3 Main Ingredients in Fertilizer and Best Natural Sources

The 3 main ingredients in fertilizer are nitrogen (N), phosphorus (P) and potassium (K). NC State Extension calls them the primary macronutrients, because plants need them more than any other soil nutrient. The three numbers on every bag give the share of each one by weight.

Natural fertilizers supply the same three nutrients. They differ in how much they hold and how fast it becomes available. This guide shows the N-P-K of each common natural source, the benefits the evidence supports, and a calculator that compares two products on the cost of the nitrogen your crop can actually use.

What the 3 main ingredients in fertilizer do

Plants need 17 essential nutrients. Carbon, hydrogen and oxygen come from air and water, and they make up about 94% of plant weight, according to NC State. The other 14 come from the soil, and N, P and K lead that list.

Nitrogen also leaches easily from soil, so timing matters, especially for N. Potassium leaches too, while phosphorus does not leach readily.

How to read the three numbers

Take a 100-pound bag labeled 0-20-10. NC State explains that it holds no nitrogen, 20 pounds of phosphorus reported as P2O5, 10 pounds of potassium reported as K2O, and 70 pounds of filler. The filler makes the product easier to spread and lowers the risk of burning plants.

One trap applies to feed-store products. Colorado State University Extension warns that bone meal bought at a feed store may list elemental phosphorus, not P2O5. The two numbers differ by a factor of 2.3, so compare like with like. For worked examples of common blends, see our guides to 13-13-13 and 30-0-10.

How much of each nutrient the world uses

Nitrogen dominates on farms as well as in plants. FAO estimates that agriculture used 185 million tonnes of the three nutrients in inorganic fertilizers in 2022, down 7% from 2021. Nitrogen took 108 million tonnes of that total. These figures cover inorganic products only, so manure and compost are not counted.

Chart

World use of the three nutrients

Million tonnes of inorganic fertilizer nutrients in agriculture, 2022 (phosphorus as P2O5, potassium as K2O)

What natural fertilizer means

A natural fertilizer is a product from plant, animal or mineral sources that holds at least one available plant nutrient. People often use “organic” for the same thing. NC State prefers “natural”, because some of these materials are mined ores, such as rock phosphate, and hold no carbon.

The label “natural” does not mean a product is allowed in certified organic farming. The USDA National List sets which substances US organic growers may use. Check with your certifier before you buy. Other countries run their own rules.

Compost and manure are also a separate legal case. Colorado State notes that they often do not meet the legal definition of a fertilizer, though they add small amounts of nutrients. NC State adds that producers need not state the nutrient content of bulk compost or manure, so a lab analysis is worth paying for before large applications.

Best natural fertilizer sources, by nutrient

There is no single best natural fertilizer for plants. The right pick depends on which of the three nutrients your soil test says is short. Start there, then match the product. The typical analyses below come from Colorado State University Extension and SARE. Your product label or lab report comes first.

Chart

Nitrogen in natural fertilizer products

Typical % N by weight, selected products

1. Blood meal and fish meal: high nitrogen

Best for: leafy crops and heavy feeders that are short of nitrogen. Blood meal typically tests 12-0-0 and fish meal 10-6-2. University of Maryland Extension lists blood meal, fish meal and cottonseed meal among its high-nitrogen choices.

2. Fish emulsion: liquid nitrogen with trace elements

Best for: quick feeding and potted plants. Colorado State gives 5-2-2, and lists added micronutrients as its main advantage. NC State notes that water-soluble products such as fish emulsion suit times when you want nutrients delivered fast, though Colorado State’s table still lists a release time of one to four months. The main drawback is the smell. Dilute it as the label directs.

3. Seed meals and alfalfa: plant-based nitrogen

Best for: building fertility ahead of a crop. Typical analyses are cottonseed meal 6-0.4-1.5, soybean meal 7-2-1 and alfalfa meal 2-1-2, each with a release time of one to four months in Colorado State’s table. Alfalfa may carry seeds. Corn gluten meal (9-0-0) is the one to know if you are searching for a fertilizer that also limits weeds: it inhibits seed germination, so products warn you to wait before seeding. It does not harm established plants.

4. Bone meal and rock phosphate: phosphorus

Best for: soils that test low in phosphorus. Bone meal typically tests 3-15-0. Rock phosphate has a catch. Colorado State research found that rock phosphorus is not available to plants unless soil pH is below 7.0, whatever the grind size. Check your pH first. Colorado State reports the same limit for bone meal: its phosphorus is only available to plants in soils below pH 7.0. If your soil is alkaline, neither product will fix a shortage.

5. Kelp and seaweed: micronutrients first

Best for: trace elements, not bulk nutrition. Kelp powder is about 1-0-4 and releases within a month. Other seaweed products have negligible N-P-K. Greensand and other rock powders can add potassium, but Colorado State says they are often unnecessary on soils already high in potassium, so test first.

6. Manure: all three, in bulk

Best for: farms with livestock nearby, or where hauling costs are low. Manure supplies all 3 main ingredients in fertilizer along with organic matter. The nutrient content depends on the animal and on its feed. SARE’s approximate totals, per ton of solid manure, are in the chart at the end of this section.

By SARE’s estimate, supplying 100 pounds of nitrogen takes about 10 tons of solid dairy or swine manure, 7 tons of beef manure, or 4 tons of chicken manure. Those amounts also bring phosphorus, so a nitrogen-based rate can build up soil P over time. The University of Kentucky walks through balancing manure with crop needs.

University of Georgia Extension advises composting fresh manure to kill pathogens and weed seed, since fresh manure can damage plants and run off. Composting has a cost too: SARE notes that considerable ammonium is usually lost in the process. Follow local manure rules, including setbacks and application dates.

7. Compost: organic matter first, nutrients second

Best for: improving soil structure over years. Treat compost as a soil builder, with a small nutrient return in the first season. Oregon State University predicts that finished compost releases less than 10% of its total nitrogen as plant-available N in the first 10 weeks. At high rates that still adds up. One inch of a 2% N compost supplies about 120 to 160 pounds of available N per acre over that period, by OSU’s estimate.

Worm castings are a form of compost made by earthworms. Their analysis varies by product, so go by the label.

8. Green manure: grow your own nitrogen

Best for: field-scale nitrogen with no spreading. Legume cover crops such as clover, vetch and peas fix nitrogen from the air. SARE reports that hairy vetch can contribute 100 pounds of nitrogen per acre or more to the next crop, and that winter and field peas fix 100 to 150 pounds or more per acre. Inoculate the seed with the right bacteria unless you have grown that legume recently.

Grasses behave differently. They scavenge leftover soil nitrogen, but if grown to maturity they tie up nitrogen as they decompose. OSU’s field work found that oat and phacelia immobilized nitrogen while vetch supplied it. Green manure also supplies no new phosphorus, as OSU points out.

Chart

Nitrogen and phosphate in solid manure

Approximate total pounds per ton, by animal

How fast natural fertilizer releases nutrients

The number on the bag is not what the crop gets this season. Soil microbes must convert organic forms first. Colorado State says this activity depends on soil above 50°F with enough moisture, so cold or dry soil delays release.

• Specialty products at 8% to 14% N released about 60% of their total N as plant-available N after 4 weeks in OSU’s trials.
• Finished compost is predicted to release less than 10% in the first 10 weeks.
• The second year adds a little more: about 5% to 10% of total N, according to OSU.

OSU’s work was done in western Oregon, so timing will differ in other climates. A soil nitrate test during the season shows what is really there. That difference in release rate is why the calculator below asks for availability as well as N-P-K.

▶ Why organic matter matters in farm soil, and what it does below the surface. · 7:33

Compare the cost of natural fertilizer

NC State cautions that natural fertilizers can be expensive when applied at rates that fully feed a crop, though they also improve soil structure. So compare products on cost per pound of available nitrogen, not per bag or per ton. Maryland’s method shows why: replacing 4.35 pounds of urea (46-0-0) with cottonseed meal (6-2-1) takes 33.3 pounds of product to supply the same total N.

Try the numbers for your own farm below.

Benefits of natural fertilizer the evidence supports

Natural fertilizer has real benefits, with limits. Here is what extension services and SARE report.

1. More soil organic matter. Compost, manure and cover crops add it. SARE’s review of many experiments found cover crops raised organic matter by 8.5% on average.
2. Better soil structure. A long-term Vermont trial described by SARE found that manure offset many of the negative effects of continuous corn silage on clay soil.
3. Slower nutrient release. NC State and Colorado State both describe this. It lowers the burn risk, but the crop may run short if the soil is cold.
4. Recycling of farm byproducts. SARE notes that over 70% of the nitrogen, 60% of the phosphorus and 80% of the potassium in feed can pass through to the manure, depending on ration and animal.
5. Soil protection from cover crops. Green manure keeps living roots in the ground, and SARE links this to less runoff and erosion.
6. Some carbon storage. Added organic matter holds carbon, but there is a limit: a SARE worked example for dairy manure assumes about 75% of the solids decompose to CO2 in the first year.
7. Trace elements. Fish and kelp products add micronutrients that a straight N-P-K product may not list.

Natural does not mean no pollution

Manure can pollute water as well. The US EPA names both chemical fertilizers and animal manure as sources of nutrient losses when crops do not use all the nitrogen and phosphorus. Its advice is the same for both: the right amount, at the right time, with the right method and placement. Year-round ground cover and reduced tillage help too. NC State also warns that natural sources can be overapplied and can damage plants.

▶ What soil organic carbon is and how it improves soil structure. · 12:28

How to choose and apply natural fertilizer

Use these steps whether you farm 500 acres or a raised bed. Always follow the product label and local rules, and ask an agronomist or extension office for advice on large applications.

1. Test the soil for N, P, K and pH.
2. Set the target from a local recommendation. Maryland, for example, suggests about 2 pounds of N per 1,000 square feet for vegetable gardens, and 3 pounds for heavy feeders such as tomatoes.
3. Pick the source that fits the shortage. Avoid manure-based products if P is already high; OSU suggests a legume cover crop or a low-P product in that case.
4. Work out the rate from the analysis and availability, then err on the low side, as NC State advises.
5. Incorporate into moist soil where you can. SARE notes that manure gives the most nitrogen benefit when worked in soon after spreading.
6. Retest during the season and adjust.

Crop, tree and houseplant notes

Vegetables: Gardeners growing peppers can see which fertilizers suit pepper plants for a crop-specific example.

Trees and forestry: Applying organic mulch and composted leaf litter recreates a forest floor and holds moisture. NC State suggests an organic mulch 1 to 3 inches deep on bare soil to limit erosion. Fertilize young trees only when a soil or leaf test shows a shortage.

Indoor plants: Worm castings, diluted fish emulsion and seaweed products are common choices because small doses are easy to measure. Feed small amounts during active growth. Follow the label dilution, because too much can still damage roots.

▶ Seven simple farming practices that protect soil and water. · 4:34

Using field data to time applications

Natural fertilizer releases slowly, so uneven fields can be hard to judge from the road. Satellite vegetation maps from Farmonaut show which parts of a field are lagging through the season. They show a change in plant growth, not its cause, so walk those areas and test soil before adding more nutrients.

Related pages on our site: soil fertility, organic farming practices, regenerative agriculture, pillars of sustainable agriculture, newer sustainable practices, climate-smart farming, farm web applications, large-farm management, carbon footprinting, product traceability, crop loans and insurance, satellite API and API developer docs.

FAQ

What are the 3 main ingredients in fertilizer?

Nitrogen, phosphorus and potassium. Labels show them as three numbers, in that order, with phosphorus reported as P2O5 and potassium as K2O. The rest of the bag can be filler, other nutrients or carrier material.

What is the best natural fertilizer?

It depends on your soil test. Blood meal or fish meal suit a nitrogen shortage, bone meal suits low phosphorus in soil below pH 7.0, and compost suits low organic matter. Legume cover crops suit field-scale nitrogen.

Does any fertilizer prevent disease?

No fertilizer prevents disease on its own. NC State links adequate phosphorus and potassium with greater disease resistance, and University of Georgia Extension says green manures can reduce certain disease problems. A shortage or an excess can both weaken a crop.

Is there a natural fertilizer that also helps prevent weeds?

Corn gluten meal is sold for this. Colorado State notes it is marketed as a pre-emergent for annual grasses in bluegrass lawns. It also stops the seeds you want from germinating, so wait as the label directs before seeding.

Which fertilizer helps root growth?

Phosphorus promotes root formation, according to NC State. Bone meal is a well-known natural source. It only helps if the soil is short of phosphorus and below pH 7.0, so test both first.

How do you apply natural fertilizer?

Work solid products into moist soil, and apply liquids as a drench or spray as the label directs. Colorado State advises foliar sprays only when temperatures are below 80°F. Apply ahead of crop demand, since release takes weeks.