Excess phosphorus in a city garden

(Question)

I have intensively used my city garden. Each year I put on shredded leaves in the fall, and plant a rye cover crop after each bed has been used. I put on as much city compost as I can as well as a small amount of home made compost.

In the last few years, i have had problems with carrots, beets not germinating, parsley not growing,raspberries with yellow leaves and little crop

A soil test showed high phosphorus, more than 4 in all beds. Have I added too much compost or too much rye turned in???

I do not know how to reduce the phosphorus levels. I have limited amounts of space too and do not have room to rotate the crops much. Suggestions???

(Answer)

It is obvious that you are a passionate and thoughtful gardener, and you seem to be doing everything that organic vegetable gardeners should do, including having your soil tested. Although phosphorus is an important soil macronutrient, it has slow solubility, which means that it does not leach away as do potassium and nitrogen, so it will build up in soils that are fertilized frequently.   An excess of phosphorus also inhibits the growth of mycorrhizae in the soil and can be the cause of deficiencies in micronutrients such as zinc and iron.

The Missouri Botanical Garden web page on Soil Fertility and Fertilizers explains it this way:

  • Phosphorus is an element important to flower and seed producing energy systems and primary root growth. It is relatively immobile in the soil even under the best conditions. When it is applied as a fertilizer, it stays in the zone of application for long periods. For this reason, it is often not necessary to reapply fertilizers containing phosphorus each season.

Another piece of the puzzle is your soil’s pH, which you may also wish to have tested as most vegetables prefer the mid-range of the pH scale and may exhibit symptoms of deficiencies in micronutrients at higher pH levels.

Part of the challenge in interpretation of soil nutrient analyses is that countries measure these levels in different ways.  I am assuming that your measurement follows the Defra Index, which is discussed extensively here: http://www.pda.org.uk/leaflets/24/leaflet24-5.html .

Recent research suggests that many organic farming operations are likely to have a phosphorus surplus because their systems rely on manures or composts as sources of nitrogen.  On the other hand, sparing use of composts and manures may produce the opposite result and supplements of phosphorus may be required to sustain crop production. So there is obviously a happy medium that needs to be found.

Here are some steps you may wish to consider:

  • Avoid adding manure, which is typically high in phosphorus, as a fertilizer to increase nitrogen levels in the soil.
  • Try to include in your crop rotation some nitrogen-fixing crops such as any members of the Fabaceae family (peas and beans).
  • If you are using fertilizer, use a formulation without phosphorus.
  • Try a different cover crop: according to the Ontario Ministry of Agriculture, Food and Rural Affairs, rye is considered to be the best cool season cereal crop for taking up leftover nitrogen from previous manure applications but can tie up nitrogen in spring when needed by the following crop. OMAFRA’s information pages on cover crops provide some alternatives, including sweetclover, a nitrogen-fixing crop credited with moving phosphorus and potassium into the root zone of following crops via its tap root system: http://www.omafra.gov.on.ca/english/crops/facts/cover_crops01/sweetclover.htm

The high level of phosphorus in your soil, and its likely consequence, low levels of the micronutrients iron and zinc, may have caused problems with your vegetables.  It is also possible that this is the cause of your raspberries’ yellow leaves and lack of fruit: the yellowing of leaves is a symptom of a condition called iron chlorosis, which occurs when a plant cannot take up enough iron from the soil to keep its leaves green.  Yellowing leaves with green veins is a typical symptom. Iron chlorosis is treated with iron supplements in the form of either powder or foliar sprays (which may also be used, along with zinc, for treating other plants affected by high phosphorus levels).

Raspberries are susceptible to many other pests and diseases, so these should be ruled out.  One disease that can cause yellowing of leaves is the Raspberry Bushy Dwarf Virus (RBDV), described fully in this bulletin: http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2013/05hrt13a1.htm

It is possible that your soil’s pH level may be inhibiting the germination of your vegetables.  Beets and carrots grow poorly in acid soil.  Beets prefer a pH of between 6.0 and 7.0 and carrots between 6.0 and 6.8 (but will tolerate a slightly wider range).  Soil pH also affects the availability of phosphorus, which is most accessible to plant roots when the pH is in this range.  Good soil aeration is important to all root vegetables, but this should not be an issue in your garden.  Carrots are slow to germinate, and often germinate unevenly over a period of several weeks.

As for parsley, it too is notoriously slow to germinate, sometimes taking weeks, and it will not germinate if the soil dries out during that period. Soaking the seeds before planting is helpful.  Once established, parsley should be watered deeply at least once a week.  It is relatively pest- and disease-free, but susceptible to lack of moisture. One significant pest, whiteflies, suck sap from the plant, resulting in a yellow mottling on the surface of the leaf, as well as leaf loss, wilting and stunting. An insecticidal soap may be used to control whiteflies.

I thought you might find this article interesting.  It is directed to rosarians, but I felt it was a good discussion of phosphorus, and one I hadn’t considered until now: http://www.rose.org/phosphorus-fallacies-too-much-of-a-good-thing/

It is amazing to ponder what makes up soil, and how its many organisms come together to support the life of a plant.  I hope you have success in restoring the balance of the macronutrients in your garden and that your 2015 harvest is back on track.