It's all about the dirt - or is it?

Garden soil management

Dirt is not the same as soil. Soil is a complex living system made up of minerals, organic materials, water, and air. Dirt is what lays beneath the soil, and will not support plant growth.

The mineral portion represents the major component of the soil. It has specific physical and chemical properties that were determined when the soil was formed thousands of years ago. Physical properties such as soil texture (soil particle size) and soil structure (how particles are held together) determine air and water movement, affecting plant growth. Soil color is determined by the amount of organic matter, drainage conditions, and oxidation or weathering.

Most soils in Fremont County, need some improvements before planting to produce large healthy plants. Depending on your location some soils are too wet due to poor drainage, others are compacted due to heavy clay soils, and some dry out too quickly due to the large amount of sand in the soil.

The “ideal soil” has about half of the total volume made up of equal parts of air and water, and the other half is minerals 45% and organic matter 5%.

Effects of tillage on soil structure and plant growth

The soil should never be tilled when too wet. If the soil is worked under wet conditions, the soil will become hard and restrict root growth, causing unproductive plants. If a handful of soil formed into a ball retains its shape, delay soil tillage until the soil dries. If a handful of soil formed into a ball crumbles when pressed with the thumb, it is ready for tilling.

Fall is the best time to till garden soil unless the site poses a danger of water or wind erosion. Fall tillage helps control certain insects and diseases that overwinter on the plant refuse. The soil is subjected to early spring, freezing, and thawing action that helps improve the soil tilth. The soil will also dry out and warm up quicker in the spring.

Organic matter improves soil quality

· Improves the physical condition or structure of the soil, increasing the tilth or ease of working

· Increases water absorption

· Increases aeration, allowing more oxygen to roots

· Increases nitrogen and other nutrient retention

· Aids soil microorganism populations

As plants grow, the amount of organic matter in the soil becomes progressively lower unless organic matter, like compost, green manures, and finely ground yard waste is added to the soil.

Animal manures – Dry, pulverized manures are a good source of humus and plant food if there is not too much litter (straw, hay, etc.) mixed in. Manure can have its nutrients leached out during heavy rains, and can introduce weeds into the garden. Poultry, rabbit, and goat manure are high in nitrogen and may burn plant roots if used too generously.

The value of manure as plant food depends on the extent to which it has been diluted or leached by water.

Percent of available nutrients in

undiluted animal and poultry waste

Undiluted Waste	% Nitrogen (N)	% Phosphorus (P)	% Potassium (K)
Cow*	.6	.15	.45
Horse*	.5	.35	.4
Poultry**	1.3	1.17	.48
Goat**	1.03	1.20	1.50
Sheep**	.95	.35	1
Rabbit**	1.1	1.5	.47

* Cow and horse manure can be applied at the rate of 500 to 1,000 pounds per 1,000 square feet of garden area.

** Poultry, goat, sheep, and rabbit manure are higher in nutrient content and should be added at the rate of 200 t0 400 pounds per 1,000 square feet of area.

Compost – Compost is a good source of humus and a great way of recycling plant refuse from the yard or garden. However improper composting that fails to generate adequate heat will not kill weed seeds, disease organisms, or roots of some plants like quack grass. Diseased plants and weeds that have gone to seed are best discarded and not placed in the compost pile.

Soil pH affects plant growth

Soil pH is a measure of soil acidity. Soil pH meters measure the hydrogen concentration in the soil solution. The concentration of hydrogen ions (h+) is compared to hydroxyl ions (OH-). When the two are equal the soil is neutral; it has a pH of 7.

Most Wyoming soils are alkaline or basic, with a soil pH above 7.1. Most plants grow best in slightly acid soil with a pH of 6 to 6.8. Within this range root growth, plant vigor, nutrient availability, and microbial activity are optimal. Above this range, plant growth can be severely limited because nutrients become unavailable to the plant, and soil microbes work less effectively.

Adding acidic organic matter or sulfur to reduce soil pH

Soil pH levels can be altered through the addition of acidic organic matter, sulfur, ammonium sulfate, or iron sulfate.

The ideal way to lower pH is to accomplish it naturally through the use of organic matter. Sphagnum peat or evergreen needles worked into the soil will gradually lower the pH and increase valuable organic matter content.

When using sulfur to reduce soil pH, the rate depends on the soil type, beginning pH level, and desired final pH level. Sulfur reacts slowly with soil and must be converted into sulfate before it can be active. Sulfur applications are long-lasting and may only be necessary every 3 to 4 years.

Gypsum

Gypsum is used as a soil amendment to help water infiltration where high sodium (Na+) soil conditions exist.

Gypsum will not improve soil drainage problems caused by compaction from traffic, or heavy clay soils low in organic matter. It is only effective when poor drainage is due to soil structure breakdown.

Gypsum requirements can be determined by tests that measure the electrical conductivity of the soil. These types of soil tests can be provided by commercial soil testing labs. The maximum application rate of gypsum is 50 pounds per 1,000 square feet in the spring and fall.

Gypsum separates into calcium and sulfate. In the soil, sulfate forms sulfuric acid that neutralizes any effect the calcium may have on raising pH. Adding gypsum does not change the soil pH. Calcium replaces the sodium on the cation exchange sites. The sodium and sulfate are leached from the soil through water application. Soil particles then come together to form aggregates or “soil crumbs” that improve soil structure (tilth) and drainage.

Soil tests analyze specific soil nutrients

A general soil test provides information about soil pH plus levels of phosphorus, potassium, and nitrogen. Yearly soil tests are not necessary; testing every three to four years is generally sufficient. More advanced soil tests can provide information on levels of organic matter, zinc, and other elements.

Before you add soil amendments other than organic matter a soil test is a must to determine what rates the amendments should be applied at. Most soil conservation districts can provide these tests.