Primary symptoms

Yield can often be reduced 10-30% by deficiencies of major nutrients before any clear symptoms of deficiency are observed in the field. Stunting or color changes are typical symptoms of deficiency. Field observations, calculations and soil analysis can indicate nutrient deficiencies.

Confirm the problem by checking the tables below.

Summary

Is mineral nutrition a problem?

Evidence: observations.

Walk through the field at different growth stages. If you find the leaf symptoms mentioned below, you can assume that the deficiency is probably important enough to cause some yield reduction. Recall that some diseases can cause symptoms similar to nutrient deficiency. If the cause is nutrient deficiency, symptoms will tend to occur in large areas of the field. If you see the symptoms on isolated plants, they are more likely the result of disease.

1. Before the V6 stage, pale yellow plants with small leaves and slow growth indicate N deficiency or, less commonly, S deficiency. Plants with reddish-purple color along the edges of the leaves can indicate P deficiency (this symptom is accentuated by cool weather).
2. During the period of rapid elongation, lower leaves with yellowing beginning at the tip and moving along the middle of the leaf in a 'v' shape indicates N deficiency. If the yellowing moves along the edges, the problem may be K shortage. At this stage other nutrient deficiency symptoms can also appear, such as pale, chlorotic stripes on the leaves. If this happens on the upper leaves, it can be due to a lack of iron or, in rare cases, copper. Broad whitish bands running along the center of new leaves or in the zone of elongation at the base of leaves may reveal zinc shortage. Yellow striping (chlorosis) on lower leaves may mean magnesium deficiency.
3. What was the previous crop? You can often tell by looking at the residue in the field, or you can ask the farmer. If maize follows a heavily fertilized cash crop, deficiencies of major nutrients are less likely.

Compare the heights of plants on the edge of the field with those having full competition. The plants with less competition will tend to be taller than the plants which are completely surrounded if drought or nutrient stress has occurred.

Evidence: calculations.

Even if you do not see clear symptoms of nitrogen deficiency in the field, you are not justified in saying that no response to N fertilizer can be expected. Ask the farmer how much N fertilizer he applies, and how much maize he harvests. You can now make some calculations to predict whether nitrogen is limiting.

First calculate the amount of N which is being supplied to the crop by sources other than the fertilizer applied by the farmer. The simplest way to do this is by calculating the N content of an unfertilized crop growing in a similar soil and environment when water is not limiting. Try to find a farmer who did not apply N either to his maize or to the previous crops in that field, and ask what yield he obtained when drought stress was not important. Assume that about 25 kg N/ha are required to produce each ton of grain.

N supplying capacity of soil/ha/season = tons of grain produced/ha x 25

>This N will come from several sources. One important source is soil organic matter, which will be abundant in a recently cleared field or after a long fallow, but which will tend to decline with continuous cropping (see Table 6 below). You can improve your estimate of the N supplying capacity of the soil in your region by finding out maize yields of unfertilized fields and relating them to their cropping histories. The other inputs of N will be from rain, dust, and non-symbiotic N fixation.

N from soil + (fertilizer N applied x 0.4)

Now add to this amount the N from the fertilizer applied by the farmer. The efficiency of recovery of applied fertilizer will vary, but you can assume for this calculation that about 40% of the fertilizer N applied is available to the plant. The total input of N will be:

Is this amount as large as that removed by a crop which yields the potential for the area? (Assume that, for a yield level near 4 tons, about 15 kg N/ha are removed for each ton of grain, and add to this another 10 kg N/ha per ton of grain if the stover is removed from the field.) If not, N supply may be limiting.

Evidence: soil analysis.

You can use soil analyses to determine whether to expect a response to P or K fertilizer. For these tests to be really accurate, however, they must be calibrated for the soils in the region.

For P. The Bray I test can be used for acid soils. A value of

For K. The results should be expressed as milliequivalents of K 1100 g soil. Depending on the soil, the sufficiency values vary widely, but in general, a value of

Table 6. Approximate amounts of N supplied annually from soil organic matter in different environments.

Possible solutions

• Increase fertilizer rate.
• Change fertilizer application method or timing so that less is lost.
• Improve drainage.
• Reduce weed competition.
• Lime to increase soil pH.
• Apply micronutrients.