Healthy soil is key factor in boosting crop yields

Plants are more likely to overcome diseases, pests and weed infestations if they are growing in healthy soil.

Speakers at the annual Sustainable Organic Production seminar for the southern San Joaquin Valley focused on soil health as a key component in not only improving crop yields, but ensuring healthier plants able to withstand environmental stress.

"Soil can absolutely make a difference," said Randy Dodds, sustainable products manager for the agriculture specialty products company Brandt. "Soil is the most important component of a good disease control program."

Plant diseases need a susceptible host, a pathogen and a favorable environment to thrive. If one is missing, Dodds said, disease could be avoided or minimized. Soil can influence both the host plant and the pathogen.

The complexity of the host-pathogen-environment interaction can make diagnosis of a disease challenging. Dodds said pathogens are able to mutate and form new strains that can be more virulent. Dodds used the acronym REPEAT—resistance, eradication, protection, exclusion, avoidance and therapy—to describe a strategy to avoid or minimize plant diseases.

Resistance involves selecting resistant or tolerant varieties. Tolerance means a plant can endure disease without suffering yield or quality losses. Resistant means a plant can suppress or retard a pathogen; common resistance targets are verticillium, fusarium and nematodes.

Eradication is removing or rouging diseased plants and also eliminating diseased plant material to suppress inoculation. Solarization of ground is also an eradication option. Planting cover crops like brassicas that inhibit soil pathogens can also help control disease.

Protection is application of biocontrol products that can thwart root pathogens and allow plants to compete with pathogens for sugars and amino acids in the soil. Using disease-free transplants and paying attention to soil movement is the exclusion component. Avoidance can be done by planting in fields with lower soil-pathogen numbers. Therapy is the application of products that improve plant nutrition. Organic solutions include soaps, salts and oils.

Integrated pest management plant pathologist Jim Stapleton, who works out of the University of California Kearney Center, said moving away from fumigation to control soil pests has been difficult. Success with alternatives to fumigants requires knowledge about soil pathogens present and trial and error. Host plant resistance and biological control will play a role in success with a crop, he said.

Biofumigation and anaerobic soil disinfestation are being studied to determine their efficacy on nematodes, Stapleton said. Biofumigation is the use of crop residues that release toxic compounds during decomposition. The compounds can reduce nematode pest populations or activity, but the outcome is unpredictable. Anaerobic soil disinfestation is a systems approach to controlling soil pests. The approach includes solarization, plus application to the soil of agricultural waste such as tomato pomace and compost. The combination of heat and decomposition increases soil acidity, making the environment for soil pests less desirable.

Poor soil management is one of the reasons pests such as the root knot nematode are able to proliferate, said Larry Parker with Westbridge Agricultural Products.

Other reasons crops such as almonds, walnuts and carrots have trouble with high nematode populations in the soil include inadequate crop rotation and use of chemicals that disrupt biological control processes.

Parker said solarization, used to kill soil pathogens and pests, also kills arthropods in the soil. Soils can be repopulated with these beneficials with the application of composts. Some arthropods can affect crop yields, but others compete with a wide variety of root and foliage feeding pests. When a healthy population of beneficial arthropods is present in the soil, Parker said, they could be effective in controlling many pest outbreaks. The population of the beneficials can only be maintained in the soil if there is a constant source of non-prey to consume—a sign of a healthy soil.

Two nematode predators found in healthy soils are mites and springtails. The one-millimeter-in-length mite is one of the most common invertebrates found in composts. Some of the predatory mites found in the soil feed on nematodes as well as other mites, eggs and larvae. The springtail is a wingless insect also found in compost. It feeds on nematodes in addition to decomposing plants, pollen and fungi. Some types of soil bacteria and fungi also feed on nematodes.

Parker advised building soil organic matter where plant parasitic nematodes are affecting crop yields. If applying crop-protection materials that will kill beneficials, he added, wait until the active ingredients have degraded before applying compost to re-inoculate soils with predatory mites.

Weed control has been one of the major issues with organic production and, while it does not usually have the devastating effects of disease and pests, weeds can rob soil of nutrients and moisture, and control can account for a large portion of the budget in organic crop production.

UC Cooperative Extension farm advisor Steve Wright in Tulare County said knowing the critical period for weed control is an important component of an integrated weed management program. Fresno State student Sarah Parry presented the results of a weed trial that showed it is not necessary to keep crops weed-free the entire growing season. The study on romaine lettuce showed that weed control more than four weeks after transplant did not add to crop yield or quality.

(Cecilia Parsons is a reporter in Ducor. She may be contacted at cecilia parsons8@gmail.com.)