The Authority in Potassium and Magnesium

Rice

Potassium – For healthy plants and higher yields

Sulphur, Boron and Manganese – To improve quality and nutritional value of rice

Rice - The most important food

For more than the half of the world’s population, rice is the most important food. About 90% of the world rice supply is produced in Asian countries. The production systems differ widely in cropping in-tensity and yield. Ranging from single-crop, rain-fed lowland and upland rice with small yields (1-3t ha^-1) to triple-crop, irrigated systems with annual yields of up to 15–18t ha^-1.

Optimum nutrient management is vital for making full use of the genetic yield potential at a particular site thereby helping to satisfy the food demands of a growing population in relation to the decreasing availability of water and land.

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Potassium – For healthy plants and higher yields

Rice yields depend on the number of spikelets per panicle, the percentage of filled grains and the thousand grain weight. These yield parameters are influenced by the management systems used and a balanced fertilisation programme is of key importance in achieving this. The use of potassium within the fertiliser program is essential because of the important functions of potassium in plant physiology:

potassium is required for a wide range of functions within the plant metabolism such as enzyme activation, osmotic turgor regulation and transportation of assimilates
an adequate potassium supply is needed to improve the integrity of cell membranes and cell walls
potassium contributes to greater canopy photosynthesis and crop growth by increasing leaf area and leaf chlorophyll content
potassium positively influences the plants health by increasing its tolerance to adverse climatic conditions, lodging, pests and diseases

K uptake and K content in modern rice varieties

Plant part	Typical observed range *	Observed average **
	uptake (kg Kt ^-1 grain-yield ^-1)
Grain + straw	14 - 20	17.0
Grain	2 - 3	2.5
Straw	12 - 17	14.5
	K content (%)
Grain	0.22 - 0.31	0.27
Straw	1.17 - 1.68	1.39
Unfilled spikelets	0.61 - 1.20	1.07

* 23 - 75 % interquartile range of farmers fields and field experiments
in Asia (n = 1300)

** Median of farmers fields and field experiments in Asia (n = 1300)

Throughout the growing season it is important to keep the leaf K concentration at certain levels. If the leaf K concentration drops below the critical level the yield potential cannot be fully achieved due to K deficiency.

Optimal ranges and critical levels of K in plant tissue

Growth stage	Plant part	Optimum (%)	Critical deficiency level (%)
Tillering to panicle initiation	Y leaf	1.8 - 2.6	< 1.5
Flowering	Flag leaf	1.4 - 2.0	<1.2
Maturity	Straw	1.5 - 2.0	<1.2

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Magnesium – Secures yield and quality

Although rice has the lowest magnesium requirements among cereal crops, magnesium fertilization to rice crops is becoming standard practice especially on high yielding cropping systems because of magnesium depletion in many soils. Magnesium plays essential roles by forming yield and quality:

magnesium is the central atom of the chlorophyll molecule
magnesium is involved in CO2 assimilation and protein synthesis and has an important impact on the nutritional value on rice
magnesium activates many enzymes
an adequate magnesium supply leads to a higher proportion of milled rice

Effect of magnesium on rice quality

Treatment	Milled rice (%)	Crude protein (% of dry matter)	Starch (%of dry matter)
Control	73.3	10.7	80.6
With Mg	76.4	12.8	85.3

Mg deficiency can be caused through low availability of soil Mg or a decrease in Mg uptake due to a sub-optimum ratio between exchangeable Ca : Mg. The optimum Ca : Mg ratio in rice shoots between tillering and panicle initiation is 1-1.5 : 1. In order to prevent magnesium deficiency, the leaf Mg con-centration should be kept within the required range. The Mg soil concentration should be > 3 cmolc Mg.

Optimum ranges and critical levels of Mg in plant tissue

Growth stage	Plant stage	Optimum (%)	Critical deficiency level (%)
Tillering to panicle initiation	Y leaf	0.15 - 0.30	< 0.12
Flowering	Flag leaf	0.15 - 0.30	< 0.13
Maturity	Straw	0.20 - 0.30	< 0.10

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Sulphur, Boron and Manganese – To improve quality and nutritional value of rice

Sulphur

the leaf S concentration should not drop below 0.10% before flowering
sulphur is a constituent of essential amino acids (cystine and methionine), which are important for the human diet
sulphur improves the nutritional value of rice
sulphur is involved in chlorophyll production and hence needed for protein synthesis, plant function and structure
sulphur increases the nitrogen-use-efficiency and has positive effects on yield formation

Boron

boron improves the stability of rice plants because of its important role for cell wall synthesis as well as for carbohydrate metabolism
boron activates the sucrose production and speeds up the transport from the leaf to grains

Manganese

manganese is required for a range of tasks within the plants, e.g. for the formation and stability of chloroplasts, protein synthesis, O₂ evolution in photosynthesis and enzyme activation
through the formation of side-roots manganese improves the nutrient-use-efficiency by rice plants

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Fertilizer recommendations

Soil application

Depending on the target yield and the existing K supply from the soil, between 100 and 180 kg ha^-1 of MOP (Muriate of Potash) is recommended to satisfy the crops potassium requirements. Ensuring adequate magnesium is available, 80-100 kg ha^-1 ESTA Kieserite is recommended to help produce better quality grains especially where yields are more than 6 t. To fully exploit yield potential of high yielding varieties grown in continuous cropping systems, it is also recommended to supply the two micronutrients boron and manganese. This can be done effectively in the form of 2-3 foliar applications with a 4-5% solution of EPSO Microtop (4-5 kg 100 l^-1 water).