IMPORTANCE Potassium (K) deficiency in California vineyards is considered third in overall importance, following that of nitrogen and zinc. This is somewhat surpris-ing, because the potassium needs of the grapevine are relatively high and are comparable to the de-mand for nitrogen. The reasons are, in part, the greater initial supply of K in most California soils and resistance of K to losses from leaching. K de-ficiency is usually confined to small areas in a vine-yard—seldom larger than 1 to 3 acres. When vines are deficient, however, the poorer vine growth and lower crop yields can be dramatic.

K-deficient areas in San Joaquin Valley vine-yards probably total no more than several thousand acres. The deficiency has not been found in Kern County vineyards and is rare in Tulare County, where it may be found only in scattered areas be-tween Kingsburg and Orosi. Fresno County has scattered areas of localized deficiency, mostly east of Dickenson Avenue north of Fresno and east of Highway 41 south of Fresno. In a tissue analysis survey of 120 vineyards throughout Fresno County, only one vineyard was found to be deficient in K, further demonstrating the infrequency and localiza-tion of the problem. K deficiency is rare in Madera County. Merced and Stanislaus counties have scat-tered deficient areas centering around Ballico, Cres-sey, and Delhi, and around Keyes and Ceres. In San Joaquin County, deficiency is occasionally found in the vineyard areas around Manteca, Ripon, and Escalon.

SOURCES IN THE SOIL

Most K in soils is derived from minerals, notably micas and feldspars. These minerals are only slightly soluble and are usually found as large-size particles. Under the influence of various weathering factors, K is gradually solubilized and becomes available to plants as positively charged ions, which become attached to clay colloids or to organic matter in the soil.

A comparatively large quantity of K is found in soils, but most of it is present in relatively insol-uble compounds. Thus, the available K level in the root zone is a more important consideration than the total supply. The ordinary range of total. K in miner-

als ranges from 0.15 percent in sands to 4 percent and higher in clay soils.

When K fertilizer is applied to the soil, part is fixed in slowly available compounds. The extent of fixation is proportional to the amounts and kinds of colloids in the soil, being greatest in clays and clay barns and smallest in sands. K fixation or holding by the soil is important, because it serves as a check against rapid leaching and provides a more continu-ous supply of available K. Even when K fertilizer dissolves in the soil moisture, it is soon attracted to the surfaces of the colloidal complex and replaces sodium, calcium, or some other element associated with the colloidal soil particles. K so fixed may move slowly in the soil, the rate dependent on the amount and nature of the colloidal complex.

The K cycle is fairly simple. It is fixed by the soil; removed by crops and, to a minor extent, by drainage water; exported by harvested crops; and returned to the land in crop residues, manures, or K fertilizers.

K levels are generally highest in the topsoil. Hence, deficiency is most likely to occur in cut areas from land leveling where the less fertile sub-soil is exposed. K deficiency is also more common in sandier soils.

ROLE AND UTILIZATION

Not much is really known about the function of K in grapevines. Much more is known about what happens to vine growth and crop yields when this element is deficient.

Plants need K for the formation of sugars and starches, for the synthesis of proteins, and for cell division. K also neutralizes organic acids, regulates the activity of other mineral nutrients in plants, acti-vates certain enzymes, and helps to adjust water relationships. It increases the oil content of certain fruits, and contributes to cold hardiness. Even though it is considered essential for the formation of carbohydrates and is somehow involved in other processes, it is not usually found as a part of organic compounds. About 1 to 4 percent of a plant by dry weight is K.

The demand for K is highest in midsummer to late summer, when greater amounts accumulate in the ripening fruit. Thus, temporary K deficiencies are sometimes associated with overcropping.

Grapevine Nutrition and

Fertilization in the San Joaquin Valley

o

DIAGNOSIS OF DEFICIENCY

The ability to recognize and identify symptoms of K deficiency is extremely important; response to K fertilization has been obtained only in vineyard areas with visible symptoms of deficiency. Fertilizer trials in vineyards with low, but not deficient, levels of K in leaf tissue have not shown a yield or growth response.

Symptoms

Leaf symptoms usually begin to show in early sum-mer and typically are seen first on leaves on the middle portions of the shoots. A fading or yellowing of leaf color begins at the margin or outer edge of the leaf.

As the season progresses, the yellowing contin-ues to progress into the areas between the main veins, leaving a central island of green extending somewhat along the main veins. (See fig. 3a.) The yellowed leaf areas of colored grape varieties may bronze or redden (fig. 3b). In all varieties, marginal burning and curling, either upward or downward, usually follows. Leaves above and below the mid-shoot section become affected as the season ad-vances, until many of the leaves show some symp-toms by harvest time.

When K deficiency is severe, shoot growth is markedly reduced, and symptoms may be present on nearly all of the leaves before blossoming time. Leaves may drop prematurely, especially if the vines are carrying a heavy crop or are stressed for moisture. If leaf drop is extensive, the fruit may fail to develop full color or to ripen normally.

Symptoms of a mild K deficiency do not appear until late summer, approaching harvest. Here, symp-toms may appear on many of the leaves on lateral or secondary shoots. In Thompson Seedless, the late symptoms produce a more blotchy or irregular pattern of chlorosis, particularly on the leaves near the ends of lateral shoots.

Vines severely deficient in K tend to have fewer and smaller, tight clusters with unevenly colored, small berries. With Thompson Seedless the lower portion of the bunch may collapse by midsummer, resulting in raisined, immature berries by harvest. (See fig. 4.)

Much of the effect of K deficiency on the fruit is the result of reduced vine growth and premature leaf fall, and accounts for lower vine yields and fruit maturity. Fruit symptoms alone are not always dis-tinctive enough to be used in diagnosis; other fac-tors, such as moisture stress, water berry, or red berry, can affect the fruit in a somewhat similar manner.

K deficiency symptoms may be confused with those caused by midsummer moisture stress during hot weather. Moisture stress, however, causes a general leaf burn of an irregular pattern that is most prominent on the older, basal leaves. The gradual fading or yellowing pattern, characteristic of K de-ficiency, is not seen.

A high water table during the spring and early summer may induce typical K deficiency symptoms. In fact, many other conditions that substantially reduce the effectiveness of the root system may also cause deficiency symptoms to appear. The diagnos-tic procedure should include digging and careful examination of the vine root system for the presence of phylloxera, nematodes, or unfavorable soil con-ditons.

Vineyard areas showing deficiency symptoms should be marked during the growing season. This will pinpoint the vines to be treated during the dor-mant season.

Using laboratory analysis

Even though keen observers may readily identify K deficiency by means of leaf symptoms, laboratory analysis of a properly collected sample of petioles may verify the diagnosis or help clear up confusion with other disorders. Based on fertilizer trials and field observations, tissue potassium values may be interpreted to determine whether the vines have an adequate level.

Soil analysis is not a reliable means of determin-ing whether a vineyard requires K treatment. Too many variables are involved: the wide range of soil depths; the varying concentrations of K in the soil profile; the many different grape varieties and root-stocks used; and the many factors influencing root health and thus K uptake.

A note of caution: the K level in the vines may be influenced by other conditions that reduce the effectiveness of the roots —overcropping, shallow-rooting, high water table, inadequate irrigation, or heavy nematode or phylloxera feeding. The appli-cation of K fertilizer cannot be expected to give a positive vine response under such conditions.

FERTILIZER PRACTICE

Research results in California for both fruit trees and vines have shown that, when a deficiency exists, massive rates of K fertilizer are necessary to obtain vine or tree recovery and yield response. Large amounts of fertilizer are needed to overcome the high fixation of K in most California soils. Fertilizer mixes or complete fertilizers are not recommended,

Z 1

Materials

The symbol "K," used for the element, is from the Latin word for potassium —Kalium. When expressed as a plant food by the fertilizer industry, potassium is usually reported in terms of the oxide, K 20, also called potash.

Potassium fertilizer is commonly available in three forms, each with a different potassium content: ■ Potassium chloride (KC1, muriate of potash)-52

percent K (62 percent K 20) ■ Potassium sulfate (K 2SO4, sulfate • of potash)-

44 percent K (53 percent K 20) ■ Potassium nitrate (KNO 3 )-37 percent K (44 per-

cent K 20) plus 13 percent total nitrogen Studies in Fresno County vineyards have

shown that one fertilizer form offers no advantage over another in terms of vine response, provided that equal amounts of actual K are used. Thus, the choice will depend on relative cost and availability of the materials and on soil and vine conditions.

Currently, potassium chloride is the most eco-nomical source of K. It must be used with caution, however, because its chloride content might con-tribute to salt injury of vines under some conditions. For example, potassium chloride should not be ap-plied to vineyard soils with an existing salinity con-dition, nor should it be used on shallow or poorly drained soils. It is best used during the dormant season and should be avoided during spring and summer. Only a low to moderate rate applied during the winter is safe for young vines.

Potassium chloride can be used safely on well-drained soils without a salinity problem. One or two heavy furrow irrigations directly over the area treated are recommended as the best means of reducing the concentration of chloride fertilizer. It is somewhat dangerous to rely on rainfall alone to move the fertilizer, because rainfall amounts are not likely to be adequate. Light rain might be ex-pected to leave a slightly dispersed, but still con-centrated fertilizer band in the root zone. To leach the chlorides in sprinkler-irrigated vineyards, one should also provide some irrigation in excess of the grapevines' water needs.

The relatively high cost of potassium nitrate all but eliminates its use in vineyards as a soil treat-ment. Potassium sulfate is slightly more expensive than potassium chloride, and is safe to use during the dormant season. However, it should be used with some caution during the growing season, es-pecially on young vines, because experience with this form is limited. Thus, from a practical standpoint,

Rates

A high rate is needed to overcome the high K-fixing power of most San Joaquin Valley soils. (See table 3.) The quickest response by far can be achieved by applying the fertilizer in a single heavy applica-tion rather than in small amounts applied annually. In general, the speed and degree of vine recovery, as well as the length of effectiveness, improve as rates are increased.

TABLE 3. SUGGESTED POTASSIUM APPLICATION RATES, BASED ON SEVERITY OF DEFICIENCY

Application rate

Per vine Per acre equivalent*

deficiency KCI

K2SO4 KCI

K2SO4

lb lb lb lb

Severe

4- 41/2 5 - 6 1,800 - 2,000 2,300 - 2,700

Moderate

3Y2 4 1,600 1,800 Mild

21/2 3 1,150 1,350

*Pounds per acre equivalent is based on 454 vines per acre (8' X 12' spacing).

Application methods

Deep placement of K fertilizer in a concentrated band close to the vine is recommended for most soils. This places the fertilizer near the root zone and also is the best means of overcoming soil fixa-tion.

The fertilizer can be applied by hand or banded with a fertilizer applicator into the bottom of 6- to 8-inch-deep furrows opened about 18 to 24 inches from the vine on each side of the row. Many growers have found a 1-pound coffee can a convenient mea-sure for applying K fertilizer by hand. A level, full can holds almost exactly 3 pounds of potassium sulfate.

Open French (row) plow furrows can also be used. In either case, the furrows should be left open to allow rainfall and irrigation water to move the fertilizer. The first irrigation should be made in these furrows to ensure deeper movement.

On sandy soils, surface banding of 2%2 pounds of potassium sulfate on each side of the vine row (total of 5 pounds per vine) was successful in limited studies in Fresno County vineyards; movement into the soil depends on winter rainfall. The vines responded fairly rapidly after surface applications, presumably because of a good concentration of actively growing roots near the soil surface in the

Vine

the choice of a potash fertilizer narrows to either potassium sulfate or potassium chloride.

because they have a relatively low K content and hence would be needed in unreasonably large amounts to supply enough K.

1

spring. Use of this method should be limited until more experience is gained; only high rates of potas-sium sulfate have been evaluated.

When subsoiling is done reasonably close to the vine rows, K fertilizer may be placed in the subsoiler slots. Good vine responses have been observed following such applications.

Foliar sprays. Potassium nitrate applied in foliar sprays in extensive Fresno County trials has shown no promise in correcting deficiency. During the growing season, repeated sprays at 4 to 5 pounds of potassium nitrate per 100 gallons of water did not measurably increase tissue K levels or reduce deficiency symptoms over a 3-year test period. Higher rates could not be used because of toxicity to young leaves.

Time of treatment

The late fall or early winter is a good time to treat, allowing maximum exposure to winter rainfall to help move the fertilizer into the soil. At the latest, the fertilizer should be applied before the first irrigation in early spring, before bud break. Some growers have found that an application in the fall

is convenient, because deficient areas can still be defined by the presence of leaf symptoms.

Symptoms are not corrected immediately after fertilizer application in the winter. A partial improve-ment in leaf color or in vine growth may not appear until midsummer; a full response usually is not attained until the second growing season, or even the third, after fertilizer treatment.

Treatment longevity

A single application of K fertilizer at a recommended high rate will sustain an adequate K tissue level and eliminate leaf symptoms for 5 years, at least. Typically, the treatment loses effectiveness after about 8 years. Higher rates than those in table 3 can last for 10 years and beyond. If soil pests, such as nematodes or phylloxera, are causing root prob-lems, the longevity of any treatment can be ex-pected to be shortened.

It would be prudent to observe the treatment area closely for reappearance of foliar symptoms and to monitor the tissue level of K by collecting petiole samples for laboratory analysis. This procedure will be helpful in anticipating the time when a follow-up fertilizer treatment may be necessary.

13

0 0 O

0 0

0 o 0 0 o

0 ° 0 0 0 0

Y

1 0

0

0

0

O 0

0

O 0 0

am:How son ro

1m wag acto 000 ono 0

M 2 : 1-type silicate clay layer

0 Potassium and ammonium ions

O Other smaller cations (W, c , etc.)

Clay minerals of the 2:1 type such as illite have the ability to fix ammonium and potassium ions. Smaller ions, such as H' . Na , and Ca' can move in and out of the internal adsorption surface and are thus exchangeable. Potassium and ammonium ions are of such size as to fit snugly between crystals, thereby holding them together. At the same tune, these larger ions are rendered at least temporarily non-exchangeable or fixed.

NONEXCHANGEABLE EXCHANGEABLE POTASSIUM POTASSIUM

Relative proportions of the total soil potassium in unavailable, slowly available, and readily available forms. Only 1 to 2 percent is rated as readily available. Of this, approximately 90 percent is exchangeable and only 10 percent appears in the soil solution at any time/

POTASSIUM MINERALS

COMMERCIAL SLOWLY AVAILABLE FERTILIZERS

CROP RESIDUES, MANURES

EROSION LOSSES FIXATION

CROP REMOVAL

LEACHING LOSSES

Gains and losses in available soil potassium under average field condi-tions. The approximate magnitude of the changes is represented by the width of the arrows.

+ir 90% 10%

I IN SOIL SOLUTION

0

0

4,

RELATIVELY UNAVAILABLE POTASSIUM

(FELDSPARS, MICAS, etc.) 90-98% of total potassium

SLOWLY AVAILABLE POTASSIUM

(NONEXCHANGEABLE (fixed) I 1-10% of total potassium

READILY AVAILABLE POTASSIUM

(EXCHANGEABLE and SOLUTION) IN SOIL

1-2% of total potassium

Potassium

Grape varieties ranked by petiole K levels Averages of Bloom and Veraison Samples

U.C. Kearney Agricultural Center

...

High, above 1.75%

Medium, 1.25-1.75%

Medium-Low, 1.0-1.25%

Low, below 1.0%

Zante Currant Ribier Emperor Muscat of Alexandria

Thompson Seedless Exotic Calmeria Carignane Cardinal Italia Queen Rubired Red Malaga Perlette Ruby Cabernet Chenin blanc

Zinfandel Sauvignon blanc Petite Sirah Flame Seedless Barbera

Ruby Seedless Grenache French Colombard Semillon Salvador

Potassium (K) increases crop yield and improves quality because it is needed for several yield-forming processes in plants.

POTASSIUM (K) is vital to so many plant processes that a review of its role involves understanding the biochemical and physiological systems of plant growth. While K does not become a part of the chemical structure of plants, it plays many important regu-latory roles.

Enzyme Activation Enzymes serve as catalysts for chem-

ical reactions — they bring together other molecules in such a way that the chemical reaction can take place. Po-tassium is required to "activate" at least 60 different enzymes involved in plant growth. The K changes the physical shape of the enzyme molecule, expos-ing the appropriate chemically active sites for the reaction.

The amount of K present in the cell determines how many of the enzymes can be activated and therefore the rate at which the chemical reaction can proceed. Thus the rate of a given reac-tion is controlled by the rate at which K enters the cell.

Enzyme activation is probably the most important function of potassium in plant growth.

Water Use The accumulation of K in plant roots

produces a gradient of osmotic pres-sure that draws water into the roots. Plants deficient in K are thus less able to absorb water and more subject to stress when water is in short supply.

Plants also depend upon K to regu-late the opening and closing of stomata (the pores through which leaves ex-change carbon dioxide, water vapor, and oxygen with the atmosphere). Proper functioning of stomata depends upon an adequate K supply.

When K moves into the guard cells around the stomata, the cells accumu-late water and swell, causing the pores to open, allowing gases to move freely in and out.When water supply is short, K is pumped out of the guard cells, the pores close tightly to prevent loss of water. If K supply is inadequate, the stomata become sluggish—slow to re-spond—and water vapor is lost. As a result, plants with an adequate supply of K are less susceptible to water stress.

Photosynthesis When the sun's energy is used to

combine carbon dioxide and water to form sugars, the initial high-energy product is adenosine triphosphate (ATP). The ATP is then used as the energy source for many other chemical reactions. The electrical charge balance at the site of ATP production is main-tained with K ions. When plants are K deficient, the rate of photosynthesis and the rate of ATP production are reduced, and all of the processes de-pendent on ATP are slowed down.

The role of K in photosynthesis is complex, but the activation of enzymes and involvement in ATP production is probably more important in regulating photosynthesis than is the role of K in stomatal activity.

Transport of Sugars Sugars produced in photosynthesis

must be transported through the phloem to other parts of the plant for utilization and storage. The plant's transport system uses energy in the form of ATP. If K is inadequate, less ATP is available, and the transport system breaks down. This causes pho-tosynthates to build up in the leaves and the rate of photosynthesis is re-duced. Normal development of energy

storage organs, such as grain, is also retarded as a result. An adequate sup-ply of K helps to keep all of these processes functioning normally.

Water and Nutrient Transport Potassium also plays a major role in

the transport of water and nutrients throughout the plant in the xylem. When K supply is reduced, transloca-tion of nitrates, phosphates, calcium, magnesium, and amino acids is de-pressed. As with phloem transport sys-tems, the role of K in xylem transport is often in conjunction with specific enzymes and plant growth hormones. An ample supply of K is essential to efficient operation of these systems.

Protein Synthesis The role of potassium in protein

synthesis is related to several of the functions discussed above. Transport of amino acids to the sites of protein synthesis, enzyme activation, and bal-ancing of electrical charges are among the key roles of K. Research has shown that K is required for every major step of protein synthesis. The "reading" of the genetic code in plant cells to pro-duce the proteins and enzymes that regulate all growth processes would be impossible without adequate K.

Starch Synthesis The enzyme responsible for synthesis

of starch in leaves is activated by K.

Thus, with inadequate K, starch accu-mulation in the leaves is reduced. The effect of the K level on photosynthesis also affects the amount of sugar avail-able for starch production. Under high K levels, starch is moved from the stems for efficient utilization.

Crop Quality High levels of available K improve

the physical quality, disease resistance, and feeding value of grain and forage crops, as well as crops used for human food. Quality is becoming an increas-ingly important market factor, so ade-quate K will become more critical for the value of the crop produced.

Other Functions of K Volumes have been written on the

specific roles of K in plants. And re-search into this subject is far from complete. This summary can only high light a few examples of how K is used in plants. Disease resistance, tolerance of water stress, rate of grain develop ment, winterhardiness, and nearly ev ery aspect of growth and development yield, and quality are dependent upor an adequate K supply.

The effects of K deficiency cause re duced yield potential long before any vis ible symptoms appear. This "hidden hun ger" robs profits from the farmer Mr( fails to keep his soil K levels in the rang) high enough to supply adequate K at al times during the growing season. ■

How potassium (K) works to increase crop yields: • Increases root growth and improves drought resistance • Builds cellulose and reduces lodging • Enhances many enzyme actions • Reduces respiration, preventing energy losses • Aids in photosynthesis and food formation • Helps translocation of sugars and starch • Produces grain rich in starch • Increases protein content of plants • Maintains turgor; reduces water loss and wilting • Helps retard crop diseases

4 Potassium for Aoriculture

Ontn ,c;o / elf r

Hybrid A

P2

Response to P

Response to P P 1 P2

bu/A K, 129 136 7 120 128 8 K2 131 147 16 125 133 8 Response

to K 2 11 5 5

Table 1. Soil test K and row applied K20 help corn overcome the effects of soil compaction.

Soil compaction level, tons

Availability and uptake of potassium (K) is often complicated by many interact-ing components. Two factors that have a predominating effect are the soil and plant characteristics involved. A third factor, improved fertilizer and manage-ment practices, can be used to modify the inherent characteristics of soils and plants involving K uptake.

Soil Test K

Row Applied K20 <5 9 19

Loss due to compaction

lb/A lb/A bu/A 204 0 151 141 129 22 45 169 164 164 5 262 0 177 153 157 20 45 173 168 163 10 469 0 169 162 149 20 45 169 167 151 18

PLANTS DIFFER in their ability to take up potassium (K) depending on several factors. The factors that affect availability of K in the soil and result-ing plant uptake are soil factors, plant factors and fertilizer and management practices.

Soil Factors... 1. The soil itself. This includes the

material from which the soil was formed, the amount and type of clay minerals in it, the vegetation under which it was formed, the topography and drainage, the climate under which it was formed and the length of time it has been forming.

2. The cation exchange capacity or CEC of the soil. This reflects the soil's ability to hold K and other cations and store them in the soil for crop uptake.

Clay minerals and soil organic mat-ter are the two parts of soil that con-tribute to CEC. In general, the higher the CEC of the soil, the greater the storage capacity and supplying power for K.

3. The quantity of available K in the soil. This is the value the K soil test measures. It is the sum of exchangeable K and water soluble K. As the level of soil test K decreases, the crop response to applied potash increases, shown in Figure 1.

4. The nonexchangeable or slowly available K. This is the K that is in equilibrium with the available K and renews the soil's supply of exchange-able K. For most soils, the more crops depend on nonexchangeable K, the lower the yields.

Figure 1. Yield responses to potash are greater on lower K soils, but yields are highest with adequate K.

5. The K fixation capacity of the soil. Some soils have clay types that can fix large amounts of K from fertilizers or other sources. This reduces the avail-ability of K to the crop.

6. The amount of K in the subsoil and the density or consistency of sub-soil layers. Some subsoils are high in K available to roots. Others, such as those formed under grass in the central Corn Belt have low K availability.

If dense layers develop in the subsoil, root penetration and rooting volumes are decreased, reducing the availability of the K and nutrients that are there.

7. Soil temperature. Low soil tem-peratures reduce K availability and up-take rate by crops. The optimum soil temperature for K uptake for a crop such as corn is about 85°F.

Effects of low temperature can be somewhat offset by increasing soil K levels. Row K can be important with

lower soil temperatures especially for early planted and minimum till crops.

8. Soil moisture. Moisture is needed for K to move to plant roots for up-take.

Moisture is needed for root growth through the soil to "new" supplies of K. It is needed for mass-flow movement of K to the plant roots with water and for the diffusion of K to the roots to resupply that taken up by the roots.

Drought stress or excess moisture reduced K availability and uptake by crops. Increasing soil K levels can help overcome the adverse effects.

9. Soil filth. This is related to the friability and ability to get air into the soil. Air is needed for root respiration for K uptake. Tillage when soils are too wet leads to compaction.

Data from Wisconsin in Table 1 show how the adverse effects of com-paction on corn yields can be modified by increasing soil test K levels and by using row-applied K2O.

Plant Factors... 1. The crop. Crops differ in their

ability to take up K from a given soil.

P,

Wisconsin, 1985-

This is associated with the type of ro( system and surface area of the root Grasses, for example, have a muc greater capacity to take up K from tl plow layer than alfalfa does. Grass , have many more fibrous, branchir roots, increasing the K absorbir surface.

2. The variety or hybrid. Crop gene ics come into play with the differenc among varieties or hybrids of a give crop.

Differences are developed throw plant breeding. They usually rela back to the type of root system, ro. density and metabolic activity that a fect K uptake and, hence, availabili of K for a given K test.

Table 2 shows an interaction betwet P x K x hybrids. The response Hybrid B to P and K was independe of the level of the other nutrient. Ho , ever, the response of Hybrid A to and K was greatest where both nut , ents were at the highest rates.

Potassium as a nutrient has a ye positive effect on root branching al density.

Hybrid B

Table 2. Corn hybrids may respond differently to applied P and K.

12 Potassium for Agriculture Potassium for Agriculture

Crop responses to placement of potassium (K) fertilizer are not as likely as for nitrogen (N) or phosphorus (P). However, some soil characteristics or growing conditions may warrant placement of high concentrations of K in the vicinity of developing plant roots.

Corn yield

bu/A

K uptake

K uptake

lb/A Ib/bu

150

140

130

120

--1,4 so0, - te:04

.6. 110

100

90

80

1

0 30 . 60 90 K20, lb/A

The other factor is that new varieties often have higher yield potentials which increase the demands placed on soil K. Additional potash will be needed under higher yields.

3. Plant populations. As plant pop-ulations increase yield of some crops are greater and demands on soil K are increased.

Yields often will not increase with higher populations unless adequate lev-els of K are in the soil, from native or fertilizer sources.

4. The crop yield level. As crop yield levels increase, total K uptake increas-es, Table 3. But the uptake per unit of crop yield, such as pounds of K per bushel or ton, may be nearly constant at optimum yield levels.

Table 3. High yielding crops need more K.

110 117 1.06 154 151 0.98 202 223 1.10 231 259 1.12 255* 304* 1.19*

'From a different experiment and location.

Fertilizer and Management Practices... 1. Increased use of N, and other lim-

iting nutrients. When adequate K is avail-able, additions of N and/or P greatly in-crease K uptake, as yields are increased. Usually the uptake of K by crops closely parallels N uptake, and may be greater. So, as limiting nutrients are added, the demands on soil K increase.

2. Applications of K in fertilizers, manures or crop residues. The major way to increase K availability is to apply adequate amounts. K is readily available for all these sources, provided they are mixed deeply enough into the soil where roots can absorb the K, Figure 2.

57

50

70 bu/A

63

I I 310 450 lb/A K Soil Test 270 290

Figure 2. Higher K soil tests can increase soybean yields.

3. Placement of K. Broadcast plow-down applications of K are more avail-able than surface applied disked-in K. Row K at moderate rates is usually twice as available as similar amounts broadcast.

Deep placement or drop irrigation help move K down. Gypsum applied with potash also helps move K down in very fine textured soils.

4. Conservation tillage limits avail-ability of surface applied K. Soil K levels should be built to high levels before shifting to minimum or conser-vation tillage. This improves K distri-bution within the plow layer. In many fine textured soils, surface applied K does not move into the soil and has low availability, particularly under dryland conditions.

5. Drainage increases K availability. Draining soils of excess moisture helps many soils warm up earlier and im-proves the aeration of the soil. This improves the availability of soil K.

6. Weed and insect control. Control-ling weeds and insects reduces compe-tition for moisture and nutrients, so that the crop being produced has rela-tively more K available. ■

BECAUSE POTASSIUM (K) is a monovalent cation and readily ad-sorbed by the soil's cation exchange capacity, it is not considered to be mobile in the soil. Thus, the K available to plants is that in close proximity to the roots. This raises the question of whether placement of supplemental K close to the plant might improve uptake and use efficiency in addition to the beneficial effects of high K soil tests on K availability.

Methods

Researchers have explored a number of K application variations including: (1) surface broadcast without incorpo-ration; (2) broadcast and disced; (3) broadcast and plowed down; (4) direct

Figure 1. Differences between K applica-tion responses are smaller as K test rises (Tennessee).

seed placement; (5) row placement (banded) — including all combinations of distances below and to the side of the seed; (6) plow sole placement; (7) deep placement or knifed; (8) surface strip; (9) fertigation; (10) combinations of the various methods.

All of these application methods can be considered as a variation or combi-nation of the two extremes: (1) banding in high concentrations with a minimum of soil contact, and (2) broadcast and more or less uniformly incorporated into the tillage layer.

Results

Responses to K placement vary among crop types, specific environ-mental conditions that exist during the growing season, and soil type.

Corn. Soil characteristics can have significant effects on how corn re-sponds to K application methods. Corn on three Illinois soils low to medium in soil test K responded differently to comparisons of broadcast and banded K. Banded K was more effective on all

Figure 2. Responses to starter K diminish as soil test K increases (Iowa).

42 41 - 40 - 39- 38

2 37- 36-

T- 35 34 - 33- 32 - 31- 30

140

240

150 160 170 180 190 200 Soil Test K, lb/A

210 220 230 250

Figure 5. Soybean yields are closely related to soil test K (Ohio).

= Corn Yield, bu/A 1.69% MI 1/4K in ear leaf

1.37% 130

120

MEM

IMO

MOM

IMO E MS

OMEN IN Row Broadcast & Strip &

Fall Plow Fall Plow

:31H

34

32

AL

tl 160 40 80

K20, lb/A Sldedressed

CC

2, 1:7 03

Gra

in Y

ield

s, bu

/A

three soils. Even at high rates of application, broadcast K was not as effective as that banded near the seed on two of the three soils.

Differences among the methods of K application usually diminish as K soil test values rise and as rates of application increase (Figure 1). Iowa studies showed less response to starter (banded) K as rates of plowdown K increased (Figure 2). But the rela-tionship of starter K re-sponse to soil test K can vary with soil type and with year. Starter K con-tinued to increase corn yields even at the highest soil test level in a Wiscon-sin experiment (Figure 3). Figure 3.

Starter K significantly increased corn yields on compacted soils in a Wisconsin study and continued to im-prove yields as K soil tests increased. Compacted, cold or extremely dry soil conditions' may favor K starter re-sponses due to slowed diffusion of soil K to plant roots even on high K soils. Large amounts of surface residue lead-ing to lower soil temperatures and higher soil bulk density under reduced till conditions may require starter K for most profitable yields.

Figure 4. Surface strip applications plowed down are effective for corn (Indiana).

Surface band (strip) applications K plowed down on a low medium K sc in Indiana were significantly more c fective than either starter (row) broadcast and plowed down K (Figu 4). Researchers concluded that op mum K application methods probat are somewhere between broadcast al banding near the row. Higher conce trations of K in a limited amount soil may produce the best balance t tween lowered K fixation and great( root-nutrient contact.

Soybeans. Soybeans require high availability for best yields and prof ability. Ohio data show a strong re tionship of soil test K to soybean yiel

Table 1. Soybeans respond well to K o deficient soils.

Soybean K20 applied, lb/A Variety 0 80 160

Yield, bu/A Dare 47 59 70 Bragg 36 45 50

(Figure 5). Responses to applied K have been good on deficient soils, whether broadcast or banded (Ta-ble 1).

Potassium should be ap-plied early for soybeans, but sidedressed rescue ap-plications can have benefi-cial effects (Figure 6).

Alfalfa. High yield al-falfa has one of the highest K needs of any crop, fre-quently exceeding 60-70 lb K20 per ton of hay. Top-dressing alfalfa with K is an adequate way of main-taining yield.. Building nu-trient levels before seeding and then topdressing for maintenance is the best approach.

Small Grains. Limited root systems, shorter growing seasons, and cooler temperatures enhance yield advantages of seed-placed over broadcast K for small grains. Barley data from Alberta showed a considerable advantage for K placed in direct seed contact at fairly low rates of application (Table 2). High rates of K in direct seed contact may cause germination damage when hoe or disc-opener drills are used.

Montana studies of K application methods have shown some advantages of knifed placement versus surface band or

Figure 6. Soybeans respond to K side-dressed at early flower (Alabama).

Table 2. Seed-placed K increased barley yield over band or broadcast on K deficient soils.

K 0 lb/A

Application Method

Yield Increase, bu/A 6 tests 13 tests

Broadcast 8.6 15 Banded 12.8 6.2

With seed 18.8 10.7

Broadcast 17.0 30 Banded 18.8 8.0

With seed 21.0 12.2

Alberta

direct seed applications for spring wheat, winter wheat and spring barley but results varied with location. Knifed applications may have resulted in better utilization be-cause of K placement in soil that re-mained moist longer.

Summary

In general, crop responses to differ-ent methods of K application are not nearly as large nor as consistent as responses to methods of application of N or P. However, cold, compacted or dry soil conditions tend to place more stress on K absorption and may war-rant placement of high concentrations of K in the vicinity of developing plant roots. ■

Starter K can be effective even at high soil test ion (Wisconsin).