Prior to fertilizing or liming, a soil sample should be taken to ensure the appropriate amounts of nutrients are applied. Ideally, this should be done well in advance of planting (at least three months) to allow any required lime time to make the necessary pH adjustments in the soil. Because applying lime and fertilizer to a garden will change the nutrient content and pH of the soil, a new soil sample should be taken each year. For information on taking a soil sample and interpreting the results, see HGIC 1652, Soil Testing.
Most soils in the Southern US are naturally acidic and often require lime to raise the pH to an optimum level for plant growth. The optimum pH range for most vegetables is 6.0 to 6.5. Within this range, all essential nutrients in the soil are relatively available for plant uptake. If the soil pH is outside of the appropriate range, plants have a hard time taking up nutrients and deficiencies may appear, even when adequate levels of nutrients are present in the soil. Below a pH of 6.0, macronutrients, such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, become less available. When pH is above 6.5, micronutrients, such as iron, magnesium, boron, copper, and zinc, become less available.
To avoid nutrient deficiencies related to pH, apply the required amount of lime according to soil sample recommendations. Lime takes a few months to adjust the pH of the soil; therefore, apply it far enough in advance that the pH will be adjusted into the appropriate range prior to planting. Lime has very limited mobility in the soil and should be incorporated to more quickly affect the pH below the surface. Tilling or discing to a depth of 6 inches is generally sufficient.
There are numerous types of liming materials available, though the two most common types are dolomitic and calcitic lime. In addition to raising the soil pH, both types of lime supply calcium to the soil. Dolomitic lime also supplies magnesium and should be used when soil sample recommendations call for magnesium. For more information on liming, see HGIC 1650, Changing the pH of Your Soil.
For most home-grown vegetables, all the recommended phosphorus and potassium nutrients should be added to the soil prior to planting, as well as 30 to 40% of the recommended nitrogen. Table 1 shows the amount of preplant fertilizer to apply, based on soil sample results.
Table 1: Preplant Fertilizer Rates Based on Soil Sample Results.
|Nutrients Required||Pounds per 1000 ft2||Fertilizer|
|N only (P and K tested high or excessive)||6.5||15.5-0-0 (Calcium Nitrate)*|
|N and P (K tested high or excessive)||6.5||15.5-0-0 (Calcium Nitrate)*|
|9||0-46-0 (Triple Superphosphate)|
|N and K (P tested high or excessive)||6.5||15.5-0-0 (Calcium Nitrate)*|
|6.5||0-0-60 (Muriate of Potash)|
|N, P, and K||10||10-10-10|
|8.5||0-46-0 (Triple Superphosphate)|
|5||0-0-60 (Muriate of Potash)|
|Note: For most fertilizers, a pint roughly equals a pound.
*To substitute ammonium sulfate and urea (34-0-0) for calcium nitrate, reduce the number of pounds per 1000 ft2 by half.
Broadcast the fertilizer evenly and incorporate it into the soil by tilling or discing to a depth of around 6 inches. Working the fertilizer into the soil ensures that the nutrients are available to the plants as soon as they are seeded or transplanted.
Occasionally, sample results will recommend applying additional nutrients prior to planting, such as boron, manganese, or zinc. Broadcast and incorporate them into the soil along with the nutrients described above.
Fertilizing After Planting
For most home-grown vegetables, nitrogen is the only nutrient needed after planting. Broadcast or side-dress the remaining required nitrogen 3 to 4 weeks after planting. On sandy soil, this may need to be split into two applications. In that case, make one application 3 weeks after planting and another 3 to 4 weeks later. Table 2 gives the amount of fertilizer to be broadcasted or side-dressed after planting.
Table 2. Broadcast or Side Dress Fertilizer Rates.
|Fertilizer||Pounds per 1000 ft2 (300 ft of row) per application||Number of Applications|
|15.5-0-0 (Calcium Nitrate)||
|2 (Sandy soil)|
|34-0-0 (Ammonium Sulfate and Urea||
|2 (Sandy soil)|
To avoid burning or damaging the roots, place side-dress fertilizer 4 to 6 inches away from the plants. To avoid burning the plant foliage when fertilizer is broadcast, water overhead after the application to wash fertilizer granules off the leaves.
Originally published 03/99