Vegetable Garden Plant Rotation

Soil testing and crop rotation help!


By: Sandy Feather ©2012
Penn State Extension

Q. I am interested in having the soil in my vegetable garden and flower beds tested. I have grown tomatoes in the same area for a number of years – they did not do well this year; neither did some of my flowers. My neighbor told me the soil could be lacking nutrients. I till in a few inches of mushroom manure to both my flower beds and vegetable garden every year. My neighbor told me to try liming the soil. Any suggestions would be appreciated.

A. While I strongly encourage you to have your soil tested, nutrient imbalances might not be the only thing troubling your garden. This has not been the easiest summer for gardeners in our area. Depending on where you live, rainfall has been sorely lacking, and keeping ornamental and vegetable gardens supplied with adequate moisture can be challenging. Also, late blight has been reported on tomatoes in our area, and if you grow impatiens, downy mildew has taken its toll on that popular annual this summer.

Soil test kits are available from your local Penn State Cooperative Extension office. In Allegheny County, soil test kits are $12 for the first kit and $9 for additional kits ordered at the same time. They come with complete information for taking representative samples and understanding your soil test results. You can send a check for the cost of the number of kits you want to:
Penn State Extension, Soil Test Kits, 400 North Lexington Street, Pittsburgh, PA 15208. Make checks payable to "Penn State Extension"


Soil test results

Once you purchase the soil test kit, take the sample and fill out the paperwork, and then send it to Penn State’s Agricultural Analytical Laboratory. The fee covers the cost of the kit and the actual testing. Your only other cost is the postage to send it to the university. The kit is a self-contained mailer with the correct address pre-printed on it.

Penn State soil tests evaluate the level of phosphate, potash, calcium and magnesium, as well as the pH (acidity or alkalinity) of your garden’s soil. The lab does not test for nitrogen since the level of nitrogen in the soil changes so rapidly. Nitrogen can be lost to leaching, volatilization and/or runoff. Nitrogen recommendations are based on the known needs of the crop you are growing.


Community vegetable garden

You should have separate tests done for the flower beds and the vegetable garden. If you wanted to have the soil in your lawn tested, that should be a separate test as well. The reason for separate tests is that different crops have different nutritional requirements. These differences determine the soil lab’s recommendations.

It is a good practice to incorporate organic matter, such as mushroom compost, into your garden soil annually. Organic matter improves the structure of the soil, which permits greater aeration, water penetration and improves drainage in our clay soils. It creates a favorable environment for microbial action and insect activity, which aerate and enrich the soil. Organic matter also increases the soil’s water and nutrient holding capacity.


Compost adds organic matter to soil

The only harm may be in using mushroom compost year after year. Mushroom compost tends to have a moderately high pH, around 8.0. Your soil test will reveal if adding mushroom compost every year has raised the pH of your soil above what most vegetables prefer, between 6.0 and 6.8. If so, your test results will include instructions to use sulfur to lower the pH into a more suitable range. I would not add any limestone until you have your soil tested to see what the pH is now, since limestone will only raise the pH more. You can alternate using other sources of organic matter, such as homemade compost, aged manure, composted grass clippings (not treated with herbicides) and/or shredded leaves. Mushroom compost is a very good soil amendment. Just temper its use with the knowledge that it’s high pH and soluble salt content can throw off the balance of your soil.


Yummy homegrown tomatoes!

It concerns me that you have been growing tomato plants in the same place for a number of years. Growing the same crop in the same place year after year does deplete the soil of crop-specific nutrients. It also creates an opportunity for soilborne diseases to become established in your garden. In the case of tomatoes, fusarium wilt and verticillium wilt are of most concern. Even in a small garden, crop rotation is an important tool to minimize the chance of such diseases becoming established in the soil.

Remember that it is important to rotate among vegetable families, rather than specific crops. For example, it would not be helpful to rotate between tomatoes and peppers, eggplants or potatoes because they all belong to the nightshade (Solanaceae) family. 

Below is a list to help plan crop rotations

List of vegetable plant families:

Amaryllidaceae (Amaryllis Family) - leeks, Egyptian onions, onions, Spanish onions, shallots, garlic, chives

Apiaceae (Parsley Family) - carrots, parsnips, celery

Chenopodiaceae (Goosefoot Family) - beets, Swiss chard, spinach

Compositae (Composite Family) - lettuce, endive, sunchokes, dandelion, chicory

Convolvulaceae (Morning Glory Family) - sweet potato

Cruciferae (Mustard Family) - broccoli, Brussels sprouts, cabbage, cauliflower, collards, kohlrabi, pak choi, radishes, kale, mustard greens, rutabaga, turnips, watercress, horseradish

Cucurbitaceae (Gourd Family) - cucumbers, winter squash, pumpkins, summer squash, melons, gourds

Fabaceae (Pea Family) - all dried & fresh beans, peas

Gramineae (Grass Family) - sweet corn

Liliaceae (Lily Family) - asparagus

Malvaceae (Mallow Family) - okra

Polygonaceae (Buckwheat Family) - rhubarb, sorrel

Solanaceae - eggplant, pepper, tomato, potato


Vegetable garden topics

How to take a soil sample

Sandy's tree tips