Managing Soil Fertility
16 | Unit 1.1
- Influences on the nutrient release from cover crops
a) Soil temperature and moisture conditions
b) Placement of the residue
i. Residue on soil surface: Will decompose more slowly due to drying
ii. Incorporation into the top 6–8 inches of the soil: Will decompose most rapidly
due to high oxygen levels and the presence of large populations of decomposing
organisms
iii. Below 6–8 inches: Will decompose more slowly due to lower oxygen levels, fewer decomposers
c) Composition/“quality” of the cover crop residue
i. The C to N ratio of the cover crop residue and N mineralization
· C:N ratios around 22:1 or less = net mineralization of N
· C:N ratios above 22:1 = net immobilization of N
ii. Optimum stage of development to incorporate cover crops = 75%–100% of full bloom
iii. The presence of lignins and tannins in cover crop residue slows decomposition
- The timing of nutrient release, crop demand, and the fate of essential plant nutrients
a) Managing the timing of nutrient release from cover crops to coincide with crop demand
b) Leaching: Nutrients (N) can become vulnerable to loss if timing is mismatched
c) Nutrient deficiencies: If timing is mismatched, nutrient deficiencies (N) may then result
- Some effects of cover crops on agricultural soils
a) Improvements to soil physical properties: Carbon and nutrient cycling through the
use of cover crops
b) The influence of cover crops on disease and pest severity
i. Rye, triticale, forage rapeseeds, mustards, and oil seed radish are known to
suppress certain plant parasitic nematodes and soil borne diseases
ii. Many legumes can actually increase pest populations
c) Weed-suppressive effects of cover crops
i. Competition for light/smothering
ii. Allelopathy
- Importance of gathering regional cover crop information
C. Composts and Animal Manures in Sustainable Agriculture (see Unit 1.7, Making and Using
Compost)
- Composts
a) How much compost to apply annually
b) The nutrient contribution of a manure-based compost: ~1N-1P-1K, i.e., balanced
contribution of N-P-K. As nutrient levels in compost vary, it is recommended that
you check with supplier or have a compost nutrient assessment done to confirm
nutrient levels and proportions.
c) Application timing: Nutrient release should ideally coincide with crop demand
i. Depending on compost quality, may be an inefficient source of N in short term
ii. Release of N may last 6 weeks–several months following incorporation,
depending on compost quality and environmental conditions
iii. Need to incorporate into root zone if applying mid season as side dress
d) Compost quality indicators
i. C:N ratio
ii. CO 2 levels
iii. Ammonia levels
iv. Smell
Instructor’s Lecture 2 Outline