Keep up to date with the latest potato and field vegetable information with this advice from Yara's field crop specialist Tom Decamp
It is well-known that sulphur deposition has decreased to almost nothing meaning no more sulphur is deposited on the land – sulphur applications are therefore a must.
When thinking about sulphur on potatoes there is, of course, the important relationship between sulphur and nitrogen. Nitrogen can’t be taken up by the plant or utilised as efficiently if there is a deficiency in sulphur. The N:S of uptake per tonne of yield in potatoes is 8.5 which is similar to cereal crops and trials have shown up to 15% yield increases from 40-75kg SO3 sulphur applications to potatoes.
However, does sulphur have another important part to play in the potato crop?
Quality is very important when growing potatoes, especially their skin finish, therefore common scab can be a real issue. Common scab is more prevalent on high pH soils (alkaline) therefore the theory is that acidifying the area around the tuber will help control common scab. Sulphur helps to acidify the soil therefore applications of sulphur should help with control.
Sounds simple then – apply sulphur and you won’t get common scab…unfortunately it’s not as easy as that. Trials work Yara did showed that there was no consistent effect of applying sulphur for common scab control at the usual 40-75 kg SO3. Further work on higher rates, 150kg SO3 +, showed more promise, but even at these very high rates there was only a marginal effect on the scab.
So, when thinking of sulphur on potatoes it should be to increase yields and nitrogen use efficiency and not common scab control. The most effective method of controlling common scab is therefore still irrigation, ensuring the moisture reaches the depth of the tubers.
With lifting about to start this is maybe a good time to reflect on potato yields and look ahead to next season before lessons are forgotten.
There are yield benefits from placing fertiliser at planting rather than broadcasting and from using compounds rather than straights as well as from optimising calcium, phosphate and zinc nutrition; here are some of the highlights from recent trials
Over the years Yara has carried out a large number of trials on the application of fertilizer to the potato crop, focusing on placement at planting. This has consistently given yield benefits, averaging 10.8% over 18 years of work. With continued legislation driving more efficient use of fertilizers such practices will become even more important. Applying fertilizer to the planted areas and not broadcast all over must be more acceptable from an environmental risk point of view.
Application accuracy is a key focus for Yara, particularly in the potato sector. YaraMila compounds can bring benefits over straights for the delivery of nutrients to the crop, with a 7.8% yield increase over straights in a long-term trial at Hanninghof Research Centre.
Recently Yara has also looked at the benefits of supplementing the basal nitrogen applications with foliar treatments. Additional nitrogen can be supplied using dilute applications of Chafer Nufol to help maintain a green leaf canopy and increase tuber size and yield . When compared with soil applied top-dressings significant improvements in nitrogen utilisation can be achieved, as we saw from this year results with an 8.3% yield increase.
Other trial work carried out by Yara includes the benefits of calcium for tuber quality, foliar phosphate applications to improve tuber numbers and tuber size and just recently, the impact of zinc, where in 2014 a 1l/ha application of YaraVita Zintrac 700 lead to a 5% yield increase.
All brassicas have a high requirement for boron and deficiency can cause poor crop establishment together with increased plant losses and ultimately reduced yield.
Boron has several key roles in plants which include cell wall biosynthesis, carbohydrate and protein metabolism, cell division/elongation and also root and shoot growing points.
Root and shoot development are obviously key to establishing the plant early on to ensure that it gets off to the best start to help combat pests and diseases. Also during this period the plant is growing very rapidly and therefore having a sufficient amount of boron present is essential to keep the root and shoot development rate optimum.
Boron is the only nutrient which, when in short supply, can accelerate physiological processes instead of reduce them and therefore abnormal growth can occur. Symptoms of boron deficiency often start on the younger leaves of the plant and they appear crinkled, deformed and brittle. Necrotic patches may appear at the leaf margins and the plants may appear stunted. Stems can also be affected with hollow lesions occurring and splitting of the stems. Some of these symptoms can be made worse by sandy, alkaline soils low in organic matter as well as high levels of calcium present.
Vegetable Brassicas require boron in quite large amounts where it helps control nutritional disorders such as tip burn and hollow heart. Find further information on the symptoms and causes of Boron Deficiency in Vegetable Brassicas
Yara brassica trials have also highlighted the synergy between calcium and boron, improving the uptake of each nutrient and improving the control of associated disorders, not only in brassicas but also in other crops showing a response to calcium and boron such as potatoes.
YaraLiva NITRABOR contains a balanced combination of soluble nitrate nitrogen, calcium and boron that can be safely applied to green crops right up to harvest without fear of scorch to enhance both the quality and marketable yield of vegetable brassicas.
The potato crop requires potassium in large quantities, 50% more, in fact, than nitrogen. A 38.5t/ha crop can remove more than 120kg/ha of nitrogen whereas it can remove over 200kg/ha of potassium. Both of these macronutrients are important throughout vegetative growth, tuber formation and bulking.
Nitrogen is important for leaf and tuber growth and is recycled from lead to the tuber during bulking, the same can be said for potassium. When thinking of potassium then as you can see from the removal values that it is key for high yields and maintaining tuber integrity.
Phosphate is also required in relatively large quantities during early growth due to its importance in root and shoot development and tuber set but also later on in the season for bulking.
Calcium is a crucial nutrient as it plays a role in important quality parameters. Potatoes need calcium to strengthen the skins of the tubers; providing better resistance to many diseases (black scurf, silver scurf and common and powdery scab) and also a better skin finish. Calcium deficiency also causes internal rust spot so it’s essential to apply calcium at the right time but also ensure the correct source of calcium is used. For example, liming materials won’t supply plant-available calcium to the crop therefore it won’t supply sufficient supply to meet the demand.
Magnesium is important during tuber bulking where it plays a major role in maintaining tuber quality and is therefore a key nutrient.
Micronutrients are also important to ensure the crop has all it needs to produce a good, quality yield. Zinc and manganese are key to help the crop’s defence against powdery and common scab as well as maintaining skin finish.
UK soils have changed a lot in the last few decades; whether it’s the 97% sulphur deficiency found in soils sampled or the low organic matters across arable land. Therefore everyone knows the importance of taking a soil sample; but should you be investing in more than just basic soil analysis?
The basic analysis will give you P, K, Mg and pH; which is a good start but what about Ca, S, Mn, Cu, B, Zn, Mo and Fe? All are important to plant nutrition and you may be unaware that your soils are low in one or more of these elements. Soil sampling is about finding the limiting factors on your farm and being able to make sure they aren’t impacting upon yield – think Liebig’s barrel!
Yara Analytical Services have processed over 20 million samples. This huge dataset has shown that after removing all the soils with limiting factors such as P < index 2, pH <6.5 and micronutrient deficiencies then less than 20% of UK arable soils growing wheat have sufficient levels. When you look at soils sent in for oilseed rape fields then even less are sufficient.
By taking a broad spectrum soil sample you’ll know about these issues before it’s too late – when symptoms appear and yield is already taking a hit. Liebig’s law of the minimum states “A deficiency of any single nutrient is enough to limit yield”. With yields being pushed further, whilst costs savings are sought (such as P and K holidays) you could be limiting the effectiveness of any increased nitrogen applications.