Soil and Bed Preparation for Growing Dahlias

Page 5

1. Dahlias prefer soil containing good amounts of humus.

"Humus"-- organic material, derived from animals and plants, that breaks down in the soil to a dark, gummy substance is the amendment by which faulty texture and tilth are best remedied. In sandy soils humus binds sand particles together retaining water that can be used by plants. In clay soils it keeps particles apart, allowing room for air and water.

How much composted material should be added to the soil?

Well composted humus material may be added to the soil in large quantities without fear of injuring plants or retarding their growth. An annual application rate of 250 to 500 pounds per 1,000 square feet or a 2 to 4 inch layer tilled into the soil would not be too much.

Contrary to popular opinion, humus soil amendments (composted yard trimmings, composted manures, and sphagnum peat moss) are not an efficient source of nutrients for plants. In most cases, commercial fertilizers will be required to provide nutrients to dahlias. See Tables I and II.

What are good sources of humus?

Partially decomposed compost is an excellent source of humus. Some examples are shown in the pictures on the right. It is important that these wastes are partly decomposed before they are added to the soil. Composting will kill most harmful pathogens. Few nutrients are available to the plants until their form has been altered by decomposition.

Moreover, the micro organisms that cause decay, burn large amounts of nitrogen in the first stages of decomposition. If these wastes are to be used raw (without composting), nitrogen fertilizer must be added to the soil to compensate for the nitrogen used by the micro organisms. The amounts of nitrogen required will vary depending on the type of material. Moreover the amount of material to be applied will vary. The assistance of a horticulturist with expertise with composting and soil amendments should be sought.


Active Compost Pile

Composted Yard Trimmings

Shredded Yard Trimmings

Shredded Hay



Grass Clippings

Partially decomposed and aged barnyard manures are another good source of humus.

As with compost, the partial decomposition of manures is important. Most harmful pathogens are killed by composting. In addition the strong odor from fresh manures is caused by excessive amounts of ammonia which is harmful to plant roots--aging and composting reduces the amount of ammonia. Aging also provides opportunity for the salts in manure to leach away. Most plants do not tolerate salt well. Feed lot and horse manures typically contain excessive amounts of salt. Dairy manures typically do not contain quite as much salt.


Composted and Aged Manure

Continued on next page.

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