High Soil Analyses Readings, Poorer Yields

(Question)

We have a vegetable garden which was started in 2010. The soil is very shallow, shale bedrock only 4 or 5 inches below. We made raised beds with good organic loam, 10 inches and have added large amounts of composted manure every year, no artificial fertilizer. Yields of garden veg were initially high, but have dropped. The garden is in full sun most of the day. We live near Belleville.

The problems- soil analyses in 2010 showed Phos 70, Potassium 200, Magnesium 150, pH 7.8.  In Oct 2016: Phos 170, Mg 550, Pot. 620, pH 7.5.

That is such a drastic increase in those elements. Could that be the cause of our poor yields? Another problem, we water with well water which is very sulfured and icy cold. Our garden is situated about 20 feet from a country road which is salted in the winter, and the road is about 15 feet higher than the garden. This area has many corn fields and we are lower than most of them, almost at river level. Could runoff from those fields be affecting our well water?

(Answer)

Thank you for contacting the Toronto Master Gardeners with your gardening inquiry.

Getting your soil tested is an excellent place to start to figure out what could be the culprit of your decreasing yields. Vegetables are kind of fussy about soil chemistry. Too much of certain nutrients and problems arise. In addition to nutrient levels, soil pH is also an important factor to plant growth.

Looking at your numbers I noticed a dramatic increase in phosphorous levels. Phosphorus is an element important to flower and seed producing energy systems and primary root growth. Although phosphorous 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.

Being a responsible gardener you are following the advice of most references and adding composted manure every year to your garden. If you use your own compost that is made mostly from plant materials the nitrogen level will be about 7 times that of phosphorus, which is what your plants want. However, if your compost is made from manure, or you use commercial compost which is based on manure the ratio of nitrogen to phosphorus is closer to 1:1. In this case, once all of the nitrogen is used up by plants, most of the phosphorus is still left in the soil.

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 from one of our earlier posts that 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: https://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. Here is an excellent article on the effects of too much phosprous in your soil : https://www.gardenmyths.com/compost-is-it-poisoning-your-garden/

It is also possible that your soil’s high pH level may be inhibiting the germination of your vegetables.  Most vegetables grow best in a soil with a pH between 6and 7. This range is the best range for bacterial growth in the soil to promote decomposition. The decomposition process releases nutrients and minerals into the soil, making them available for the plants to use. When the pH rating is outside this range, both of these extremely important processes become more and more inhibited, thus locking up the nutrients in the soil so that the plant cannot take them up and use them to their full advantage. Sulphur or moistened peat moss can be added to your soil to lower it’s pH.

You might be interested in this excellent article on perfect soil mix for raised beds:

www.gardeninginraisedbeds.com/raised-bed-soil-mix/the-perfect-soil-mixture-for-filling-your-raised-bed

I hope you have success in restoring the balance of the macronutrients in your garden and that your  harvest is back on track.