Soil Preparation

By Jonathan Wooten

Besides supporting the plant, the primary functions of soil are to retain moisture, to allow air and oxygen to reach the roots, and to make various nutrients available to the plant.  Dahlias require abundant moisture, good aeration and drainage, a near neutral PH of 6.0 to 6.5, and high fertility in terms of Nitrate, Phosphate and Potash.

One of the best ways to find out the suitability of a particular soil for Dahlias is to dig a sample for a soil test. For Connecticut residents this can be accomplished by sending a representative soil sample to the Connecticut Agricultural Experiment Station in New Haven. A good sample should be mixed from several locations from several inches down in the soil. The soil test results will typically report the soil texture, organic matter content, ph, Nitrogen, Phosphorus, Potassium, Calcium and Magnesium. The results should also recommend remedial action such as application of lime to raise ph and calcium, sulfur to lower ph, and specified amounts of fertilizer to address shortages of Nitrogen, Phosphorus and Potassium.

Soil Texture, Drainage and Aeration

Soil texture is defined primarily by the proportions of various sizes of soil particles ranging from sand at the largest, silt at the mid range, and clay which consists of tiny particles similar to flour. Ideal soils are combinations of all particle sizes and are usually labeled in the “loam” category. A loam soil will usually provide good moisture retention, good drainage and good porosity to air.  

The USDA has constructed a Soil Triangle which defines soil type according to percentages of sand, silt and loam. A good practical series of tests to roughly determine soil type is at the bottom of the page.

A clay soil has excellent water retention but extremely poor drainage due to the inability of water to flow through it. In fact, certain types of clay are actually used as water barriers in the bottom of landfills. Clay soils are typically hard when dry, and become putty like when it rains. Drainage is extremely poor. Dahlia roots are forced to sit in water for extended periods and will likely rot. Clay soils need to be amended to increase porosity to both air and water. While intuition would suggest that sand would be the best remedy, anything less than huge quantities of sand would not help much because the small clay particles fill the large spaces between sand particles and the porosity is little better than before.

 Sandy soils have excellent porosity and drainage, but too much of a good thing. Porosity is so good that water is not retained and fertilizers tend to leach out very quickly. Addition of silt or clay based soils can help, but the amounts required to change the soil in a meaningful way would be large and impractical for many people to implement.

 The best solution to both clay and sandy soils is to add organic matter. In a clay soil, the large vegetation particles create spaces between clay particles for air and water to penetrate. In a sandy soil, the compost acts like a sponge and retains water and fertilizer which otherwise would drain off. Even loam soils benefit from organic amendments which further increase the water retention of the soil and improve porosity.  It is important to continue to add organic matter each year, as the composting process eventually proceeds to complete destruction of cellulose ending as carbon which no longer helps the soil much.

 An ideal organic amendment is composted manure. Another good amendment is almost any type of garden compost, although this may be hard to obtain in meaningfully large quantities. Many towns provide free or low cost compost make from leaf pickups to town residents. I have had excellent success with uncomposted fresh leaves raked up in the fall and tilled in with a large rototiller. Unlike woody amendments like sawdust and wood shavings, leaves compost rapidly in the spring and even make the soil more resistant to compaction.

 One great source of organic matter which is often overlooked is mulches, which see. One advantage is the benefits the mulch provides during the current year. The ability to till the mulch in as a soil amendment at the end of the year is a free bonus. Most mulches can be tilled in safely after a year of exposure to the elements, where there might be problems if they were tilled in at first. Some examples include sawdust, wood shavings, most barks, and small wood chips. These examples might require somewhat higher nitrogen application in the next year, but this ordinarily is not a problem.

 The best time to work the soil is in the autumn before it becomes wet and soggy over the winter. The fall weather is more predictable and one can relax in the early spring when there are lots of other things to do. Typically, the workability of the soil in the spring is poor and there may not be adequate time before planting time.

 A Simple Soil Texture Test

 Gather a small walnut-sized ball of soil and moisten it until it has a putty-like consistency. Roll the soil in your hand into a ball and feel the texture between your fingers.

1. Soil does not form a ball à sand

2. Soil forms a ball, but not a ribbon à loamy sand

3. Soil forms a dull ribbon that breaks when 1” long and

a) Grinding noise is audible, gritty feel à sandy loam

b) No grinding noise is audible, smooth, floury feel à silt loam

c) Grinding not clearly audible, grittiness and smoothness à loam

4. Soil has moderate stickiness and firmness, forms a ribbon 1” - 2” long and

a) Grinding is audible, gritty feel à sandy clay loam

b) No grinding audible, smooth floury feel à silty clay loam

c) Grinding not clearly audible, grittiness and smoothness à clay loam

5. Soil has dominant stickiness and firmness, forms shiny ribbon >2” and

a) Grinding is audible, gritty feel à sandy clay

b) No grinding audible, smooth floury feel à silty clay

c) Grinding not clearly audible, grittiness and smoothness à clay