Feed the soil and the plants

What N, P and K do, how to read a fertilizer label, how to turn a soil-test recommendation into pounds of product, side-dressing and compost rates.

1. What nitrogen, phosphorus and potassium do in a garden Understand

Plants build leaves, stems, roots and fruit from water, air and a set of elements they take up from the soil through their roots. Three of those elements are needed in amounts large enough that fertilizer is sold by them: nitrogen (N), phosphorus (P) and potassium (K). We call them the macronutrients in this course, following the usage on fertilizer labels. Other elements matter too, but a beginner's fertilizer decisions are almost always about these three.

This module does not teach plant chemistry. It teaches the decisions a home gardener makes with these three elements: whether to add any, how much, in what product, and when. For that, what matters is how each one behaves in the soil and what goes wrong when there is too much or too little.

Nitrogen: the one plants use most and lose most easily

Convention The University of Nebraska-Lincoln's guide G945 describes nitrogen as the nutrient plants need from the soil in the largest quantity. It is also the nutrient that moves most readily with water, which is why vegetable fertilizer programs center on it. The University of Maryland Extension's page on fertilizing vegetable gardens gives a general rate of 2 lb of nitrogen per 1,000 sq ft for vegetables and 3 lb for heavy feeders such as tomatoes, broccoli and beets. A heavy feeder is a crop that extension sources single out for a higher nutrient rate than the general garden figure. Those rates are Maryland's general guidance for a garden with no soil test; a lab report written for your soil replaces them.

Physical principle Too much nitrogen is not a safe surplus. Nebraska's G945 says it can push leafy growth and delay fruit. Maryland describes the same effect as lush leaves at the expense of fruit, and adds that excess nitrogen can leave plants more open to pests and disease. The UC Statewide IPM Program's aphid Pest Note says high nitrogen favors aphid reproduction and advises never using more than necessary. Module 10 returns to that link between feeding and pests.

Convention Soil already releases some nitrogen on its own as organic matter breaks down. Maryland estimates about 0.4 lb of nitrogen per 1,000 sq ft per year for each 1 percent of organic matter, and calls that conservative. On that estimate, a soil at 5 percent organic matter releases about 2 lb per 1,000 sq ft, which is Maryland's whole general recommendation. That is one reason a soil test with an organic matter figure changes the fertilizer answer.

Phosphorus: stays where you put it

Physical principle Phosphorus behaves almost the opposite way. Nebraska's G945 calls it a non-mobile nutrient in the soil and says to apply it before or at planting, worked into the root zone, rather than scattered on top later. New Mexico State University's Guide H-120 says phosphorus becomes available to plants gradually, so it can go on all at once before planting. Because it stays put, phosphorus builds up over years of fertilizing and composting. Many long-used gardens test high in it, and Maryland says that when a soil test shows high phosphorus and potassium, adding more does not benefit the plants.

Phosphorus that does leave the garden, carried by water off bare or overloaded soil, ends up in streams and lakes, where Oregon State's EM 9308 notes that it promotes algae. Section 5 covers that, and the local rules some places have about it.

Potassium: often already there

Convention Clemson University's Home and Garden Information Center (HGIC 1228) notes that plants do not use the three nutrients in equal quantities, and that nitrogen and potassium are needed in larger amounts than phosphorus. Whether a garden needs added potassium depends on the soil. Nebraska's G945 says potassium is usually high in Nebraska soils, and New Mexico's H-120 says it is usually adequate in New Mexico. Those are statements about two states, not a national rule. Your lab report says whether yours is low.

Why leaf color is not a soil test

Test it Maryland lists stunted or slow growth, pale leaves and low yields as general signs of a nutrient shortage, and then warns that crowding, poor light, compacted soil and root-knot nematodes produce the same signs. A pale tomato in a shaded bed may need sun, not nitrogen. Nebraska's G945 adds that a soil pH far from the ideal (it gives 6.5 as ideal for most vegetables, and says soils below 5.5 or above 7.5 often need modification) can lock up nutrients that are present, so more fertilizer does not fix the problem. Module 3 taught the soil test because it is the only tool in this course that measures what the soil holds. This module turns its recommendations into action.

2. Read the guaranteed analysis on a fertilizer label

A fertilizer bag, box or jug carries three numbers separated by hyphens. Clemson's HGIC 1228 explains what they mean, and everything else in this module rests on reading them correctly.

Convention The three numbers are the grade: the percent by weight of nitrogen (N), phosphorus expressed as phosphate (P₂O₅), and potassium expressed as potash (K₂O), always in that order. The block of text on the label that lists them, with the forms and sources of each nutrient, is the guaranteed analysis. Clemson explains that the law requires the manufacturer to guarantee the nutrient claims on the label, which is why that block is called the guaranteed analysis.

Clemson's example is a 100-lb bag with the grade 18-4-10. It holds 18 lb of nitrogen, 4 lb of phosphate and 10 lb of potash. The other 68 lb is carrier and filler that helps the product spread evenly. A 50-lb bag of the same grade holds half of each. That is the whole idea behind every calculation in Section 3: percent by weight tells you how many pounds of nutrient are in a pound of product.

Complete, incomplete and ratio

Clemson calls a fertilizer with all three numbers above zero a complete fertilizer, and advises choosing an incomplete one (with a zero in one or two places) when the soil test recommends only one or two of the three. Maryland makes the same point: if the test shows high phosphorus and potassium, use a nitrogen-only product. The common habit of buying one bag with three equal numbers for every crop and every bed adds phosphorus and potassium whether the soil needs them or not.

The rest of the label

Beyond the grade, a label lists the nutrient sources (which tells you whether the nitrogen is fast or slow, Section 4), directions for use, cautions and storage. Clemson notes that under South Carolina law a product called slow-release must have at least a third of its nitrogen in a slow-release form. That is one state's legal definition, cited here as an example of how labeling terms are regulated state by state. Read the whole label before buying, and keep it: the directions and cautions are your record of what the maker says the product is for.

3. Turn a recommendation into pounds of product

A soil report gives a recommendation as pounds of a nutrient per 1,000 sq ft (some labs use per 100 sq ft; the math is the same once the area matches). A bag gives a percent. The conversion between them is one division, worked by the University of Maryland Extension and stated as a rule in New Mexico's H-120.

Physical principle Pounds of product = pounds of nutrient ÷ (percent on the label ÷ 100). New Mexico writes the same thing as 100 ÷ percent × pounds of nutrient needed. Both say: if a pound of product holds 0.12 lb of nitrogen, you need 1 ÷ 0.12, or about 8.3, pounds of product for each pound of nitrogen.

Then scale to your area. Soil reports use 1,000 sq ft as their unit because it is a round number, not because gardens are that size. A 4 × 8 ft bed is 32 sq ft, which is 0.032 of 1,000. Multiply the per-1,000 figure by your area and divide by 1,000.

The steps, in order

1. Write down the recommendation exactly as the report gives it: the nutrient, the number, the unit and the area unit. "2 lb N per 1,000 sq ft" or "0.4 lb P per 1,000 sq ft."
2. Convert to the label's form if needed. Elemental P × 2.29; elemental K × 1.2 (NMSU H-120). Nitrogen needs no conversion.
3. Find the matching number on the bag. First number for N, middle for P₂O₅, last for K₂O.
4. Divide. Pounds of nutrient ÷ (percent ÷ 100) = pounds of product per 1,000 sq ft.
5. Scale to your area. × your square feet ÷ 1,000. Multiply by 16 for ounces.
6. Check what else the product brings. Multiply the pounds of product by each of the other two percents. If the product supplies a nutrient your report says not to add, choose a different product.
7. Weigh it. A kitchen scale kept for garden use is the reliable tool. A cup of one product does not weigh the same as a cup of another.

The fertilizer calculator does steps 4 and 5 for you and prefills your area from setup. It does steps 2, 3 and 6 only if you enter the right numbers, so do those by hand first.

When a product carries more than one nutrient: size by nitrogen

Convention A product with two or three nonzero numbers cannot match two recommendations at once unless its percents happen to be in the same proportion as the report's. New Mexico's H-120 says to size a mixed fertilizer by the nitrogen requirement. Its tomato example shows why: sizing by phosphorus would have applied about 28 lb of product instead of about 21, and supplied too much nitrogen. Too much nitrogen has the effects in Section 1; a small phosphorus shortfall can be made up separately before planting. The worked example in Section 6 shows this with Plot 14's numbers.

One pound at a time

Convention New Mexico's H-120 also says to split nitrogen into two or three applications, and to apply no more than 1 lb of nitrogen per 1,000 sq ft at one time, to avoid salt burn. It says the same splitting applies to manures and other organic materials that carry salts. If your report recommends 2 lb of nitrogen for the season, that becomes two applications of 1 lb each unless the report itself says otherwise. A report's own instructions take precedence over a general rule from another state's guide.

Containers

Convention Containers of potting mix do not use per-area recommendations. Penn State Extension's container guide says plants in pots need more fertilizer than plants in the ground because they can reach only what is in the pot, and it gives a pattern: a slow-release fertilizer at planting, then a soluble fertilizer every 2 to 4 weeks. The amount comes from the product label, since a soil test of potting mix is rarely how container gardeners work. Marisol's balcony, in this course's fictional cases, follows that pattern with the label's rate for container vegetables.

4. Organic and synthetic, fast and slow, and side-dressing

The plant takes up the same nitrate whether it came from a bag of synthetic fertilizer or from compost. What differs is how fast the nutrient becomes available, how much of it there is per pound, and what else comes with it.

Side-dressing

Side-dressing is applying fertilizer beside growing plants, in a band along the row or a ring around each plant, partway through the season, rather than mixing it in before planting. Maryland says to side-dress granular fertilizer after plants are established, not at planting, and to keep it off stems and leaves.

Convention Kansas State University Extension's article on side-dressing garden vegetables gives timing by crop. We use it for timing only.

The article also warns against weed-and-feed products in a vegetable garden. Those combine fertilizer with an herbicide and can injure vegetables.

5. Compost, too much of a good thing, and runoff

Compost is an amendment first and a fertilizer second. It adds organic matter, which Module 3 described as improving how soil holds water and air. It also carries nutrients, at low and variable concentrations, which is why it counts in the fertilizer picture.

How much compost

Convention Oregon State University Extension's EM 9308, on using compost in gardens, gives the rates this course uses:

The arithmetic for any depth: area in sq ft × depth in inches ÷ 12 = cubic feet. A 4 × 8 ft bed at 1/2 in needs 32 × 0.5 ÷ 12 = 1.33 cu ft, a little more than one 1 cu ft bag.

Compost counts toward the nutrients

Physical principle OSU EM 9308 warns that too much compost can reduce water quality: excess phosphorus and potassium can move into surface water and groundwater and feed algae, and excess soluble salts can injure seeds, transplants and sensitive plants, with damage that looks like fertilizer burn. Oregon State's EC 1561 adds that many manures and manure-based composts are high in soluble nitrogen, ammonia or salts, and that composting concentrates salts; it names chicken and feedlot manure composts and mushroom compost as typically high in salts. It advises checking a compost's analysis for soluble salts and ammonium nitrogen, applying 1 in or less of materials high in ammonium plus nitrate, and allowing about a month between applying high-ammonium material and planting.

So a gardener who adds a thick layer of compost every year and a full fertilizer program on top can end up with more phosphorus than the soil needs. The soil test is how you notice. Nebraska's G945 suggests retesting every fifth year unless a major amendment has been added since the last test. A heavy compost habit counts as a major amendment.

The EPA page lists what does not belong in a home compost pile, including meat, dairy, pet waste, diseased or pest-infested plants and plants treated with herbicide. Module 11 covers manure timing before harvest, which is a food safety question rather than a nutrient one.

Over-fertilizing

The extension sources in this module agree on what too much does:

• Root and leaf burn. Maryland and New Mexico both say excess fertilizer can burn plants; New Mexico ties the burn to fertilizer salts.
• Leaves instead of fruit. Excess nitrogen gives lush leaves and delays or reduces fruit (Nebraska G945, Maryland).
• More pest and disease pressure. Maryland says excess nitrogen can make plants more vulnerable. UC IPM ties high nitrogen to aphid reproduction, and UConn's fact sheet on powdery mildew of cucurbits lists avoiding excess fertilizer as a cultural control.
• Nutrients that leave the garden. Maryland says leaching and runoff carry nutrients away from roots and pollute surface water and groundwater, and advises addressing erosion and standing water.

Physical principle "More" is not a remedy. If a plant looks poor and you do not know why, adding fertilizer adds one more unknown. The diagnosis order in Module 10 (check water, light, pests, then nutrients, with a test) applies to feeding too.

Runoff, and local rules on phosphorus

Runoff is water that flows over the soil surface instead of soaking in, carrying soil particles, and whatever nutrients are attached or dissolved, toward drains and streams. Granules spilled on a sidewalk or driveway go straight into a storm drain with the next rain. To keep fertilizer where it belongs: sweep stray granules back onto the bed, do not apply before heavy rain, water in gently rather than until water runs off, keep bare soil covered with mulch (Module 8), and keep a strip between the bed and any paved surface.

Ask the authority Some states, counties and towns restrict when and where phosphorus fertilizer may be applied, especially near water, often aimed at lawns but sometimes written broadly enough to cover gardens. Some also set dates or setbacks for any fertilizer. We do not list these rules because they differ by place and change. Ask your state department of agriculture or your county extension office (find yours through the Extension Foundation's state directory) whether any apply to you. A community garden may also have its own rules about what can be added to plots.

6. See it See it

Both worked examples use this course's fictional cases. The soil test results are invented for the examples, as are the product percents, which were chosen to show the arithmetic and are not product recommendations.

• Fertilizing Vegetable Gardens (University of Maryland Extension): the product math worked three ways, organic matter's nitrogen contribution, and the signs of over-fertilizing.
• Reading a Fertilizer Label (Clemson HGIC 1228): the three numbers, the 100-lb bag example, ratios and slow-release nitrogen.
• Home and Market Garden Fertilization (New Mexico State Guide H-120): the P and K conversion factors, the 100 ÷ percent rule and why mixed products are sized by nitrogen.
• How to use compost in gardens and landscapes (Oregon State EM 9308): compost rates for new and existing beds, bag coverage, and what too much compost does.

7. Practice Practice

Minimum: Find your area, copy the recommendation or the label directions, read the label, calculate the amount and plan the compost; skip the side-dressing dates and the amendments sheet. Time: about 45 minutes, plus a few minutes at each application. Materials: your soil test report from Module 3 (or the date you sent the sample), the label of any fertilizer you own or are considering, a calculator or the fertilizer calculator, your bed measurements, and a kitchen scale kept for garden use.

1. Find your area. Add up the square feet of every bed you will feed, from your bed sheet. Divide by 1,000 and write that fraction down; it is the multiplier for every recommendation.
2. Copy the recommendations exactly. For each nutrient on your report, write the number, the form (N, P or P₂O₅, K or K₂O) and the area unit. If the report gives a recommendation by crop, copy the one for each crop group you grow. If you have no report, write "none yet" and go to step 7.
3. Convert forms. Elemental P × 2.29, elemental K × 1.2. Skip if the report already uses the oxide forms.
4. Read your label. Write the three numbers. Note whether any nutrient the report says not to add has a nonzero number.
5. Calculate. Enter the recommendation, label percent and area in the fertilizer calculator, or divide by hand. If the product carries more than one recommended nutrient, size it by nitrogen and compute what it supplies of the others.
6. Go deeper Plan the split and the timing. If the season's nitrogen is more than 1 lb per 1,000 sq ft, split it. Mark side-dressing dates from the Kansas State table for the crops it names, and write "ask extension" beside any crop it does not.
7. If you have no report: read the label's directions for vegetables, and write down the amount it gives for your area. That is the fallback until the report arrives. Do not use this module's example rates in its place.
8. Plan compost. Using Oregon State's rates, compute the cubic feet for your beds at the depth you choose. Convert to bags or cubic yards.
9. Go deeper Record it. Enter each planned application in the amendments sheet: date, bed, material, amount, unit, and which recommendation it answers. Update the row with the actual date and amount when you apply it.

8. Your garden Your garden

9. Check your work Check your work

Criteria

• Can you point to the line on your soil report (or the label direction) that each amount came from?
• Did you match the report's form (P or P₂O₅) to the label's form before dividing?
• Does your area multiplier match the beds you will actually feed, not the whole yard?
• Did you check what else the product supplies, and is any of it something the report says not to add?
• Is any single nitrogen application above 1 lb per 1,000 sq ft, and if so, does your report say to do that?
• Does every planned application appear in the amendments sheet with its reason?
• Is your compost depth inside the range for your bed type, and have you counted it as part of the nutrient picture?

Worked example

The Okafors' plan passes each criterion. Their amounts trace to one lab report line. No form conversion was needed because the report gives nitrogen only. The multiplier, 0.064, matches the two beds. The 12-0-0 product adds nothing the report excludes, and they set aside the three-equal-numbers bag for that reason. Each application is 1 lb N per 1,000 sq ft, at New Mexico's limit, as the report's split instructs. The amendments sheet has two planned rows. The one gap in their plan is honest and named: the tomato side-dressing question is with extension. Their compost plan uses the low end of Oregon State's range because the fill is already one-third compost.

Plot 14's plan shows the multi-nutrient case: conversion done (0.92 lb P₂O₅), sizing by nitrogen, the phosphorus shortfall computed rather than ignored, and the decision about it handed to the people who interpret the test.

Common mistakes

• Dividing by the wrong number. Using the first number for a phosphorus recommendation. Say the nutrient out loud as you find its number on the bag.
• Forgetting the percent sign. Dividing 2 by 12 instead of 0.12 gives 0.17 lb where the answer is 16.7 lb per 1,000 sq ft. If the result looks a hundred times too small, this is why.
• Mixing P and P₂O₅. Applying a P recommendation as if it were P₂O₅ supplies less than half of what was recommended. Check the report's units.
• Scaling to the wrong area. Using the whole yard, or forgetting to divide by 1,000. A 64 sq ft garden needs 6.4 percent of the per-1,000 figure.
• Measuring by volume. Scooping "a cup" of product. Weigh it.
• Treating a side-dressing as extra. Side-dressings share the season total unless the report says otherwise.
• Feeding a symptom. Adding fertilizer to a pale or slow plant before checking water, light and pests. Module 10 is the order to check in.

What is still unresolved

This module cannot tell you whether your garden needs fertilizer at all; your soil report does. It cannot settle side-dressing for crops Kansas State does not name, or whether a small shortfall matters; your extension office can. It cannot tell you local rules on phosphorus; your state or county can. And it cannot tell you how your plants responded; your harvest log and next soil test, reviewed in Module 12, can.

10. Knowledge check and scenario

Before you move on

• I can say what each of the three label numbers means and which form (oxide or element) my report uses.
• I converted each recommendation on my report into pounds or ounces of a product for my actual area, or wrote down the label directions as the fallback.
• I checked what else the product supplies and that no single nitrogen application exceeds what my report allows.
• I planned side-dressing dates for the crops Kansas State names, and noted the others as questions for extension. (Go deeper; optional)
• I computed a compost amount and entered planned applications in the amendments sheet.
• I know where to ask whether local rules limit phosphorus or fertilizer timing.