From the Director’s Desk: Building Better Soils for Better Results

Leonar­do da Vin­ci once said, ​“We know more about the move­ment of celes­tial bod­ies than about the soil underfoot.”

The aver­age soil’s com­po­si­tion con­sists of approx­i­mate­ly 50% solids and 50% pore space. The solids con­sist of about 45% inor­gan­ic min­er­al par­ti­cles (sand, silt, clay), and 5% organ­ic par­ti­cles. The pore space is made up of 25% water and 25% air, but varies some based on rainy weath­er and dry periods. 

Soil tex­ture is the per­cent­ages of sand, silt, and clay in the matrix. By par­ti­cle size, sand is the largest, clay is the small­est, and silt is in between. Tex­ture can­not be changed eas­i­ly, so it is best to work with what is present. 

Soil struc­ture has a major influ­ence on water and air move­ment, root and plant growth, and seedling emer­gence. Soil struc­ture describes the arrange­ment of soil solids and the pore space between them. This arrange­ment is deter­mined by the type of soil par­ti­cles and how they bind togeth­er to form aggre­gates. Struc­ture can be improved or degrad­ed by soil man­age­ment. When the pro­por­tion of large to small aggre­gates increas­es, soil qual­i­ty gen­er­al­ly increas­es. In soils con­sist­ing of main­ly small aggre­gates, filled pores and sur­face crusts occur. Filled pores low­er water and air capac­i­ty dimin­ish­ing the con­di­tions for root growth. Sur­face crusts pre­vent infil­tra­tion and pro­mote erosion. 

Some prac­tices such as poor tillage meth­ods and soil dis­tur­bance activ­i­ties lead to poor soil struc­ture. Pes­ti­cide and fungi­cide usage and soil com­paction, in par­tic­u­lar dur­ing wet con­di­tions, will result in poor aggre­gate sta­bil­i­ty. Soil struc­ture can be improved by the addi­tion of organ­ic mat­ter – com­post and mulch – and by pre­vent­ing com­paction and soil dis­tur­bance. Unfor­tu­nate­ly it can take sev­er­al grow­ing years for sig­nif­i­cant organ­ic mat­ter gains, while activ­i­ties that degrade soils can result in rapid, long-term damage.

With­in the soil struc­ture are soil nutri­ents. These include macro- and micronu­tri­ents as well as trace ele­ments. The pri­ma­ry macronu­tri­ents are nitro­gen , phos­pho­rous, and potas­si­um. The three sec­ondary macronu­tri­ents are cal­ci­um, sul­fur, and mag­ne­sium. The micronutrients/​trace min­er­als include chlo­rine, man­ganese, iron, zinc, and cop­per. A soil lab analy­sis, read­i­ly avail­able from your local Coop­er­a­tive Exten­sion, will reveal basic nutri­ent lev­els and pH. 

Soil pH refers to the acid­i­ty or alka­lin­i­ty of the soil. Low­er num­bers are more acidic, high­er num­bers are more alka­line. Most plants grow in a pH of 6.5−7 (slight­ly acidic to neu­tral), the range in which soil nutri­ents are most avail­able. Many plants grow in a low­er or high­er pH, but the list grows short­er as one gets fur­ther away from neu­tral in either direction.

A major com­po­nent of soils that is sel­dom con­sid­ered is ben­e­fi­cial microor­gan­isms. Healthy for­est soils have abun­dant com­mu­ni­ties of microbes, but in most urban soils the pop­u­la­tion is deplet­ed or even absent. Myc­or­rhizae are one such sym­bi­ot­ic microbe. These fun­gi reach out into the sur­round­ing soils and extract min­er­al ele­ments and water for their host plant, then live off the plan­t’s sug­ars. Plants with such sys­tems are bet­ter able to sur­vive in stress­ful envi­ron­ments. Fun­gi also help to form aggre­gates in the soil. The fun­gi strands enmesh soil par­ti­cles and form cross-links between them. Anoth­er required microbe is ben­e­fi­cial bac­te­ria. It improves plant resis­tance to many root dis­eases, helps make essen­tial soil min­er­al ele­ments avail­able, decom­pos­es organ­ic mat­ter, and improves phys­i­cal prop­er­ties of the soil.

In essence, good plants, whether in a yard, park, gar­den or wood­land, require good soils. And good knowl­edge results in good soils.