VAI TRÒ QUAN TRỌNG CỦA PHÂN KALI ĐỐI VỚI CÂY NGÔ – AGRICULTURE
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VAI TRÒ QUAN TRỌNG CỦA PHÂN KALI ĐỐI VỚI CÂY NGÔ

Posted On October 19, 2016 at 10:36 am by / 346 Comments

Ngô là cây lấy hạt như cây lúa, nhưng lại được trồng trên đất màu, có độ cao cao hơn đất lúa nước. Ngô có cường độ quang hợp cao hơn cây lúa, điểm bù ánh sáng thấp hơn nhưng điểm bão hòa ánh sáng lại cao hơn…

Ngô là cây lấy hạt như cây lúa, nhưng lại được trồng trên đất màu, có độ cao cao hơn đất lúa nước. Ngô có cường độ quang hợp cao hơn cây lúa, điểm bù ánh sáng thấp hơn nhưng điểm bão hòa ánh sáng lại cao hơn, trong lúc đó không có hô hấp ánh sáng nên sản phẩm quang hợp không bị tiêu phí nhiều như cây lúa.

Nhờ vậy, cây ngô thường cho năng suất sinh vật và năng suất kinh tế cao hơn. Do đó, ngô cần được cung cấp dinh dưỡng cao hơn lúa, thường là gấp 1,5 – 1,8 lần.

Ví dụ, cây lúa mức đạm chỉ cần bón trong khoảng 80 – 100kg N/ha, trong khi cây ngô cần cung cấp từ 130 – 180kg N/ha, tùy loại đất, trung bình khoảng 140 – 160kg N/ha mới thỏa mãn nhu cầu sinh lý của ngô, nhất là ngô lai.

Ví dụ, một thí nghiệm bón phân cho ngô lai của Viện Nghiên cứu ngô thực hiện trên đất đó Đồng Nai có 7 công thức khác nhau, thứ tự:

1/120kg N – 80kg P205 – 80kg K20;

2/120kg N – 80kg P205 – 100kg K20;

3/150kg N – 80kg P205 – 80kg K20;

4/150kg N – 80kg P205 – 100kg K20;

5/180kg N – 80kg P205 – 80kg K20;

6/180kg N – 80kg P205 – 100kg K20;

7/150kg N – 80kg P205 – 60kg K20 làm đối chứng.

Như vậy tác giả có 3 mức N, chỉ có 1 mức P và 2 mức K so với mức 60kg K làm đối chứng. Tác giả đã đo đếm, tính toán hơn thiệt và cho thấy ở công thức bón 180kg N – 80kg P205 và 80kg K20/ha cho năng suất ngô hạt khô cao nhất là 8.053kg/ha; công thức 6 có liều K cao hơn (100kg K20/ha) nhưng năng suất vẫn thua công thức 5 (7.772kg/ha).

Công thức 1 cùng mức P và K nhưng liều đạm chỉ có 120kg N thì năng suất thấp hơn các công thức thí nghiệm khác (6.299kg/ha), nhưng vẫn cao hơn công thức đối chứng chỉ có 5.972kg ngô hạt/ha dù mức N cao hơn công thức 1, nhưng lượng K chỉ có 60kg/ha.

Từ đó tác giả rút ra kết luận là trồng ngô lai LVN666 trên đất đỏ Đồng Nai, cần đến 180kg N, 80kg P và cũng chỉ cần đến 80kg K/ha là thích hợp không cần bón đến 100 hay 120, 130kg K/ha.

Để đánh giá rõ thêm về nhu cầu K đối với cây ngô lai cũng trồng trên đất đỏ Đồng Nai, bạn đọc cùng tham khảo kết quả thí nghiệm của TS Đỗ Trung Bình và cộng sự đã sơ kết thí nghiệm vào năm 2014, sau 8 vụ thí nghiệm với ô khuyết trên nền phân NPK là 150-100-120/ha.

Sau vụ thứ nhất kết quả cho thấy thiếu K chưa gây ảnh hưởng xấu đến năng suất so với thiếu N và P.

Ví dụ, nền trồng chay không có phân chỉ hưởng chất dinh dưỡng tồn lại trong vụ trước, đạt được 6.080kg ngô hạt, nền thiếu N cho năng suất 6.340kg, thiếu P cho năng suất 6.730kg, còn thiếu K thì năng suất thu được 7.230kg.

So với nền phân đầy đủ là 7.410kg ngô hạt thì chỉ giảm có 180kg/ha, sự hơn kém này chỉ nằm trong phạm vi sai số.

Nhưng trồng đến vụ thứ hai mà không bón K thì năng suất đã giảm so với nền bón phân đầy đủ khá rõ. Thiếu K cho năng suất 5.950kg/ha, còn bón phân đầy đủ cho năng suất 7.240kg/ha, giảm đến 1.290kg ngô hạt/ha.

Từ vụ thứ 2 trở đi nếu tiếp tục không bón K thì năng suất ngô tiếp tục giảm rõ rệt. Đến vụ thứ 8, thiếu K năng suất ngô chỉ còn khoảng một nửa so với nền bón phân đầy đủ. Nền bón phân đầy đủ thu được 6.350kg/ha thì nền thiếu K chỉ thu được 3,150kg/ha bằng 49,6% so với nền bón phân đầy đủ (giảm hơn 50% năng suất).

Trong thí nghiệm này không có ý so sánh liều lượng K mà chỉ đặt trọng tâm đến vai trò K đối với ngô là chính. Kết quả trong thí nghiệm để xác định thêm vai trò K đối với cây ngô trên đất đỏ, dù là đất có độ phì cao, nhưng K dễ tiêu không nhiều thì vụ nào cũng cần bón K cho ngô, khác với lúa trồng trên đất ĐBSCL được ngập nước và được nhận phù sa cũng như vụ nào cũng trả lại gốc rạ cho đất nên không thiếu K.

Trên đất đỏ, nhu cầu K đối với cây ngô là vậy, còn trên đất xám thì sao? Cũng chung với thí nghiệm trên đất đỏ Hưng Lộc, Đồng Nai. Tác giả đồng thời tiến hành thí nghiệm trên đất xám Tây Ninh, nền phân khoáng là 140kg N + 120kg P205 và 130kg K20/ha.

Nhận thức được đặc tính dinh dưỡng của cây ngô, Cty CP Phân bón Bình Điền đã cho ra sản xuất cặp phân NPK bón cho ngô trên nhiều loại đất là Đầu Trâu tăng trưởng NPK 19-12-6+TE bón thúc 2 đợt đầu và Đầu Trâu chắc hạt NPK 16-6-19+TE bón thúc trước khi ngô xoắn nõn rất phù hợp.

Kết quả thí nghiệm cho thấy, ngay vụ đầu tiên, công thức không bón phân hay các thí nghiệm ô khuyết (thiếu N, P và K) đều làm giảm năng suất ngô rất nghiêm trọng. Ô đối chứng, không bón phân, do tính chất đất nghèo dinh dưỡng nên ngay vụ đầu đã giảm năng suất so với nền bón phân đầy đủ đến 4.010kg ngô hạt (giảm 60%). Các ô khuyết thì thiếu N giảm năng suất nhiều hơn thiếu K, và thiếu K giảm nhiều hơn thiếu P.

Trên đất xám đối với ngô lai, ở vụ đầu thì N là yếu tố hạn chế số 1, tiếp đến là K, còn P mức độ hạn chế thấp hơn. Trong thí nghiệm này thiếu P chỉ giảm năng suất so với đối chứng 390kg hạt/ha (giảm 5,6%). Còn thiếu K ngay vụ đầu giảm năng suất đến 28,3%, giảm nhiều hơn thiếu P. Nhưng nếu không bón K cũng như P, bình quân 8 vụ thì thứ tự trên được thay đổi.

Nghĩa là năng suất ngô giảm do thiếu P nhiều hơn, giảm 74,8% so với nền bón đầy đủ NPK. Còn thiếu K, năng suất giảm sau công thức thiếu P, nhưng cũng giảm đến 61,6% so với nền bón đầy đủ NPK.

Như vậy, bức tranh về dinh dưỡng đối vơi cây ngô lai trên đất xám, bình quân 8 vụ, ô thiếu N cũng giống như trồng chay không bón tí phân nào. Còn mức giảm năng suất được xếp là thiếu P > thiếu K > thiếu N > không bón phân.

Ở thí nghiệm này, đến vụ thứ 8 thì khái niệm bón phân cân đối thể hiện rất rõ. Chỉ nói đến trong phạm vi 3 yếu tố đa lượng, trên nền đất vốn có độ phì nhiêu thấp, càng bón mất cân đối thì các vụ sau những nguyên tố bị thiếu trở thành nhân tố quan trọng làm giảm năng suất cây trồng càng rõ.

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