Gary A. Moulton, WSU – NWREC Mount Vernon
A good fertility program is just one component of quality fruit production. Good fertility practices may be ineffective, for example, if water management is neglected, particularly in the early season. Furthermore, poor management of pests can further compound the problem. However, maintaining soil fertility is essential to good quality fruit.
What do those numbers mean?
The label on a bag of fertilizer contains 3 basic numbers. The first number represents nitrogen (N), the second phosphorus (P, measured in P2O5), and the third potassium (K, measured in K2O). For instance, 16–25–33 would mean 16% nitrogen, 25% P2O5, and 33% K2O. This is true no matter what the source, whether animal manures or inorganic compounds. Soil testing is the ideal way to determine exactly what your soil needs, but it can be expensive and impractical for the home gardener. The following are general recommendations that we have found to be common in western Washington.
- Nitrogen is not needed in most of western Washington since we have such high levels of organic matter in our soil, and it is continually released during the summers. Nitrogen controls growth. With excess we get rank growth. Fruit maturity is delayed; and storage life of apples and pears is reduced. Peaches need more nitrogen so applications may be necessary. Sandy soils sometimes may need more nitrogen. Monitor growth closely; if it is slow and general leaf color is yellow some nitrogen may be required, but remember fruit quality will be reduced by over- application Nitrogen is very mobile in the soil and can be leached readily. Applications during the dormant period will be lost by leaching in western Washington.
- Phosphorus is usually recommended at planting. It is a very immobile element and should be incorporated into the soil. Surface applications of phosphorus are ineffective.
- Potassium should be applied each year. It is associated with better fruit color, more sugars, and better flavor. Its rate of use can be higher than nitrogen. In fact, a good crop of apples can pull 150 to 240 pounds of K2O per year from the soil, of which at least half is removed with the crop. Yearly applications of potassium are necessary to return what is taken. Since potassium does not move quickly in the soil, surface applications should be made in the fall or early winter so that it can be taken down into the soil profile by the winter rains.
- Micronutrients can be added to a fall fertilizer mix. Boron can be added to the potassium mix in the fall. It can also be added as a spray to the soil surface, mixed with a herbicide such as Roundup when killing weeds. Care must be taken not to over apply boron. In our soils we will need about 1 to 2 pounds of actual boron applied per acre every year. Boron can also be applied as a foliar spray during bloom to enhance set. Solubor, which is 20% boron, can be mixed with fungicides at this time. Zinc is applied most effectively as a delayed dormant spray. Other materials such as manganese can be applied in a foliar spray about a month after bloom.
- Liming is the addition of calcium (Ca) alone or calcium and magnesuim (Mg). For soils west of the Cascades, in most cases it is desirable to add calcium with magnesium. This type of lime is called dolomite. The best time to lime is at planting, when the lime can be incorporated throughout the soil profile. In established plantings that have not been limed for many years, it will take years for the lime to work down through the soil profile. Surface applications should probably be made for the first 3 years and then adjusted to every other year for the next 4 years. Taking a sample and measuring the soil pH about 8 inches deep in the soil profile will help you determine if you are adding too much or not enough. Liming should be done in the fall after harvest so that the winter rains will help wash it down. Just by adding lime, nutrients such as potassium become more available.
Other Essential Terms
- CEC (cation exchange capacity) defines negatively charged soil sites which hold onto cations. Clay soils have more exchange sites than sandier soils, in fact about twice as many. It is desirable that about 80% of those sites be occupied by bases. The ideal ratio is about 65% calcium (Ca), 12% Magnesium(Mg) and about 3% potassium(K). The rest of the sites are occupied by other cations such as sodium, hydrogen and micronutrients such as zinc, aluminum, and iron.
- Base saturation is the amount of bases such Ca, Mg, K , and Na that are positively charged and that occupy the negatively charged sites in the soil. Without liming and with a lot of rain over the years these cations become depleted and are replaced with the positively charged hydrogen ion (H+). This makes the soil more acid. We can determine this by checking the pH level of the soil. Ideally the pH should be in the 6–7 range. When it falls below that level we need to add lime. Adding too much lime, however, causes our pH to rise too high and micronutrient availability becomes limited. Small soil testing kits can give you a reading of soil pH.
Determining the amount of fertilizer to apply
All fertilizers, whether organic or inorganic, have a certain formulation of the essential elements. If the percentage of the targeted element is low, then more of that fertilizer will have to be applied in order reach your goal. This can be extrapolated down from a per acre recommendation using simple arithmetic. After calculating for the target element, check the amount of the other elements in the fertilizer to be sure that other elements or salt in the fertilizer are not reaching toxic levels at the application level you have set for the target element.
For instance, let’s say we have a formulation with 17% potassium as K2O. That means that a 100-pound sack contains 17 pounds of K2O. If we want to apply 130-140 pounds of K2O per acre, that means we’d need 800 pounds of this fertilizer formulation per acre to obtain the amount of K2O we want (17% K2O per sack times 8 sacks =136 pounds per acre of K2O).
To get this down to an application level that is the size of my yard, I need to know how much to apply per square foot. Dividing 800 pounds by 43560 (the number of square feet per acre) I’ll get how much needs to be applied per square foot, which equals .018 of a pound. We could convert this to grams by multiplying the pounds .018 times 454 (grams per pound) to equal about 8 grams per square foot. Now let’s say we will be fertilizing a 10′ X 10′ area around a fruit tree, which equals a 100 square foot area. 8 grams times 100 equals 800 grams or about 2 pounds.
When liming we can use a rule of thumb that 1 ton of lime raises the pH 1 point. In heavier soils that would probably be less, while in sandy soils it would be more. A ton of lime equals 2000 pounds per acre or .045 pounds per square foot (2000 divided by 43560). On a 10′ by 10′ area, which equals 100 square feet, we would apply about 4.5 pounds of lime.
Incorporating these materials throughout the soil profile at planting is ideal. Surface applications that can be scratched in enhance filtrations through the soil profile. However, care must be taken so as to minimize root damage. Make applications as uniform as possible over the treated area.
Our pages provide links to external sites for the convenience of users. WSU Extension does not manage these external sites, nor does Extension review, control, or take responsibility for the content of these sites. These external sites do not implicitly or explicitly represent official positions and policies of WSU Extension.