Home and Market Garden Fertilization
Esteban Herrera, Extension Horticulturist
College of Agricultural, Consumer and Environmental Sciences, New Mexico State University
Vegetable gardening has become so popular in recent
years that about half the nation’s homeowners now grow
some vegetables. In particular, New Mexico has experienced
a tremendous upsurge in vegetable gardening.
Success in gardening depends much upon soil fertility. If soil lacks fertility, gardeners must add plant nutrients in the form of fertilizer. This guide will help gardeners select and use fertilizers to obtain high yields of good quality vegetables.
Having the soil tested is an excellent way to know its
fertility level and how much fertilizer to add. In every
New Mexico county, an Extension agent can provide
information about how to have soil tested.
Major Plant Nutrients
The major plant nutrients are nitrogen (N), phosphorus
(P) and potassium (K). New Mexico soils are commonly
deficient in nitrogen and phosphorus, but potassium is
Commercial fertilizer labels usually show, in percentages, how much actual nitrogen (N), phosphorus as available phosphoric acid (P2O5) and potassium as potassium oxide (K2O) they contain. Fertilizer recommendations often are given in terms of N, P and K.
To convert a recommendation for P into the P2O5 form, which is most common, multiply the P value by 2.29. Similarly, multiply the K value by 1.2 to obtain the recommended amount with K2O (although this is rarely needed in New Mexico). Thus, a 20–20–20 (N-P-K) recommendation would mean the fertilizer contained 20 units of nitrogen, 45.8 units of phosphorous and 24 units of potassium.
Amounts for Small Areas
Fertilizer recommendations often are made in terms of
pounds per acre. However, most gardeners plant areas
much smaller than an acre. Table 1 shows the nitrogen
and phosphorus requirements of some common garden
vegetables for 1,000 ft2. Table 2 shows the requirements
for the same vegetables for 10 foot row.
For easy measuring, use the following equivalents, which are accurate with most granular fertilizers.
1 pint = 1 lb
1 cup = 8 oz
1 tablespoon = 1/2 oz
Table 1. Nutrient requirements of some common garden vegetables (lb per 1,000 ft2).
|Cabbage, broccoli, cauliflower||5.0||2.0|
Table 2. Nutrient requirements of some common garden vegetables, given in oz per 10 linear ft.
|Vegetable||Nitrogen (N)||Phosphorus (P2O5)|
|2-ft spacing**||3-ft spacing||2-ft spacing||3-ft spacing|
|--------------- oz/10 ft of row ---------------|
|Cabbage, broccoli, cauliflower||1.67||2.50||0.67||1.00|
|* Multiply the above amounts by the appropriate fertilizer ratios in tables 3 and 4. For example, if you have planted cabbage in rows with
2-ft spacing, its nitrogen requirement (according to table 2) is 1.67 oz per 10 ft of. You have decided to fertilize with urea.
According to table 3, the factor for N in urea is 2.17. So multiply 1.67 oz/10 ft of row by 2.17: 1.67 × 2.17 = 3.62 oz/10 ft.
**Spacing between rows.
Matching Nutrient Needs and Fertilizers
Commercial fertilizer comes in many different formulations that supply differentnutrients amounts. Therefore, calculations are necessary to determine how much product is required to supply needed nutrients. Table 3 shows some fertilizer formulations and the proportion of fertilizer needed for one unit of nitrogen (N) or phosphoric acid (P2O5). Use table 3 to calculate how much fertilizer to supply a certain nutrient requirement (see examples below).
Table 3. Nutrient content of selected commercially available fertilizers and the proportion of each needed for one unit of nitrogen (N) or phosphoric acid (P2O5).
|Fertilizer**||Nutrient content||Proportion needed
for one unit*
|Percent N||Percent P2O5||N||P2O5|
|* To calculate the amount of fertilizer to apply, multiply this number by the appropriate nutrient recommendation listed in table 1 or table 2.
**For materials not listed, obtain the amount (in pounds) of fertilizer needed by dividing 100 by the fertilizer percentage; then multiply by the amount of nutrient desired.
Example: Broccoli needs 5 lb/1,000 ft2 of nitrogen and 2.0 lb/1,000 ft2 of phosphorous (table 1). The fertilizer is 15–5–0. Therefore,
Nitrogen: 100 ÷ 15 × 5.0 = 33.3 lb/1,000 ft2 of 15–5–0 fertilizer
Phosphorous: 100 ÷ 15 × 2.0 = 13.3 lb/1,000 ft2 of 15–15–0 fertilizer.
Example 1: A gardener has planted chile. Table 1 lists the nitrogen requirement for chile as 1.8 lb per 1,000 ft2. The gardener plans to use urea to fertilize. According to table 3, 2.17 is the proportion needed for one unit of N. Therefore, multiply 1.8 lb/1,000 ft2 by 2.17.
1.8 × 2.17 = 3.91ul>
For chile, the gardener will need to apply 3.91 lb of
urea per 1,000 ft2.
Example 2: For the same chile crop, the gardener needs to know how much P2O5 to apply. Table 1 recommends applying 1 lb of P2O5 per 1,000 ft2. The gardener is going to use triple superphosphate (44% P2O5) to fertilize. According to table 3, 2.27 is the proportion needed for one unit of P2O5. Therefore multiply 1 lb/1,000 ft2 by 2.27.
1 × 2.27 = 2.27ul>
Apply 2.27 lb of triple superphosphate per 1,000 ft2 for chile.
Too much nitrogen can burn plants. If you are applying a mixed fertilizer, apply amounts according to the nitrogen requirement to avoid applying too much. For example, tomatoes. the N requirement for tomatoes (table 1), is 3.7 lb/1,000 ft2. You have decided to use 18–12–6 fertilizer. According to table 3, the proportion needed for one unit of N is 5.6. Multiply the fertilizer recommendation (3.7) by the proportion needed (5.7).
3.7 × 5.7 = 21.09ul>
You will need to apply about 21 lb of 18–12–6 fertilizer per 1,000 ft2 for tomatoes.
What if you had applied the fertilizer according to the P2O5 recommendation? According to table 1, the P2O5 recommendation for tomatoes is 3.4 lb/1,000 ft2. For 18–12–6 fertilizer, the proportion needed for one unit of P2O5 is 8.3. Multiply the recommendation by the proportion needed:
3.4 × 8.3 = 28.22 lb/1,000 ft2.ul>
If you had figured the amount to apply by the P2O5 recommendation, you would have applied about 28 lb
of the fertilizer, which would have provided too much
nitrogen and risked burning the tomatoes.
For vegetable gardening, do not use fertilizers that are mixed with herbicides or pesticides, as plants are easily burned when these are applied to the garden.
Some gardeners may want to use organic fertilizers. Table 4 shows the nitrogen and phosphorus content of selected organic materials and the proportion of each material needed for one unit of nitrogen or one unit of phosphoric acid.
Table 4. Nutrient content of selected organic materials and the proportion of each needed to supply one unit of nitrogen (N) or phosphoric acid (P2O5).
|Organic materials*||Nutrient content**||Proportion
for one unit*
|Percent N||Percent P2O5||N||P2O5|
|Phosphate, rock colloidal||0.0||25.0||0.0||4.0|
|Sunflower seed-oil cake||5.5||1.0||18.1||100.0|
|Wood ashes||-Do not use-||1.0||-Do not use-||100.0|
|* Some organic materials, such as manures, contain salts that may be detrimental to plants ifadded in large quantities. Apply these fertilizers in split
applications (see the section “When to Apply Fertilizers”).
** Average percentages. Actual percentage may vary.
***To calculate the amount of fertilizer to apply, multiply this number by the appropriate nutrient recommendation listed in table 1 or table 2.
When to Apply Fertilizers
Phosphorus becomes available to plants gradually, so it
can be applied all at one time before planting. Nitrogen
applications, however, should be split. Provide nitrogen
in two or three applications, which allows plants to feed
more evenly and decreases the possibility of salt burn.
As a general rule, apply no more than 1 lb of nitrogen at
one time for 1,000 ft2 of garden space. When the total
nitrogen requirement is more than 1 lb, supply it in two
or more applications.
Apply about one-third of the required nitrogen before planting, apply one-third when plants reach 4–5 inches tall, and apply the remaining one-third about a month later. This system of split applications also is good for applying mixed fertilizers.
Planting Vegetables by Group
Home vegetable gardeners tend to have a potpourri
of vegetables in a small area, so fertilizing different
crops with differing amounts can be tedious. However,
if the vegetables are grouped according to their fertilizer
needs, then only two or three different fertilizer
amounts have to be applied.
Table 5 shows vegetable groups and the amount of fertilizer per 1,000 ft2 of any single crop or combination of crops within the group. Use this table to help you group vegetables according to their nutrient requirements.
Table 5. Vegetables with similar nutrient requirements grouped by pounds of nitrogen (N) and phosphorus (P2O5) needed per 1,000 ft2.
|4.5 lb N and
2.0 lb P2O5
|2.5 lb N and
1.5 lb P2O5
|1.5 lb N and
1.5 lb P2O5
|0.5 lb N and
1.0 lb P2O5
|Brussels sprouts||Carrots||Cantaloupe||Peas, all|
|Cabbage (incl. Chinese)||Eggplant||Chile|
|Corn, sweet||Peppers, bell||Radishes|
|Kohlrabi||Squash, summer||Sweet potatoes|
|Lettuce (all)||Tomato||Squash, winter|
Fertilizing Small Garden Areas
If the garden area is small, a compromise fertilizer application can be made. The application will not provide the correct amount of nutrients for each crop, but it should provide enough nutrients for adequate growth and development. For most gardens and vegetables, 3 lb of nitrogen and 2 lb of phosphoric acid per 1,000 ft2 is suitable. Because applied phosphorus is released slowly in most New Mexico soils, the amount applied in the compromise formulation may seem high, but it should satisfy the nutrient need.
To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at aces.nmsu.edu.
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Revised and electronicaly distributed February 2004, Las Cruces, NM.