Can too much compost hurt plants? How much is too much?

It is common for people to overuse compost. We often think that since its natural, we can use it as much as we want. But then we learn it the hard way; that’s not the case. Too much of anything can be bad.

So Can too much compost hurt plants? Yes, Using too much compost for a long time leads to an excess phosphorous buildup in the soil. The high levels of phosphorous results in several mineral deficiencies that can cause interveinal chlorosis, stunted growth, low output of fruits and flowers, among other things

How much do plants need?

Compost can be a boon for the plants if appropriately applied. The correct dose depends on the type of field, size, soil composition, etc. Here is an estimated amount required for each type. However, if you want to get the exact amount, a soil test is the way to go

Type Amount of compost required
New Lawns Add 1-2 inch layer of compost 6-8 inches deep before planting
Already grown lawns  Apply 1/4th to 1/2 inch layer on the topsoil
Trees Remove the mulch and apply 2 inches of compost around the canopy. Keep it away from the trunk. Once it is done, cover the compost with mulch.

Composition of compost

To understand why too much compost is terrible, we need to go into a little chemistry. Let’s look at the chemical composition of compos

Chemical Composition in Compost
Nitrogen 0.95% to 1.68%
Phosphorous 0.27% to 1.13%
Carbon 16.6% to 23.89%

The important thing here is to look at the percentage of nitrogen and phosphorous. Based on your raw materials, the level of phosphorus can close to nitrogen. Generally, compost made from adding manure has more phosphorous.

Plants need nitrogen for growth. They use nitrogen to make branches and leave. Hence, the plants deficient in nitrogen have low growth and discolored leaves.

Phosphorous is needed for root growth and seed germination. Without phosphorous, plants develop whithered and purple leaves.

Appropriate quantities are required for a healthy plant. Compost by itself is rich in both and sufficiently fulfills the need(If used in correct doses. But when you apply more compost than required, many issues emerge.

The problems associated with too much

Phosphorous is highly reactive and easily binds with other materials in the soil. This reactivity reduces its solubility in the water. Due to reduced solubility, it doesnt move with soil water. This can cause phosphorous build-up in the soil. When you add compost to the soil, other minerals flow faster than phosphorous. This does not cause problems if compost is added in prescribed amounts, as eventually, it is utilized. But if you add too much, the entire balance gets disturbed

The high phosphorous inhibits the plants from taking iron and zinc. The deficiencies of these two vital minerals can cause a host of problems

  • Zinc deficiency emerge as bleaching of the plant tissue
  • Iron deficiency can cause yellowing between leaf veins
  • Acid loving plants like azaleas and blueberries can die in excess phosphorous
  • Lawn grasses are especially vulnerable as they need high levels of iron
  • Both deficiencies first affect the youngest tissues first
  • High phosphorous can also enter rivers resulting in the growth of many undesirable plants

 

What can you do about it?

  • Regular soil tests are a good idea. They help you determine how much compost do you need to add to the soil.
  • To resolve the issues caused by excess phosphorus, stop applying additional fertilizers to the soil. Yes, this also includes compost and manure.
  • If you do have to add fertilizers, opt for low phosphorous options like pine bark mulch
  • The plants require iron and zinc. But direct addition will not work in this case. The phosphorous will convert them into forms that plants cannot utilize.
  • Foliar (On leaves) application of iron and zinc gives better results.
  • The time required to solve the problem entirely depends on several factors. It can take somewhere from a few months to five years for the soil to completely stabilize excess phosphorous.

How proper use can benefit the soil

I understand if you are a little worried about using compost after reading all these things. There is no need to worry as long as you are using it in moderation. The effects we described above arise when compost is overused for a long time. If used in the recommended dose, it can do so many great things for the soil. Let’s talk about them as well.

  • Compost is an excellent source of carbon, nitrogen, and potassium( And phosphorous!). It also contains several micronutrients that plants desperately need, like cobalt, iron, iodine, molybdenum, manganese, iron, zinc, etc. Most gardeners often forget about these. The more varied materials are used to make the compost, the better mineral profile it gets
  • Nutrients are released from compost in a controlled manner. In spring, when the plant growth is slow, the microorganisms in compost also work slowly. When the summer comes, plants start to grow faster, and microorganisms ramp up the process to furnish minerals required by the plant to grow.
  • The organic matter in the compost nicely forms aggregates with soil. These aggregates are so much better for the plant. The sandy soil is too coarse to form aggregates, and the clay soil turns into a cement-like structure when it gets wet. Aggregates, on the other hand, nicely hold water while maintaining the space for oxygen. They hold both oxygen and water wonderfully, and plants love that!
  • As we just discussed, the water holding capacity of compost is good. But you would be amazed to know that it can hold water 200 percent of its dry weight compared to just 20 percent for ow humus soil.
  • Compost adds beneficial microorganisms and worms to the soil. These are natural soil builders. The teamwork of these creatures benefits everything, including soil, environment, pants, and us.
  • Compost can even neutralize several toxic chemicals by bonding with them. These include toxins like cadmium and lead. Once these are bonded, they cannot be absorbed by the plants.
  • Compost acts as a natural pH buffer. The earthworms constantly pass the soil through their body and balance the pH in the process. You can also change the pH of the compost yourself according to the soil using acidic raw materials like oak, sawdust, and pine needles to get a lower pH and wood Ash to get higher pH.

Final Thoughts

The basic rule of nature is that excess of anything is wrong. The same thing can be beneficial and harmful based on the amount. The same principle applies to compost. It would be best if you always did comprehensive research before using compost. Check your soil composition, compost composition, and type of field you have. If you follow all the principles, you will have an outstanding result

Sources

  • https://agrilifeextension.tamu.edu/library/gardening/phosphorus-too-much-and-plants-may-suffer/
  • https://counties.agrilife.org/valverde/files/2014/11/Phosphorus-Too-Much-Plants-May-Suffer.pdf
  • https://www.mofga.org/resources/soil/phosphorus/
  • https://mcgillcompost.com/mcgill-compost-products/compost-calculator#toggle-id-5
  • https://www.longdom.org/open-access/some-physical-and-chemical-properties-of-compost-2252-5211-1000172.pdf
  • https://pubs.acs.org/doi/abs/10.1021/bk-1997-0668.ch010
  • https://iopscience.iop.org/article/10.1088/2515-7620/ab3b8c/meta Gaston Small et al 2019 Environ. Res. Commun. 1 091007

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