WHEAT

Introduction

Wheat (Triticum aestivum) is used for human and livestock consumption. In Zimbabwe it is mainly used

as human food in the form of bread. Wheat is also used in the form of paste products, breakfast cereals,

cake, tortilla, etc.

Wheat is regarded primarily as a source of carbohydrates because starch is the main chemicals

constituent (79%) whilst its valuable contribution of protein is ignored. Wheat is the only cereals that

contains gluten in its flour which makes it suitable for breadmaking.

Wheat (Triticum aestivum) production in Zimbabwe started as early as 1925. The economic value

relative to the total value of agricultural commodities is very low, ranging from 5% in 1980 to 3% in 1984

and to 6% in 1989/90 (A.M.A, 1985). However, it is becoming an essential part of the daily diet of an

increasing portion of the country’s population. Wheat may become even more important as

urbanization (urban migration and growth point development), the general standard of living and family

incomes increase. It is one of the few crops that can be economically grown in winter. Many farmers

have developed irrigation infrastructure and wheat provides a means of maximizing returns from their

investments.

Wheat production was initially very low. In 1956 it was about 12 000 tonnes but dropped to 500 tonnes

by 1960. In 1966, the government in response to the threat of sanctions after UBI, initialed a farm

Irrigation Fund to help farmers enter into irrigation wheat production. This paid off and by 1971 the

wheat area increased to 20 600 hectares production about 86 500 tonnes. After independence wheat

production reached a record post – independence low 0f 98 000t in 1984. This was a result of a serious

drought during the proceeding summers (1982/83 and 1983/84) which caused a reduction in the wheat

area to 21 000 hectares. In response to the drought the government started a National Irrigation Fund

to stimulate election of national water reservoir as security against drought and to motivate interested

farmers to enter into irrigated wheat production. In 1986 the area and production reached, a level

which was then, a debt time record of 47 600 hectares and 246 000 tonnes, respectively. This record has

since been surpassed. In 1989 the total wheat produced rose to about 284 00t produced from about 50

000 hectares from which was obtained a record 326 823 tonnes of wheat grain. These increases are a

consequence of the national irrigation fund and incentive producer prices that led to increases in farm

count and wheat area per farm.

Although production of irrigation wheat has increased over the years it has never exceeded

consumption except in 1978. The level of consumption is under – estimated because of rationing of

wheat supply to millers. Production of wheat from the summer season and soils with residual moisture

remains insignificant.

Wheat

Origin

Wheat probably originated from the Mediterranean region and parts of Russia, the Middle East, Ethiopia

and the Far East. At present wheat is found in many countries of the world. There are three genomic

groups of wheat, namely, the diploid, tetraploid and hexaploid groups. The one cultivated in Zimbabwe

belongs to the hexaploid group and is generally referred to as common or break wheat.

Botany

All wheat belong to the genus Triticum of the Graminae, the grass family. Wheat is a self-pollinating

plant. The plant is a spring annual or winter annual grass with flat blades and survive under snow while

winter wheats require vernalisation for floral initiation and can survive under snow.

Environmental Requirements

Soil requirements: the best soils for wheat production are considered to be heavy textured well drained

soils with a good organic matter status. It is possible to obtain good wheat yields on a large range of

soils, provided they are well-drained, but the irrigation and fertilization management should be of a high

class. Wheat is susceptible to water – logging, so poorly drained and badly aerated soils should be

avoided.

Temperature: Cultivated wheat was originally a natural grass growing in the wild in temperate areas.

There are two types of wheat, spring and winter wheat, that differ in their response to temperature. It

has generally been observed that spring and winter wheat grow best at relatively low temperature.

Temperature is the main climatic factor affecting development and yield of wheat in Zimbabwe. Wheat

grows and develops more slowly the cooler the conditions (hence traps more solar radiation needed for

photosynthesis), and in general the yields are better under cooler conditions.

Seedlings emerge most rapidly and shoots develop earliest at about 24 to 28 oC, the largest root system

was produced at about 12 to 160oC. a comparison of root and plumule development in wheat seedlings

at low and high soil temperature indicated that a distinctly different growth pattern was operative at the

lower extreme. At low temperature (8 to 12oC), the roots developed first and were several centimeters

long before the plumule began to grow, at higher temperature (28 to 32 oC), the plumule was well out of

the soil before the roots developed.

Low temperature in the early stages of growth promotes strong, vegetative growth and free tillering –

thereby increasing the number of productive ears per unit area. Generally, the number of productive

ears per unit area are influenced by the planting density the tillering capacity of the variety and survival

rate of the tillers.

Temperature influences the number of grains per ear during the spikelet initiation period. The lower

temperature in the Highveld results in slower morphological development which in turn lead to

increased duration of spikelet initiation with a corresponding increase in the number of spikelets per ear

and grains per spikelet in the mature ear. For example, the higher the May temperature, the lower the

number of spikeletes per head as illustrated below:

May

Temperature

Optimum temperature for wheat growth is 20 to 25oC. High temperature at flowering result in reduced

grain set and that during grain filling period result in reduced grain weight due to accelerated

senescence and poorly filled grain.

In general, the cool winter season in the midveld and highveld of Zimbabwe have proved ideal for wheat

production and exceptionally high yields can be obtained. However, the short duration of the cool

period in the lowveld, together with high day temperatures which occur, limit the potential maximum

yields in the area. This region experiences relatively high temperature at the beginning and the end of

the growing season.

Although low temperature may favour high yields, very low temperature may be detrimental. During the

early stages of growth, frost does little damage to the wheat plant and can, in fact induce tillering. After

booting, frost can adversely affect the crop.

1) During flowering, floral sterility may occur with an exposure of 4 hours at – 4oC

2) Severe frost may cause grain moisture to freeze resulting in grain damage and distortion – hence

quality reduction results.

Moisture supply -: moisture supply on wheat in Zimbabwe is not directly dependent on rainfall but

on irrigation. Mention summer wheat and vlei wheat.

The field could be irrigated to field capacity before or after planting. After about five to seven days,

depending on soil temperature, the wheat seed has germinated, developed roots and the shoot

emerges at about this time. It is necessary to apply an emergence irrigation at this stage to break

the surface crust and facilitate emergence of the shoot.

After this emergence irrigation the wheat crop can be irrigated basing on evaporative demand as

determined Meterlerkany (1975). Wheat can tolerate some moisture stress at this stage and the

normally encourages rapid downward development of roots. Wheat roots have been found to reach

a depth of 150cm by the booting stage and is at the maximum at the leading stage.

During the shooting phase (stem elongation) the plant places heavy water and nutrient demands on

the root system. The crop should, therefore, be adequately supplied with water. Water stress at this

stage and during flowering can result in poor grain set.

The wheat plant tolerates water stress after anthesis and milky dough stage more than at any stage.

Severe water stress during this stage (grain filling period) could however, lead to early leaf

sevescence and premature sevescence, shriveled grains and low grain yields and quality.

Time of Planting

The choice of appropriate time of planting for the area is important for high yield. The time of

planting should be selected such :

I. That spikelet initiation and tillering occur during cool conditions

II. The crop matures under moderately high temperatures and dries early enough to allow for

harvesting to be completed before rains fall.

The lowveld – experiments to determine the most appropriate planting time were done from 1968 to

1970. The results of direct effects of time of planting on mean yield performance in the lowveld were as

follows (means of 9 varieties at 3 sites, kg/ha):

Table 1: Effect of planting time of yield of wheat in the Lowveld (t/ha)

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Planting time 1968 1969 1970 mean

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May 2 4555 4215 4337 4369

May 23 4345 4420 3971 4245

May 13 3915 3439 2893 3416

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The results showed that planting early may give higher grain yields than later planting times. Planting

during the first half of May is recommended as this tries and ensures that both ear formation and grain

filling stages avoid periods of extreme heat. More recent experiments (1980’s) indicated that these

recommendation still apply today with current cultivators.

The midveld and Highveld – Experiments done in the midvield and Highveld during the second half of

May and not later than 31st ensures that high yields are obtained (Table 1). Earlier or later planting times

tend to result in reduced yields.

However, optimum planting dates vary with the location (as shown in table 2) and cultivar (Fig 1) must

be assessed on each farm from past experience, taking into account the following points:

a) Temperature – planting should be timed so that the temperature – sensitive period of spikelet

initiation occurs during cool weather.

b) Frost – in frost – prone areas the planting date should be selected to ensure anthesis occurs

after the period of greatest frost risk (fig 1). The actual time of planting will depend on the

variety grown and the estimated time of anthesis at a specific altitude.

Table 2: Effect of planting time of yield of wheat in the Highveld and middleveld (t/ha)

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Planting time Highveld Middleveld mean

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May 6 7.4 7.0 7.2

May 15 8.2 7.2 7.7

May 24 8.8 7.6 8.2

June 3 8.5 6.3 7.4

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Table 3: Effect of planting time on yield of wheat at different sites in the Highveld and midveld

(t/ha)

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Planting time Harare Matopos Kadoma mean

(1506m) (1338m) (1157m)

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May 6 7.4 1.9 6.9 5.4

May 15 8.2 2.7 7.6 6.2

May 24 8.8 5.5 7.7 7.3

June 3 8.5 6.4 5.8 6.9

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c) Rust – do not plant before May 1st in order to avoid infecting the winter crop with the rust

carried on summer volunteers. Recent recommended varieties have high levels of resistance to

stem rust but varying levels of resistance to leaf rust.

d) Maize Streak Virus – can be a problem if the crop is planted too early i.e before May 1st.

e) Variety – plant short duration varieties when delayed planting is unavoidable, as yields and test

density of the long duration varieties are more sensitive to delayed planting.

f) Late planting – avoid late planting because yield and quality losses occur from too rapid

senescence, leaf introduced from earlier plantings and damage from early rains.

Seeding Rates

The choice of the proper seedrate is aimed at producing the optimum number of productive tillers,

spaces evenly throughout the land. Wheat has the ability to compensate for low populations by tillering

or increasing the size and weight of the ears and grain. Seeding rates can therefore vary within relatively

wide limits without effect on yield.

Experiments done have not shown any significant yield differences between seeding rate of 30kg/ha and

200kg/ha (Table 5). One should aim to establish 180 to 260 plants per m2 and manage the crop so as to

get 400 to 500 ears m2. The same results were obtained from wheat grown using reduced irrigation

techniques. Optimum seedrates should be assessed from past experience, taking into account the

following points:

1. Seed viability – the minimum germination percentage for certified seed is 85%. The viability of

seed retained on the farm can fall below this level. However, a farmer can improve the viability

of seed retained by rogueing a portion of a field thoroughly and harvesting this separately from

the rest of the wheat crop and storing the seed under good conditions.

2. Variety – varieties vary in their tillering capacity, tendency to lodge and seed size. Experiments

done have not shown any seedrate and variety interaction effect on yield.

3. Seedbed tilth – determines, to a large extent the germination obtained and hence it will

influence the choice of seeding rates. A poorly prepared field with a poor tilth increases field

losses. One should increase seedrate under these circumstances.

Table 4: Effect of seeding rate on yield of wheat at different sites.

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Seed rate (kg/ha) Mutare1 Lowveld2 A.R.T.3 T.T.I.4 Mean

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30 97 97

35 99 99

60 100 100

67 99 99

80 110 107 109

90 104 104

100 100 100

101 101 101

120 98 100 96 98

135 102 102

140 95 95

150 99 99

160 97 98 98

180 100 101 101

200 100 99 100

220 100 100

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1. Unpublished data, Wheat Trials, C.B.I., R and SS 1986

2. Wall, PC 1972 Seedrate and spacing studies, lowveld irrigated wheat

3. A.R.T Witer Report, 1988

4. A.R.T Winter Report 1986

5. A.R.T Winter Report, 1990

4. Time of planting and climate – Temperature are higher in April than in May. In May the

temperature is initially high but decreases with time. Thus, the earlier the planting is advanced

the higher the seedrate in order to compensate for reduced tillering capacity and ear size. Late

plantings tend to tiler more profusely than early planting because of low temperature that

favour tillering. The ear size also tends to be bigger.

5. The recommended planting depth is 4cm. no benefit is accrued by planting deeper, as the crop

is grown under a controlled water reime. The tillering of wheat planted below 5cm may be

affected.

6. Spacing – Experiments done on row spacing indicated no advantage of a row spacing wider or

narrow than 20 or 25cm (Table 6), but yield losses occur when wider row spacings are used.

Narrower spacings may lead to higher levels of lodging depending on variety Interrows spacing

has an effect on weed control, i.e the closer the rows, the quicker the cover, the better the

initial weed control.

7. Methods of planting – Adequate stands can be obtained either by broadcasting the seed and

covering it with a light harrow or by use of a seed drill. The adequate rate needed for drilling

should be increased by 25% when broadcasting or aerial seeding of wheat.

8. Soil type – wheat grown on light textured soils tends to tiller less than on heavy soils. Thus,

when planting wheat in sandy soils one should increase seed rate so as to compensate for

reduced tillering.

Comment: In conclusion – although there were no yield gains obtained from increasing plant

establishement from 60 to 260 plants per m2 one should ensure very good management during the

tillering period when using the low seedrates. Seedrates generally recommended are 120kg/ha and

160kg/ha when drilling and broadcasting respectively.

Table 6: Effect of between – row spacing on yield response of wheat

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Row spacing (cm) % Mean yield

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13 101

15 100

18 104

25 100

30 101

36 99

38 98

46 98

53 95

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Fertilizer Requirements of Wheat

Wheat is best grown on the heavier textured, well drained soils with a slightly acid to neutral reaction

(pH CaC12 5,1 =6.0) and good organic matter status.

Fertilizers are applied to increase the supply of plant nutrients to crop thereby increasing its potential

yield. The amount of fertilizer required depends, among other factors, on the financial return a farmer

can expect form the potential yield increase. Too little or too much of a particular nutrient can decrease

profit margins. Fertilizer applications, therefore, should contain the correct combination of nitrogen,

phosphate and potash, and where necessary other essential nutrients. This is dependent on by how

much and how easily the soil can supply each nutrient, either from natural reserves or residues from

previous fertiliser applications. The soil pH is too low or too high maximum uptake will not be achieved.

Nitrogen

The amount of available nitrogen has the greatest effect on attainable yields providing the soil status is

good, good management is practiced and favourable moisture conditions exist. Previous work has

shown that for every unit of N removed per hectare 30kg/ha of wheat grain is produced. About 70% of

the N absorbed by the crop is in the grain and the remainder in the straw. Thus application levels of N

should be based on target yield and, of course, the soil mineral status. Excessive amounts of N result in

delayed maturity, increased lodging, and plants become extremely sensitive to frost, disease, or insect

pests.

Absorption of nitrogen corresponds to water consumption which reaches a peak during booting (81/2-

11) weeks after planting). It is also dependent on the root system which begins to develop rapidly 2-3

weeks after planting and attains a maximum depth about four weeks later. Consequently, to maximum

N uptake on sandy soils top dressings should be given at three weeks 6-7 weeks, and 8-10 weeks after

planting or more frequently. With the later top dressing sufficient time should be allowed before the

peak uptake period for ammonium in the nitrogenous fertilizer to be oxidized to nitrate, the form of N

most readily available to wheat.

On heavy clay soils N can reasonably be assumed that the bulk applied in the earlier stages of growth

will be available to the plant when required. The total N is normally applied in three stages, i.e at

planting, at 3-6 weeks and at 6-10 weeks on heavy soils. On sandy soils, the N may be split into 5 or 6

lighter applications are best made through the irrigation or by air to minimize damage to the crop due to

tractor wheels.

Symptoms of deficiency

Plants suffering from N deficiency are pale in comparison to healthy plants due to the breakdown in

chlorophyll. Specific symptoms of N deficiency first appear on the oldest leaves with the new leaves

remaning relatively green. The older leaves become paler that newer leaves with chlorosis beginning at

the tip and gradually merging into light green further down the leaf.

Phosphate

Much less phosphate is taken up by wheat than either nitrogen or potash. However, large amounts of

fertilizer phosphate are required for maximum economic yields. This is because the amount of available

P2 O5 in the soil is probably less important that the ease with which it is available. In Zimbabwean soils

especially red clay soils fertilizer phosphate is quickly converted into progressively more insoluble forms.

All fertilizer phosphate must be incorporated below the level of the seed – any phosphate above the

seed is wasted because for practical purposes phosphate is immobile in most Zimbabwean soils, moving

only slightly in certain sands. Phosphate is vital for early root development and hence it should be

applied at or before planting.

Although only small amounts of phosphate are taken up by the wheat plant, a linear response to P2o5

up to 100kg/ha p2o5 has been obtained at Harare Research Station.

Symptoms of deficiency

During the early stages of vegetative development, the most noticeable feature of phosphorus

deficiency in wheat is the reduced growth and vigor of the plant. The colour of all leaves of P deficient

plants becomes a dull dark green with slight mottling of the oldest leaf. Leaves appear coiled to a

greater degree that normal, old leaves sometimes encase young leaves.

Specific symptoms are, however, on the old leaves. Chlorosis begins at the trip of the old leaf and moves

down the leaf. The base of the leaf, like the remainder of the plant, stays dark green. Other common

symptoms of phosphate deficiency are delayed and irregular plant maturity and small heads. P

deficiency is usually more general across a field than N deficiency and usually results in stunted plants

with fewer shoots if the deficiency is mild.

Potash

Potassium is important in the synthesis of plant tissues and possibly straw strength and disease

resistance. Zimbabwean soils are rich in potash. However, it is necessary to apply some potash so as to

avoid mining the soil of this nutrient. This is made more important by that wheat is a luxuriant extractor

of potassium. Wheat removes large amounts of potash from the soil of which nearly all about 85% is in

the straw.

Where wheat is grown regularly in the rotation and the trash is not returned into the soil it is essential

to apply maintenance dressings of potash.

Specific potash deficiency symptoms always appear in the oldest leaves of wheat, although growth of

the whole plant can be affected prior to appearance of symptoms with all leaves having an unthrifty and

spindly appearance. Under severe K deficiency, necrosis in the oldest leaves begins as a necrotic

speckling along the length of the leaf, spreading quickly to the tip and margins. As a result of this spread

of necrotic tissue, an arrow of green tissue from the base upwards the centre can remain.

Chlorotic tissues, generally seen as a mottling, turn necrotic rapidly with K deficiency in contrast to N

deficiency. Complete death of old leaves is common and plants in the field may appear to have dried

prematurely due to drought – stress. Crops severely affected appear drought – stressed with large

numbers of prematurely dead, old leaves and sprindly growth.

Sulphur

Sulphur is essential in protein synthesis and thus an adequate supply is essential for optimal results.

Although there have been no proven responses to sulphur all compound fertilizers in Zimbabwe contain

sulphur in sufficient amounts to supply wheat when applied at recommended rates.

Since S is also important in Chlorophyll formation, deficiency symptoms in wheat are similar to N

chlorosis (i.e. a. general chlorosis of the leaf). However, S deficiency does differ from N deficiency in that

the whole plant is pale with a greater degree of chlorosis in the young leaves.

Copper and Zinc

Deficiencies of copper and zinc have been reported on wheat in Zimbabwe. The first usual symptoms of

copper deficiency in wheat is a general wilting of the whole plant occurring at early tillering, even

though the soil may be at field capacity. If the deficiency is severe enough it can affect tillering. Although

there is initially a decrease in tillering as in other deficiencies, there may be a proliferation of tillers in

severely Cu-deficient plants that occur when Cu-adequate plants are experiencing stem elongation.

Plants also appear lighter in colour where Cu is deficient. These symptoms are generally only noticeable

when comparisons are made with Cu-adequate plants. Cu – deficiency has been found to cause poor

growth and head discoloration and sterility of wheat on some sandy soils. In some cases, soils with

acceptable levels of Cu (0.75 ppm) showed Cu-deficiency systems because of an abundant availability of

zinc in the soil.

Withertip, the first charactereristic sysmptom of Cu deficiency, appears on young leaves. It shows as a

sudden dying and withering (curling) of the tip end of the leaf blade, sometimes up to half the length of

the blade. The base of the leaf can remain green until normal senescence occurs. Under severe

deficiency, the new growth can wither before unfolding and few heads will emerge. With less severe

deficiency, heads may emerge but gains may not form in the spikelets at the tip of the head. The whole

head takes on the appearance of the ‘rat tail’ with full grain in the base of the head, shriveled grains in

the middle of the head, and a withered necrotic tip. This withered tip ‘rat tail’ appearance is sometimes

confused with frost damage.

Copper deficiency can be rectified by applying copper sulphate or the oxychloride as soil or foiliar spray.

Zinc

The first symptomsof Zn deficiency in wheat appear on the middle aged leaves, but considerable

variation exists between individual plants, some plants produced symptoms in both old and middle-aged

leaves simultaneously. These initial symptoms show a change in colour from a healthy green to a muddy

grey green, generally in the central regions of the leaf.

Zn deficiency in wheat can occur in soils inherently low in Zn, in soils where Zn is unavailable to plants,

and in soils where plant – available Zn has declined. Severe Zn deficiency in the field can result in

stunted chlorotic plants with many collapsed leaves due to necrosis in the centre of the leaves.

Zinc deficiency can be corrected by using a zinc containing compound fertilizer in the rotation and this is

normally adequate.

Baron

Baron deficiency symptoms on wheat have been noticed on some varieties in Zimbabwe. Due to the

passive nature of baron uptake and the nonmobility of boron in the plant, expression of sysmtoms

under marginal deficiency conditions is extremely variable, influenced largely by the weather prevailing

at particular plant growth stages. The first characteristic sign of boron deficiency is a splitting of the

newer leaves close to the midrib. This is accompanied by some unusual indentations, also along the

length of the leaf, but on the opposite side of the midrib to the splitting. Although not always present,

another characteristic symptoms of boron deficiency is the development of a saw-tooth effect on young

leaves, reflecting abnormal cellula development. Head sterility is also a characteristic symptom of boron

deficiency. Reduced 1000 grain weight and grain shriveling can also result from boron deficiency.

Correction of boron deficiency can be done by use of a boron containing compound fertilizer in the

rotation.

Magnesium

Magnesium deficiency can occur where magnesium has become unavailable or on sandy acid soils in

high rainfall areas. Symptoms of Mg deficiency in the field appear as patchy yellowing and stunting of

the crop. Examination below the ground would reveal a poorly developed root system.

On magnesium deficiency soils and sandy soils the lime used should contain some magnesium.

Calcium

Calcium is immobile in the phloem of plants and, since it plays an important role in the growth of

meristematic tissue, symptoms of Ca deficiency always appear in new growth. Plant roots are generally

the first tissues to show Ca deficiency. The main roots become shortened with a proliferation of stunted

laterals. Wheat leaves do not become chlorotic with Ca deficiency, old leaves, particularly, retain their

dark green colour.

The first definite symptoms is a necrotic spotting about the middle of the leaf on the newest growth.

This area quickly expands and the leaf collapses mid way before unrolling. However, Ca deficiency in the

field is very uncommon.

Calcium is normally applied in line applied for correction of soil pH.

Rates of application

A well planned fertilizer programme is needed to ensure that the soil can supply adequate nutrients to

obtain the targeted yields. The required rates of fertilizer vary according to soil type, land history,

cultivar and climate. To determine the correct rate of fertilizer application it is advisable to have a soil

sample analysed by the Soil Testing Section of the Chemistry and Soils Research Institute.

Experimentation on nitrogen and phosphate amounts by the Agronomy Institute, Department of

Research and Specialist Services over seven years on a very wide range of soil types at a large number of

locations in the midveld and Highveld indicated that:

a) Large responses to N and P2o5 are only obtained when both nutrients are applied together

b) The optmum application rate of N ranges from 120 to 180 kg/ha N (Fig 3)

c) High levels of P2o5 increase yield at any particular level of N (Fig 3). As P2o5 increase yield at

any particular level of N (Fig 3). As P2O5 mobility in the soil is relatively low, and hence is less

leached from the root zone such applications may be desirable.

d) Application rates of N above 180kg/ha N and P2O5 above 150kg/ha P2O5 are likely to reduce

yields.

Experimentation on the yield response of wheat to N on paragneiss, alluvial and basalt soils in the

lowveld showed a consistency in response pattern also applied where previous crop were different. The

previous crop only affected the attainable yield at given N rates. The optimum N rate ranged from 90 to

135 kg/ha N.

The phosphate status of the granitic alluvium is very high and no responses to P2O5 were obtained even

after 17 years of continuous cropping. On the basalt the phosphate reserves are less extreme and hence

phosphate may be required. The phosphate status of the paragneiss is herently low so that maintenance

applications are necessary.

All three lowveld soils have a relatively high potash status. No yield responses have been obtained on

basalt or on paragneiss, but small responses to potash have been obtained on the alluvium.

Extra reading

1) Irrigation scheduling in wheat:

a) Pan Et

b) Deficit irrigation

c) Irrigation scheduling using phonological growth stages

2) Diseases in wheat

3) Cropping programmes in wheat (rotations)

4) Factors used in determining choice of variety for different areas.