by Sandy Hollingsworth, Gilpin County Master Gardener
“N, P, K, Fe, Ca, Mg, pH…” Listening to gardeners talk about their soil can make you wonder if you need a science degree to understand it all! Since soil health is the foundation of a successful garden, amending it to its best before planting will help your garden grow. Spring is a good time to test the soil and see what minerals and nutrients it needs. CSU offers reduced cost soil tests which give you a detailed report of information about what is too high, too low and just right in your garden soil. Too much of a good thing like compost will interfere with your plants’ ability to take up nutrients needed for growth. I was surprised to learn from my vegetable garden soil test last year that I did not need to, and should not, add any more compost in the spring or manure in the fall as that was my annual routine in my home garden. Yet in the Gilpin County community garden where I volunteer the test recommended adding more compost for 2-3 years, plus certain minerals and nutrients in each garden.
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| pH and soil type chart |
In Colorado, the soil is generally high pH (although this rule of thumb does not always hold in the mountains), and the goal is to build the soil to 6 to 7.2 pH for growing vegetables. The overall organic content is best in the 4-5% range which is ideal for the release of nitrogen from the soil. That is why I do not need to add yet more compost since mine was over 5%. You may find that you need to continue to slowly build your organic matter an inch or two at a time over several years, like in the community garden, which was at 4.4%. “Organic matter is also an important energy source for bacteria, fungi and earthworms that live in the soil.” (CSU Fact Sheet #7.235)
Compost and aged manure well mixed into your garden help improve the soil texture, tilth, aeration, drainage, and water retention so is well worth the work. Organic soil amendments and compost tea can be used to increase minerals and micronutrients. Gardeners can avoid chemical fertilizers or choose synthetics as preferred.
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| Soil Nutrients (credit Lumen Learning) |
Primary nutrients for plants include Nitrogen (N), phosphorus (P), and potassium (K). Aside from carbon (C), hydrogen (H), and oxygen (O) these are used by plants in the greatest amounts. C, O, H are generally gained from air and water aided by potassium in the plant. N/P/K help with foliage, root, flower, and fruit growth, plus disease protection. Packaged fertilizers list these 3 on labels to help you pick what you need in higher or lower percentages. Nitrogen helps green growth while phosphorus promotes fruits and flowers so you give the veggies or flowers what they need throughout their growth stages.
Secondary nutrients, needed in lesser amounts, include Magnesium (Mg), calcium (Ca), and sulfur (S). They are just as important to the overall health of the plant.
Micronutrients are needed in even smaller amounts and most soil amendments will contain some amount of them. Micronutrients include zinc (Zn) iron (Fe), manganese (Mn), copper (Cu), boron (B) as listed on my CSU soil test reports, plus molybdenum (Mo), nickel (Ni), Chlorine (Cl) and cobalt (Co).
Locally we can readily find amendments for minerals and micronutrients which include:
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| Alfalfa pellets (S. Hollinsworth) |
- Alfalfa Meal pellets for readily available nitrogen; they also feed soil organisms. It’s better to get ones that only contain alfalfa pellets.
- Ammonium Sulfate to add nitrogen. Can also be used for side dressing after plants are up and growing.
- Bio char which may increase soil fertility, help plants use nitrogen and sequester carbon. (CSU Fact sheet 0.509 details the research: https://extension.colostate.edu/topic-areas/agriculture/biochar-in-colorado-0-509/)
- Blood meal as a fast release nitrogen which purportedly also may repel deer if you need that benefit.
- Bone meal or bone char to add phosphorus and calcium.
- Fish emulsion or fish meal as a source of nitrogen and potassium. It is a byproduct of fish farming and has an odor, which may attract certain animals until it settles in.
- Kelp Meal from dried, ground up seaweed provides trace minerals, amino acids, and enzymes.
- Mycorrhiza stimulate plant and root growth and are beneficial to soil life.
- Phosphate and superphosphate which promote flowering and fruiting. Best incorporated into the soil in spring or fall before planting if needed, as most Colorado soils have adequate phosphorus.
- Soybean meal (non-GMO if you prefer) with high amounts of slow-release nitrogen and potassium.
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| Blood Meal (S. Hollinsworth) |
According to CSU research and Plant Talk information, Green sand, Rock powders, Gypsum and Lime are often not needed amendments in most areas of Colorado (although a soil test may indicate otherwise) – they may be beneficial in other parts of the USA.
Also “in areas like Colorado, where the entire growing season is used for vegetable production, a green manure is less practical.” For additional information, refer to CMG Garden Notes #244, Cover Crops and Green Manure Crops (http://cmg.colostate.edu/Gardennotes/244.pdf). But if you can squeeze in a cover crop or rotate beds to allow it every other year it can be quite beneficial for soil health plus may attract pollinators while in bloom. Till it into the soil before it is 4 inches tall to add the plant material into the garden and plant into the amended soil.
If you have not done a soil test to get recommendations, amendment products packaging usually includes suggested application rates.
A few important amendment and soil tips are:
– Mix any amendments well into the top 6-8 inches of soil where plants and roots grow.
– Avoid working soil when it’s too wet as it can damage soil.
– Once your soil is amended, avoid stepping on it to prevent soil compaction.
– Side dressing with more fertilizers as needed means mixing it into the soil next to and around plants without disturbing them too much.
– Retest soil after the growing season or early the next year to know what adjustments are needed before the next planting.
– In general manure is best added in the fall to allow several months for it to break down in the soil, and only use well aged manure as fresh manure can have high salts, ammonia, and even e-coli.
For more information CSU Fact Sheet 7.235 (https://extension.colostate.edu/topic-areas/yard-garden/choosing-a-soil-amendment/) provides details on wood, manure, peats, biosolids, plant- based soil amendments, both types to use and avoid, plus application tips for building up your garden. Garden Notes #234 (http://cmg.colostate.edu/Gardennotes/234.pdf) explains more about fertilizing and fertilizers. CSU soil test information http://www.soiltestinglab.colostate.edu/
After you make the time to amend, you’ll be ready for planting and sowing seeds, or planting seedlings when the soil and weather warms up for a yummy harvest!
