Problems with Garden Soils

A number of garden soil problems can prevent the growth of healthy flowers and vegetables. Among these are various soil-borne fungal diseases and microscopic worms called nematodes. Herbaceous plants, with their relatively small, fibrous root systems are most susceptible to these soil pest problems.

Fungal diseases are most commonly found in heavy, poorly drained garden soils. Commonly referred to as water mold fungi, they include the species fusarium, pythium and phytophora. Small amounts of these organisms can be found in most soils, however, when conditions are right they multiply and infect plant roots. Infected roots die, leaving the plant unable to take up sufficient amounts of water and mineral nutrients. If the infection is severe, plants rapidly wilt and die. If the infection is less severe, plants show show various degrees of wilting even when adequately watered. Yellowing leaves may also develop as plants become starved for nutrients.

Conditions which favor the development of soil-borne diseases include poor drainage and over watering. In-ground vegetable and flower beds should be tilled to incorporate large amounts of organic matter such as compost or manure. This will help to loosen the structure of our heavy desert soils, thus improving drainage. Keeping the soil too wet, even if it has been properly loosened, can also result in root rot.

One of the most common ways root rot fungi become established in garden soils is when they are introduced on diseased plants. Always check the roots of garden plants purchased at nurseries and garden centers. Roots should be light colored, usually creamy white, and numerous. If even part of the roots growing on the outside of the root ball are brown, or black and mushy choose another plant.

Improving soils for planting vegetable and flower gardens can be a slow process. An alternative is to garden in raised beds. Simple frames can be used to hold high quality garden soils, rich in organic matter and possessing excellent drainage.

As well as fungi pests, many garden soils also contain damaging nematodes. These are microscopic worm-like organisms, ranging in size from 1/100 to 1/8 inch in length. They feed on roots either externally or internally, causing stunting, wilting, and slow growth.

Root-knot nematodes are the most damaging and common kinds in home gardens. They enter roots near the young tips, where their feeding stimulates growth of the root tissues, resulting in swollen galls. Nearly all common vegetables and flowers can be injured by root-knot nematodes.

Because damage from nematodes and water mold fungi are to the roots, above-ground symptoms will be similar. Abnormal coloration of leaves, stunted growth, unexpected wilting, and little or no response to watering and fertilizing are the general symptoms of root injury. In the case of nematodes, damage is often sporadic. One or two plants in a row of vegetables may be stunted and off-colored while others remain heathy. That's because nematodes move very slowly in the soil. Hot spots develop where nematode populations are high enough to cause significant injury.

As with root rot fungi, the addition of organic matter to garden soils can also help prevent nematode problems. The reason being that organic matter encourages the growth of beneficial soil micro-organisms which, in turn, feed on parasitic nematodes.

Plentiful amounts of organic matter must be incorporated into garden soils on a regular basis. The best time is just prior to the spring and fall planting seasons. Periodically moving the location of the garden, or crops within the garden, will also help prevent the build-up of harmful fungi and nematodes.

If a garden area cannot be relocated, then consider soil solarization. This is a method by which the soil is heated to temperatures that will kill most soil pests including; harmful insects, nematodes, disease organisms and even weed seeds. The how-tos' of soil solarization will be the feature of next weeks gardening column.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on May 31, 1998.
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