaction.  Compacted soil hampers root spread and impedes the plant’s ability to obtain phosphorus from new locations.
• Moisture.  Insufficient water reduces phosphorus availability and uptake. On the other hand, excess watering also diminishes the amount of oxygen getting to roots and thus reduces phosphorus uptake.
• Mycorrhizas.  Mycorrhizal fungi can increase the amount of phosphorus available to plants.
• Organic matter.  Organic matter contains phosphorus and can increase the amount of phosphorus available to plants.
• pH.  At a pH below 6.5 or above 7.5, phosphorus will bind with calcium or aluminum, respectively, and become less available to plants.
• Temperature.  Cold soil hinders the uptake of phosphorus.

Plants are most likely to suffer from a phosphorus deficiency when they are grown in compacted, poorly aerated soil that is low in organic matter. Symptoms are also more prevalent early in the season when the soil is cold.

SIGNS OF A PHOSPHORUS DEFICIENCY

Phosphorus really moves around in plants—it will move from old leaves to new growth quite readily.  For this reason, signs of phosphorus deficiency are most often exhibited on older leaves first.  The signs vary widely among the species, but the common signs include the following:

• Plant growth will be stunted or dwarfed.
• Leaf tips appear burned.
• Older leaves may turn a dark blue-green or reddish purple; they may go yellow first and drop off instead of turning color.
• The rest of the leaves may become spotted with dead tissue.

Species-Specific Signs

• Brassicas—the stems and undersides of the leaves often turn purple.
• Corn—the edges of the leaves may turn purple.  The stems and undersides may also turn purple.
• Legumes—often show signs of nitrogen deficiency because a lack of phosphorus diminishes the ability to use nitrogen-fixing bacteria to produce nitrogen.
• Lettuce—the stems and undersides of leaves often turn purple. 
• Strawberries—leaves turn red or reddish brown, usually from the outside in.
• Tomatoes (and other nightshades)—the stems and undersides often turn purple.

Quick Steps for Correcting a Phosphorus Deficiency

• Neutralize the soil pH—aim for a pH between 6.5 and 7.5.
• Loosen and aerate the soil.
• Warm the soil, if it is early in the season. 
• Maintain adequate moisture levels.
• Apply a thick layer of organic mulch.

If the above steps fail to completely rectify a phosphorus deficiency and a soil test (if available) confirms that the problem is due to low phosphorus availability in the soil, you will need to dig some organic matter into the soil.  And you will need to add organic matter periodically to prevent future deficiencies.  Organic fertilizers high in phosphorus include bone meal and animal manures, especially poultry manure.  Rock dust and rock phosphate are other natural soil amendments. (They aren’t organic—organic matter, by definition, comes only from plants or animals).  There is also a variety of synthetic high phosphorus fertilizers. 

Links to related posts:

Fertilizer by the Numbers, Part 1

Fertilizer by the Numbers, Part 2

References:

https://plantprobs.net/plant/nutrientImbalances/phosphorus.html