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How to Fertilize Raised Garden Beds

The Alaskan Cooperative Extension published a very useful guide to fertilizing raised garden beds and know how to manage nutrients and minerals in your beds.


“Without a soil test, its hard to know which nutrients your soil needs, but there are a few basic characteristics of Anchorage soils. Most topsoil purchased in bulk is acidic and is low in the primary nutrients, nitrogen (N), phosphorus (P) and potassium (K). Even if you don’t have to buy topsoil, your soil will likely be low in nutrients if fertilizer has never been applied. If you are planting a garden in soil that used to be lawn and the lawn was limed and fertilized on a regular basis, the pH might already be in the ideal range for vegetables and flowers. If a high phosphorus fertilizer such as 8-32-16 was used on the soil, there may be a reserve of phosphorus in the soil and also some potassium. Nitrogen leaches out with watering and is often lacking in soils that have not had compost or other organic soil amendments high in nitrogen added to them on a regular basis. Cooperative Extension Service’s standard fertilizer recommendation for a vegetable garden on soil that has never been fertilized is 4 lbs./100 square feet of 8-32-16. The recommendation is to use half this amount of fertilizer if the garden has been previously fertilized.

To calculate the ACTUAL amount of nutrients you are applying, multiply the amount of fertilizer by each number on the bag which is given as a percentage. For example, to find out how much actual nitrogen you are applying with the above recommendation, multiply 4 X .08 which gives you 0.32 pounds of actual N; 4 X .32 gives you 1.28 pounds of phosphorus (P205); and 4 X.16 to give you 0.64 pounds of potassium (K20). Knowing how to apply this simple math allows you to determine how much of a fertilizer with a different analysis to apply to get the same amount of nutrients.

The fertilizer used in the example above, 8-32- 16, is an inorganic fertilizer. There are organic fertilizer products sold in Anchorage that have half this amount of nitrogen. Two of these products include 4-6-2 and 4-5-3. To apply the same about of nitrogen as in the above example, 0.32 #/100 sq. feet, you’d have to add twice the amount of 4-6-2 fertilizer because the fertilizer is 4% N instead of 8% N. However, if you add 8 # of 4-6-2 fertilizer per 100 square feet, you are putting down only 0.48 # of actual P (8 X0.6) and 0.16 # of actual K (8 X .02). Without additional sources of P and K your plants might not thrive. Bone meal is a good organic source of P and kelp, greensand or wood ashes are a good source of K. Wood ashes also raisesthe pH of the soil, so they need to be added with caution. To boost soil P add 3 lbs. of bone meal or 6 lbs of rock phosphate per 100 sq. feet. For an organic source of K apply 1 lb. kelp and 4 lbs of greensand.

Most topsoil purchased in bulk

is acidic and is low in the primary

nutrients, nitrogen (N), phosphorus

(P) and potassium (K).

An organic grower is continually adding compost and organic sources of nutrients to the soil. This is a good practice for gardeners using inorganic sources of fertilizers as well. With organic fertilizers, not all of the nutrients are readily available to plants. The microorganisms in the soil must work to release these nutrients.

Nutrient break down in warmer climates is faster than in Anchorage and we can not say for sure when organic nutrients applied to the soil will be available for plant growth. In determining how much of an organic fertilizer to apply, it is usually taken into account that some of the nutrients in the fertilizer material will be available the following season and some the season after that. An average analysis for compost is 1-1-1, so you can see although it includes N,P,K and important micronutrients, the fertilizer value of compost is not real high.

The square footage of a standard-size raised

bed garden, 4 ft, wide X 8 ft. long, is 32 square

feet. If the recommended amount of 8-32-16

fertilizer for new garden soil is 4 #/100 sq. feet,

the amount of this fertilizer needed for a 4 ft.

X 8 ft. raised bed is 1.25 pounds. One pound of

granular fertilizer equals two cups. The fertilizer

should be spread on the soil before planting

and raked in. You’ll also need to apply additional

fertilizer midseason and to areas where

you planting a second crop during the season.

When using a fertilizer that is 4% N, apply 2.5

pounds of fertilizer. If using an organic fertilizer

that is slow to break down, apply a little

more or water regularly with fish emulsion

which is usually about 5-1-1. Soil preparation

prior to planting should also include working in

1 lb. of bone meal and 0.5 lbs of kelp along with

1.25 lbs of greensand.

Lime should also be applied prior to planting

and worked into the soil. Apply according to

pH test. If soil has never been limed and will

not be tested, apply 3.5 lbs/ 32 sq. feet (rate, 10

lbs/100 sq. feet).

Without a soil test and a mineralization rate

for organic nutrients, the above recommendations

are an educated guess. Watch for signs of

nutrient deficiency throughout the season.

Nitrogen deficiency shows up as lower yellow

leaves. Plants deficient in phosphorus will exhibit

a purple discoloration. Early in the season

P deficiency may be evident even though here

is enough P is present because of the cold soil.

P deficiency is often seen on the lower leaves of cabbage and broccoli. Potassium deficiency is more difficult to notice. It sometimes starts as a brown stippling of the leaves along the outer edges. Mid-season fertilization will likely be necessary for most crops. An easy inorganic water soluble fertilizer to use is 15-30-15 or for an organic product, use fish emulsion (5-1-1).”

 

Written by Julie Riley, Extension Horticulture Agent

 

The original guide can be found at: http://www.uaf.edu/files/ces/districts/anchorage/horticulture/fertilizing-raised-beds.pdf?__toolbar=1



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