Getting the N supply correct is the most critical and difficult aspect of the maize fertiliser programme. Nitrogen is made available from three sources:
1. Livestock manure (slurry and FYM)
2. Soil N
3. Fertiliser N applied
Livestock manures have the potential to supply all the N requirements of the crop. An application of 33 t/ha of cattle slurry(3,000 gal/ac) will provide from 50 to 60 kg/ha of N depending on the quality of the slurry and the time and method of application. The soil can supply anything from 50 to 250 kg/ha of N; however, even with detailed soil measurements, this level cannot be established with any accuracy until mid-June, which time the crop is well established. Teagasc advice is given in Table 1 below. Applying too much N will delay crop maturity and is often more damaging than applying too little. Thus, the grower or advisor should consider the nutrient programme and N application rates very carefully.
Phosphorus and Potassium
Phosphorus and K advice is also given in Table 1. An application of 33 t/ha of cattle slurry will provide about 70% of the P and K needed at soil at Index 3 and above (i.e. soil levels at or above 6 mg/l P and 100 mg/l K). Apply the slurry in spring and plough in immediately to retain the N content. For soils with low K levels, apply 125 kg/ha of potassium chloride (muriate of potash). It is advised that approximately 24 kg/ha N and 20 kg/ha P be combined-drilled at sowing to provide a supply of easily soluble P at the very critical establishment phase. This additional P will make up any shortfall in that supplied from slurry.
|N, P and K for maize (kg/ha), assuming a dry matter yield of 15 t/ha and not accounting for slurry application|
|1 No fertiliser K is required when soil K is above 250 mg/l.
*Must be incorporated prior to or during sowing.
Magnesium deficiency can be a problem when the soil Mg is less than Index 2. Use magnesium limestone where lime is applied or Kieserite otherwise. Crops deficient in Mg should be treated with Mg chelate or a recommended inorganic formulation. During cold weather in April/May, most maize crops will show deficiency symptoms. These will often be evident even where soil trace element levels are adequate. Always check soil analysis results before applying trace elements.
Maize is very sensitive to Zn deficiency during May and June. When deficiency is identified, spray with Zn Sulphate (5 kg/ha), Zn chelate or inorganic formulation as required.
Manganese deficiency often occurs in maize, especially at high pH for example if maize follows heavily limed sugar beet in a rotation. Yield responses are expected in maize growing in soils of pH 7 or more when the easily reducible Mn level falls below 50 mg/l. However, leaf analysis is more reliable than soil analysis for diagnosis of possible Mn deficiencies.
Manganese is controlled by the application of foliar sprays of Mn compounds such as manganese sulphate at 5 kg/ha in 450 l of water. Application should be repeated if the symptoms persist. Chelates and inorganic compounds of Mn are also effective. These have the added advantage of being applied in lower volumes of water, and of being compatible with certain fungicide and pesticide sprays. Applications of manganese sulphate to the soil are not effective in correcting deficiencies.