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Soil samples taken by Kiwi Fertiliser consultants are sent to Perry Agricultural Lab (PAL) in Missouri. After analysis, Kinsey Agricultural Services (KAS) (who receive samples from 70 countries world-wide,) make the appropriate recommendations. Regarding the order of deficiencies found Potassium and Sulphur are invariably at the top of the list.

Potassium (K) is involved in nearly every aspect of plant growth. It does not become an integral part of the plant, unlike silicon and calcium that become the main components of the cell wall; and magnesium and nitrogen that are the main components of the chlorophyll molecule. This is the reason why cut hay is so vulnerable to potassium loss due to rainfall.

Potassium is second only to nitrogen in terms of quantity taken up by a plant. K is involved in the regulation of around 50 enzymes in a plant. It is needed to convert nitrogen into protein thereby reducing the amount of non-protein nitrogen (nitrate) in the system. It facilitates the movements of sugar and starches and is important in sizing up fruit and grain and preventing build-up of sugars in chloroplasts. K influences stomata regulation. Stomata can open to seven times their original size, therefore influencing gas exchange for photosynthesis and drought resistance and/or water efficiency.

The positive benefits of adequate K fertility are:

Deeper rooting. K helps plant roots penetrate to access deeper soil water.

Faster closing of the crop canopy. When the crop canopy closes, the ratio of transpiration to evaporation increases, which means more of the available water is used by the crop.

Greater osmotic gradient. The more K inside the plant cell, the better it can attract water from the soil, and control water loss.

Earlier maturity. Adequate K ensures plants get through to the critical pollination period before possible drought.

Stomata can open approximately seven times their original size. Therefore if potassium is deficient the plant will not receive adequate CO2, moisture and nutrients from the environment or foliar sprays. That will also affect the plants ability to cool itself. K is quite mobile in the plant and moves to the young leaves, therefore symptoms show in the old leaves first.

Potassium nutrients image
Potassium nutrients image

Only a fraction of potassium is available at one time

 

The visual signs of K deficiency are similar to nitrogen deficiency; yellowing of the pasture, browning off of the tips of grass blades, and prominent urine patches. If clover growth in urine patches is much stronger than in the rest of the pasture, K deficiency is likely.

Potassium is an alkalising mineral, so when potassium is low the fruit can be acidic and therefore taste sour. It is also due to the fact that potassium is not sufficient enough to transport good levels of sugars and nutrients into the fruit. The plants resistance to pathogens is reduced. Fungi tend to attack crops at low pH. Low pH simply means too much hydrogen and not enough nutrients.

About 90% of the potassium found in soils is insoluble. This source is released by microbial activity and weathering over time. K+ is attracted to, and hence stored on, the negatively charged clay colloid. The humus colloid has less attraction, but is also capable of storing potassium.

Potassium nutrients image

Severe potassium deficiency in Lucerne

Potassium nutrients image

Maize just starting to show potassium deficiency

 

Plants access potassium via the soil water. In sandy soils, there are very few storage surfaces for potassium, so some is carried away in the soil water. In light soils, (i.e. soils with a CEC<10) humus can make a significant difference in increasing the potential for potassium retention and storage. Leaching is particularly prevalent in light sandy soils where spoon-feeding potassium may be the better strategy. Animal urine contains 80-90% of the potassium excreted by animals.

Excessive magnesium leads to tight soils which in turn traps potassium in the clay layers. Improving the Ca% in the soil alleviates this problem. In high Mg soils, more K is required to reach that soils higher production potential. The problem of tight soils is alleviated by opening up the soil structure, leaving more space on the external colloid for K to attach to. When pH exceeds 6.5 there is no space for increasing K levels on the colloid. All spaces are filled up with existing cations rather than hydrogen, which can be readily bumped off. The same problem essentially occurs in dry soils where the soil solution and diffusion needed does not occur. K levels cannot be built when pH is 6.5 or higher; unless you use compost or manure. If you have a high pH, just use maintenance levels of K until the pH comes below 6.5.

When the base saturation of potassium gets greater than 7.5%, weed pressure increases, boron is tied up and often pastures become less palatable. Soil structure tends to decline because the K ion is dispersive (much like sodium), which contributes to poor soil structure and eventual collapse of the soil particles. Crops will struggle (particularly wheat) when the K% is greater than Mg.

Adding K to the soil is easily achieved for crops, but can be problematic where livestock are concerned. It needs to be done with caution and after magnesium and calcium levels have been addressed. It may be safer to apply up to 60 units after mating, but this does depend on individual farms’ soil balance, particularly having correct Mg & Ca levels.

With PAL test results, sodium should never be higher than K. If K + Na exceed 10% of base saturation, manganese uptake will be blocked. That may show up as excess bull calves born over heifer calves. When the soil is properly balanced, the ratio can be 60:40 in favour of heifers.

Potassium Sulphate is superior to Potassium Chloride! Potassium chloride raises the brix of many weed species, lowers the brix of many crop species and acts as a biocide.

More Energy. It costs more per unit, but delivers better value. Money spent on potassium sulphate (K2SO4) will purchase less fertilizer by weight than will one dollar spent on potassium chloride (KCl) but the dollars spent on potassium sulphate will buy significantly more crop growth energy. 47% of KCl is choride and of little use for crop growth. In contrast almost 100% of potassium sulphate is useable by the plant.

Lower Salt Index. K2SO4 has a salt index of 46. KCl is 116. The higher the index, the greater the chance of damaging germinating seeds, seedlings and soil biology.

Better Uptake of Potassium. Uptake of potassium requires it to be in the phosphate of potassium form. When there is an excess of chlorides the bonding of potassium with phosphate is blocked. The end result is less potassium uptake into the plant in the preferred form. The sulphate form does not overwhelm the soil solution with chloride ions and consequently more potassium is taken up by the plant.

Microbial Stimulation vs. Microbial Suppression Sulphates have a stimulating effect on the microbial system in the soil whereas chlorides at high levels are very hard on soil biology and are never recommended by creditable sources. A small amount of chloride is actually beneficial for soil microbes. This modest requirement is easily met by the 1-2% in potassium sulphate. High rates of chlorides can destroy soil carbons. Humus destruction leads to greater N leaching as soil N levels are dependent on humus levels. The better the humus percent, the more anions (particularly nitrate, sulphur and boron), held in the soil.

Plants and soils need Sulphur. Most soils are sulphur deficient. In order for plants to make oils and sulphur bearing amino acids such as cysteine and methionine the plants need an adequate supply of sulphur in the sulphate form. This is exactly what potassium sulphate supplies.

Better palatability. Pastures, crops, vegetables and fruit taste poorly when the potassium comes from potassium chloride (that’s right, cows prefer K2SO4). This happens because chlorides are also taken up by the plants. Fruit and vegetables grown with calcium chloride taste bitter; this is the main reason why children reject vegetables.

Less is more. The application of 100 kg of potassium sulphate will give a greater plant response than 200 kg of potassium chloride. This relates to a soil where the cations are balanced and soil potassium is adequate.

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